A111D2 Mfi71E7

A1 1102487127

Hanaon, D. Wayna/lnduatrlal time service OC100 USfl NO.B6-3042 V1986 C.1 NBS-PUB- lAL TIME SERVICE STUDY

NBS

Reference publications

D. Wayne Hanson

David A. Howe

National Bureau of Standards

U.S. Department of Commerce

Boulder, Colorado 80303

February 1986

-QC

100

.056

86-3042

1986

NBS sesearch information

CRiYTER NBSIR 86-3042

INDUSTRIAL TIME SERVICE STUDY

D. Wayne Hanson

David A. Howe

Time and Frequency Division

Center for Basic Standards

National Measurement Laboratory

National Bureau of Standards

Boulder, Colorado 80303

February 1986

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

CONTENTS

Page

INTRODUCTION 1

PHASE I: REQUIREMENTS FOR A HIGH ACCURACY 3

TIME AND FREQUENCY SYSTEM

PHASE II: A REVIEW OF POSSIBLE TIME SERVICES 8

PHASE III, PART 1: THE FIXED SATELLITE SERVICE, 6l

A PRELIMINARY DESIGN

PART 2: THE GLOBAL POSITIONING SYSTEM 125

OPTION

PART 3: the GEOSTAR OPTION U3

SUMMARY 1 76

REFERENCES 180

APPENDIXES 181

iii

INDUSTRIAL TIME SERVICE STUDY

D. Wayne Hanson and David A. Howe

Time and Frequency Division

National Bureau of Standards

Boulder, Colorado 80303

This study examines options for delivery of accurate time and

frequency information to industrial users. The study is sponsored by the

Bonneville Power Administration (BPA) who finds a need for accurate timing

to the one microsecond level. Prospective existing and future

dissemination methods (Loran-C, GOES, USRDSS, GPS, etc.) are discussed in

detail. The study produces a system architecture and preliminary design

for a new time service using the widely available U.S. fixed satellite

service (FSS) in which customers shall assume full costs of its operation

through subscriber fees.

The study elaborates on three viable options: (1 ) FSS, ( 2 ) GPS, and

( 3 ) USRDSS. Based on this study, conclusions can be drawn regarding a timing system which will most satisfactorily meet the long range goals of

most industrial users.

Key words: accuracy; frequency; satellite; time; time broadcast services;

time dissemination

Commercial companies are identified in this report in order to adequately

discuss issues. Such identification does not imply recommendation or endorsement by the National Bureau of Standards, nor does it imply that any

identified entity is the only or the best available for the purpose.

V

INDUSTRIAL TIME SERVICE STUDY

INTRODUCTION

This report is a comprehensive study of accurate time dissemination

options which can satisfy present and future requirements for the

Bonneville Power Administration (BPA) . Over the years, the BPA has

explored the use of precise time synchronization as a means of better

fault location and improved system stability. In addition to its own

continuing analyses, BPA contracted with the National Bureau of Standards,

Time and Frequency Division, to study suitable time distribution systems

and asked NBS to make recommendations based on the study.

Many of the existing time services are unsatisfactory solutions to

the BPA for reasons discussed in this study. Furthermore, the future

communication and navigation services, which directly affect prospective

options for solving the time transfer problem for the BPA, are in a state

of continuing technical and policy changes at this time. Nevertheless,

the study has done a useful evaluation of what can be done for the BPA.

The report consists of four phases of study. It starts (Phase I)

with a brief review of requirements for the BPA and NBS. That is followed

by general discussions of any and all viable or closely viable timing

systems (Phase II). Since no system will be a perfect solution, this

phase narrows the options to those which have the least risk and

1
fewest problems. Considerably more detail is presented concerning three

possible options in the following phase (Phase III) . The report then

concludes with a summary.

This study has gone into great detail in some areas, particularly

Phase III, which itself has three parts. This was done because Phase III

presents three separate and viable options in enough detail that the

sponsor (BPA) could arrive at its own conclusions. In addition, a

preliminary design for a timing system using the Fixed Satellite Service

(FSS) is extensively discussed. A combination of market demands,

technical advances, FCC policy changes, and satellite receiver cost

reductions led to further investigation of a FSS-based system.

2
PHASE I. REQUIREMENTS FOR A

HIGH ACCURACY TIME AND FREQUENCY SYSTEM

CONTENTS

A. PHASE I INTRODUCTION AND OBJECTIVES

Page

B. POLICY REQUIREMENTS 4

C. FUNCTIONAL AND OTHER TECHNICAL REQUIREMENTS 6

3
A. PHASE I INTRODUCTION AND OBJECTIVES

The following is a statement of requirements for the "high accuracy time

and frequency system" as provided for in the Agreement between the

National Bureau of Standards (NBS) and the Bonneville Power Administration

(BPA) - Agreement No. DE-AI79-84BP19476 . The statements of requirements

are divided into two parts -- a policy section and a functional section.

The policy section outlines those requirements imposed upon services

provided by government by administration policy and by NBS management to

control unexpected financial liability. The functional requirements are a

composite derived from those requirements by BPA for such a service as

provided in the agreement, by NBS technical considerations given its

experience with similar systems, and through long term and extensive

contacts between time and frequency users and NBS.

Throughout this document the system of interest will be referred to as the

Industrial Time Service or the ITS.

B. POLICY REQUIREMENTS

Certain administration requirements must be met in order for the NBS to

enter into a time and frequency service such as the ITS.

1 . The ITS must generate through user charges sufficient funds to

completely recover all costs associated with the establishment,

operation and maintenance of the service.

4
2. The ITS must be designed, implemented and operated in such a

manner that no significant potential exists for large, unexpected

cost to be incurred through failures or other uncontrollable

events

3 . The ITS must be configured so that, at the discretion of NBS,

direct NBS involvement may be substituted by contract to or

transfer to the private sector.

These are policy matters which protect NBS from any financial risk and

inability to control the service. Short term funding is needed to start

the service, but it is the long term projections of operating funds which

must be in line with NBS expenses.

Policy requirements as they are do not restrict the development of the ITS

in any way given adequate demand, and hence funding, for such a service.

Policy is in place to reduce financial risk and assure that the benefitting sector of the economy bears the cost of service received. NBS

expertise and history are precisely in the area of time and frequency

generation and dissemination, and all efforts will go toward proper

implementation of the ITS.

With regard to item 3, ideally the removal of NBS involvement would not

seriously affect the continuing operation of the ITS and service to its

users. It must be pointed out that financial considerations alone do not

affect the decision concerning ongoing involvement by NBS. NBS must

5
maintain full control of its’ own resources, and its ongoing direct

involvement cannot be guaranteed. Again, this does not take anything away

from development of the service.

C. FUNCTIONAL AND OTHER TECHNICAL REQUIREMENTS

The following are functional and technical requirements for the ITS.

Specific receiver requirements as provided by BPA in the Agreement can be

guaranteed by these requirements and conventional and economical receiver

design.

1 . PROVISIONS OF THE ITS

a. Time of Year information in the form of a time code repeated

at a rate of at least once per minute.

b. The ITS shall provide synchronization to UTC(NBS) throughout

CONUS to a precision of 0.1 ps or better.

c. The ITS shall operate continuously with availability of

99.7/S averaged over one year.

d. The ITS shall provide information relating to the differences

between UTC(NBS) and UT1 and shall provide notification of a

leap second during the month of occurence.

6
e. The ITS shall require the use of an antenna no greater than

1 meter in any dimension to achieve all stated requirements.

f. The ITS shall require of the user no operator or other

assistance once a properly set up receiving system is in

place (outside of user equipment failure).

7
PHASE II: A REVIEW OF

POSSIBLE TIME SERVICES

CONTENTS

D. PHASE II INTRODUCTION AND OBJECTIVES

1 . Accuracy

2. Unambiguous UTC Time Code

3. Coverage

4. Status

5. Operational Time

6. Receiver System Costs

7. Operator

8 . Antenna Package

9. NBS Operation and Maintenance

Costs for ITS

10. Subscriber Fees

11. Long Term Prospects

E. FIXED SATELLITE SERVICE 16

F. BROADCAST SATELLITE SERVICE (BSS) 23

G. NAVSTAR GPS 28

H. RADIODETERMINATION SATELLITE SERVICE 31

I. LORAN-C SYSTEM 37

J. TRANSIT 41

K. GOES 50

L. CONCLUSIONS OF PHASE II 59

Page

9

8
D. PHASE II INTRODUCTION AND OBJECTIVES

The purpose of this phase of the study is to examine systems that have the

potential of meeting some or all of the requirements for the Industrial

Time Service (ITS). The system or systems that appear to meet all

requirements for the ITS will be identified and will be the subject of

more study and development in the next phase of this study. The systems

under examination include those existing, under physical development, or

in the planning stages. These systems either already provide signals

intended for time dissemination, signals that inherently contain

synchronization information that can be related to the NBS time scale, or

systems to which time dissemination service can easily be added. These

systems include navigation or position location systems, data transmission

systems and broadcast systems. The results of this phase of the study are

summarized at the conclusion section of this report as well as in Table 1.

Many of the options discussed in this phase for the industrial time

service are developed to a degree that allows a fair evaluation of their

capabilities. Such systems are Loran-C, OMEGA, Transit and GPS. However,

important options for the ITS involve the use of geostationary satellites,

and in this area there is considerable change and growth in markets. As a

result of future needs a new segment in communications is emerging at a

9
rapid rate. This is the Radiodetermination Satellite Service (RDSS). RDSS

based systems are discussed in this phase but in a preliminary manner.

RDSS systems appear to be in only a planning stage, and no such systems

may evolve. In addition, if such services evolve, they may not be able to

carry appropriate time and frequency information for any one of several

reasons. Therefore the discussion of RDSS-based ITS is exemplary only.

The RDSS for the United States is described in preliminary form in

Appendix II, CCIR Study Draft of 518~3, April, 1985.

Except for RDSS systems, well developed ITS options are discussed in this

phase of work. Each will be described and compared against the technical

and operational requirements determined in Phase I of this study.

By itself, the market for precise time and frequency distribution is

limited. However, a large market exists for systems which allow precise

navigation and a variety of navigation systems are in service or are

proposed. Future high accuracy navigation systems are directed toward

techniques involving precise time distribution from several locations in

order to do position location. The better services for time and frequency

distribution are usually linked to some sort of navigation system. Since

the two disciplines work together, the fact that proposed time and

frequency distribution systems are actually derived from radio navigation

systems adds uncertainty to the long term viability of a time and

frequency distribution service. This is so because the objective of the

radio systems is navigation first with only secondary regard for the

10
timekeeping capability to the end user of such systems. In fact, the need

to periodically coordinate with some form of time reference is considered

an added maintenance function for navigation systems. Unfortunately radio

navigation systems find it counterproductive to service the time and

frequency users because of the limited market and added maintenance of

time coordination with a reference such as NBS or USNO. One concludes

then that the most suitable kind of industrial time service is one

specifically designed for time and frequency distribution which is

technically workable and at the lowest possible cost overall.

The systems examined in this study are evaluated against some of the

requirements and other parameters relevant to choosing between various

system options for the ITS. The requirements of major importance are

shown in the first column of Table I. They include:

D1 . ACCURACY

Accuracy is the measure of how closely the system will deliver

UTC(NBS) time to the user. This specification has been set at one

microsecond as minimum acceptable performance. This level applies

under all operating conditions or requirements that follow; i.e. on a

continuous basis, over the CONUS, using a receiving system costing

$3,000 or less, etc. In cases where a manufacturer or system

operator quotes an accuracy figure, that figure is used for this

aspect of the study.

11
D2. UNAMBIGUOUS UTC TIME CODE

This provision requires that the system transmits a time code giving

the date including days, hours, minutes and seconds that is

unambiguous to at least one year. It must be a uniform time code in

that discontinuities such as the leap second do not occur without

prior notification in the code during the month of its occurrence.

D3. COVERAGE

The coverage of the signal must be the continental United States

(CONUS) as a minimum with greater than 99^ availability.

D4. STATUS

The "status" is not a requirement but is simply a comment giving

information about a system’s capability to provide its intended

purpose and when.

D5. OPERATIONAL TIME

The operational time is the time that the system can perform and

deliver all of its applicable characteristics and specifications.

The ITS requirement calls for full time operation with a small

allowance for signal loss for any reason {< 1 % on the average

and always less than a few hours at the worst at any one time)

12
d6. receiver system costs

Receiver system costs include the costs for antenna, preamplifier,

cables, receiver and external oscillator if required. It represents

an estimate of the complete cost of the receiver hardware reflecting

current prices.

D7. OPERATOR

The operator is the organization responsible for the operation of the

source or delivery portion of a system. The objective of a system

may or may not include time and frequency dissemination. A second

operator may be required to operate any time and frequency capability

or subsystem. For example, this is the situation for GOES, where NBS

is responsible for the operation of the time code and NOAA, Department

of Commerce, is responsible for the overall operation of the

satellites and its uplinking facilities. A similar situation would

exist for the FSS and BSS entries.

D8. ANTENNA PACKAGE

This description includes the weight and physical size of the antenna

system. Many of the antennas used in the systems discussed must be

mounted on masts or similar structures. The dimensions of all

supporting structures are not included here.

13
D9. NBS OPERATION AND MAINTENANCE COSTS FOR ITS

The annual costs for operating the time and frequency portion of the

system are estimated here. For most systems, the time and frequency

information is a by-product of a navigation system and is available

at essentially no cost to the user. In many cases, the cost is

not separable from the costs of operating the navigation portion of

the system. This aspect is important to this study only when NBS or

another such agency provides the time and frequency function.

DIO. SUBSCRIBER FEES

If the private sector provides time-and-frequency information, it

must recover costs plus profit through subscriber fees. It is policy

now that government also realize full cost recovery through user

fees. This will apply to new and proposed time and frequency

services. At present no time and frequency broadcasts have user fees

associated with them mainly because of the difficulty in denying

access to those who do not pay any fees. New systems, however, can

be designed to make charges possible to specific users with no access

available to non-users.

D1 1 . LONG TERM PROSPECTS

Presented here are brief comments on the viability of each system and

on where and what the system may be in the foreseeable future. The

ITS study requires a reasonable expectation that the system have a

14
lifetime of 20 years. In some situations this lifetime refers to the

principal service of the system assuming that the addition of a time

and frequency capability is always possible throughout that lifetime.

For instance, this is true with regard to the lease of a channel on a

domestic communications satellite (the FSS option).

15

Table

1.

Comparison

of

Time

Services

15 -a
E. FIXED SATELLITE SERVICE

The Fixed Satellite Service is a radio communication service between earth

stations at specified fixed points where one or more satellites are used.

In some cases this service includes satellite-to-satellite links which may

be configured in the intersatellite service. The fixed satellite service

may also include feeder links for other space radio communication

services.

This service is mainly for telephone, telegraph, telegram and telex trunk

services as well as television distribution. More recently, new services

have included high speed transmission of documents, data transmission

between computers, audio, and video teleconferencing.

International examples of the use of the FSS are INTELSAT and INTERSPUTNIK. Neither of these international systems provide CONUS coverage

from one satellite. Furthermore, CONUS coverage is not possible with

either system using any number of their operational satellites.

A number of domestic satellite communication systems have been put into

operation since its inception in the U.S. in 1975. Many more are planned

for the future. Table 2 shows the presently authorized FSS systems

serving the U.S. and Table 3 shows those that applied with the FCC before

the November 1983 cutoff. More systems are expected to be added once the

backlog of applicants from the 1983 action has been acted upon.

16
TABLE 2.

AUTHORIZED SYSTEMS (FIXED)

Advanced Business Communications Inc.

American Satellite Company

American Telephone and Telegraph

GTE Satellite/Spacenet

Hughes Communications

Mobile Satellite Corporation

Rainbow Satellite Inc.

RCA Americom

Satellite Business Systems

United States Satellite Systems

Western Union

17
TABLE 3.

PROPOSED SYSTEMS AND SATELLITES

(November 1983 Applicants)

General

Alascoran, Inc.

American Satellite

American Telephone & Telegraph

Cablesat General Corp.

Columbia Communications Corp.

Comsat General Corp.

Digital Telesat

Equatorial Communications Systems

Federal Express

Ford Aerospace Satellite Services Corp.

GTE Satellite Corp.

GTE Spacenet Corp.

Hughes Communications Galaxy, Inc.

Martin Marietta Communications Systems, Inc.

Mobile Satellite Corp.

National Exchange, Inc.

Rainbow Satellite, Inc.

RCA Americom

Satellite Business Systems

Systematics General Corp.

18
The FSS is a service intended to support the point-to-point transmission

of communications on a bulk basis. It is not intended for broadcast

services, although recent applications are using it for broadcast service.

No time services exist today using the FSS although some audio services do

carry time information on a par with the typical time-and-temperature

messages available on the telephone in urban areas accompanied by either

commercial messages or operated on a pay basis, typically 25 cents per call.

Satellites operating in the other services have been used both

experimentally and operationally for time and frequency dissemination

with very acceptable levels of performance. In the early 1970's an

experimental NASA satellite, ATS-3, was used to transmit a slightly

modified WWV format to large geographical areas of the Western Hemisphere

with results for the first time demonstrating superior time performance

related to accuracy, reliability of signal transmission, simplicity, low

cost of user equipment, and signal recovery. These early experiments led

to the GOES service (discussed elsewhere in this report) which has been

providing a full time code to the Western Hemisphere since 1975. Both of

these examples illustrate the use of dedicated facilities for which there

is very limited or no backup (redundancy).

The use of the FSS is much like the simple lease of a telephone line.

The FSS has many operators competing for users' business including long

term protected leases. This means a time service can be implemented

through a long term lease of a channel on someone's existing satellite.

The lease can be the result of a competitive procurement guaranteeing the

19
lowest price for a specified level of service. No special equipment

needs to be designed or placed into orbit over what is already there in

abundance.

Through (1) a careful examination of the technical characteristics of the

FSS transponders, (2) a review of work done on satellite tracking

techniques applicable to a time service concept, (3) noting the existing

telecommunications and broadcasting applications now implemented in the

FSS, (4) a review of equipment costs and trends, and (5) past experiences

with implementing satellite services, we at NBS feel, with reasonable

certainty, that the performances summarized in Table 1 for a time service

using the FSS can be achieved. These expectations do, however, require

detailed study and experimentation to verify, some of which will be done

in the next phase of this study assuming enough merit in the FSS option

relative to the other options is found to continue.

The FSS provides facilities for telecommunications activities. These

facilities are highly reliable and can be provided on a continuous basis

even during periods when the sun would be eclipsed by the earth, thereby

cutting off the source of energy from the satellite's solar collectors.

All satellites in the FSS serving the U.S. provide at the minimum CONUS

coverage with many giving additional coverage of Hawaii, Puerto Rico and

Alaska. Since the antenna pattern cannot provide sharp cutoffs in its

pattern, parts of Canada and Mexico can also experience similar signal

levels near our borders with decreasing levels as one moves away from

these boundaries. Continuity of service is guaranteed by use of backup

satellite circuitry and, if necessary, a backup satellite.

20
The principal accuracy of this time signal is a matter of final signal-to-noise ratio in the user's receiver and accurate information on the

position of the FSS satellite. (Equipment systematlcs can be calibrated

and are comparatively small.) Preliminary calculations have shown that

sufficient S/N ratios can be achieved for certain signal structures to

yield better than one tenth microsecond precision using antennas of less

than one meter in diameter. These calculations hold true for both the C

band and the KU band of the FSS. Recent NBS analysis of tracking

experiments done by General Electric, under contract to NASA, on some of

NASA's Applied Technology Satellites (ATS) in the 1970's indicate that

adequate tracking can be accomplished by a tracking system suitable for

the ITS concept and mode of operation. This analysis coupled with more

recent filtering techniques leads the authors to believe a low cost,

simple, and reliable tracking system can be developed for the ITS to

guarantee nearly 0.1 microsecond delivery accuracy. Preliminary

examination of the use of the Data Encryption Standard (DES) in the ITS to

deny delivery of time to nonpaying users has been favorable. The DES has

been implemented on a single chip costing twenty to fifty dollars

(depending upon speed) to which a "key" (decode authorization) can be sold

periodically to each user, thus making cost recovery feasible. The number

of customers one can reasonably expect remains unanswered and is certainly

a function of overall annual costs. We think that for a receiver costing

$2500, a 10 % annual charge of $250 per year would be acceptable to most

users. The GOES broadcast has nearly 2000 receivers in the field without

21
any NBS promotion for its use. A similar number of ITS customers should

be possible with

that the service

active NBS promotion of its use especially considering

will be meeting many more users' needs.

22
F. BROADCAST SATELLITE SERVICE (BSS)

The Broadcast Satellite Service is a system of direct broadcast of signals

to the general public. Signals are relayed by satellites generally at

geostationary orbital altitude and intended for direct reception to the

recipient. In the BSS, signals may either be received directly by

individuals or may be received by a community receiver for distribution to

other recipients. In the literature this system is also referred to as

the Direct Broadcasting Satellite Service or DBS Service. These systems

are generally designed to handle NTSC video signals since the primary

objective is to provide information and entertainment program services.

Given the wide available bandwidth other signals may be added (multiplexed) with the normal video and audio transmissions on a noninterfering

basis given appropriate approvals. Thus an opportunity exists for

additional transmission of a time code.

For home reception the receiver antenna will be small for aesthetic,

structural, and cost reasons. A home antenna of less than 1 meter in

diameter is the system goal. With a small receiver antenna a burden

is placed on the relay satellite to produce enough transmitted power so

that the energy incident upon receiving antenna is sufficient for full

reception. Larger receiving antennas may be used in other situations.

For example a community receiver may be used to distribute a signal to

more than one individual. In this way the costs are spread over the

number of recipients and there is more flexibility in choosing an optimum

antenna location. Community receivers may be used in cable systems,

educational facilities, and hotels or apartment houses.

23
Although considerable groundwork has been laid in the development of a

specific BSS planned for the CONUS, there is no routine service in place

now. Small systems have been used in the United States to validate

concepts. Operational systems do exist in Japan and parts of Europe,

however. A specific BSS planned for the United States would have the

following major aspects based on the primary objectives of the system.

(Other planned systems may have somewhat different characteristics.) Independently programmable satellites would be used for each of the four

time zones of the contiguous U.S. The four operating satellites would

provide downlink transmissions in the 12 GHz BSS band with a typical EIRP

of 57 dBW. Uplinks are in the 17 GHz BSS band. Satellite orbital

locations would be selected to maximize relay capacity and spectrum use

while retaining acceptable eclipse times and elevation angles. Operationally, additional satellites would be in orbit as spares in a full

nationwide implementation of the BSS. The spares would allow restoration

of service within a few minutes to highly populated areas if any primary

satellite failure should occur. The spares would be maintained in such a

way that they could be relocated and reoriented to provide full backup to

any failed satellite.

A central uplink location would be used for the majority of transmitted

programs. Although it is possible to have multiple access it is not

entirely desirable during high viewing periods. Multiple access may be

used during low viewing periods. The most favorable plan calls for all

24
access to the BSS to be done through a central dispatch. Real time

programming outside the central location could be linked via other

satellites (i.e., FSS) or terrestrial microwave links.

Although the BSS calls for telemetry tracking and command control of its

relay satellites, only very coarse positioning information will be

provided. The BSS is not deemed as any sort of navigation facility. Its

emphasis in command and control is for system testing, activating home

equipment of subscribers, changing levels of programming to existing

customers, and coordinating customer subscriber billing functions.

Because of the subscription nature of the video program material,

broadcast transmissions from satellites are scrambled for security

purposes. The purpose of the security scrambling is to discourage piracy

and to filter out nonpaying customers. Normal decoding information will

be done in real time over the air by periodic transmission of the unique

data key for recipients. In addition special scrambling techniques

suitable for special programs or special interest material will be on

line.

Standard broadcast transmissions of video signals use a single sideband

amplitude modulated scheme for the video information and frequency

modulation (FM) for the audio portion. To minimize costs the BSS will use

only FM in the transmission of video and audio information. Not only

cost, but system complexity are also reduced. The overall signal

transmission parameters for the BSS are outlined in Table

25
TABLE 4.

Video Baseband:

Audio/Control Subcarrier:

Inputs:

Audio Encoding:

Composite Bit Rate:

Modulation:

Frequency:

Amplitude:

Emphasis:

Video Deviation:

IF Bandwidth:

Uplink Frequency:

Downlink Frequency:

CCIR Standard M with NTSC color

1 ea. Basic Program; Audio

Bandwidth = 13 kHz

1 ea. Stereo or Second Language;

Audio Bandwidth = 13

kHz

1 ea. Access Control Channel; Bit

Rate = 62 kbps

PCM, Bit Rate = 315 kbps/channel

692 kbps

QPSK

5.5MHz

0.12 volts r.m.s. before emphasis at

1 volt p-p video reference

point

525 line per CCIR Rec. 405-1

10 MHz p-p

16 MHz

In the band 17.3 - 18.1 GHz

In the band 12.2 - 12.7 GHz

26
Because of the higher output power needed by the BSS satellites, the

onboard battery power system is insufficient for full time operation.

Periods when the satellite solar panels are not receiving solar power

(eclipse periods) exist for the satellites and cause periods in which

operation is not possible. This is a serious limitation in view of the

ITS requirement for full time unattended reception of an accurate time

signal. In addition to this limitation the BSS is not fundamentally a

navigation system and satellite position information will not be

calculated to a precise enough level. It is possible for the ITS to bear

the full burden of precise satellite position information. However, the

principle mission of the BSS allows for scrambled video signals and there

is no direct provision for a "clear" channel available for time and

position information which needs to be relayed. Since non-BSS signals are

not a provision of the BSS service the addition of time and frequency

dissemination at the 1 ys level is more complicated.

On the surface there appear to be some benefits available from the use of

the BSS for time-and-frequency distribution. However, the BSS must be

ruled out on two key issues. The first is that transmission is not

available on a full time basis (full time is not needed to meet the goals

of the BSS) . Secondly, the spectrum allocation in the 12 GHz frequency

range is strictly assigned for meeting the objectives of the BSS, and no

provisions exist for other signals. This would make the plan to

distribute a time and frequency signal complicated by unknown factors at

this point since none of the approved BSS applications are in service.

27
G. NAVSTAR GPS

The NAVSTAR Global Positioning System (GPS) is a worldwide navigation

system using satellites and has been under development since 1973. GPS is

a Department of Defense (DoD) program intended to satisfy DoD requirements. During the past few years the use of GPS by non-DoD applications

has been encouraged by the DoD. The system is expected to be fully

operational by 1988 or 1989 and will provide worldwide continuous

navigation capabilities to all users. Only six satellites are in orbit

(May 1985) now and provide less than continuous worldwide coverage.

Position fixing with GPS is accomplished as follows. The satellites

broadcast a precisely timed signal modulated with a ranging code and a

navigation message consisting of statements on where the satellite was at

the time of broadcast. A receiver performs a tr ilateration to three

separated satellites, computing satellite range from the ranging code and

decoding satellite location from the navigation message. The range to one

satellite establishes a sphere around the satellite upon which the user is

located. Range to two satellites defines a circle of intersection of two

spheres on which the user is located. A third satellite range reduces

the user's location to two points one of which can be rejected as

impossible. The final key element to this position location procedure is

that to perform accurate ranging, the user must determine the time of

reception which he differences with the broadcast time of transmission

(given in the satellite message). The user’s receiver clock is not

28
synchronized with the satellite clock (each satellite carries atomic

clocks, all mutually synchronized). This lack of synchronization adds a

clock bias (range bias) to each satellite signal as it arrives. The bias

is resolved by ranging to a fourth satellite and in effect produces four

equations involving four unknowns (the user's three dimensions of position

and the user's clock bias).

When fully operational the satellite constellation will consist of 18

satellites in low eccentricity (20,169 kilometer) orbits inclined at 55

degrees with 6 satellites equally spaced within three orbit planes each

equally spaced at 120 degrees around the equatorial plane. The satellites

are at half-synchronous altitude making two revolutions of the earth each

sidereal day (23 hour, 56 minutes and 4.09 seconds).

The GPS satellites broadcast continuous navigation messages at

approximately 1575 and 1228 MHz called the LI and L2 frequencies

respectively. The LI frequency is modulated by two codes called the P

code or precise code and the C/A code or coarse acquisition code. The L2

frequency has only the P code. The P code runs at a rate of 10.23 Mbit/s

and the C/A code at 1.023 Mbit/s. The P code is very long, repeating

every 26? days. The P code sequence is changed every seven days for the

purpose of transmission security. The P code is intended for the highest

precision work and will be available only to the DoD and other users

approved for use on the basis of "national security interests." The C/A

code is shorter, with a one millisecond period, and has been stated by DoD

to be available to the general public. Each satellite has a unique C/A

and P code.

29
There are plans by the DoD to degrade the accuracy of the C/A code and

limit the navigation accuracy to 100 meters. Present performance without

degradation is much better.

As mentioned before, each GPS satellite carries an atomic clock synchronized to a time scale called GPS time. The message from the satellite

provides the relationship between GPS time and UTC time and therefore

provides the means of UTG time dissemination. The time transfer process

is as follows. The position of the satellite is given in the transmitted

message as a function of time. The position of the user’s receiver is

known either as a result of the user entering a position or as a result of

the receiver operating in the navigation mode with the GPS system. These

two pieces of information provide the distance between the satellite and

the user’s receiver as a function of time (it is assumed that the user’s

receiver is stationary). The propagation delay is determined using the

velocity of light. The user’s clock can then be synchronized to UTC

through simple calculations handled by the receiver itself.

A commercial, relatively low cost GPS timing receiver has been used to

evaluate the GPS system for satisfying the requirements of the ITS. The

system was found capable of synchronizing external clocks to within

0.1 microsecond. The receiver costs $25,000, but significant cost

reductions are expected over the next 10 years after GPS becomes

operational. The antenna is small and lightweight, and requires no

pointing. Although subscriber fees have been mentioned, the most recent

statements by the DoD on GPS policy have indicated that DoD will allow

free access to the C/A code.

30
H. RADIODETERMINATION SATELLITE SERVICE

The Radi ode term! nation Satellite Service is a radiocommunication service

for the purpose of radiodetermination involving the use of one of more

space stations. The radiodetermination satellite service for the United

States, USRDSS, is being developed to provide position and navigation

information and a limited digital message capability to users with small,

low cost transceivers. The system is intended to be used for maritime,

aeronaut ical , and land based mobile applications. Four companies have now

revealed plans to launch satellites to handle services ranging from

locating and navigating trucks, boats, and planes to handling nationwide

cellular radio, paging and digital messages.

Several companies have applied to the FCC to launch a radiodetermination

system. Geostar Corporation was the first to file with the FCC (March

1983) and has made some proposals of particular interest to this study.

Geostar is in fact the only system at this time for which we know details

on its future plans. It will therefore be used to further discuss what

the RDSS is and how it might be useful to the ITS. The system proposes to

provide the following functions: positioning, directional guidance,

collision and terrain voidance for aircraft and boats, position reporting

to fleet dispatch centers, sending messages, receiving messages, and

interconnection to the telephone system for all services that can be

provided through a modem. Geostar will provide these services through the

use of a ground station with a computer, two or more geostationary

31
satellites, and transceivers carried by aircraft, surface vehicles, and

possibly individuals.

In September 1984 the FCC issued a Notice of Proposed Rulemaking proposing

the allocation of frequency bands in the microwave region in the United

States for the RDSS. The FCC’s final action in the RDSS will determine

technical details and standards for the RDSS and which entities may begin

business in the U.S. If the FCC acts in a timely manner. Geostar expects

to begin service to CONUS in 1987.

Additional Geostar plans that have been advanced recently include time and

frequency dissemination. Drawing directly from a report prepared for

Geostar by the Systematics General Corporation we found Geostar’s plans to

be described as follows:

The USRDSS is a proposed system envisioned in the radiodetermination

satellite service which will consist of a number of operational satellites

in geostationary orbits, one or more fixed location control centers, and a

large population of mobile users (subscribers) located within the system

coverage zone. In the envisioned system, a periodic time reference PN

code, originating at a control center, is relayed through one satellite to

all user stations within a coverage zone. The user stations are expected

to be relatively inexpensive, preprogrammed tranceivers designed to

respond with uniquely identified signal bursts back to the control center

via two of the satellites in the system. Responses may include messages

for other users or requests for other services. The control center will

determine the two associated roundtrip propagation path delays and, using

32
a stored terrain map or altitude information from the user station,

compute the precise location of the user station using redundant high

speed computers. Computed positions along with pending messages are

addressed and transmitted back to the appropriate user via the original

signal containing the embedded periodic reference PN code. During

operation, the system will be continuously calibrated using known,

fixed location "benchmark" transceivers. Outbound links from a satellite

to the user stations are planned for the frequency range 2483.5-2500.0 MHz

while inbound links from the users to the satellites are planned for the

frequency 1610.0-1626.5 MHz.

As a complementary by-product of its primary service of precision

radiodetermination, the USRDSS System is expected to provide both general

time dissemination to a large number of users and high accuracy time

transfer. The capability for wide area coverage and the design emphasis

on low cost, automated user transceivers enable a large user base for

these functions. For general time dissemination the operational mode of

the envisioned USRDSS includes corrections for the user position performed

by a microprocessor within the user transceiver. Computed position

parameters from the control center will be used and the estimated accuracy

for users in this mode is 150 microseconds.

The operational mode for a high accuracy time transfer service with the

envisioned USRDSS involves a method analogous to the simultaneous exchange

of timing signals through a communications satellite link. Two users

desiring to correlate their local clocks will respond to a specified

33
epoch on the outbound USRDSS signal. The time of receipt of the epoch at

the user station is labeled with the local clock value. The control

center can estimate the difference in the outbound signal’s time of

arrival at the two stations by measuring the time difference in the

arrival of their responses and incorporating calibration factors derived

from the "benchmarks." By comparing the clock value labels and the

calculated times of arrival of the outbound signals, the offset between

the two user clocks can be determined. The estimated accuracy for this

service is 10 nanoseconds.

A special case of the high accuracy time transfer service makes use of an

accurate clock at the central control center. The clock is periodically

recalibrated by transferring time from a reference standard clock such as

that available at the United States National Bureau of Standards. The

control center can then transfer precise time, upon request, to any user

located within the system coverage zone. The estimated accuracy for this

special case is also 10 nanoseconds.

The following Tables 5 and 6 that follow summarize the characteristics,

costs and advantages to Geostar’s plans for time dissemination.

34
TABLE

5.

_

»— re

re c c 0 o •— ~ O' 4-1 C7< *J c re c

re

—

u

—

k- *—U 4-1 4) *J o <v 4J u a u

4.1 & re • o re • c c O 4J oo 4.4 00 0 o in CO p w oo — E o o

4.1— 4> E —

C P —

IV k. c C ^ C 4) u 0> c 4J r>. re 4J r>»

0)a re 1/) 00

— in CO

a

X

— O' Q. >* O'

O uj Q. Irt

—

m

u.

1) o > <*- 4)

V) in 0» 3 in V

n

V > c ^ 4) >

L.

— *J

V in

—

(0 V) •— 4> C

u.

O

V

p>»V

V

V

tm! • o o «

LT) in ^ > >

UJ »-

—

u

—

u

i_ CO

— 4J

— 4J o 4-> QN re V) o re u) m E O O w >. o w >. c o . o — 4) ^ —

O o v> wr\ w ^ o> </> m ^ CT

u. </> 3 *J

u 3 4J

u

4> 1/1

— c re—

C re

(0

w 3 Q.

O j:

a o JZ

B 13 ^ E o —' E o

L-o

Vt-cV>

4-1 >

IV 4J

t_ >. -—

u o

fO o — 4; 4J a IV — in m p U ^ c Q 3 (V o u a o o u re V «s o >u

lA in

4)

V

ID 4-1 4-> »<n re re

4-1 4-1

u 00

CT

re *o

p

u 4J 4)

4; 4-1 > — •— o C c o =5 o c0

4) •

>

4)

_

C re c

4J 4J o c 0 u — re -1— — 4J — c 1/1 E irt c OO lA l4- ^ u 00 3 ~ 4i OO — in U 4J 0—0 4) m O .C o 4J C

4-1 4-1 CC T3 4; E w cr O' o E re re — oo t) u 00 — u u

i/i *£ 3 5: CL

K

O Z>

X Q.

35
1/1 o;><V

VI

0)a>cn>o

lA ‘oaoc

ooCO

O' \r>^ ooO'

jO

(0

V)>. to

00ooO'

00O'—

JS

<0

tv>

tvocs

V) to

to

UJ

_JCO<c

hu

IV to

v>

4Jo>

fVc

IV>

•o

IV(A ooco

V)o

o>c

IV>

•o<

IVa.uc

VI

c;D1

»0 ucm>

•oc

(TJ Q.OC

l. Q. • V)

t- 4)

u E

0) •o

— • > —

£ V)

4) > Ql tAk

4) •D o re >-

V

O •—

k

k 00 V)

lA

a oi 3 C o

<0 4/ 4J • OI • 3 Oi c c

»- c O o o o • re — g u •— 4) c 4/ 4J re

0/ Ol £ t- re V) H3 fO o re k VI =3 u l_ 4) 4J 4) 4/ tJ V a c a > •o o » — o 0 > — u re 4; m E » 4-1 V

4-1 4) o 1 — o>

l_w o • V) re o i/) £ 5 >• c VI o. >- a H- o o VI

(A re re 4?

k.

k E • o Oi • 3 • o 4-1 U 4) VI «A 4-» 4J VI y o> 4) o (Q Oi 0 re re D o C o k — 5 re j= 4> > 5 4/ J O' > O o 3 L. o — O k

re _) .c o Q. > co V c re c

4-/ 4^ o c o u 2

•• OJ OO E iA E 00 CA wu 00 3 4; 00 — lA

4) o u 4J 4A o JZ o V

C

4-> 4J QC o 4) E iA e: CT' a; E

ro OO

V

w 00 1.

—

00 < 3 X. Cl

O :3 CL. 1— »—

36
I. LORAN-C SYSTEM

Loran-C is a low frequency radio navigation service which operates at a

frequency of 100 kHz. The bandwidth of all Loran-C transmissions with

modulation is 20 kHz. Low frequency radio signals (long wavelength

signals) have transmission characteristics which are stable for distribution of frequency and time as well as its Intended purpose of navigation

for the military. Because the radio signal follows the curvature of the

earth, the usual HF skip conditions are reduced or eliminated. Low

frequency radio transmissions propagate almost exclusively by ground wave

characteristics.

For satisfying navigation needs Loran-C is made up of many synchronized

transmitters forming a network or chain of stations. One station in each

chain is a master station and it serves as the originator of frequency and

timing information. The other transmitters in the network retransmit

signals received from a master station. Retransmission from a slave

station in the network occurs at a precisely delayed interval of time. A

Loran-C navigation receiver requires reception from three transmitters (a

master and two slaves, for example) to establish a position fix. For

frequency and time reception only a single Loran-C transmitter needs to be

used.

Loran-C is a well developed navigation system that has demonstrated time

accuracy to the 1 microsecond level for measurements in long term (several

days). Performance is not expected to exceed these numbers. For

frequency dissemination the stability is of the order of 1 x 10 at

37
one day averaging. One must realize that these performance characteristics are the best published and LORAN-C has been given very extensive

development by large agencies such as the military. Indeed, given the

signal -to-noise ratios typically available from Loran-C and the limited 20

kHz bandwidth, the performance specifications published represent

remarkable results.

Table 7 shows the Loran-C network and other parameters of interest with

respect to this study. Although Loran-C satisfies most of the prerequisites of the ITS a few issues are not satisfied. The most serious

problem with the use of Loran-C is that its format does not transmit a

time code. Time interval measurements are possible using Loran-C but no

absolute time is actually transmitted. One could, of course, maintain

coarse time at the receiver to resolve time ambiguity. But given a

complete receiver failure, the receiver would not be able to automatically

resynchronize. Some other method of received time would be necessary and

would need to be downloaded to the coarse clock.

A second area of concern is the high cost (approximately $10,000) of a

suitable automatic time receiver. Automatic in this sense means that the

receiver can be operated completely unattended. Although Loran-C

navigation receivers can be purchased for under $2,000, the need for the

additional automation to make it a timing receiver (i.e., the ability to

identify the proper cycle and decode pulse groups) adds considerably to

the cost of the basic 100 kHz receiver.

38
CHAIN

C(r^cc*l PaClIlC

C *ntd 1 «n tail

Co< s c

StA ot Japin

(Co'^.T jndo Hon)*

CariddiAn West

Cod s i

Not th Atlantic

Coif ot AlaSAa

Norwegian Sea

Southeast U.S.

Meditettanean Sea

Great Lakes

West Coast U.S.

Northeast U.S.

Northwest Pacific

North Pacific

CHAIN

CRI TRANSMITTER

COORDINATES

TED

PWR (kw) SOURCE

Table 7, LORAN-C NETWORK

GRI transmitter COORDINATES TED PWR

49900 M Johns ton Is . , HI 16 44 44.0 N 169 30 31.2 w

(USEC) (kw) 275

X Upolu, PT., HI 20 14 49.2 N 155 53 09.7 w 15972.23 275

Y Kure Is . , HI 28 23 41.8 N 178 17 30.2 w 34253.17 275

59 300 M Caribou, ME 46 48 27.2 N 67 55 37.7 w 350

X Nantudket, MA 41 15 11.9 N 69 56 39.1 w 13131.88 275

V Ca(>e Race, NFLD 46 46 32.2 N 53 10 28.2 H 28755.02 1500 59700 M Pohang, Korea 36 11 02.1 N 129 20 25.8 E 30 W Hok ka i do , Japan 4 2 44 37.1 N 143 43 09.2 E 15783.68 1000

X Kwang J u , Kor ea 35 02 27.5 N 126 32 31.4 E 31947.02 30

Y Gesashi, Okinawa 26 36 25.0 N 128 08 56.4 E 45565.56 1000 59900 M Williams Lake, BC 51 57 58.8 N 122 22 02.2 W 400

X Shoal Cove, AK 55 26 20.9 N 131 15 19.7 w 13343.60 540

Y George, WA 47 03 48.0 N 119 44 39.5 H 28927.36 1600

Z Port Hardy, UC 50 36 29.7 N 127 21 29.0 W 42266.63 400

79 300 M Ang i s soq 59 59 17.3 N 45 10 27.5 w 760

W Greenland

Sandur , I cel and 64 54 26.6 N 23 55 21.8 W 15068.03 1500

X E)de , Faeroe Is., 62 17 59.6 N 07 04 26.1 w 27803.77 325

z Deiima r k Cape Race, NFLD 46 46 32.2 N 53 10- 28.2 w 48212.20 1500 79600 M Tok, AK 63 19 42.8 N 142 48 31.9 w 540

X Narrow Cape, AK 57 26 20.2 N 152 22 11.3 H 13804 .45 400

Y Shoal Cove, AK 55 26 20.9 N 131 15 19.7 W 29651.14 540 79700 M E)de , Faeroe Is . , 62 17 59.6 N 07 04 26.1 W 325

w Oenma r k Sy 1 t , Germany 54 48 29.9 N 08 17 36.3 E 30065.64 275

X BO, Norway 68 38 06.2 N 14 27 47.0 E 15048.10 165

Y Sandur , Iceland 64 54 26.6 N 23 55 21.8 w 48944.53 1500

z Jan Mayen, Norway 70 54 52.6 N 08 43 58.7 w 63216.30 165 79800 M Malone, FL 30 59 38.7 N 85 10 09.3 w 800

W Orangeville, LA 30 43 33.0 N 90 49 43.6 w 12809.54 800

X Raymondvi lie , TX 26 31 55.0 N 97 50 00.1 w 27443.38 400

Y Jupi ter , FL 27 01 58.5 N 80 06 53.5 w 45201.88 275

Z Carolina Be . , NC 34 03 46.0 N 77 54 46.8 H 61542.72 550 79900 M Sel 1 i a Marina, It 38 52 20.6 N 16 43 06.2 E 165

X Lanipedusa, It 35 31 20.8 N 12 31 30.3 E 12755.98 325

Y Kargabarun, Turk. 40 58 21.0 N 27 52 01.5 E 32273.30 165

Z Es tar t i t , Spain 42 03 36.5 N 03 12 15.5 E 50999.71 165 89700 M Dana, IN 39 51 07.5 N 87 29 12.1 W 400

W Malone, FL 30 59 38.7 N 85 10 09.3 W 14355.11 800

X Seneca, NY 42 42 50.6 N 76 49 33.9 w 31162.06 800

Y Baudette, MN 48 36 49.8 N 94 33 18.5 w 47753.74 500

99400 M Fallon, NV 39 33 06.6 N 118 49 56.4 w 400

W George, WA 47 03 48.0 N 119 44 39.5 w 13796.90 1600

X Middletown, CA 38 46 57.0 N 122 29 44.5 w 28094.50 400

Y Searchlight, NV 35 19 18.2 N 114 48 17.4 w 41967.30 540 99600 M Seneca, NY 42 42 50.6 N 76 49 33.9 w 800 W Caribou, HE 46 48 27.2 N 67 55 37.7 w 13797.20 350

X Nantucket, MA 41 15 11.9 N 69 58 39.1 w 26969.93 275

Y Carolina Be. , NC 34 03 46.0 N 77 54 46.8 w 42221.65 550

Z Dana, IN 39 51 07.5 N 87 29 12.1 H 57162.06 400

99700 M Iwo Jima, Japan 24 48 03.6 N 141 19 30.3 E 1800

• • W Marcus Is., Japan 24 17 07.9 N 153 58 53.2 E 15283.94 1800

X Hokkaido, Japan 42 44 37.1 N 143 43 09.3 E 36685.12 1000

Y Gesashi , Japan 26 36 25.0 N 128 08 56.5 E 59463.18 1000

Z Yap Island, USA 09 32 45.8 N 138 09 55.0 E 80746.79 1000

99900 M St. Paul, AK 57 09 12.3 N 170 15 06.8 W 275

X Attu, AK 52 49 44.0 N 173 10 49.0 E 14875.25 275

Y Pt. Clarence, AK 65 14 40.3 N 166 53 12.6 W 32068.95 1000

Z Narrow Cape, AK 57 26 20.2 N 152 22 11.3 w 46590.45 400

The Loran-C transmitters and the basic chain configuration.

The Loran-C Group Repetition Interval (transmission rate) in microseconds.

M • Master Transmitter of the Loran-C chain. W-2 » Secondary Transmitter identification letter within a given

cha i n

Location of transmitter antenna. Total emission delay. This is the sum of the coding delay and baseline length in microseconds.

Transmitter radiated power in kilowatts.

United States Coast Guard Specification of the Transmitted Loran-C Sional

July 1981, COMDTINST Ml6562.k (with the exception of Commanoo Lion).

Expected to be operational in 1981. Chain reconfigures when H antenna is down for maintenance. This

occurs approximately every 3-4 years tor a 30-60 day period, Marcus

Island becomes M and the GRI is temporarily changed to 79300. Last temporary reconfiguration was FEB 1981. 39
Although the prospects for continued viability of Loran-C as a navigation

system are good, other navigation systems of higher precision and

generally better performance are now becoming available. This trend will

generally take away from future technological upgrades of Loran-C. Loran-C

represents the best that systems using radio propagation on the earth's

surface can realize for precise timing. This otherwise good system

suffers from only two major objections; the inability to receive

unambiguous time and the high cost of a fully automatic receiver. The cost

of the receiver may fall in time, but this is not a predictable trend, and

the market size may not be sufficient to drive production up and cost down

substantially.

40
J. TRANSIT

The U.S. Navy Navigation Satellite System called Transit became

operational in 196^1. It answered the need of the Polaris ballistic

missile submarines for an accurate global navigation system. Transit

provides intermittent position fixes in that the satellite is only in view

of a fixed position on the earth for approximately 20 minutes.

There are usually five operational Transit satellites in orbit with many

more on the ground, all ready for launch to replace failures in orbit.

Each satellite transmits carrier frequencies, coherently derived from an

onboard oscillator, at approximately 150 and 400 MHz. The transmitted

signals provide, during a pass, a source of constant transmitted

frequency, a navigation message, and timing information.

The navigation message is controlled to begin and end at the instant of

every even minute. An updated navigation message and time correction is

obtained periodically by uploading from the ground. The time correction

data are stored in the satellite’s memory and applied in steps as small as

9.7 microseconds.

The five satellites are in nearly circular polar orbit at an altitude of

approximately 1075 kilometers. At this altitude the satellites and their

orbits would look something like Figure 1. The earth rotates underneath

the constellation of satellites. With a satellite above the horizon, one

can make a position fix. The interval between fixes (passes of the

41
SOUTH

POLE

Fig. Jl. Five Transit Satellites in Polar Orbit

42
satellite) can vary from a half hour to nearly two hours depending upon

your position relative to the orbit planes.

The navigation message, transmitted every two minutes, begins on the even

minute. The end of the message contains a synchronization word that

identifies the time mark and beginning of the next two minute message.

The satellite’s orbital parameters in the first part of the message

define an orbit from which the satellite's position can be calculated.

To determine one's position on the earth, one has to relate his position

to the known satellite orbit through the process of measuring the Doppler

shift which is a unique function of the observer's position and motion

relative to the satellite's motion through a known orbit.

The frequencies received from the satellite consist of the frequencies

transmitted plus a Doppler frequency shift of up to plus or minus 8 kHz.

The Doppler-shifted frequencies are due to the relative motion between the

satellite's transmitter and the user's ground-based receiver. A position

fix is obtained from the following process: (1) an informed estimate is

made of the user's position; (2) the slant ranges from this position to

the known satellite positions are computed; (3) the computed ranges are

compared with the actual measured ranges; and (4) the assumed position is

systematically varied and the process repeated until the sum of the

squares of the residuals is minimized.

43
As is with any navigation system, there is a certain time order involved.

In the case of Transit, a clock is involved, being carried in the

satellite which can be used to synchronize clocks on the ground, a

by-product of the navigation function.

A Transit timing receiver needs to only receive the Transit signal,

demodulate it, and process the data and timing mark called the Fudical

Timing Mark or simply the FTM as shown in Figure 2. The FTM is the timing

mark at the end of the two minute Transit message that begins on the even

minute. It is the transition from the synchronization word consisting of

23 consecutive 1 's followed by a single 0 to the modulation burst of 400

Hz as shown on Figure 3* The timing mark and ephemeris data are handed to

the micro computer which calculates the delay between the satellite and

the receiver, corrects the arrival of the FTM, and sets an internal clock

to agree approximately with UTC, including hours, minutes and seconds.

Commercial versions of this receiver usually have the ability to average

the results over many passes of selected Transit satellites and to filter

the data for poor quality due to signal level, low elevation angles, poor

performance as stated through DoD bulletins or other criteria.

During most of 1979, MBS used two commercial Transit timing receivers to

acquire the 400 MHz Transit signals. These receivers computed the path

delays and corrected a 1 pps output to be in synchronization with the

satellite clock. Reference to DoD bulletins were then used to reference

the received time to UTC. The block diagram of Figure 4 indicates how the

commercial TRANSIT receivers were used for these evaluations. Although

44
Fig. J2. A Position Fix Using a Transit Satellite

45
1 PPS

HHMMSS

TIME OF DAY = 15 MINUTES

FTM

Fig. J3. Block Diagram of Transit Receiver and Transit Signal

46
USNO SERIES TRANSIT ELEV.

17 ANGLE PREDICTIONS

FIGURE J4. Block Diagram of Transit Monitoring System

47
these particular receivers include capabilities for averaging over any

number of satellite passes from 1 to 100 and for selectively deleting one

or more of the 5 operating satellites from the ensemble used to correct

the output 1 pps, NBS chose to use a multichannel data logger to

accumulate data separately from each successful satellite pass. The data

from each pass were recorded providing a measurement of the Transit

receiver 1 pps relative to UTC(NBS), identification numbers for the

particular satellite and receiver involved, the amount of correction

computed and applied by the receiver, the data and time of correction, and

the standard deviation of the individual 2 minute points as supplied by

the receiver. After the fact the data file was completed by adding a

"Transit clock-UTC(USNO) ” correction as published by USNO and the

elevation angle for each pass. These data were then analyzed in various

ways to show the dependence on the particular satellite ensemble used, the

number of passes averaged, the particular receivers used, the application

of the USNO corrections, and satellite elevation angle.

These tests revealed that Transit provided synchronization to UTC(NBS) to

within 25 ps the majority of the time given that proper support was

present. This support included eliminating data from consideration when

advised by USNO bulletins that there were problems with any particular

satellites. There is also the requirement to maintain a clock as a

"flywheel" to keep time within tolerances when no suitable satellites

were available for data collection.

48
The receiving equipment costs run between $12,000 and $21,000 depending

upon features. The antenna is small and lightweight and requires no

pointing. General consensus for the future of Transit is that it may be

discontinued once GPS is fully operational making Transit’s existence

unlikely after 1990. There is some possibility that private industry may

continue the service.

49
K. GOES

The Geostationary Operational Environmental Satellites (GOES) are

operated by the National Environmental Satellite Data Information Service

(NESDIS) of the National Oceanic and Atmospheric Administration (NOAA).

The GOES satellites are primarily for meteorological purposes including

the production of images of cloud systems for improving the accuracy of

short term weather forecasts. GOES is not a navigation system and has no

need for high accuracy position information. Additionally, a data

collection system on the spacecraft, illustrated in Figure K1 , receives

and relays environmental data sensed by widely dispersed surface data

collection platforms (DCP’s) such as rain and river gauges, seismometers,

tide gauges, buoys, ships, and automatic weather stations. The DCP’s

transmit data to the satellite at regular intervals, upon interrogation by

the satellite, or in an emergency alarm mode.

A time code is sent continuously through the satellite so the DCP’s may

label the environmental data with the date and time as it is collected,

thereby enhancing its usefulness. The time code and its associated

generation and monitoring systems are provided by the National Bureau of

Standards. The time code is referenced to the NBS time scale UTC(NBS)

The interrogation and time code messages comprise a continuous data stream

sent through two GOES satellites. These messages originate at Wallops

Island, Virginia, where two l8.3m (60 foot) diameter parabolic antennas

maintain the continuous data link with the satellites. The signals are

50
-t V V

- '>

FIGURE K1 . Collection of environmental data from data collection platforms.

FIGURE K2. Coverage of GOES Satellites

51
sent to the satellites on a carrier of approximately 1.68 GHz. The

1.68 GHz carrier is changed in frequency at the satellite to nominally 469

MHz and is retransmitted to the earth through antennas with global

coverage beamwidths. See Figure K2 for approximate coverages for the GOES

satellites.

The time code is contained within the interrogation channel of the GOES

satellites. The interrogation channel is used to command remote DCP's to

send their collected data to the GOES satellites. The satellites relay

these data to the Command and Data Acquisition (CDA) facility at Wallops

Island, Virginia, for processing and dissemination to users.

Interrogation messages are continuously being sent through the GOES

satellites. The format of the interrogation messages is shown in

Figure K3.

The interrogation message is 0.5 s in length, or 50 bits. The data

rate, controlled by atomic frequency standards, is 100 bit/s (bits per

second). An interrogation message consists of 4 bits representing a

binary coded decimal (BCD) word of time code beginning on the half second

of UTC, followed by a maximum length sequence (MLS) 15 bits in length for

message synchronization, and ending with 31 bits as an address for a

particular remote DCP. Sixty interrogation messages are required to send

the 60 BCD time code words constituting a time code frame. The time code

frame begins on the half minute of UTC and repeats every 30 s (see

Figure K4) . The time code frame contains a synchronization word, a time

message (UTC), the UT1 correction, indicators for current system accuracy

and daylight savings time, and the satellite’s position in terms

52
1/? s 1/? s 1/2 S 1/2 S

TC "l5 OTh£B TC MLS other TC MLS other TC MLS other

mO^O STNC DATA WORD SYNC DATA WORD SYNC DATA WORD SYNC DATA

30 S

TIME CODE frame CONSISTING OF:

• synchronization word

• OATS, HOURS, minutes, seconds •universal time correction •SATELLITE POSITION

tc time COOE MLS • MAIIMUM length sequence

FIGURE K3 Interrogation channel format.

40 32 8 20 1 6 16 108 BITS

sync time of WORD YEAR LONG LAT RAO experimental use —V

UTl SATELLITE

time position corrections

-30 SECONDS^ —

begins on

00 AND 30 SECONDS UTC

Figure K4. Time code format

53
WESTERN SATELLITE EASTERN SATELLITE

EREOUENCY 468 B2S0 MHj 468 8376 MHz

POLARIZATION RHCP RHCP

MODULATION CPSK {- 60*) CPSK (- 60*1

DATA RATE 100 BPS 100 BPS

SATELLITE LOCATION 135* W 75* W

SIGNAL STRENGTH

(OOTPUl «OW iSOTKffIt VriNHil -139 dBm -139 dBm

CODING MANCHESTER MANCHESTER

BANDWIDTH 400 Hz 400 Hz

FIGURE K5. Interrogation channel signal characteristics.

TO WESTERN

SATELLITE

TOEASTERN

SATELLITE

FIGURE K6. Time code generation system

54
of its longitude, latitude, and height above the surface of the earth

minus a bias of 119,300 us. The position information is updated

frequently, presently every 1 min.

The GOES time signals from both satellites are summarized in Figure K5

The signals are right hand circularly polarized (RHCP), coherent phase

shift keyed (CPSK), and separated in frequency by 12.5 kHz. The data

rate, being 100 b/s, occupies only 400 Hz of bandwidth. The data are

Manchester coded and phase modulate the carrier ±60°, thus providing a

carrier for conventional phase- locked loop demodulation.

N0AA*s CDA ground stations are located at Wallops Island, Virginia, where

all interrogation signals sent to the satellite are originated. NBS

maintains an ensemble of atomic clocks at that site. These clocks

provide the time and frequency reference for the time code. The time

code generation system, partly shown in Figure K6 , is completely redundant

and fully supported by an uninterruptable power supply. There is a

communication interface between the equipment and NBS, Boulder, using a

telephone line. Over the telephone line, satellite position information

is sent to the CDA and stored in memory for eventual incorporation into

the time code and interrogation message.

Data are also retrieved from the CDA via the telephone line to Boulder.

These data include the frequency of the atomic oscillators and the time of

the clocks relative to UTC compared via GPS satellite signals and Loran-C

55
MICROSECONDS

FIGURE K7. Typical corrected timing data from GOES.

56
transmissions from Cape Fear, North Carolina. Besides the time-andfrequency monitoring functions the NBS equipment at Wallops Island

provides the information necessary for NBS staff at Boulder to remotely

determine if and where malfunctions exist and to correct them through the

switching of redundant system components.

The satellite position information is generated at Boulder by using a

large scientific computer and orbital elements furnished by NESDIS.

NESDIS generates these orbital elements weekly using data obtained from

their tracking facilities at the CDA.

Commercial receiving equipment is available from three manufacturers with

costs in the range of $3,000 to $7,500 depending upon features. The

antennas are typically of the helix type with overall lengths of about one

meter, diameters of approximately 1/H meter, and weight of 10 kilograms.

The performance for GOES has been typical of that shown in Figure K7

Except for rare occasions when the satellite ephemeris is of poor

quality, the GOES signals provide time to within 100 ys of UTC(NBS)

throughout the entire coverage areas of both east and west satellites.

Since May 1974 the NBS has operated time code broadcasts from NOAA's GOES

satellites. There are three or more satellites in orbit; two are

operational, the other serving as the 'in-orbit* 'hot' spare ready to

replace either satellite in case of failure. Other satellites are usually

57
ready for launch when needed. The present complement of satellites is

expected to insure two satellites in continuous operation. Plans exist to

continue the program beyond the year 2000.

Considering the long life of the GOES system, the important mission of the

GOES program, and the wide geographical coverage of the satellites the

GOES time signals are finding many applications where reliability,

automatic signal recovery, and accuracy are desirable. Being geostationary, GOES is a source of continuous synchronization, offering a

significant advantage over a non-geostationary satellite which can only

provide exposure to the user during brief periods of time. Also, the GOES

time signals are not subject to the fading and unpredictable path delays

that limit terrestrial systems operating in the low to high frequency

ranges

The time code information is a convenient addition to GOES but not a

requirement. The present level of accuracy is 100 ps with no provision

for improvement given the low resolution and precision of position

information.

58
L. CONCLUSIONS OF PHASE II

Seven systems have been examined against criteria designed to select the

best system or systems that have reasonable potential for satisfying the

requirements of the ITS. The BSS has been rejected for further study or

development because it did not offer continuous operation time and

presents tracking problems that are not supportive of the one microsecond

requirement with reasonable costs. LORAN-C was not acceptable because of

receiver costs, lack of a time code, and uncertain future.

The Transit system was rejected for basically the same reasons as for

LORAN-C with the additional problem of inadequate accuracy. GOES was not

acceptable for accuracy reasons.

The FSS, GPS and RDSS will be examined further in the next phase of this

study. FSS will be studied because of its excellent potential to meet all

requirements and have time and frequency dissemination as its sole

mission. An ITS system will be partially developed using the FSS so

issues of cost, accuracy and feasibility can be examined in a more

realistic environment. GPS is a system that is already working and

satisfies or will satisfy all requirements except cost. Long term DoD

policy adds some uncertainty which will be explored in the next phase along

with some technological forecasting of equipment costs for the near

future. The RDSS is a very new system and was a surprise option for this

study. If the direction for Geostar continues as planned it may serve as

an excellent ITS. We will attempt to establish a dialog with the Geostar

organization to provide a clearer picture of their plans. We will assist

59
in developing those plans through a limited cooperative effort if such an

effort is found desirable by both parties.

The final phase of the ITS study converges on examining a private sector

offering, a DoD navigation system with a mission different from time and

frequency dissemination, and the exclusive use of a portion of a

telecommunication service (the FSS option). Each option has technical and

political advantages and disadvantages. This next phase will attempt to

further identify and analyze all pertinent issues needed to make informed

decisions for future use of high precision synchronization services.

60
PHASE III, PART 1:

THE FIXED SATELLITE SERVICE

A PRELIMINARY DESIGN

CONTENTS

Page

M. PHASE III, PART 1 INTRODUCTION 62

N. ELEMENTS OF THE ITS 65

1 . THE SPACE SEGMENT 65

2. THE GROUND SEGMENT 76

a) . The master earth station 77

b) . The tracking system 80

c) . The user's equipment 86

3. SIGNAL STRUCTURE 90

4. ORBIT DETERMINATION AND EPHEMERIS

PREDICTION 99

5. DENIAL OF SIGNALS TO UNAUTHORIZED USERS 111

6. LINK CALCULATIONS 121

O. SUMMARY OF PRELIMINARY DESIGN 122

61
M. PHASE III, Part 1 INTRODUCTION

The FSS (Fixed Satellite Service) option represents the design of

the ITS as a satellite broadcast system using part of a transponder

carried by a U.S. domestic communications satellite operating in the Fixed

Satellite Service. The FSS is generally intended for point-to-point

telecommunications. However, recent footnotes added to the ITU regulations now allows FSS bands to operate on a secondary basis in a broadcast

mode. There are several very successful examples of this in use today,

most notably that of the Equatorial Communications Corporation who

distributes data to small earth stations in a point-to-multipoint or

broadcast mode.

The ITS, using a FSS satellite, could be implemented in close

approximation to that described in this section. A moderately sized earth

station located at the source of time and frequency standards would uplink

or transmit the time and frequency signals to a communications satellite.

This moderately sized earth station, the diameter of which would be on the

order of 6 meters, would guarantee that the total communications link would

be downlink limited. In other words the uplink would constitute no

liability in the total system noise budget.

The satellite, upon receiving the uplink signals, would change the

carrier frequency, amplify, and reradiate them back to the earth’s

surface. These signals would be received by users equipped with small

microwave antennas (likely to be a dish with a diameter of one meter or

less) and a receiver which would amplify, down convert and decode the

62
timing signals. The timing signals would provide the information required

to set and control a clock to within better than one microsecond of the

NBS time scale denoted as UTC(NBS). That information would include the

day number, hour, minute, second, and the satellite’s current position in

longitude, latitude, and earth center distance. The receiver could also

generate one or more frequencies such as 1 MHz and 5 MHz disciplined to

the NBS frequency standard via this ITS service link.

The tracking of the satellite’s position relative to the earth and

the generation of its ephemeris would be accomplished using the timing

signals. The ephemeris of the satellite is information required by the

user’s receiver in the conputation and correction of signal delay from the

NBS earth station through the satellite to the user’s receiver. The

position of the satellite would be determined by receiving the timing

signals relayed by the satellite at three carefully selected points on the

earth within the satellite’s antenna coverage. One point will be NBS,

Boulder. The other two sites will be selected to provide the best

geometries for determining the satellite’s position as well as for

political, economic and other technical reasons. These two sites will

communicate their data in real time to NBS, Boulder either through the same

satellite or via terrestrial communications facilities. An orbit will be

fit to the data. This orbit will then be used to generate the satellite’s

ephemeris algorithm for deriving ephemeris data.

The timing signal will be encrypted to deny its use by unauthorized

or nonsubscribing users. The key for encryption will be provided to the

user according to a subscription list. Subscription fees will pay for the

63
operation and maintenance of the ITS. Each user's receiver will have a

unique master encryption key built into the receiver. Each key shall be

generated, authorized and registered by NBS prior to being installed in

the receiver by the manufacturer. A working key could be distributed by

NBS or its contractors on a periodic basis and according to subscription

fees paid. The key may be delivered either in the form of an electronic

apparatus (such as PROMs) which is inserted into the receiver or, more

preferably, sent via the satellite to each paid-up user's receiver.

Delinquent user's receivers could be made inoperative, as well, by sending

an invalid working key. The intent of the encryption procedure is to

render cheating a poor use of time and effort when compared to paying a

relatively small annual subscription fee for the service delivery of time

and frequency information.

The cost of user's equipment must be reasonable enough to guarantee a

sufficient population of users to cover the service's cost of operation

and maintenance. In the last few years the costs of small microwave

antennas, low noise amplifiers and down converters designed for satellite

receive-only application have decreased significantly. This cost

reduction has been particularly true due to the emerging Direct Broadcast

Satellite (DBS) industry in the U.S., Europe, and Japan. The ITS, being

broadcast in nature, can enjoy the economies of scale to be realized from

the DBS industry. Since tens of millions of units are to be produced,

this should particularly benefit the ITS user. The user's antenna

must be physically small but not necessarily classified as mobile.

Typically, DBS antennas are being designed and manufactured with diameters

as small as 0.6 meters. Complete DBS receiving systems not including the

TV set are expected to be in $400-500 range.

64
The use of the FSS can be extremely reliable. One need only notice

that the major TV networks are now routing programming to local distributors via satellite and that many major sporting events from around the

world are watched live with little interruption. Due to the numerous

sources for satellite bandwidth and time, especially in the U.S., one can

be assured of a very competitive environment and thus realize the lowest

costs possible for satellite bandwidth and time.

The discussion above was a brief statement of one model for the ITS

using the Fixed Satellite Service and is intended as an introduction to

the discussions of the main issues relevant to the ITS. They are more

fully developed and described below.

N. ELEMENTS OF THE ITS

N1 . THE SPACE SEGMENT

The space segment is that part of the ITS that will be located in

space. In this case, it is that part of a satellite that is directly

involved with the ITS.

The FSS as operated in the United States is a nonmonopoly satellite

service and is priced on an open market basis. It is driven by costs and

profit incentive. The costs per channel have been steadily dropping

relative to inflation since its inception. This has resulted in a steady

growth. Growth has stimulated the design of larger and more cost

effective satellites, further reducing costs through economies of scale.

65
This situation is expected to continue for some years to come, at least

until the growth of fiber optics communications begins to assume a major

role in the point-to-point telecommunications markets.

The FSS within the United States can be partially understood by

looking at the numbers of transponders in space today and the numbers

expected in the near term. A transponder is simply a satellite repeater.

The transponders have mainly been 36 MHz in bandwidth but other

bandwidths are becoming common, especially in the KU band. It has become

standard to discuss satellite capacity in terms of numbers of 36 MHz

transponders. Recent studies have placed the number of transponders in

space at 732 for 1985, 1212 for 1986, and 1336 for 1987. The capacity for

1985 in terms of bandwidth is therefore 732 x 36 MHz, or 26,352 MHz,

available to support various telecommunication customers. A typical

KU band transponder and frequency plan are shown in Figures N1 and N2.

Satellite services may be acquired at a number of levels. Satellite

owners will lease or sell transponders (36 MHz units). Full time holders

(owners or lessors) of transponders may lease out the transponder on a

part time basis. This may be scheduled, for example, on a "sametime-every-day" or "only-on-Mondays" basis or on an occasional basis for a

special event. Fractional transponder users, like the ITS, could obtain

their necessary bandwidths from transponder owners or lease holders. An

example of a fractional transponder use is a business needing only a

narrowband data service or one that does not need a full transponder.

66
RECEIVE 14.0—14.5 GHz

IRS: 6 to 7 Input Redundancy Switch.

ORS: 7 to 6 Output Redundancy Switch. DA: Driver Amplifier

PA: Power Amplifier

Fig. Nl. Typical KU-BAND Transponder Configuration

67
DOWNLINK

FREQ

(MHz)

FREQ

(MHz)

UPLINK

Fig. N2. Typical KU Band Frequency Plan

68

14440

-

12140
The FSS for US domestic use operates in the 5.925 to 6.425 GHz uplink

and 3.7 to 4.2 GHz downlink band and is commonly referred to as the

C-band. The other FSS domestic assignment is at KU-band with the 14.0 to

14.5 GHz band for the uplink and 11.7 to 12.2 GHz band for the downlink.

Additional frequency bands were allocated at the 1979 World Administrative

Radio Conference (WARC-1979). These additional bands will not become a

factor for the ITS for several years because of technical and economic

reasons.

The U.S. domestic satellites operating in the FSS are assigned

positions by the FCC in the geostationary arc between 62 degrees West

longitude and 146 degrees west longitude (except for the arc between 105

and 120 degrees which is assigned to Canadian satellites). These

satellites have antenna patterns that cover the CONUS with some satellites

also providing spot beams to Hawaii, Alaska, and Puerto Rico and Virgin

Islands. The recent assignments of the geostationary arc are shown in

Table 8. These satellites, due to antenna size, provide reduced, but

usable, signal levels far into adjacent areas of Canada, Mexico, the

Pacific and Atlantic Oceans. A typical foot print is shown in Figure N3

where contours of equal ERIP are shown.

Recent efforts to increase the capacity of the geostationary orbit

have led the FCC to require that satellites be spaced at two and three

degrees apart rather than the old standard of four degrees. This new

spacing means that ground antennas will experience more interference from

adjacent satellites through their side lobes. These ground antennas, when

transmitting, will interfere with the reception by adjacent satellites

69
TABLE 8. SATELLITE ORBITAL POSITION ASSIGNMENTS

POSITIONS SATELLITE KU BAND C BAND

1 ne AURORA 2 X

WESTAR 7 X

142 AURORA 1 X

140 GALAXY 4 X

138 SATCOM 1R X

136 SPACENET 4 X X

136

134

GSTAR 3

UNASSIGNED

X X

134 COMSAT GEN B X

132 GALAXY 1 X

132 WESTAR B X

130 SATCOM 3-R X

130 GALAXY B X

128 ASC-1 X X

126 TELSTAR X

126 MARTIN MARIETTA B X

124 WESTAR 5 X

124

122

FEDERAL EXPRESS B

UNASSIGNED

X

122 SBS 5 X

120 SPACENET 1 X X

POSITION BETWEEN 120 AND

ASSIGNED TO CANADIAN

105 DEGREES ARE

SATELLITES

105 GSTAR X

103 GSTAR X

101 FORD 1 X

99 WESTAR 4 X

99 SBS 1 X

97 TELESTAR X

97 SBS 2 X

95 SBS 3 X

95 GALAXY 3 X

93 FORD 2 X X

91 WESTAR 3 X

91 SBS 4 X

89 UNASSIGNED X

89 UNASSIGNED X

87 SPACENET 3 X

85 TELESTAR X

85 RCA A X

83 ASC 2 X X

81 SATCOM 4 X

81 RCA B X

79 MARTIN MARIETTA A X

79 WESTAR 2 X

70
77 FEDERAL EXPRESS A

76 COMSTAR D4

75 COMSAT GENERAL A

7M GALAXY 2

73 WESTAR A

72 SATCOM 2-R

71 GALAXY A

69 SPACENET 2

67 SATCOM 6

67 RCA C

611 ASC 11 62 SATCOM 7 62 SBS 6

71

XX

XX
I5l 1 1 l—isL— — — I<^_Z1 170 165 160 155 150 145 140 135 130 125 120 115 110 105 100 95 90 85 80 75 70 65 60

Fig. N3. Typical EIRP Contours for U.S. FSS Satellites

72
as well. The interference geometry for adjacent satellite systems is

illustrated in Figure N^l. New sidelobe standards have been imposed by the

FCC for the earth station. The effect has been the use of larger and more

expensive antennas on the ground. These sidelobe standards are mandatory

for all transmit antennas and for all receiver antennas wanting the

protection afforded by a FCC license but must accept whatever

interference adjacent satellites and terrestrial microwave may inflict.

The interference will be particularly severe for the very small earth

station whose side lobes are high within two or four degrees from its main

lobe axis. Techniques such as the use of spread spectrum are available to

reduce the effect this interference and are discussed elsewhere.

The costs of transponders or their long term leases can only be

estimated. The reasons are the nearly complete deregulation of satellite

tariffs. The current procompetit ive policies of the Federal Communications Commission (FCC) have resulted in a number of actions that directly

affect domestic satellite firm’s charges for their transponder services.

In the FCC’s Competitive Carrier decision ( CC Docket 79-252) two

regulatory tiers for nondominant common carriers were established and

include satellite firms providing domestic interstate services,

specialized common carriers, and resellers. Nondominant carriers are

those not possessing sufficient market power or unable to raise prices

significantly above costs for an appreciable period of time or to

discriminate against any class of customers.

73
ADJACENT ADJACENT

SATELLITE SATELLITE

Fig. N4 . Interference Geometry for Adjacent Satellites

74
Under the streamlined regulation order (adopted October 1983) tariff

requirements were substantially reduced for these non-dominant carriers.

Tariffs could be filed on 14 days notice rather than 90. Carriers were

permitted to expand services subject to nominal reporting requirements and

to discontinue services subject to a relatively short approval period and

customer notice obligation. This order has made for a more dynamic

environment where services can more easily meet market demands.

Under the "forbearance" doctrine adopted in August 1984, these

carriers were now able to change prices or rate structures, or to expand

or discontinue a service without any waiting period requirement or

reporting to the FCC. In an action taken in November 1984 the FCC

directed common carriers subject to the regulatory forbearance to cancel

their tariffs on file and not to file other tariffs in the future. All

tariffs are now cancelled. All carriers have an obligation, however, to

keep material on file at their offices that can be produced readily upon

inquiry from the FCC, in order to substantiate reasonableness of rates,

terms and conditions.

This competitive environment has been credited with an increased

array of services and choices. In addition to communication services,

satellite common carriers, transponder resellers and brokers offer a

variety of operational and ownership alternatives. The customer can

select from end-to-end services provided by one entity, or customers may

contract with a satellite or transponder owner for space segment only and

build and own the ground segment.

75
In this deregulated environment it is difficult to predict the

space segment costs accurately without a formal bidding process or direct

negotiation with the carriers. A recent example can be informative

however. The ITS, in this study, requires a small part of one

transponder, approximately 200 kHz of the transponder's 36 MHz or roughly

one percent. For a fixed term, protected* service (two years or more),

the costs have recently been in the $130,000 to $170,000 per month range.

Assuming a partial transponder use of one percent and a mark up of 100/S by

a broker due to reduced transponder efficiency and brokerage fees, these

costs for the ITS would be $2600 to $3^00 per month. These costs are for

satellite transmission time only.

N2. THE GROUND SEGMENT

The ground segment will consist of three major components including

(1) the master earth station with the time and frequency reference, system

tracking control, and the information generation system, (2) the remote

tracking earth stations consisting of two or more smaller antennas, and (3)

the user receiving system each consisting of a small antenna and receiver.

*Two forms of protected transponder services are common. There is the

"Protected-dedicated, " in which a company will reserve a specific

replacement transponder, consisting of an unassigned transponder or a

transponder subject to interruption in a separate satellite or in the same

satellite, for use in the event of transponder failure. With "Protecteddesignated," typically, no specific replacement transponder is identified.

76
N2a. THE MASTER EARTH STATION

The master earth station will transmit the signals to the satellite

for relay to the users. The earth station will be co-located with the

time and frequency standards which generate UTC(NBS) at the NBS

facility in Boulder, Colorado. A master earth station already exists at

Boulder and can serve as the ITS master earth station. It is a 6.1 meter

KU band antenna roof mounted using a kingpost mount. A photo of the

antenna is shown in Figure N5. The antenna is within 300 feet of the time

and frequency standards, i.e., the NBS time scale and the U.S. frequency

standard.

The required ITS earth station is represented by a block diagram in

Figure N6 . The NBS time and frequency standards provide the reference for

the time code generator, to the time interval counters (TIC) which output

the range measurements data between the satellite and the NBS master and

two other smaller earth stations. The conputer receives these range

measurements from the TIC’s and computes an orbit for the satellite.

Using this computed orbit, the computer outputs the satellite's current

predicted position for inclusion into the next message frame to be sent to

the satellite. The conputer also looks at the data relayed by the

satellite for the purposes of checking operations.

The time code and satellite position are encrypted. The encrypted

output is joined with the initiation vector used in the encryption and

decryption process, and both are encoded for forward error correction

77
78

Fig.

N5

.

NBS

Earth

Station
Fig. N6. NBS Master Earth Station

79
using a rate 1/2 code. The data are then joined with a frame sync word and

applied to a spreader where each bit is expanded by approximately one

thousand times to give a processing gain of at least 30 dB.

The spread data are then passed through a modulator, upconverted to

KU band, amplified and input to the antenna through the ortho-mode

transducer ( OMT )

The received signals at the master station consist of the signal

transponded through the satellite and two others that were transponded by

remote earth stations and relayed back to NBS via the satellite. The

signals are despread at the remote sites and respread with orthogonal

spreading codes for separation at the Boulder earth station.

Upon reception at Boulder, the signals are amplified by a low noise

amplifier (LNA), down converted and demodulated. The three signals are

despread in their respective channels, the result of which is applied to

the TIC's for the range measurements.

N2b. THE TRACKING SYSTEM

An accurate estimate of the satellite position at all times is a

requirement for the ITS. This position information is required by each

user's receiver to conpensate for the time delay of the signal between its

source (NBS) and the user's receiver. This time delay is constantly

changing since the satellite never maintains a perfect "geostationary"

80
orbit. All geostationary satellites have an orbit with a small

eccentricity and a small degree of inclination. Figure N7 illustrates the

orbit geometry with respect to the earth.

Communication satellites are maneuvered every week or two to keep

their positions, relative to the earth’s surface, reasonably constant.

Typical requirements for communications satellites are to keep them within

a box in space of ± 0.05 degrees north and south of the equatorial plane

and ± 0.05 degrees east and west of an assigned position in longitude.

Assignments in longitude for U.S. satellites are now made at every two

degrees by the FCC. Keeping the satellites at these positions requires

tracking procedures by the satellite’s operators that are not sufficient

to meet the ITS requirements. The operators need only track the

satellites at infrequent intervals rather than on a continuous basis as

required by the ITS to produce orbital elements accurate enough to produce

long term, accurate predictions of satellite position.

The use of communication satellites for the ITS, therefore,

necessitates the ITS system to include its own tracking system. A

tracking system for the ITS can be generally specified with reasonable

conf idence.

The ITS tracking system would ideally include three earth stations,

each performing ranging measurements to the satellite on a continuous

basis. The earth stations would include the same master earth station as

NBS used to uplink the time and frequency signals to the satellite. Two

81
z

Fig. N7 Satellite Orbit Geometry

82
other smaller earth stations would be located on the earth’s surface at

locations to produce the best tracking results. Generally, the

coordinates of the three earth stations should define a triangle of the

greatest areas possible within the beam of the satellite. If the beam

covers CONUS then the triangle would be constrained to lie within CONUS

with one vertex located at NBS , Boulder. These three earth stations

providing range measurements to the satellite constitute a process called

trilateration of the satellite.

Ranging signals will originate at NBS, Boulder, and be sent to the

satellite. The satellite will receive these signals, change their

frequency, and send them back to earth. Reception of the signals at

Boulder will allow a measurement of the range from Boulder to the

satellite. Range measurements made through the other two earth stations

will produce the combined ranges between Boulder and these sites via the

satellite.

The three range measurements from known positions on the earth's

surface define the satellite's position. These data taken over a

sufficient period of time will be used to fit an orbit in a least squares

manner. The computed orbit is then used to predict future positions of

the satellite using Kepler's laws alone. The data gathering, processing,

and orbit prediction will be at the NBS, Boulder, site. The current

satellite's position will be part of the data sent to the user's receiver.

This process will be handled using a small computer at the Boulder site

and constitutes part of the tracking system. The tracking system is

illustrated in Figures N8 and N9.

83
SATELLITE

Fig. N8. ITS Tracking System

84
Fig. N9. Remote Turnaround Station

85
N2c. THE USER’S EQUIPMENT

The equipment will consist of (1) an antenna system, (2) the receiver

performing the downconversion, demodulation, timing, and decoding

functions, and (3) the processing system for computation of path delays,

Doppler shift, control of outputs, and the generation of standard time

codes and corrected outputs including displays and 1 pps.

The antenna will consist of the physical antenna and supporting

structures and a suitable low noise amplifier (LNA) or low noise block

downconverter (LNBC). Making use of economies of scale, a significant

cost benefit is expected through the use of the same antenna being

manufactured for the direct broadcast satellite (DBS) industry. These

antennas are designed to work at the same frequency band, KU band, as is

assigned to the ESS and have many new features built in. Included are

offset feeds for lower antenna noise temperatures and near vertical

mounting (for locations within CONUS) which reduces its snow and ice

loading potential. DBS antennas are now being produced with 2 to 4 foot

diameters.

Located at the antenna feed is a low noise amplifier or a low noise

block downconverter. The LNA amplifies the 11.7 to 12.2 GHz band by

approximately 50 dB with a noise figure of approximately 2.5 dB or about

260 K noise temperature. The low noise block downconverter has similar

gain and noise figures but has an output typically at 950 to 1450 MHz. A

commercial LNBC for the DBS is illustrated in Figure N10. The costs for

86
87

Low-Noise

Block

Converter

for

Direct

Broadcast

Satellite

Application
the antenna and LNBC (or LNA) are expected to be less than $500 since tens

of millions will be manufactured for a worldwide market. Direct

application of these antenna systems to the ITS should make a $2500 ITS

receiver system possible.

The receiving system block diagram is shown in Figure Nil. The

antenna, LNA, and downconverter including its local oscillators will be

the same as used in the Direct Broadcast Satellite (DBS) industry for home

roof top use. The cost for this system is expected to be less than $500.

The output of the downconverter will be the input to a phase lock loop

demodulator. Phase lock detection will be possible since the spectrum

will be constructed to leave a residual carrier for this purpose. The

signal will be phase shift keyed approximately ± 70 degrees to guarantee

the residual carrier thus eliminating the need for a Costas or squaring

loop demodulator. The output of the PLL is fed to a delay lock loop (DLL)

to remove the spreading code from the data. A locally generated PN

sequence is used in the DLL to correlate against the received sequence.

The DLL shifts its phase until it phase matches that of the received

sequence and then locks to it. The phase of the locally generated PNB

sequence then is used as a precision timing marker.

The collapsed spectrum is now the data which are run through the FEC

circuitry to correct for errors and then through the DES machine for

decryption. The decrypted data containing the time code, satellite

position, and related data are input to a microprocessor for computation

of path delays and correction of an output one pps to be in synchronism

with UTC(NBS).

88
Fig. Nil. ITS User Receiving System

89
Figure Nil shows the technique which generates the PN sequence and

provides the key for decryption. This may be a virtual chip in that the

PN sequence and decryption key will be sent to the receiver via the

satellite rather than chips mailed to the user for insertion into the

receiver.

N3. SIGNAL STRUCTURE

The ITS signal structure discussed here is based on careful

consideration of all the ITS system requirements as discussed in phase one

of this study. Selection of a good signal structure is important since it

has been decided that it must serve for both timing and tracking purposes

as well as having a major impact for receiver design and reduced costs.

This section describes a favorable signal structure and provides a brief

discussion of its merits relative to various ITS requirements.

OBJECTIVES FOR THE SIGNAL STRUCTURE

The primary objectives are listed below.

- provide a time code and ephemeris message and other related

data to the user on a reliable basis.

- provide an accurate tracking capability.

- provide a high precision timing capability.

- tolerate substantial interference from adjacent satellite and

other terrestrial sources.

90
- allow user equipment to operate in a completely passive

(one way) mode.

- support the capability to deny access to data by unauthorized

users.

Spread spectrum techniques have been chosen as the basic signal

concept for the ITS. This is because a spread spectrum signal will

provide good timing resolution with arbitrary ambiguity and at the same

time serve as an excellent tracking signal. Spread spectrum also will

provide the necessary adjacent satellite interference rejection that will

be experienced with the use of small user antennas and can provide an

additional protection against unauthorized (unpaid) users. These features

are "bought'* in exchange for using more bandwidth than is generally

necessary to accomplish the same tasks. Spread spectrum also has

advantages of requiring relatively low signal -to-noise ratios at the

receiver. Additionally, since the signal has a noise-like spectrum it is

less likely to interfere with other terrestrial and satellite based

systems.

Some of the advantages of spread spectrum transmission may be seen

from Shannon's theorem, which relates the capacity of a channel (C) to the

bandwidth it occupies, B, and the signal-to-noise ratio. The theorem is

as follows:

C = Bln( 1 + I

) .

For S/N ratio small the expression can be approximated as

C . B(f)

91
which shows that if the capacity is maintained constant, the bandwidth can

be increased while decreasing the S/N.

Spread spectrum essentially stretches in time each bit of

information. This is done by transforming each bit into a series of many

elements or chips called a pseudo-noise (PN) sequence. These elements

make up two distinct patterns that are defined as a one or a zero. While

the receiving system may make mistakes in receiving all the elements, due

to interference or noise, it will see enough of the pattern to understand

whether the element represented a one or a zero. An example of a good

series of elements, a ’’one” may be represented as 10001001 10101 1 1 and a

"zero" as its inverse or 011101100101000. In this case the chip rate is

fifteen times that of the bit rate, thereby requiring a corresponding

increase in bandwidth to transmit.

The generation of the signal is as follows. As shown in Figure N12,

a data stream is input to a modulo-two adder where each bit of data is

expanded into a PN sequence that is N bits long occupying a duration T,

which is the duration of one of the data bits. The modulo-two adder

inverts the sequence unchanged when a zero is present. The data in the

case for the ITS will be made up of a synch word, an injection vector and

encrypted data via the DES. This data structure is explained in more

detail in Section 5. The output of the modulo-two adder is passed to a

phase modulator where the phase of the carrier is shifted according to the

input. The amount of phase shift is adjusted to leave a residual carrier

92
Fig. N12. ITS Signal Generation

93
for simple phase lock loop recovery in the receiver. The spectrum is

expanded by N as shown due to the multiplication in the modulo-two adder

and shifted up in frequency in the mixing process.

Detection of this signal is by correlation where the received signal

is modulo-two added, bit by bit, with an identical replica of this signal

and integrated over the entire signal element duration as is illustrated

in Figure N13- Not shown are the demodulation and downconversion

processes. Here the received signal, when modulo-two added to the

in-phase reference signal, is reduced in spectrum width and the spreading

code is removed from the data. The reference signal must be in phase with

the received signal in order to collapse the spectrum and is accomplished

with the aid of a delay lock loop which seeks out this correct phase

relationship and locks to it. Figure N14 shows the same process but also

illustrates the correlator’s processing of a CW and wide band interference

signal which is spread by the reference code and then narrow band filtered.

The benefit of this process is called processing gain and can be expressed

as follows: Processing Gain (dB) = 10 log [number of chips per bit]

= 10 log [chip rate/data rate]

= 10 log [Bg/B^]

where Bg is the spread spectrum bandwidth and Bj^ is the bandwidth of the

narrowband filter following the correlator.

The interference is reduced by the amount of the processing gain.

The ability to reduce interference is useful since the user of small

diameter antennas will receive more adjacent satellite interference than

conventional signal will allow. New FCC rules place satellites at two

94
T

T T

Fig. N13. ITS Signal Detect

NARROW BAND

FILTER

I

I

I _L

T

ion

95
RECEIVED SIGNAL MOD-2 ADDER OUTPUT NARROW BAND

FILTER OUTPUT

INTENDED SIGNAL

INTERFERING CW SIGNAL

rrm

^-Tnnn-~w<TTTT I I I I I I ITTTVr-rTTTN

WIDE BAND

Fig. N14. Interfering Signal Rejection

96
degree spacings. At KU band a 0.6 m antenna has its first sidelobe

at five degrees and the half power beamwidth is 2.8 degrees. Figure N15

shows 0.6 in dish antennas operating at C band and eliminating adjacent

satellite interference through the use of spread spectrum techniques.

The use of the PN code also lends itself well to the ranging process.

The transmission of a binary sequence of digits (ranging code) as a signal

whose period can be designated to be arbitrarily long can reduce the range

ambiguity problem to any level desired. Range resolution is a problem of

how precise the returned code can be matched to a delayed reference code.

In practice this amounts to a few percent of one code chip. The

measurement of range is, in reality, the same problem as the measurement

of time delay or the time synchronization of clocks. The same PN sequence

serves both measurements.

The chip rate must accommodate the data to be sent at the repetition

rate found necessary and yield a reasonable processing gain. It must also

support the tracking and synchronization precision requirements. The

basic data include the time code, the satellite position, synchronization

words and other related information. The entire code should repeat at

least every minute.

The PN sequence has been selected to run at a 102.3 kHz rate, the

chip rate, with a period of 1023 bits or every 10 milliseconds. The

processing gain will be approximately 33 dB for this case. The modulation

will be phase shift keying at ± 70 degrees to allow for a residual

carrier. This will allow direct carrier recovery of the signal and a

97
98

Fig.

N15.

C-BAND

Earth

Station

Using

Spread

Spectrum

Signals
coherent delay-lock loop for the PN code tracking. Direct carrier

recovery and coherent code tracking will be simpler and more cost

effective to implement in the user’s receiver but probably introduce more

offset errors than non-coherent tracking. We think, however, that these

errors will not produce a noticeable impact on the ITS requirements.

m. ORBIT DETERMINATION AND EPHEMERIS PREDICTION

The requirement to provide the user of the ITS with an accurate value

of position of the satellite has been mentioned a number of times in this

report. This section discusses NBS’s experiences in attenpting to obtain

this kind of information, the quality of the information obtained, and why

the ITS system must provide the information itself rather than relying on

outside sources. Arguments are also made that a simple tri lateration

network can support better than a one microsecond delivery by the ITS.

These arguments are based upon results with the ATS-5 satellite, applied

to a hypothetical tracking network for the ITS [1].

In the process of generating a set of orbital elements, some or all

of the measured quantities, including range, range rate, and angle

information, are gathered from one, two, three, and sometimes more widely

dispersed sites on the earth’s surface. After some minimum amount of data

is taken, an orbit is calculated from these data. This orbit is described

by six parameters or state vectors. Two of the parameters describe the

orbit’s size and shape (semimajor axis and eccentricity) since the orbit

is theoretically an ellipse for two body point sources. Three of the four

99
remaining parameters describe the orbit's orientation with respect to the

earth. They are the inclination, the argument of perigee, and the right

ascension of the ascending node. The final parameter describes the

satellite's position in the orbit at the epoch of the elements (the mean

anoraoly )

These elements are then used as input to a prediction program to

determine the satellite's position at any time before or after the

elements epoch. The sophistication of the prediction program needed

depends upon how accurately prediction is required. Also, the greater the

time difference between epoch and prediction time the greater the

sophistication of the prediction program that will be required.

NBS first experienced and dealt with the position determination

problem in 1969 using DoD experimental satellites. DoD-supplied satellite

position predictions were sufficiently inaccurate for NBS purposes that

NBS chose to determine satellites position using an NBS-operated

tr ilateration network. Results of 1-10 microseconds, without prediction,

throughout North and South America were obtained.

During the period of 1970 to 1973 while working with NASA's ATS

series of satellites, NBS depended upon NASA-generated predictions using

NASA's Goddard Trajectory Determination System (GTDS) program and NASA-run

computers. GTDS is a large and sophisticated program that accounts for

the earth's geopotential expressed in a 15x15 matrix, the gravitational

attraction of the planets, solar radiation pressure on the satellite, and

the effects of the moon. This procedure generally provided 1 to 1

0

100
microsecond predictions over a one week period. These results were

obtained from a tracking network far too complex for consideration with the

ITS concept.

Using GOES satellites between 1975 and 1983, NBS has generated its

own ephemeris using NOAA orbital elements derived from NOAA's

tr ilateration network and NBS’s version of NASA's GTDS. The NBS version

of the program requires 40,000 Fortran statements, 250 subroutines, and 19

overlays on a large mainframe conputer. It has proved effective but is

expensive to run and maintain. Results of 1-10 microseconds for

prediction over a one week period were usual.

Recent cost-cutting efforts have forced NOAA to drop the trilateration network associated with the GOES satellite and to make position

estimates based upon the earth’s image which is transmitted by the

satellite every twenty minutes. The resulting orbital elements have

generally been of a poorer quality; this resulted, at times, in 100 microsecond prediction errors of the GOES time code service.

Discussions with communication satellite operators in the FSS have

revealed that they also do not produce orbital elements of a quality or

frequency that can satisfy the needs of the ITS. The conclusions drawn

from these experiences are that the ITS service must determine the

satellite's position using information gathered from its own ranging

system.

101
The tracking and prediction process for the ITS must be cost

effective and simple as well as accurate enough to support the delivery of

better than one microsecond to the user. Because of the unique functions

inherent with the ITS, such as continuous tracking by using the timing

signals, these goals will be argued to be possible with surprising

simplicity as compared to the efforts and facilities normally expended in

the orbit determination and prediction process.

The work done in Ref. 1 using a trilateration tracking network

provided the experimental results on which we base our expectations for a

trilateration tracking network for the ITS. The geographical positioning

of the ITS trilateration network will be limited by the satellite

footprint to the northern hemisphere and probably to CONUS, Hawaii, and

Puerto Rico. Other locations are either of little benefit or would be on

foreign soil (Canada, Mexico, or islands of the Caribbean) and may

represent logistical and political problems requiring formal agreements

that probably would not justify any technical benefits derived. The NBS

does, however, enjoy the advantages of having facilities at Gaithersburg,

Maryland and Kauii, Hawaii that may host tracking earth stations of the

ITS.

Whatever the trilateration configuration becomes, it will be

considerably smaller than that used by Ref. 1 i.e. Schenectady, NY,

Hawaii, and Buenos Aires. The results obtained and verified by

NASA’s sophisticated tracking network has been a valuable tool to support

102
the analysis of a ITS tracking system. The issue is whether the reduced

geometry of the ITS tr i laterat ion system will affect the precision of the

ITS tr ilateration system. The NASA results provide some indication and an

extended analysis was performed to answer the question posed above.

TRILATERATION STUDY:

The tr ilateration work of Ref. 1 was supported by NASA and accomplished in 197^-75. The purposes of the NASA-supported work were stated

among others to be..." to provide a tr ilateration network capable of

location of the ATS-5 satellite including the following tasks:

1 . To investigate and report on:

a) A system of predicting satellite position using measurements

made at short intervals spaced an hour apart and prediction

for the next hour.

b) A method of verifying prediction capability (both precision

and accuracy).

f) The design of computer software required to determine near

real time satellite position in latitude, longitude, and

earth center distance and to predict satellite position.

2 . .

3 . . . 4 . .

5. Develop computer software to determine the nearly real time

satellite position in latitude, longitude, and earth center

distance. The software will also compute the orbital elements of

the satellite, range measurement standard deviation, and short

term predictions of satellite position.

103
6. Conduct cooperative experiments with NASA using ATS-5. These

tests will accomplish the following;

a) An exercise of real-time satellite tr ilateration ranging,

position and predicted position computation, and data

transmission to NASA and MARAD (Maritime Administration).

b) Establish basis for predicting satellite positions up to

several hours. The prediction will be an extrapolation based

on Kepler's laws from a sequence of trilateration positions.

c) Verify the accuracy of trilateration position measurements

and predicted position..."

Only the objectives of importance to the ITS were quoted here. These

objectives were met and carefully documented in a number of reports and

papers including (1-2). The following is a description and summary of the

work that had direct impact on the ITS. It is indeed fortunate that this

body of work was successfully accomplished, verified, and documented in

the detail that directly benefits the ITS. Without it, much of that work

would have to be repeated before definitive statements on the feasibility

of an ITS trilateration system could be made to the degree afforded at

this time. It will be argued by direct analogy to this trilateration work

and by further supporting analysis by NBS that a trilateration network can

be established that will support 0 . 1 -microsecond-precision time dissemination simply and inexpensively using domestic communication satellites

while, at a minimum, serving CONUS and probably most of North America.

104
PROGRAM DESCRIPTION:

Using signals consisting of short sequences at audio frequency, 9.8 kHz,

followed by a digital address in which the audio cycles are inhibited for

zeros and transmitted for ones, range measurements were made between the

ATS-5 satellite and three locations on the earth's surface. These ranging

signals were sent (originated) at Schenectady, NY and transmitted to the

ATS-5 satellite at L band (1651 MHz). The audio signal was frequency

modulated onto the carrier and occupied a total RF bandwidth of 60 kHz.

The tri lateration network consisted of the master ground station near

Schenectady, NY and remote, unmanned transponders in Buenos Aires,

Argentina and a location near Wahiawa, Hawaii. In the operation, NY

master sequentially sent ranging signals to the satellite which

translated them in frequency and retransmitted them back to earth. Each of

the three ground stations had a unique address and responded only to that

address. The two remote, unmanned transponders responded by retransmitting

the ranging signal to the satellite, after a fixed delay, for eventual

reception by the master. In this way the slant ranges r^ , r

2

» and r^ were

measured. In actuality 2r^ , 2(r^+r2) and 2(r^+r^) were measured from which

r^ , r

2

, and r^ were obtained.

The accuracy of the tr ilateration network was determined by

comparison with NASA's C-band range and range rate measurements to ATS-5

from the NASA Rosman, NC and Mojave, CA tracking stations. A 2^ hour test

was run where both NASA's C band range and range rate tracking network and

L band tr ilateration tracking network were operated simultaneously.

The position for ATS-5 computed agreed very well with the NASA

105
derived positions. The accuracy of the tr ilateration positions were

checked further by computing slant ranges from the ATS-5 satellite to

NASA’s Rosman and Mojave sites and conparing these with the NASA measured

slant ranges.

Additionally, the study determined the positions of ATS-5 on 17

separate days. A comparison of these positions with NASA predictions

generated from biweekly range and range rate measurements showed

excellent agreement within the bounds of the gradual degradation of the

NASA predictions as a function of time from the epoch date of the orbital

elements.

The experiment also used three 10 minute ranging periods separated by

an hour to define the orbit of the ATS-5 satellite. The position of the

satellite was then predicted for the next two hours. The predictions were

verified by the NASA network as well as against trilateration measurements.

The conclusion and findings of the trilateration program, particularly during the 24 hour tests directly relevant to the ITS are:

1. L band trilateration positions of ATS-5 agreed with NASA’s

computed positions based upon their C band range and range rate

measurements to within 0.00050 degrees in longitude, 0.00025

degrees in latitude, and 50 meters in earth center distance.

2. Precision of single observatory-satellite slant range

measurements were on the order of 12 meters; for single remote

transponder-satellite slant range measurements, 23 meters.

106
3. Computed slant ranges based on trilateration and NASA measured

slant ranges to the NASA Mojave tracking station agree to within

1.6 ± 9.6 meters and to the NASA Rosman tracking station to

within 15.7 ± 8.7 meters.

11. ..5. . .

6. Satellite position predictions degraded by less than 0.00075

degrees in longitude and latitude and 200 meters in earth center

distance in one hour from the last range measurement.

The findings can be condensed into the following facts pertinent to

the ITS tracking problem. Position data of an accuracy and precision

stated above when used to establish a single Keplerian orbit can be used

for short periods of time (up to one hour) to predict the satellite's

position to better than 0.00075 degrees in longitude and latitude and

better than 200 meters in earth center distance. The issue for the ITS

is: What will the performance be for a trilateration system that must be

contained, by virtue of its footprint, to CONUS and possibly Hawaii and

Puerto Rico? The trilateration network included Schenectady, NY, Hawaii,

and Buenos Aires, Argentina. To attempt to answer that question an

analysis was made of the geometrical effect on position determination for

a general trilateration network.

Using the analysis tool developed in Appendix III, the geometric

dilution of position (GDOP) for various satellite and tracking sites were

calculated. The GDOP's show that the effects of moving a tracking site

from the southern hemisphere to the northern hemisphere greatly magnifies

the GDOP's in the Z direction. Likewise, allowing the satellite's

107
longitude to be outside of the tracking site’s meridian rather than

between them will result in greatly increased GDOP's in the X and Y

directions. By careful design of the network it is shown that the ITS can

be tracked with sufficient precision that satellite position fixing

results of a domestic satellites similar to the tr ilateration study can be

expected. For instance, referring to Table 9, a CO-HA-FL tracking

configuration produces a GDOP of 32.6 or just a little more than three

times the GDOP computed for the tracking configuration of HA-NY-Argentina.

The position error variances for the two tracking configurations can be

made equal by improving the range error variances by this same factor.

The study had quoted their range error variances as between 80 and 120

nanoseconds. This means the CO-HA-FL tracking configuration should work

equally as well for prediction purposes if the range error variances are

reduced to approximately 30 nanoseconds. In Section B6, the range

resolution is calculated to be less than 27 nanoseconds with less than one

second averaging times. This will be possible because of greater C/N^'s

expected to be available to the ITS and therefore higher resolution for

the slant range measurements. Additionally, benefits should be realized

by:

1) Simultaneous slant range measurements. (the study used

sequential measurements).

2) More averaging of data.

3) Reduced uncertainties in tracking sites locations.

4) Consideration of the effects of the moon and sun in determination

of a set of orbital elements and in the prediction process.

(This was not used by the study)

108
Table 9. Calculated GDOP ’

TRACKING

SITE

LOCATIONS

SATELLITE

AND

LOCATION

Ox/

/oi

V V

(GDOP)

+ 0/ + 0,^

HA-NY-Argentina 6.3 2.0 7.3 9,8

NY-HA-FL ATS-5 8.6 2.3 37.7 38.7

NY-CA-Argentina 105°W 13.2 4.4 8.2 16.2

NY-CA-FL 10.0 5.7 46.4 47.8

CO-MD-PR 89°W 23.8 3.56 38.5 45.5

CO-HA-FL 7.39 3.61 31.5 32.6 119°W

CO-MD-FL 27.8 18.8 48.5 56.8

109
5) Continuous tracking of the satellite.

6) Reduced prediction times from one hour to 10 or 20 minutes.

7) Use of Kalman filter techniques to make better use of our

understanding of the mechanics involved.

Based upon the work with the tr ilateration of the ATS-5 spacecraft,

it has been concluded that the ITS tracking of domestic satellites at

either C or KU band can be accomplished as well as or better than in the

GE case. This is especially true since by providing continuous timing

signals there is a continuous source for trilateration. As with GE, the

three measurements of satellite range once each hour is satisfied. The

results of prediction can be monitored and a new orbit fit to the data as

needed. In effect, a sliding set of data can be envisioned, i.e., three

sets of positions one hour (or any other interval) apart at any time may

be used to generate orbital elements from which predictions are made based

upon a Keplerian orbit. As that prediction degrades and falls below some

threshhold, a new set (three position values) of data is picked and

processed. Other algorithms with other sets of input data may provide

better results and should be investigated. However, we can be confident

that, provided the ranging resolution and accuracy are available, a

better-than-one-microsecond prediction system can be maintained. The

problem of orbit maneuvers is still present but its effect can be greatly

reduced in magnitude and duration due to the continuous real time tracking

of the satellite. Simple techniques such as the ’’adjustment" of the

output ephemeris to keep each tracking station "on time" could be used

until a valid new orbit could be calculated. "Adjustment" could be the use

110
of satellite position just determined. Typically the satellite might be

moving by 20 ns/s. Measurement of the range rate could be used to aid the

prediction process.

N5. DENIAL OF SIGNALS TO UNAUTHORIZED USERS

The NBS has for many years offered time and frequency information

free by way of their radio broadcasts from WWV, WWVB, WWVH, and the GOES

satellites. Government policy essentially mandates or strongly encourages

that new services such as the ITS operate on a "full cost recovery through

user charges" basis. Responding to this policy means that the usefulness

of the ITS signals must be denied to unauthorized users or at least made

unreliable or difficult to use by them. Unauthorized users are those

users who have not paid for the use of the signals. A user becomes

authorized through subscription after which his receiver becomes useful

through some periodic upgrading.

For this part of the study, we consider the use of the Data

Encryption Standard (DES) in denying signals to unauthorized users. The

DES is a complex ciphering algorithm based upon both substitution and

transposition techniques. The DES was first published by NBS in 1975 and

it became a Federal Information Processing Standard in 1977 for the

protection of non-national security information.

The DES defines a set of operations to be performed on a 6^1 bit block

of information, the plain text, to encrypt it into a 64 bit block of

111
scrambled information or encrypted text. This is done under the control

of a 56 bit key block chosen by the sender and known by the receiver.

Data protected by the DES, the experts say, may be recovered by

unauthorized individuals or organizations through the use of the large

computers expected to be available in about ten years. Even then the cost

of using these new supercomputers would make such tasks quite expensive

and impractical when as in the case for the ITS, the key may be purchased

for perhaps only a few hundred dollars. The DES is presently a very

secure encryption device^ and its use in the ITS may seem to be a large

overkill. The logic in using the DES, however, lies with its low cost.

Today the DES may be purchased in the form of a single chip in single

quantities for as little as $20 each. Also certain modes of operation of

the DES make technical sense for its use in the ITS.

The use of the DES in the ITS would be as follows: The user's

receiver would have an access chip which is provided on a subscription

basis by the ITS operator. The chip is a complex LSI device and contains

the cryptographic key and PN sequence code. Unless it is installed in the

receiver, the receiver will not decode the satellite data correctly nor

lock to the incoming PN sequence. Each access chip could be unique or

customized to a particular receiver with its own unique serial number or

encryption key. In other words, each receiver would have a unique master

key requiring a unique working key. The unique access code, which changes

every year, must be installed in the user's receiver. This access code

could be mailed as a chip and installed by the user. Alternatively, the

112
access code could be sent as part of the data from the satellite and

loaded automatically into the receiver.

The annual renewal process would be very much like that for a

magazine subscription. A month or two before the subscription expires, a

renewel notice is sent. The user must send his annual fee to the operator

who in turn sends the access code as a chip by mail or by satellite

transmission.

An LSI device is an excellent media for the access code. Its

production costs can be very low but its cost to copy would be far too

difficult and costly to be any significant threat to the ITS support base.

Development of the access chip and billing procedure can be subcontracted

to a manufacturer. Transmission by satellite is possible but would

require a modified data format over that presented in this study.

The subscription fees must cover the costs of operation and

maintenance of the ITS. About 2000 timing receivers for the GOES system

have been sold, each costing between $3000 and $5000. We estimate that

the ITS will have as many as 3000 customers. If each paid $200 per year,

there would be $600,000 per year income, easily covering operational and

maintenance costs.

The DES has a number of modes of operation. An analysis, not covered

here, has determined that the Output Feedback Mode (OFB) will work best

113
with the ITS. Generally there are three functions the receiver must

perform to operate properly and to synchronize to encrypted data. These

functions are:

1 . The receiver must use the same key for decryption as was used for

encryption.

2. The receiver must use the same initialization vector (IV) as was

used in transmitting.

3. The receiver must find the start of the cipher text in the

received data.

In the case of the ITS there will be a continuous transmission of

data. The data must, therefore, be constructed to have a beginning and an

end. This process must be repeated continuously. The determination of

which bit in the received data starts the first cipher text is mandatory.

Once the receiver correctly finds the first bit of the cipher text,

decryption synchronization is reduced to counting received data bits.

The most common solution to the problem is to precede the cipher text with

a synchronizing data pattern. This is a pattern which has an

autocorrelation function which peaks at the in-phase position and is low

elsewhere. Once the pattern is detected, it is used to point to the start

of the cipher text. The beginning of an encrypted transmission in the OFB

mode is also preceded by a fixed length initialization vector (IV). The

position of the IV is also determined from the sync pattern.

114
In the OFB mode, the DES is used to generate a pseudorandom binary

stream which is modulo-two added to plain text to produce cipher text.

This DES output is fed back to form all or part of the next input to the

DES. The number of bits fed back may be as few as one and as many as 64.

Encryption begins by loading an IV as an input block to the DES. The IV

is processed through the DES device operating in the encrypt mode and the

process yields a block of pseudorandom data. This block of pseudorandom

data is added on a bit-for-bit basis to the plain text to produce cipher

text. This block of pseudorandom data from the DES is fed back to form a

new input to the DES and the block is again encrypted, yielding another

block of pseudorandom data. This new pseudorandom block is added to the

next block of plain text to form cipher text. This process is illustrated

in Figure N16. This process is repeated for each block of plain text to

be encrypted.

The decryption process is identical to the process of encryption.

The pseudorandom stream is generated in exactly the same manner as that

used for encryption. The keystream is modulo-two added to the received

cipher text to create the plain text. In order to decrypt correctly, the

decryption keystream generation process must be synchronized with the

encryption process. That is, the same bit of the keystream used to

encrypt a particular bit of plain text must be used to decrypt the

corresponding cipher text bit.

The OFB mode had one major advantage over the other modes. Since the

two keystreams are generated independently and do not depend upon the

115
K BITS K BITS

SHIFT FEEDBACK SHIFT FEEDBACK

Fig. N16. K-bit Output Feedback Mode (OFB)

116
integrity of the received data, OFB mode does not cause error extension.

A single bit error in the received cipher text results in a single bit

error in the plain text. This property is appropriate for the ITS

application.

The DES operated in OFB modes can be thought of as a pseudorandom

sequence generator. The IV in OFB mode determines a starting point in the

particular output sequence. Each time the transmission is started, the

encryption process should begin with a new randomly derived IV. This will

minimize the possibility of multiple use of the same portion of the

pseudorandom output sequence. If the receiver started each time with the

same IV, the identical sequence would be generated and used to encrypt

plain text. Synchronization between the transmitter and receiver requires

that both start at the same point in the pseudo random sequence. In

addition both must generate the sequence in step with one another. In

other words, the same bit of keystream used to encrypt a bit of plain text

at the transmitter must be used to decrypt the corresponding cipher text

bit at the receiver. The receiver must generate the same sequence as the

transmitter but delayed by any propagation delays between them. If the

receiver does not generate keystream in synchronism with the transmitter,

approximately 50 % of the decrypted plain text bits will be in error.

Starting at the same point in the sequence requires that both

transmitter and receiver begin with the same IV. In OFB mode, this

requirement is much more important than in other modes. Other modes will

eventually synchronize even if both the transmitter and receiver start

117
ONE FRAME

SYNC

WORD ////

V y V V Y ^ <ENCRYPTED

<XPr^>CK

SYNC

WORD

//// Y Y Y Y V

.ENCRYPTED*

>Ck5^Jax>

SYNC

WORD

///J

VIV3/

'Y//

y Y Y Y -/ V ENCRYPTED/

V^DATAyM Y YV^

j

'N*"

ENCRYPTED FEC APPLIED

Fig. N17. ITS Data Format

118
with different IV' s, given that the block boundaries are correctly

established. However, if the transmitter and receiver in OFB mode are

started with different IV’ s and are clocked at the same rate, they will

never synchronize.

The data stream for the ITS is shown in Figure 17. One section of

the encrypted message contains the sync word, and IV and the encrypted

data. This structure will allow any receiver that has either just been

turned on or has lost sync to properly sync or re sync to the data stream,

load the IV, and decrypt the data. The sync word is a fixed word that is

repeated frequently, possibly every second. The frequency of the

occurences of the sync word is a fixed word that is repeated frequently,

possibly every second. The frequency of the occurrences of the sync word

would be a function of the error rates that can be tolerated for the IV

and data. The IV and encrypted data may be protected from errors by

adding Forward Error Correction (FEC) if needed such as a rate 1/2 code.

Having the sync word in the clear and repeated at a fixed rate provides

its own error correction or error forgiveness. If the receiver is in sync

it should see the same sync word repeated at a given rate. If the sync

pattern shows up in the IV data field it is highly unlikely it would appear

again at the proper interval. Sync to the message frame is assured then

when three or four sync words are detected at the fixed intervals of the

data stream. If this test fails, sync is assumed to have been lost and

the receiver reverts to the mode of resyncing to it.

119
TOD

IV

Fig. N18. Generation of Initialization Vector (IV)

120
At the transmitter, the IV s might be generated in the following

manner. The time of day is the input or seed to the DES machine operated

as shown in Figure N18. The TOD is also the key to the DES initially.

After one cycle of the DES machine, a switch is thrown so the output of

the DES becomes the input and the key. Each cycle of the DES produces a

random 64 bit output which may be used for an IV in the ITS transmission.

N6. LINK CALCULATIONS

The objective for the ITS is to deliver to the user a signal which is

synchronized to the NBS(UTC) time scale at a level of one microsecond or

better accuracy. The two main factors are accurate satellite position and

a signal strength to yield sufficient precision. The issue on satellite

predictions has been discussed in Section N4. The results of those

investigations were favorable and it has been concluded in this report

that a simple trilateration network will support the one microsecond

accuracy for the ITS. In this section link calculations are made to

determine the synchronization precision that will be available using a

transponder in a typical communications satellite operating in the FSS at

KU band.

The uplink from the master earth station to the satellite requires 89

dBW EIRP at the earth's surface to fully saturate this typical KU band

transponder on a geostationary satellite operating in the Fixed Satellite

Service. The transponder is assumed to be supporting 100 separate users

in a single channel per carrier (SCPC) basis. The required input backoff

121
in the transponder necessary for SCPC operation and for reduction of

intermodulation products, is assumed to be 10 dB. The maximum EIRP per

channel is then 53.4 dBW.

Using a 1 .6 dB/K spacecraft quality factor for G/T and including

reasonable allowances for the path loss, tracking loss etc. , a C/N^ for

the uplink is calculated to be 74.9 dB-Hz. The downlink, satellite to the

user, is calculated to have an EIRP of 13.4 dBW. The user's receiving

system noise temperature is taken to be 250K or 50K for the antenna and

200K for the LNA . The C/N^ for the downlink is then 53-3 dB-Hz. The link

is therefore downlink limited as expected when uplinking with a large

antenna and placing the burden on a small receiving antenna on the other

end. The combined C/N^ is 53.3 dB-Hz. A tracking bandwidth of 3 Hz was

used to compute the timing-error variance in the delay-lock loop. The

result was 27 nanoseconds, nearly a factor of 40 better than the required

accuracy of one microsecond. The complete link calculation is shown in

Table 10.

0. SUMMARY OF PRELIMINARY DESIGN

A preliminary design for the ITS using a small portion of a typical

FSS transponder has been presented. All technical and cost requirements

for the ITS appear to be achievable through this design. The tracking of

the satellite by means of a simple trilateration network should produce

orbit fits of sufficient accuracy to predict satellite position at the

accuracy levels necessary to support the ITS goals of one microsecond or

122
Table 10.

PARAMETER

UPLINK, 14.25 GHz

Uplink EIRP for Transponder Saturation 89.0 dBW

Total Number of Carriers -25.6 dB

Required Input Backoff -10.0 dB

Required EIRP per Channel 53*4 dBW

Total Path Loss -207.5 dB

Tracking Loss - 1.2 dB

Spacecraft G/T + 1.6 dB/K

Boltzmann’s Constant -228.6 dBW/K-Hz

(C/Nq) up 74.9 dB-Hz

DOWNLINK, 11.95 GHz

Spacecraft EIRP 44.0 dBW

Total Number of Carriers -25.6 dB

Required Output backoff - 5 dB

EIRP per Channel 13*4 dBW

Total Path Loss -205.9 dB

Receive Antenna Gain 42.0 dB

Tracking Loss - 0.8 dB

Receive Carrier Power -151.3 dBW

Boltzmann's Constant -228.6 dBW/K-Hz

System Noise Temperature 24.0 dB-K

Spectral Noise Density -204.6 dBW/Hz

(C/Nq) down 53.3 dB-Hz

Combined (C/N^) 53.3 dB-Hz

Timing Error Variance. . (T^ = 10 ys, Bj__ = 20 Hz/3 Hz) 70ns/27ns

123
better synchronization to UTC(NBS). These conclusions on the tracking

system were arrived at through direct comparisons with extensive

experimental work done in Ref. 1 and NASA during the mid 1970’s.

The partial transponder typical of the FSS was shown to be capable

of supporting the one microsecond objective while working into small user

antennas. Through use of antennas, amplifiers, and downconverters being

manufactured for the Direct Broadcast Satellite industry on a very large

scale, acceptable cost user equipment appears to be feasible. The use of

the Data Encryption Standard (DES) for denying the signals to unauthorized

users will guarantee that only fully subscribed users will benefit from

the ITS signals. The DES will be a cost effective solution to insuring

the "full cost recovery through user's fees" policy.

A spread spectrum signal will p-rovide the tracking and timing

precision necessary for the proper functioning of the ITS while protecting

the data from interference from adjacent satellites and terrestrial

sources. The spread spectrum signal will make a small user antenna

possible while adding an additional inhibitor to its use by unsubscribed

recei'^ers.

The preliminary design requires additional study efforts in order to

optimize all parameters and to insure proper interaction of all system

components. Final verification of the ITS operation can be accomplished

through limited experimental use of a partial satellite transponder and

full tracking system deployment accompanied by the testing of a prototype

user receiver.

124
PHASE III, Part 2:

THE GLOBAL POSITIONING SYSTEM OPTION

CONTENTS

P. PHASE III, Part 2 INTRODUCTION

Page

126

Q. GPS SYSTEM OVERVIEW 128

1 . THE SPACE SEGMENT 1 30

2. USER EQUIPMENT SEGMENT 133

R. TIME FROM GPS 137

S. RECEIVER SYSTEM COST 138

T. CONCLUSION REGARDING GPS 141

125
P. PHASE III, Part 2 INTRODUCTION

The NAVSTAR Global Positioning System (GPS) is a worldwide navigation

system developed by the U.S. Department of Defense for use by the Armed

Forces and our Allies. Completion of the system is targetted for between

1988 and 1990. Time information is transmitted in the system in two

forms, one for DoD use only (the P-code) and one for DoD and civilian use

(the C/A code). The P-code is a high precision mode and is not available

to civilian users. However, the C/A code used for receiver coarse

acquisition will have time transfer accuracy to 250 ns or better. The C/A

code would meet the 1 ps criterion needed by BPA.

Seven GPS satellites are currently in orbit and are being used for

evaluation purposes. Eighteen satellites make up full implementation of

the next phase called BLOCK II (full coverage using nonstationary,

low orbit satellites). Atomic standards (rubidium or cesium) are carried

on each.

The current DoD policy regarding civilian access to the C/A code

is available from the U.S. Naval Observatory automated data service. The

latest statement is shown below (12/20/85):

Revised Comprehensive GPS User Policy (Effective 22 May 85):

"The GPS is a continuous worldwide satellite-based radio

navigation system currently in full-scale development by DoD.

The system will provide properly equipped users the capability to

obtain navigation and geodetic positions in three dimensions,

velocity in three dimensions, plus highly accurate time. The

126
system will simultaneously transmit navigation information that

permits positioning accuracy at two levels. The higher level of

accuracy can be obtained from the standard positioning service

(SPS)

This policy provides that DoD intends that the SPS signal will

be broadcast in the clear and will be available for use by any

properly equipped user. There will be no annual or other direct

fee associated with the use of this signal. The SPS will be made

available to civil, commercial, and other users on an international basis at the highest level of accuracy consistent with

U.S. national security interests. It should be noted that at the

direction of the defense subcommittee of the senate appropriations committee, the GPS has been designed and engineered in a

manner to protect the user-fee option should it be appropriate in

the future. If Congress does decide to require a user-fee

implementation in the future, an appropriate time would be

allowed for the transition of user equipment into a user-fee

configuration.

Some key issues regarding the C/A code service are in the

process of being evaluated as pointed out in the last CCIR concluding

report

1. The first priority of GPS is for military tactical missions, and

as such civilian users have no control, of the

C/A service.

127
2. Possible degraded service of the C/A code is 250 ns (two sigma

estimate), but the level of degradation could change depending on

prevailing military policy.

3 . GPS contains an implementation of the service using a user fee,

but present policy is to allow free access.

This section of the BPA study will elaborate on the NAVSTAR GPS. In

spite of key issues, it is useful to review GPS as a viable solution for

BPA’s requirements because some of the service is available now.

Furthermore, the full military implementation is virtually assured in the

coming years.

Q. GPS SYSTEM OVERVIEW

The NAVSTAR GPS concept evolved from a DoD Joint Program Office which

was established in 1973- It's purpose was to consolidate separate

navigation systems which were in service (or proposed) to form a single

military -based, highly survivable system to serve all armed services. The

GPS is jointly supported by the Army, Navy, Marine Corps, Air Force, and

Defense Mapping Agency. The Air Force is designated as lead agency to

develop, test, acquire, and deploy the GPS. Bradford Parkinson and

Stephen Gilbert were among the leaders of the GPS planning and development, and much of the material in the section comes from their

literature [ 2]

128
The GPS is a satellite-based radio navigation system intended to

provide highly accurate three dimensional position and precise time on a

continuous global basis. When the system becomes fully operational, it

will consist of 18 satellites in six orbital planes inclined at 55°. Each

plane will contain three satellites spaced 120° apart in 12 hour orbits.

The relative phasing of the satellites from one orbital plane to the next

is 40°. Each satellite will continually transmit navigation signals at LI

= 1575.42 MHz and L2 = 1227.6 MHz consisting of the P-code ranging signal

(10.23 MBPS), the C/A code ranging signal (1.023 MBPS), and 50 BPS data

providing satellite ephemeris and clock bias information.

Navigation using GPS is accomplished by passive triangulation. The

GPS user equipment measures the pseudorange to four satellites, computes

the position of the four satellites using the received ephemeris data; and

processes the pseudorange measurements and satellite positions to

estimate three dimensional user position and precise time as depicted in

Figure Q1

There are situations where the user only has three satellites

available and still wishes to utilize GPS. This can be accomplished by

equipping the user with a precise clock synchronized to GPS time.

129
Q1 . THE SPACE SEGMENT

As currently planned, the operational NAVSTAR GPS will consist of 18

primary satellites deployed in six orbital planes. These will be

augmented by three on-orbit spares to insure a high degree of system

availability. To accomplish this, the Air Force awarded a 1.17 billion

dollar multi-year procurement contract, one of the first of its kind, for

production of 28 NAVSTAR GPS satellites. The first four of these

satellites are to be delivered in 1986; eight more in 1987; nine in 1988;

and the remaining seven in 1989. The planned launches aboard the Shuttle

closely follow the satellite delivery schedule.

Table 11 gives the reference orbit values for this baseline

deployment, including the three on-orbit spares which are located in every

other orbit plane to provide quick reaction and recovery in the event of a

primary satellite failure. The satellites are all in circular orbits at

altitudes of approximately 20183 km (10898 nmi)

This selection of parameters results in a repeating daily ground

trace or, in other words, an orbital period that causes each satellite to

pass over the same point on the earth every 23 hours 55 minutes and 56.6

seconds, i.e., a sidereal day. This could have been accomplished by

placing the satellites at synchronous altitude —the concept embodied in

the FSS and GEOSTAR approaches. However, to avoid the need for a global

tracking and control station network that could continuously "see" each

satellite in the system, the orbital altitude was selected to provide a

130
TABLE 1 1 . REFERENCE ORBIT PARAMETERS:

BASELINE SATELLITE DEPLOYMENT

Satellite

Number

Orbit

Plane

Longitude

of Ascending

Node (deg)

Right Ascension

of the Ascending

Node (deg)*

1 1 0.180 30

2 1 240.60 30

3 1 300.120 30

4 2 260.80 90

5 2 320.140 90

6 2 20.200 90

7 3 340.160 150

8 3 40.220 150

9 3 100.280 150

10 60.240 210

1

il 120.300 210

12 4 180.00 210

13 5 140.320 270

U 5 200.20 270

15 5 80.260 270

16 6 220.40 330

17 6 280.100 220

18 6 1 60.340 330

Spares

19 1 95.15 30

20 5 215.35 270

21 3 25.105 150

*Referenced to Astronomical Coordinates of 1950.0 as of July 1 , 1985 0 h 0 min GMT

and regressing at -0. 0^009 °/day.

131
period exactly one half synchronous— 11 hours 57 minutes and 58.3

seconds. Another way of looking at this is to imagine that each satellite

completes exactly two orbits while the earth turns one complete revolution

on its axis thereby causing the satellite to pass directly over the same

spot once a day. The difference between this and the solar day of exactly

24 hours causes the satellites to pass over the same spot about 4 minutes

earlier each day. The real importance of this feature of the system

concept is to allow each satellite to be viewed by a single control

station at least once each day, thereby eliminating or at least minimizing

dependence on stations outside of the direct control of the United States.

The minimum number of satellites required to provide continuous

global coverage with at least four satellites in view at a given location

is 18. Thus a satellite failure in any of the six orbital planes could

disrupt service (for position determination) over one or more critical

areas of the globe. The time required to launch, check out, and place into

operation a spare satellite is likely to be unacceptable from an

operational standpoint, and this will become even more critical as civil

and international users of the system become more plentiful and dependent

upon the system. The ability to quickly reposition one of the on-orbit

spares, which has already been checked out and placed into full operation

will be a major asset to the system reliability and availability. In

addition, while the baseline constellation of 18 satellites is fully

operational, careful selection of the "storage location" for each of the

three on-orbit spares will result in significantly enhanced coverage and

accuracy over any specific area —e.g., the continental United States.

132
The widely dispersed monitor stations employ extremely stable GPS

receivers to gather transmitted navigation data from each of the GPS

satellites as they pass overhead. The location of each monitor station is

precisely surveyed and the navigation measurements are combined with

additional data on atmospheric conditions, etc. This information is

transmitted back to the Master Control Station where precise predictions

of satellite ephemerides and clock offsets are made.

The Master Control Station, to be colocated in Colorado Springs, CO,

with the Consolidated Space Operations Center (CSOC), will process the

data received from all of the monitor stations to determine the predicted

satellite ephemerides and clock bias parameters for each satellite in the

system. These data are then used to generate upload messages for each

satellite to correct the satellites’ navigation messages describing those

parameters to the users. The upload messages are then relayed to the

appropriate upload station for subsequent transmission to the satellites.

Q2. USER EQUIPMENT SEGMENT

The NAVSTAR GPS User Equipment Segment is a separately operated

department which provides development and testing of military receiver

hardware. It also provides information regarding nonsecure

specifications for civilian users. The information here is derived

directly from the GPS literature.

133
/

I

Fig. Q19. GPS Four Satellite Position and Time Recovery

134
Each satellite continuously transmits a unique navigation message

that contains the information that the user equipment requires to perform

the measurements and computation to effect the navigation solution. All

satellites transmit on the same two frequencies, LI at 1575.^2 MHz and L2

at 1227.6 MHz, coherently derived from the 10.23 MHz atomic frequency

standards on-board each satellite. The LI signal is modulated with two

pseudo-random noise codes in phase quadrature, historically called the

Precise (P) and the Coarse/Acquisition (C/A) codes. The L2 signal is

modulated only by the P-code during normal system operation, but for

special applications and testing the P-code modulation on L2 can be

switched to the C/A code. Both the LI and L2 signals are also

continuously modulated with a navigation data bit stream at 50 bit/s.

The P-code operates at 10.23 Mbit/s and completes a code generation

cycle in 267 days. It is the modulo-2 sum of the output of two pseudorandom code generators, each of which employs an input from the sum of the

output of two subsidiary generators. Each satellite uses the same basic

code generator, and each is assigned a unique and mutually exclusive 7 day

segment of the 267 day cycle. The initial state of the code in each

satellite is made to occur every 7 days by resetting the two code

generators to their initial state at the end of each week. In this

manner, using a code-division, multiple-access scheme up to 36 satellites

could transmit synchronized navigation signals simultaneously on the same

two frequencies and each satellite can be uniquely identified by the 7 day

phase segment of the long code it is transmitting.

135
The P-code provides the fundamental accuracy of GPS. However, unless

the user equipment has been provided with both a precision time reference

synchronized to GPS time and an estimate of current position (within 3~6

km), direct acquisition and tracking of the code segment for any specific

satellite will be extremely difficult. It is necessary in most instances

to resort to initial acquisition on the shorter C/A code. They operate at

1.023 Mbit/s, as compared to the 10.23 Mbit/s rate of the P-code and

they repeat each millisecond as opposed to the 267 day cycle of the

P-code. Thus the C/A codes are much easier to acquire without prior

knowledge of system time and user position. As part of the data stream

contained in the modulated C/A code, a Handover Word (HOW) is transmitted

every 6 s, indicating the correct phase point in the incoming P-code

associated with the transmitting satellite. Based on this information,

the user equipment P-code generator is shifted in phase to synchronize

with the incoming P-code at the next change of the HOW.

The navigation message transmitted by each satellite includes the

following information:

1 ) Status of the satellite so the user can either accept or reject

the data for use in the navigation solution.

2) The HOW used to determine time synchronization for transfer from

the C/A to the P-code.

3) Satellite clock correction and ephemeris parameters.

136
4) Parameters for correcting propagation delays through the

ionosphere.

5) Almanac information including ephemerides and status of all other

satellites in the system.

Obtaining time from GPS is a three step process. One must first

determine the time kept by the satellite being tracked. Next, one applies

corrections to the satellite time to derive UTC(GPS) . The reference for

GPS time is an ensemble of three cesium clocks at the Master Control

Station at Vandenberg AFB . Then, one applies corrections to UTC(GPS) to

obtain UTC(USNO) which in turn allows calibration to UTC(NBS) through

published bulletins. A complete navigation message consists of 1500 bits

of information. A subframe of 300 bits is transmitted every 6 s. From

Subframe 1 of the navigation message one computes the relationship

R. TIME FROM GPS [3]

^ = ^sv ^fO ^fl^t - t^^) - a^2(t - t^^)2 + (At^)

where

t GPS system time

t sv space vehicle PRN code phase time

afQ,afi,af2 polynomial coefficients of Kalman filter determined by

MGS

t oc clock data reference time

At

r relativistic term which user must now calculate

F

= Fe(A)^^2sinEj^

constant = -4.443 x 10~^^ s/(m)^^^

137
One corrects t for several factors, e.g., ionospheric correction,

tropospheric correction, dither (if present), etc.

tg = t + corrections.

One then gets UTC from

UTC = tgCmod 86400) - At^s ~ ~ “

'‘^OT^’ where

Atj^^g leap second correction (Subframe 4),

Aq,A^ coefficients of polynomial (Subframe 4) provided by USNO,

tQY reference time for Aq,A^.

Presently, At^^g is 3 s, i.e., UTC(USNO) = (GPS time) -3 s. It will

increase by 1 s every time a leap second is introduced.

The received-time marks (one every 6s) adjusted by these corrections

are now within 100 ns of UTC(USNO). Present steering is such that the rms

error of this adjusted time reference is 25 ns.

S. RECEIVER SYSTEM COST

GPS receivers intended for time transfer using the C/A code are

currently priced from approximately $15,000 to $25,000 from commercial

manufacturers. Examples of manufacturers at the time of this report are

listed below:

138
Allen Osborne Associates

756 Lakefield Road, Bldg. J

Westlake Village, CA 91361

Tel: 805-495-8420

TWX: 910-494-1710

Austron Inc.

P. 0. Box 14766

Austin, TX 78761

Tel: 51 2-251 -2341

Ball/Efratom Division

18851 Bordeen Avenue

Irvine, CA 92715

Frequency and Time Systems, Inc.

343 Tozer Road

Beverly, MA 01914

Tel: 617 - 927-8220

TWX: 94-0518 FTS BVLY

Stanford Telecommunications Inc.

1195 Bordeaux Dr.

Sunnyvale, CA 94086

Tel: 408-734-5300

139
Kinemetrics/True Time

3243 Santa Rosa Avenue

Santa Rosa, CA 95^07

Tel: 707 - 528-1230

Telex: 176-687

Interstate Electronics Corp.

1001 E. Ball Road

P. 0. Box 3117

Anaheim, CA 92803

Tel: 7U-758-0500

TWX 910 591 1197

There are more manufacturers who supply receivers for navigation use only.

Prices are coming down as more receivers are built and sold as a result of

lower manufacturing costs. Competition is also causing some price

reductions. Although a reducing price trend should continue for a number

of years, the market for timing receivers is very limited. The navigation

market on the other hand is considerably larger (by possibly 1000 times or

more) so the predictions by manufacturers for prices to ultimately fall to

approximately $1000 are probably valid. Even though the time transfer

capability is closely tied to the navigation capability within GPS, the

support, added circuitry, reliability aspects, and special timing

requirements all suggest that the cost of timing receivers will not be nearly

as low.

140
T. CONCLUSION REGARDING USE OF GPS

The Global Positioning System in its present phase of study has

demonstrated its normal time transfer capability to better than 100 ns.

The military has stated that the civilian time transfer capability may be

degraded to 250 ns (2 sigma estimate) in the final (Block II)

implementation which will be operational by 1988-1989. This level of

performance will meet BPA's 1 ys accuracy goal.

There are no technical stumbling blocks to using GPS to satisfy BPA's

timing requirements. Receiver costs are high at this time

($15 ,000-$25 ,000) with a possibility of future reductions, although we do

not believe the costs will drop to the same degree the navigation receiver

costs are projected to drop.

An important issue still is that there is an unsatisfactory statement

of policy regarding a civilian (nonmilitary) time service via GPS.

Realizing BPA will invest a great deal of time and money toward an

internal timing system, the use of GPS carries a risk if it is to be its

source of timing. Although the service is now freely available, there is

also no military statement regarding future subscription fees in the

actual Block II implementation.

141
The GPS policy as stated by DoD is a strong indicator of an intent to

accommodate civilian users to the fullest extent possible given a

peacetime situation. However, any level of military need which is greater

than that of the prevailing peacetime situation can change DoD’s policy.

There is no policy now regarding civilian use of GPS in a national alert

situation. Certainly a full scale national emergency situation would put

military use as a top priority.

142
PHASE III, Part 3:

THE GEOSTAR OPTION

CONTENTS

U. PHASE III, Part 3 INTRODUCTION 144

V. GEOSTAR SYSTEM OVERVIEW 145

1 . GEOSTAR MARKET 1 48

2. GEOSTAR FCC AUTHORIZATION 150

3. SYSTEM CONFIGURATION 151

4. EXPERIMENTAL TESTS '55

5. GEOSTAR TECHNICAL SPECIFICATIONS 157

W. ESTIMATED TIME TRANSFER ACCURACY OF GEOSTAR 1 62

X. PROSPECTS FOR HIGH ACCURACY TIMING 171

Y. CONCLUSION REGARDING USE OF GEOSTAR 175

143
U. PHASE III, Part 3, INTRODUCTION

During the course of this contract, NBS has gotten information regarding a

new navigation system based on geostationary satellites. This system is

called GEOSTAR, and because of the additional potential for precise time

dissemination GEOSTAR has the potential for satisfying BPA system

requirements. It is important to keep in mind a number of factors:

1) The GEOSTAR system is not an operational service. It is a system

which is projected to have the qualities which will be described

in this section.

2) The system has as its principal feature navigation and message

handling capability. An enhanced mode allows GEOSTAR to deliver

time to a high precision to a special GEOSTAR receiver.

3) Much of the information regarding GEOSTAR is proprietary. NBS,

given access to much of this information, has attempted to draw

clear technical conclusions. Information presented here will not

be detailed on those proprietary matters.

4) The GEOSTAR system is intended strictly for civilian applications.

There are no military sources of funds nor any support funds that

we are aware of other than those ultimately derived from end

users. In this sense, the GEOSTAR system is a risk venture for

those people investing in the system at this early stage.

144
We have tried to evaluate the GEOSTAR prospect in enough detail to get a

clear understanding of the advantages and disadvantages of this future

system. Although there are a number of questions still remaining, we

still believe that GEOSTAR needs to be explored for satisfying the BPA

requirements. We have seen enough documentation and evidence to warrant

the time for this phase of the study.

V. GEOSTAR SYSTEM OVERVIEW [4]

GEOSTAR Corporation was founded in February 1983 by Dr. Gerard K. O'Neill.

The company was formed to look at the feasibility of a satellite based

system to provide position determination, navigation, two way digital

message service and emergency location services to transceivers used by

the recipients of the service. The original feasibility study stemmed

from the airline industry’s desire to move in a direction of automated

aircraft navigation. Some work was done prior to 1983 but O'Neill at that

time gathered enough support that the GEOSTAR system was created for

general civilian use. The work done by GEOSTAR has passed much of the

preliminary feasibility stage. A key experiment has been completed in the

vicinity of the Sierra Mountains (Nevada, U.S.A.). This experiment used a

ground station, a user transceiver and satellite emulators located on

mountain peaks. From this technical feasibility study, there emerged a

number of other studies and directions which further proved the viability

of such a system. More recently the FCC has approved the use of the

145
USRDSS system which will consist of authorization for a number of operational satellites in geostationary orbit to provide information to a large

population of mobile subscribers.

The GEOSTAR system will provide the following services;

1. Navigational positioning

2. Radio location

3. Emergency location

4. Terrain warnings to pilots

5. Warnings of potential collisions between GEOSTAR equipped

aircraft

6. Precise Time Dissemination

7. Two way digital message service

8. Interconnection to ground data bases

As the system evolved, it found additional markets. The market appears to

be very large for items 1, 7 and 8. The system as it has been planned

provides for particular needs which are not met by any other existing

system, the strongest of which is the two way digital message service in

which large volumes of information can be distributed to users and small

amounts of information can be sent from a user to a GEOSTAR central

dispatch or another user. This system of high level multi-point

distribution coupled with information "trickle back" is a desirable

feature of the USRDSS service. A powerful aspect of the GEOSTAR system is

that subscribers are given both a receiver and a transmitter (transceiver) . This ability to transmit data to a satellite on a trickle back

146
basis opens up opportunities not available in most other multi-point

distribution services. It is in fact this transceiver aspect which allows

for precise navigation, hence precise time distribution.

The transceivers are designed to be low in cost. Each uses the same

bandwidth for transmitting as well as receiving. All transceivers are on

the same frequency. The single channel can accommodate a large number of

subscribers for three reasons.

1. Each transceiver is silent most of the time, transmitting only

in short, occasional bursts.

2. Transceivers use spread spectrum transmissions, which permit many

messages to occupy the same channel simultaneously without

interference.

3 . Different satellites of the GEOSTAR system will cover different

geographical areas using controlled spot beam techniques. It is

proposed that several satellites be used ultimately. This will

provide wide area coverage and only three ground stations are

seen now as being necessary to provide all services worldwide,

excluding only the extreme polar latitudes. Having a multisatellite system not only allows for space diversity but also

provides more redundancy to back up system capabilities if any

single satellite should fail. A strong point is that

GEOSTAR-like systems could coexist using the same frequency bands

because of the noninterference among various spread spectrum

modem schemes.

147
VI . GEOSTAR MARKET

The market for the GEOSTAR system has changed since the original

feasibility studies were done. Indeed, the market has changed because of

other available services such as the Global Positioning System. The GPS

for example has surfaced many users by drawing attention to new navigation

and timing performance levels never before seen. In a way, the GEOSTAR

system has become considerably more credible because of the GPS system.

There was original skepticism since GEOSTAR in its original concept

offered even more capability (two way message communication). The market

studies to date done by GEOSTAR indicate a very large, worldwide market.

In fact the original market (civil aviation) is diminishingly small and

nearly 0 % compared to over a half dozen other viable markets. The GEOSTAR

marketing reports to date indicate favorable response to the system given

its low cost per user (around $45 per month). Coupled with the ability to

add enhancements, the end user acceptance has particularly grown strong on

two fronts: (1) general navigation, and (2) multi-point data distribution

and information trickle back.

The possible applications of the GEOSTAR system extend to a variety of

areas in today’s society. A few of the more notable applications are

listed below:

Positioning and Navigation. Any GEOSTAR-equipped aircraft could be

located, in three dimensions, to within at most 7 meters. For more

accuracy, as in terminal navigation, the presence of a stationary

ground -mounted "benchmark” transceiver at or near an airport could

148
improve accuracy to 1 meter —far better than current Cat 3 ILS

standards. Guidance can be provided direct from one point to another

or along any path stored in the almost limitless memory of the

central computer, including curved approaches and/or stepped -down

glidepaths. Other diverse uses for the positioning/navigation

capability include taxicabs, fire and police services, trucking and

freight lines, and emergency, service-oriented professional

individuals such as doctors and medical dispatch personnel.

Aircraft collision avoidance. Since the central computer knows the

location of every user and can send messages to any of them either

individually or simultaneously, it can monitor potential collision

hazards and advise the aircraft not just of the hazard, but of its

exact location and the required evasive action. By the same token

traffic conflicts could be avoided by issuance of enroute holding

advisories when necessary.

Terrain avoidance. Among the information in the central computer's

database will be a complete terrain map of the United States, which

will continually be crosschecked against aircraft position and

altitude. Messages such as "pull up--radio tower 3 miles ahead" can

be generated and transmitted automatically to aircraft as required,

without disturbing other user transceivers.

149
Communications. Since each transceiver has a unique address code,

messages can be addressed to it without being registered by other

units. Moreover, the user transceiver can originate messages.

Weather services. The system's communication capability can be used

to transmit routine or special weather information.

Emergency help. The press of a single key places a user transceiver

in emergency mode. If this signal is transmitted by any unit, the

central computer can alert local authorities and respond with

immediate nearby help.

V2. GEOSTAR FCC AUTHORIZATION

On March 31 > 1983 the GEOSTAR Corporation petitioned the Federal

Communications Commission to begin a proceeding to allow spectrum use for

developmental purposes for this new type of satellite system. Developmental licensing was requested for GEOSTAR's original radio determination and

associated messages transfer services to subscribers throughout the

continental United States. At this time no allowable frequency allocations existed for any such services. During the proceedings a substantial amount of support came in the form of comments to the FCC for

the GEOSTAR proposal. Sufficient interest was demonstrated by a very

broad cross section of potential users to suggest that a nationwide radio

determination satellite service would be beneficial and a spectrum should

150
be assigned for this purpose. Only two companies, ONI (Offshore

Navigation Inc.) and MOBILSAT, objected to the overall proposals in the

proceedings.

After one and one half years of deliberation the FCC began a general rule

making procedure to allow spectrum for the USRDSS proposal. The procedure

which took place at the middle of 1984 was abnormal for the FCC but not

without precedent. The FCC typically refuses applications for spectrum

which is not allocated within the FCC rules. An exception was made here

because of the interest in adopting the USRDSS concept. General rule

making was then carried out which created the spectrum for the USRDSS

system and that rule making was complete in the summer of 1985. Needless

to say the GEOSTAR corporation was in a first hand position to gain

licensing and full authorization for the construction of their system.

The general rule making proceeding, consolidated with the application

process, was unacceptable to Commissioner Dawson of the FCC. However, the

other commissioners were generally inclined to move expeditiously on

behalf of a new USRDSS service because of the large number of positive

comments in the initial phases of the proceedings.

V3. SYSTEM CONFIGURATION

The GEOSTAR system is based on several concepts that were unavailable at

the time the Transit and GPS systems were designed. Central to GEOSTAR '

system is the idea of keeping the sophisticated hardware on the ground

while Transit and especially GPS require advanced computational capability

in both the satellites and user equipment. GEOSTAR fundamentally uses

151
transponders and keeps system control located on the ground. Three

satellites plus a fourth inactive on-orbit spare are proposed for the

GEOSTAR system. The satellites are in geosynchronous equatorial orbit

aligned with the Eastern, Central, and Western United States. All three

are above the horizon anywhere in North America and appear to be

stationary. User equipment is a digital transceiver with a function of

exchanging messages with the satellites. They in turn are relay stations

that exchange data with the user's transceivers and retransmit it to the

central computer on the ground.

The central computer transmits an interrogation signal analogous to that

used by today's transponders; one hundred times per second. Relayed by

tight microwave beams to the three satellites the signal from the central

computer is rebroadcast by them on a single frequency of 2492 MHz in a

pattern which covers CONUS.

The user transceivers each have a unique digital signature code which

identifies it. These codes are built in at the factory. When a

transceiver receives the interrogation signal from the satellite, it

answers with its identifier on a frequency of 1618 MHz. A small antenna

system on the user equipment is all that is required for the relatively

low power needs of its transmitter. The satellite antenna system and

modulation technique carries the necessary sophistication to properly

receive any and all identifier signals.

152
The transceiver's answer back identifier is received by all of the

satellites and relayed by microwave to the ground station. There the

computer compares its time of arrival at each of the satellites and

computes using a triangulation technique the location of the user

transceiver. This information is encoded and addressed to the

transceiver and relayed back to the satellites. The total time for the

entire transaction is about 1/2 second.

An important aspect of GEOSTAR 's probable success lies in the tremendous

computational capability which will be available on the ground. Tests of

software on a CRAY-1 supercomputer similar to those planned for the system

indicate that a single such computer could handle the entire projected

maximum system load of 100,000,000 users with relative ease. By

addressing each user individually, the system can handle all of them using

the two frequencies, one frequency used for interrogations and data

transmitted from the satellite and one for identifications and data

transmitted by the user transceivers. The limit on the number of users is

statistically derived, based on a 90 % chance of deciphering signal

collisions from the user transceivers. Because of the nature of the

signals, many transceiver messages can occur simultaneously and the system

will properly decipher them. The system has a reasonable upper limit of

350,000 users per satellite. This number of users allows for channel

contention and still meets the 9055 success within a one second interrogation time. Keep in mind that messages are polled during the uplink and

confirmed during the downlink so that all messages conceivably will be

deciphered, but not all may be deciphered within one second. Because the

153
system software identifies which users are associated with aircraft and

which, for example, are associated with light users such as pedestrians,

it can budget its time and update aircraft position twice per second while

pedestrians may only require updating once every few minutes. As the need

changes the system can adjust its response rate for aircraft to meet en

route or terminal requirements.

GEOSTAR has the potential to provide its stated accuracies with inexpensive equipment because it is a two way, interactive system. Most of the

errors in other satellite locations systems cancel out in GEOSTAR because

positions are measured relative to fixed-site "benchmark” transceivers.

GEOSTAR is a strictly differential, closed-loop system rather than

open-loop system. The basic transceiver has a projected cost of $^50 and

the size is projected to be smaller than checkbook size, having a space

for keypad and LCD display and about an inch of thickness including a

built-in antenna. The system will consume low power much like a present

day calculator, and only two wrist watch batteries are required to power

the unit for about a year. The projected monthly lease of the basic

transceiver unit is expected to be about $45 per month for the basic

services. In all probability, the bulk of the system would serve users

such as fire and police services, taxicabs, freight lines, and individual

pedestrians. Virtually all of these users would subscribe to the

basic service consisting of the navigation and message-handling capability.

154
Vl4. EXPERIMENTAL TESTS

The first test satellite equipment is scheduled to be orbited at the end

of 1985. The GEOSTAR equipment is an add-on package using a proprietarycentral encoding and decoding technique of burst spread spectrum. This

add-on package properly implements the multi-point distribution and

trickle back transceiver capability aboard the satellite. The add-on

package is called the LINK 1 system. Testing of the LINK 1 system will be

carried out through most of 1986 . GEOSTAR has been performing a series of

tests near Lake Tahoe on the California-Nevada border. This site was

chosen because it combines good logistic support including an all weather

airport with surrounding mountains on the summits of which satellite

emulators can be placed. These units are called EMEs (electromagnetic

equivalents) . Each EME is solar powered and simulates the actual

operation of the satellites and their representatively low signal levels.

In 1983 a complete test for the GEOSTAR system was put in place consisting

of a computer control ground station, some user transceivers, and the

EME's. Since the EME's were located on the peaks in the Sierra Mountains

the conditions simulated operation in extreme northern latitudes near the

limits of the proposed satellite coverage. The system demonstrations

included guiding a pedestrian to a hidden marker in a field, guiding a

vehicle to a precise street address, and guiding an aircraft to a

precision landing. In good conditions, positions were repeatable to

approximately one meter. This test was the first full scale test, and

some technical adjustments were made to the system to enhance overall

capability.

155
In 1984 the transmissions were done with high elevation angles similar to

those of a real satellite system. With the EME’s located on two light

aircraft to altitudes of about 3»000 meters the system more closely

approximated projected kinds of satellite angles. The ground station had

additional computing power and calculated the changing positions of the

emulators as they were not in fixed positions. This configuration allowed

more realistic testing, although with precision degraded from that of the

actual satellite system because of the motion of the emulators in

turbulent air. In spite of these effects, the 1984 tests confirmed the

positive result of having a higher elevation angle for the EMEs. The

improvement in position was of the order of 1 meter under a larger variety

of conditions. This is because the prevailing limit of the 1983 tests

was multipath due to terrain clutter around the low lying EMEs.

Throughout the tests the hand-held display was able to transmit its

messages back and forth to the central computer system. The hand-held

display was transferred to the top of an instrument panel on board a light

aircraft. In the approach mode it indicated distance and bearing to a

waypoint, ground track, ground speed, and time to waypoint. During the

final approach of the aircraft, the display changed to include left and

right deviation and glide slope. Accuracy and stability of the position

and steering indications were above the capabilities of any other system

for an aircraft approach. "Distance to go" zeroed out properly over the

threshhold and left and right guidance were accurate to a couple of feet.

Holding vertical speed called for by the display units resulted in

altitude which corresponded exactly with the normal on board vertical

positioning system to within one half meter.

156
One can derive normal levels of expected timing accuracy given the tests

of the differential navigation capability of GEOSTAR. Figure VI shows raw

timing data from the GEOSTAR airborne test system. Standard deviation of

the 1984 tests was 15.9 ns. The projected improvement in timing for the

full system is expected to be to about 5.9 ns standard deviation. This

corresponds to a positioning error of about 1 .6 meters for a typical

location in view of the three satellites.

V5. GEOSTAR TECHNICAL SPECIFICATIONS

The information here outlines the major operational specifications for

Geostar based on its FCC application for licensing in the USRDSS Satellite

Service. A developmental license has been authorized; the information

here, however, refers to the full system configuration which is targetted

for 1987.

1. The Geostar system's space segment is to consist of three

geostationary satellites located at 70° west, 100° west and 130° west

longitude to provide coverage to the contiguous 48 states. Each of the

satellites is to be identical in design. The proposed satellite system

would use random access time division multiplex with slotted ALOHA

protocol and differential phase shift keying (DPSK). The satellites are

to relay information between the system's users and a control center. The

control center will contain facilities to communicate with each satellite

157
Time (nanoseconds)

Fig. VI. Raw Timing Data of Geostar Airborne Test System

158
and computers required to perform position determination and other system

functions. Geostar has proposed to locate its control center at

Princeton, New Jersey.

2. Four communication links are required for the radio-determination

system proposed by Geostar: two space-to-earth links and two earth-tospace links. The users’ radiodetermination uplink would operate within

the 1610-1626.5 MHz band. The users’ radiodetermination downlink would

operate within the 2M83. 5-2500 MHz band. The control center would be

linked to each satellite using a single data uplink bandwidth of 16. 5 MHz

located within the 6M25-7075 MHz band. For the data downlink, the band

5117-5183 MHz would be used to link each of the satellites to the control

center.

3. Operationally, the control center continually transmits via each of

the three satellites what Geostar terms an Interrogation Pulse Group (IPG)

available to all users throughout the continental U.S. The IPG’s are

transmitted at a 100 Hz rate and consist of a preset pattern of 6^ bits

of 80 nanoseconds duration each. The user may respond from what Geostar

calls an Automatic Beacon Transfer (ABT) by transmitting a message which

will be in one of several possible formats, depending upon the needs of

the user. The message always contains timing and identification

information and may contain a user message. Based on elapsed time from

the emission of an IPG from the control center to the receipt of an ABT’s

response via each of the 3 different satellites, the user’s position is

calculated using direct range measurements. The control center then

159
transmits an addressed message, which may include the user's position,

through one of the satellites for relay to the user intended to receive

the message.

4. A user may access the system through three basic types of responses

to the IPG: a position request, an emergency signal or a message signal.

Based on the user's selection for the type of response, a varying length

digital message transmission will be relayed to the control center through

the satellite system. After responding, the user awaits a system reply.

The reply will be either the user position location information or

confirmation that the user message has been received by the control

center and sent out as instructed. If the user does not receive a reply

within 0.6 seconds, either due to coincidence of the user's response with

that of another user's at the satellite antenna, or bit errors on the

user-to-control-center communication link, the ABT will automatically

repeat the response message. Randomization is introduced into the time of

repeat to avoid consecutive coincidences on the uplink.

5. Geostar claims its radiodetermination system will provide position

information with accuracies in the range from 1 to 7 meters. To

accomplish such accuracy. Geostar will employ fixed "benchmark"

transceivers at known locations. Accordingly, systematic errors for each

mobile user's measured range can be corrected by subtracting range errors

to fixed benchmark transceivers in the same geographical area. Such

errors include ionospheric delay variations, drifts in the electronic

delays through the satellites, variations in the positions of the

satellite antennas, and lack of knowledge of details of the earth's

160
shape. The only equipment error which cannot be subtracted, according to

Geostar, is drifting of the electronic delay within an individual user

transceiver

6. At the present time, the 1610-1727.5 MHz band, requested for use to

provide the user radiodetermination uplink, is allocated internationally

and domestically to aeronautical radionavigation. International Footnote

732 provides for satellite-based facilities associated with aids to air

navigation. Within the U.S., no telecommunication use is presently being

made of this band. However, the I 6 IO. 6-1613.8 MHz band is used by the

radio astronomy service for observing the hydroxyl spectral line.

7. The 2483.5-2500 MHz band has been proposed by Geostar to provide for

the radiodetermination downlink to users. The band 2483.5-2500 MHz is

part of the 2400-2500 MHz industrial, scientific, and medical (ISM) band.

Geostar has provided a technical analysis regarding the potential

interference that would result from ISM equipment operating in this band

to Geostar user equipments. In particular, it carried out a statistical

analysis and performed on site field measurements for interference

resulting from operation of microwave ovens. From its study. Geostar has

concluded that this interference source should not pose a serious

operational problem to its proposed system. The sole effect of possible

microwave oven interference to Geostar user equipments would be to cause a

modest increase in the "retransmit” rate.

161
8. The next proposed band used by Geostar is 5117~5l83 MHz. This band,

like the 1610-1626.5 MHz band, is allocated primarily for aeronautical

radionavigation. This band would be used by the Geostar system to provide

a downlink to the earth station located at Princeton, N.J. and would

contain seven channels, condensed into the width of four channels by use

of dual polarizations; this accounts for a total bandwidth of 66 MHz

(4 X 16.5 MHz).

9. Last, a 16 MHz bandwidth within the 6425-7075 MHz band is proposed to

provide for an uplink to transmit commands and message traffic from its

proposed computer center and earth station at Princeton, N.J. to each of

the three geosynchronous satellites. This band's allocation status is

currently being used by licensees in the Domestic Public Fixed, Private

Operational Fixed, and Auxiliary Broadcast services.

W. ESTIMATED TIME TRANSFER ACCURACY OF GEOSTAR

Although the primary purpose of the USRDSS service is navigational

position determination for a user, a by-product could be the precise

transfer of time to end users. There are two modes of time transfer

claimed for Geostar users: (1) general time transfer to 150 ys accuracy

and (2) high accuracy time transfer to as high as 10 ns accuracy.

For general time dissemination, an operational mode of Geostar includes

corrections to a time code which correct for the user position. This is

performed by a microprocessor within the user's transceiver. Computed

position parameters from the control center will be used, and the

162
estimated accuracy for users in this mode is 150 us. This is intended to

be a minimum cost service and, for this reason, ignores errors due to

ionospheric delay, tropospheric delay, and satellite latitude and

longitude wander. In contrast to the high accuracy mode, this is a

broadcast mode and the computational load on the control station is

independent of the number of users accessing the time information.

The high-accuracy capability uses a comparison between two users who each

receive a specific code transmitted from a common satellite. The

comparison is done back through the satellites, and the control center

estimates the difference of the outbound signal’s time of arrival at the

two user sites. A comparison of the actual time code arrivals and the

calculated times of arrival of the outbound signals allows the offset

between the two users to be determined. Brian Tunstall studied the errors

of the high accuracy mode for Geostar in a report of July, 1985 [5]. The

following information is derived from that report.

The high accuracy mode is intended to provide time transfer services with

accuracies in the nanosecond range. The suggested technique compensates

for ionospheric delays and is independent of user and Geostar satellite

positions and of tropospheric delays. The mode requires some Geostar

control station processing for each time transfer executed. The suggested

high accuracy mode is a round trip technique in which time can be

transferred from the Geostar control station to User A or between User A

and User B via Geostar. Consider first the more general case in which

User A wishes to compare his clock with the clock of User B.

163
Assume that the Geostar control station broadcasts the outbound signal

through the Geostar central satellite. By prearrangement Users A and B

read their clocks upon receipt of a particular numbered frame and transmit

a response back to the control station via the central satellite. The

response is synchronized to the outbound frame just as in the usual

navigational mode. The control station then knows the round trip time

delay to User A and to User B. To a first approximation, the time at

which each user reads his clock is just the midpoint of his round trip

time delay. The time difference between the midpoints of the round trip

delays, as observed at the control station, should equal the difference in

the clock readings observed by the users. If not, the discrepancy equals

the relative clock offsets.

Several observations may be made at this point. No position information

is required concerning the satellite or either user. Further, the Geostar

control station need not maintain an absolute time reference; it need only

measure time differences of less than a second.

To refine the technique description somewhat and to develop an accuracy

estimate consider the time delay diagram shown in Figure W1 . In Figure

W1(a) (not drawn to scale) the signal delays encountered in a round trip

between the Geostar control station and User A are portrayed, assuming

zero ionospheric delay. The delays encountered are labeled as follows:

164
UA

Li

\1/

T2T •-2

/N

MP

(a) No ionospheric delay

UA

T1 '1 '2^2 T3 '3

/N

MP

(b) With ionospheric delay

Fig. W1 . Time Delay Diagram

165
= tropospheric delay on control station to satellite link

= geometric path delay from control station to satellite to User A

T

2

= tropospheric delay on satellite to User A link

UA = Point at which the signal is received by User A

= tropospheric delay on User A to satellite link

L

2

= geometric path delay from User A to satellite control station

= (assuming quasi-stationary satellite position)

= tropospheric delay on satellite to control station link

Tropospheric delay is dependent on temperature, pressure, water vapor,

elevation angle, and user altitude but not on RF frequency. Therefore T^

= Tj^ and T

2

= T^. As a result of this symmetry, the signal reception at

User A, point UA, is coincident with the round trip midpoint, point MP.

Also Figure W1 does not illustrate delay lock loop jitter. This is

treated in subsequent paragraphs.

Figure W1(b) adds the effect of ionospheric delay to the delays shown in

Figure W1(a). Because of the 1 /f frequency dependence of ionospheric

delay the delays on each of the four links involved will, in general,

differ. The symmetry of Figure W1(a) is no longer present and the point UA

will be shifted to the left of the midpoint, point MP . The suggested

technique attempts to minimize and to compensate for this offset.

166
Ionospheric delay follows a 24 hour daily cycle reaching a minimum shortly

before dawn. In order to maximize accuracy the predawn hour will be

utilized for time transfers. Within CONUS, for a zenith trajectory (90°

elevation) and a frequency of 1.6 GHz, the maximum pre-dawn delay has been

measured and shown to be less than 10 ns. Nonzenith trajectories show

increased delays with a maximum of 35 ns at 0° elevation.

Assuming the worst case elevation and the 1/f frequency dependence, the

ionospheric delays in Figure W1 (b) have the following values:

I^ = ionospheric delay on the control-station-to-satellite link

= 2.1 ns at 6533 MHz

I 2

= ionospheric delay on the satellite-to-User A link

= 14.8 ns at 2492 MHz

I^ = ionospheric delay on the User A to satellite link

= 35 ns at I 6 I 8 MHz

Iil = ionospheric delay on the satellite-to-control station link

= 3-5 ns at 5150 MHz

The resulting offset between points MP and UA is calculated as follows

MP - UA = 0.5 [T^ + 1

^ + + I 2

+ T

2

+ + I 3

+ L

2

+

- [T^ +1^ + ^2 "^2^

167
= 0.5 [1^ +12 + 13 -^ Ii|] - di +

= 10.8 ns.

Figures W1(a) and W1(b) represent the two extremes of measurement error due

to ionospheric delay. In order to minimize the average error, the point

UA will always be assumed to be 10.8/2 = 5.4 ns prior to point MP. This

results in an ionospheric delay error, Ej, of

Ej = ±5.4 ns

on the link between the control station and User A.

Assuming that the customer for high accuracy time transfer will use the

higher performance transceiver for geophysical survey applications, the

round trip timing error due to delay lock loop jitter will be ± 2.8 ns.

This implies the error, Ej , in locating the midpoint will be

Ej = ±2.8/2

= ±1.4 ns.

Root sum squaring of the two error components, the overall error, E^,

in measuring User A's time of reception is

168
= ±5.6 ns.

The analysis of the link between the control station and User B is

identical to that given above for User A, i.e.

E

B

EA*

The overall error in transfering time from User A to User B, i.e.,

the error, E^^g, in measuring the offset between the midpoints of the

signal paths to User A and B, is therefore given by

^AB Eb2)1/2

= ±7.9 ns.

Two extensions of the high accuracy mode described above are (a)

release of the predawn operating restriction and (b) maintaining accurate

absolute time at the control station which can be transfered to users on

an individual basis.

169
For extension (a), no restriction on time of day of operation, the

dominant error component, Ej, increases by a factor of 5 resulting in

an overall accuracy of

E^B = ±38.2 ns.

For case (b), (maintaining absolute time at the control station) it

is assumed that the control station clock is updated by means of a Geostar

link to the National Bureau of Standards. Then the control station clock

is accurate to the level of E^,

E^ = ±5.6 ns.

When this time is transfered to an individual user, the overall

accuracy of the user’s update is again E^g = ±7.9 ns.

170
X. PROSPECTS FOR HIGH-ACCURACY TIMING

For high accuracy time transfer, no cost estimates have been put forward

in any of the Geostar documents to this date. Medium accuracy broadcasts

of time transfer to the 150 ys level is suggested as being available at a

small or no additional charge. A special user ABT is required for time

transfer in the high accuracy mode to the level of 50 ns or better.

Geostar at this point is putting all of its emphasis on other concepts and

services of the system and the size, scope, and costs of just the

preliminary 1986 test (Link One test) has left no resource for any high

accuracy time transfer cost analysis or commitment. To add justification

for licensing. Geostar has done a preliminary analysis of the highaccuracy potential capabilities (Tunstall, 1985) and has not committed to

servicing users at any level of time transfer.

It is assumed that full costs of the Geostar high accuracy mode will be

borne by the users of such a service just as in the navigation and

telecommunication services. In our opinion, it is unlikely Geostar would

themselves invest in deployment of a high accuracy time transfer service.

This is because (1) it is not implied or stated in Geostar's prospectus

that the ground computer software is easily or directly extendable to

high accuracy time transfer, (2) a "special" ABT of unknown size and cost

is necessary by the user, and (3) the market for high accuracy time

transfer is only a few percent of Geostar's navigation and telecommunications market. Given the current Geostar prospectus and timetable,

171
if funding were available outside of Geostar Corporation and if high

accuracy timing capability were targetted for 1988 or later, such a

capability could be in place.

Geostar could provide its advertised high accuracy capability with

appropriate actions. First, we suggest that Geostar go ahead with

installation and maintenance of a suitable cesium clock at its central

ground station in Princeton, NJ . Although this eliminates the requirement

to do time transfers at "predawn" times to reduce ionospheric delay

uncertainties, these uncertainties are not a significant problem for BPA's

one microsecond accuracy. The intent hoped for here is to simplify the

overall operation by having a master clock at Geostar's Central dispatch.

Second, NBS would monitor the accuracy of time transfer using a satellite

receiver-decoder at Boulder, Colorado by comparison against UTC. Next,

corrections would be applied to the Geostar clock as needed to steer its

time to within preset limits relative to UTC. A practical limit would be

±100 ns (one sigma estimate). This is ten times better than BPA's

specification, yet coincides with rugged, commercial clock instabilities

and reasonable correction updates of say, once per day or less. Along

with this, equipment would need to be made which automatically did the UTC

comparison and Geostar clock updates. Correction data would need to be

sent from Boulder to Princeton via a telephone link or Geostar's own

communications system.

172
To make the system operate in the high accuracy mode, additional

computations need to be executed at Geostar's central computer at

Princeton, NJ. The cost of the additional software development cannot be

estimated now. As mentioned earlier. Geostar Corporation has made no

commitment to do this development.

173
A rough estimate of costs is shown below.

Initial Cost Estimate

Cesium Clock $ 45k

Automatic Correction Equipment $200k

Total $245k

Unknown Initial Costs

High accuracy ABT (at Boulder, CO)

High accuracy Software at Geostar Central

Ongoing Cost Estimate

NBS Geostar-UTC Comparison

Maintenance, travel, misc. (1/3 man year)

Total

Unknown Ongoing Cost Estimate

Geostar monthly subscription fee (high accuracy timing)

The items listed above are given as a basis only. They do not constitute

a proposal. Little can be concluded about actual overall costs until the

high accuracy equipment and software is priced and Geostar’s ongoing fee

for high accuracy time transfer is determined.

$30k/yr

$33k/yr

$63k/yr

174
Y. CONCLUSION REGARDING USE OF GEOSTAR

We believe GEOSTAR will technically and financially realize its

objectives in the radio determination satellite service (RDSS) of

navigation and message handling capability. However, it is difficult to

say whether GEOSTAR will meet its own timetable of being fully operational

by 1988.

More important is the fact that GEOSTAR does not appear to have put in

place adequate measures to provide a high accuracy time transfer

capability at this time. However, the potential exists and has been

evaluated to provide as good as 10 ns accuracy. Development of

high accuracy timing capability may occur later, but there is no clear

direction or policy regarding this.

Our analysis of the USRDSS concept has focussed on the GEOSTAR proposal

since it is now the only one in which detailed information pertinent to

this study is readily available. Other companies may arise in the future

and should be considered if the USRDSS option is deemed appropriate for

accurate time dissemination.

175
SUMMARY

This report has studied ways in which time transfer could be achieved

to meet all requirements of the Bonneville Power Administration and NBS.

This work has concentrated on systems which disseminate time in a

broadcast mode and are independent of the existing internal communications

links of the BPA. The desired level of accuracy throughout the power

transmission system was established as one microsecond.

The first phase of study gave policy guidelines for this work. The

NBS provides several time-and-frequency broadcast services. Although it

was not true in the past, current U.S. administration policy is that any

new work or services should contain a method of full cost recovery for

NBS. The second phase concentrated on existing and potential services.

Pertinent issues such as accuracy, cost to users, reliability, coverage,

and other items were discussed. In all, seven time distribution systems

were considered and details on them were given. Six of the systems were

satellite based, and one (Loran-C) was not. Phase III then presented

extensive review of three systems which were likely to serve the needs of

BPA based on the phase II study. The three systems determined as viable

were

1. The Fixed Satellite Service (FSS) used as a time broadcast link

directly from NBS,

176
2. the military's Global Positioning System (GPS), and

3. GEOSTAR, a private sector Radio Determination Satellite Service

(RDSS) under development

There are advantages and disadvantages associated with each of these

systems. Table 12 shows a breakdown of the main features of each of these

three systems with regard to system issues outlined at the beginning of

Phase II.

The Phase III portion of the study elaborated on the FSS, GPS, and

GEOSTAR options and the conclusions are summarized as follows:

1. FSS option: All technical and cost requirements appear to be

achievable using the FSS preliminary design presented in Phase III.

Possibly the most important factor to successful FSS operation is

accurate tracking of the satellite which NBS proposes as integrated

into the time distribution system. Based on experimental satellite

tracking done by NASA during the mid-1970's, time synchronization

to less than one microsecond should be achievable. No technical or

policy obstacles prevent NBS from developing the FSS as an

"Industrial Time Service" which could meet BPA's objectives and

could serve a variety of users on a full cost recovery basis.

2. GPS option: Technical requirements can be met using GPS. Receiver

costs are high now with an average cost of $20,000 and a range of

$15,000-25,000 with a possibility of future reductions, but we

177
Radio

SYSTEM

Fixed

NAVSTAR

Determination

Study

Satellite

Global

Positioning

Satellite

ISSUE

Requirements

Service

System

Service

178
do not believe the costs will drop to the same degree the GPS

navigation receiver costs are projected to drop. Of special

importance is that the military policy regarding a civilian time

service carries some risk which leaves open the possibility for

altered, degraded, or discontinued service at the future disposition of the military administration. Furthermore, the service is

now considered to be freely available, but the option for future

civilian user fees exists in the final Block II implementation.

3 . GEOSTAR option: Although technically and financially strong,

GEOSTAR Corporation does not appear to have put in place adequate

measures for providing a high accuracy time transfer capability.

However, the potential exists for accurate time transfer and has

been evaluated to provide accuracy as good as 10 ns. Development

of high accuracy timing capability may occur later, but there is no

clear direction or policy regarding this. In fact, the GEOSTAR

system is operated entirely by the private sector, and future

operational aspects are difficult, if not impossible, for NBS to

predict at this time.

179
REFERENCES

1. ATS-5 Trilateration Support Final Report, 12 November 1973~31 December

1975. Prepared under NASA Contract NAS5-20034 by General Electric

Company, Schenedtady, NY 12301. Report No. 76-004.

2. "NAVSTAR: Global Positioning System - Ten Years Later," B. W.

Parkinson and S. W. Gilbert, Proc. IEEE, Vol. 71, no. 10, pp.

11 77-1 186, Oct. 1 983.

3. "Modern Navigation Systems and Their Relation to Timekeeping," W.

Klepczynski, Proc. IEEE, Vol. 71, no. 10, pp. 1193-1198, Oct. 1983.

4. Geostar Radiodetermination Satellite System, Federal Communications

Commission File nos. 21 91 -DSS-P/LA-83 through 21 94-DSS-P/LA-83

,

April 5, 1985.

5. "Time Dissemination Accuracies Using Geostar," B. Turnstall,

Systematic General Corporation memorandum to Geostar, July 3» 1985

(Copy submitted to ITU-CCIR Library, October, 1985).

180
APPENDIX I

PHASE 2: TECHNICAL APPROACHOPTIONS FOR SERVICE DELIVERY

Below are some of the options to be studied and considered as the delivery

vehicle of a TOD and frequency system. The list includes most of the

present thinking at NBS but may be expanded depending upon new information

received prior to or during the actual study.

System Configurations:

a. The use of a partial transponder aboard a communications satellite

operating in the Fixed Satellite Service with downlinks of 3.7 to

H.2 GHz or 11.7 to 12.2 GHz. This option has the advantages of

high reliability and low costs due to the extremely competitive

environment existing in this industry. Special techniques will

have to be used, however, to compensate for the rather low power

flux densities available and the potential interference from

adjacent satellites and other terrestrial services.

b. The use of the Direct Broadcast Satellites (DBS) operating in the

bands allocated to the Broadcast Satellites Service (BSS) with

downlinks at 12.2 to 12.7 GHZ. This option has advantages of high

flux densities and inexpensive equipment due to high production

quantities. Several tens of millions of small earth stations will

be built by the DBS industry in the next five years.

181
c. The use of the time and frequency service space-to-earth broadcast

channel allocation at 400.1 MHz. This allocation is exclusive in

that it is a primary allocation and will not be sharing the channel

with other services thus assuring interference free operation. It

also has the advantage of not being limited in power flux density,

it may broadcast to the earth's surface.

d. The service may be integrated into an already existing system.

Although dedicated to other purposes or other users, systems such

as LORAN-C, OMEGA, TRANSIT, GPS, and the terrestrial distributions

of telecommunications services do provide certain useful levels of

time and frequency information. These systems suffer the

disadvantage of being subject to modification, with notice, in such

ways that nullify or reduce its utility as a source of time and

frequency information.

These above options shall be examined from the standpoint of how well they

might meet technical and operational requirements determined in phase 1 of

this study.

182
APPENDIX II Documents

CCIR Study Groups

Period I 982 -I 986

Doc. USSG 7/19-1

April 15 , 1985

Page 1

UNITED STATES OF AMERICA

DRAFT REVISION OF REPORT 518-3

Time/Frequency Disseminations and Coordination Via Satellites

(Question 2/7)

1. Add new section A. 3 . A as follows:

i*.3.A USRDSS

The USRDSS is a proposed system envisioned in the radiodeterminationsatellite service which will consist of a number of operational satellites in

geostationary orbits, one or more fixed location control centers, and a large

population of mobile users (subscribers) located within the system coverage

zone. In the envisioned system, a periodic time reference PN code, originating

at a control center, is relayed through one satellite to all user stations

within a coverage zone. The user stations are expected to be relatively

inexpensive, pre-programmed transceivers designed to respond with uniquely

identified signal bursts back to the control center via two of the satellites

in the system. Responses may include messages for other users or requests for

other services. The control center will determine the two associated roundtrip

propagation path delays and, using a stored terrain map or altitude information

from the user station, compute the precise location of the user station using

redundant high speed computers. Computed positions along with pending messages

are addressed and transmitted back to the appropriate user via the original

signal containing the embedded periodic reference PN code. During operation,

the system will be continuously calibrated using known, fixed-location

"benchmark” transceivers. Outbound links from a satellite to the user stations

are planned for the frequency range 2 A 83 . 5~2500.0 MHz while inbound links from

the users to the satellites are planned for the frequency 1610.0-1626.5 MHz.

As a complementary byproduct of its primary service of precision

radiodetermination, the USRDSS System is expected to provide both general time

dissemination to a large number of users and high-accuracy time transfer. The

capability for wide area coverage and the design emphasis on low cost,

automated user transceivers enables a large user base for these functions. For

general time dissemination the operational mode of the envisioned USRDSS

includes corrections for the user position performed by a microprocessor within

the user transceiver. Computed position parameters from the control center

will be used and the estimated accuracy for users in this mode is 150

microseconds [Tunstall, 1985 ].

The operational mode for high accuracy time transfer service with the

envisioned USRDSS involves a method analogous to the simultaneous exchange of

183
-2 -

timing signals through a communications satellite link. Two users desiring to

correlate their local clocks will respond to a specified epoch on the outbound

USRDSS signal. The time of receipt of the epoch at the user station is labeled

with the local clock value. The control center can estimate the difference in

the outbound signal's time of arrival at the two stations by measuring the time

difference in the arrival of their responses and incorporating calibration

factors derived from the "benchmarks". By comparing the clock value labels and

the calculated times of arrival of the outbound signals, the offset between the

two user clocks can be determined. The estimated accuracy for this service is

10 nanoseconds [Tunstall, 1985]»

A special case of the high accuracy time transfer service makes use of an accurate clock at the central control center. The clock is periodicaly

recalibrated by transfering time from a reference standard clock such as that

available at the United States National Bureau of Standards, The control

center can then transfer precise time, upon request, to any user located within

the system coverage zone. The estimated accuracy for this special case is also

10 nanoseconds [Tunstall, 1985]»

2, Make the additions to Table 1 as attached.

3. Make the additions to Table 2 as attached.

A. Add to the list of references the following:

Tunstall, B. [October, 1985 ] Time Dissemination Accuracies Using Geostar.

Prepared for Geostar Corp, by Systematics General Corporation, March, 1985*

(Copy submitted to ITU — CCIR Library, October, 1985)<>

184
APPENDIX III: GEOMETRICAL DILUTION OF PRECISION FOR TRILATERATION

An equation relating the range error variances with position error

variances is derived for the simple tr ilateration model illustrated in

Figure A1 . Three tracking stations on the earth's surface are shown with

coordinates (Xj^, Y^, Z^) for i = 1, 2, and 3* The satellite is located at

(X, Y, Z). The ranges to the satellite are I 2

, and

The range error variances are shown to be directly proportional to

the position error variances by a constant generally known as the

geometrical dilution of precision or simply the GDOP.

The equations for the ranges, 1^, to the satellite are:

1

, - /(Xg-X, )2 t (yg-yi)2 - (Zg-Z,)^

12 - /(Xg-xg)^ t (yg-yj)^

13 - /(Xg-Xj)^ + (yg-yj)^ • (Zg-zj)^

It is assumed that the satellite coordinates and ranges

expressed as:

’'s -=‘sn * **

ys ysn

* ^y

^sn

*

*1 "

*'ln ''*'1

^2 = it +AX/2

where y^^, 2,^^^ 2,2n ^3n stochastic variables

A2,^ , A 2,2 and are errors and Xg = Ejxg^j, y^ Ely^^} etc.

(1 )

(2)

(3)

may be

(4)

(5)

( 6 )

(7)

( 8 )

(9)

Ax, Ay, Az,

185
z

Fig. APPl. Trilateration Geometry

186
Therefore

'In hn - (ysn-y2>^ *

••Bn y^sn-='3’^ * * '^sn’^s'^

Substituting Equations 4-9 into 1 yields

^1n

^ = y(Xsn+Ax-Xi )2 + (yg^+Ay-y^ )^ + (Zg^+Az-z^ )^

Squaring both sides and ignoring 2nd order terms yields

( 10 )

(11 )

( 12 )

^sn'^1 . ^sn-^l

"^^1 = " “T Ay

'In 'In

^sn~^1

^1n

Likewise

X -x„ y -y^ z -z^

AO 2 . -^sn •'2, sn 2

Ailp = Ax + Ay + '2n '2n '2n

and

X -x_

A n sn 3 . AHo = Ax +

^ ^3n

Def ining

A E

^sn-y

^3n

1 Ay

z -z_ sn 3

+ Az

^3n

ai ^ ai2 ai 2

^21 ^22 ^23 , where

^31 ^32 ^33

X -X, sn 1 ^11

2,^ ^ and soforth

187
We have

X = [Ax Ay Az]"^

L E [AJL^ ^2

L = Ax

X = a‘”'l

Since the above equation is linear, the relationship holds for errors

in positions and ranges also.

s

-

To obtain the position and range error variances, the covariance

matrix is formed as

cov(x) =

and cov(L) =

COV(X) = E{A~^ £j^(A”^

= E{A~''ej^eg^'^A”'^}

= A"''E{e^ej^'^|A’^

= A“”'cov(L)A"'^

p However cov(L) = 0 ^

I assuming the range residual errors are

uncorrelated and equal for each log or range.

Thus cov(x) = o^[A~^A or

cov(x) = Ol[a"^A]~^

The geometrical dilution of precision, GDOP , is defined as

GDOP ^ yTRACEEA^A]"’'

188
Thus

GDOP

2

0 z

GDOP is then the amplification factor of range error variances to the

position error variances.

189
NBS-1 14A (REV. 2-BC)

BIBLIOGRAPHIC DATA

SHEET (See instructions)

PUBLICATION OR

REPORT NO.

2. Performing Organ. Report No. 3. PubI ication Date

NBSIR 86-3042 February 1986

4. TITLE AND SUBTITLE

Industrial Time Service Study

5. AUTHOR(S)

D. Wayne Hanson and David A. Howe

6. PERFORMING ORGANIZATION (If joint or other than N BS, see in struction s) 7. Contract/Grant No. NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

DE-AI 79-84BP19476

8. Type of Report & Period Covered

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City. State. ZIP)

Bonneville Power Administration

Portland, OR 97208

10.

SUPPLEMENTARY NOTES

Document describes a computer program; SF-185, FIPS Software Summary, is attached.

11.

ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here) This study examines options for delivery of accurate time and frequency

information to industrial users. The study is sponsored by the Bonneville Power

Administration (BPA) who finds a need for accurate timing to the one microsecond

level. Prospective existing and future dissemination methods (Loran-C, GOES,

USRDSS, GPS, etc.) are discussed in detail. The study produces a system architecture and preliminary design for a new time service using the widely available

U.S. fixed satellite service (FSS) in which customers shall assume full costs

of its operation through subscriber fees.

The study elaborates on three viable options: (1) FSS, (2) GPS, and

3) USRDSS. Based on this study, conclusions can be drawn regarding a timing

system which will most satisfactorily meet the long range goals of most industrial

users.

12.

KEY WORDS (S/x to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolons)

accuracy; frequency; satellite; time; time broadcast services; time dissemination

13. AVAILABILITY 14. NO. OF

PRINTED PAGES

[2^ Unl imited

I I For Official Distribution. Do Not Release to NTIS

I I Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C.

20402.

[X] Order From National Technical Information Service (NTIS), Springfield, VA. 22161

198

15. Price

USCOMM-DC 6043-P80

U.S. GOVERNMENT PRINTING OFFICE:1986— 677-909/45126