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i cASIBILITY STUDY OF

ORBITING STANDARDS PLATFORM

A. J. Estin

R. C. Baird

Electromagnetic Fields Division

Center for Electronics and Electrical Engineering

National Bureau of Standards

Boulder, Colorado 80303

June 1978
I
National Bureau of Standards APR 1? 1979

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IMBSIR 78-886

FEASIBILITY STUDY OF

ORBITING STiriDARDS PLATFORM

A. J. Estin

R. C. Baird

Electromagnetic Fields Division

Center for Electronics and Electrical Engineering

National Bureau of Standards

Boulder, Colorado 80303

June 1978

U.S. DEPARTMENT OF COMMERCE, Juanita M>kreps, Secretary

Sidney Harman, Under Secretary

Jordan J. Baruch, Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director
FEASIBILITY STUDY OF ORBITING STANDARDS PLATFORM

A. J. Estin and R= C. Baird

This report consists of four components of a feasibility

study for a satellite-based measurement system for determining

important operational parameters of satellite communications

systems and its major sub-systems. We have addressed the

questions of required accuracy, methods of attaining and

maintaining measurement accuracy and traceability , system

tradeoffs, and economic impacts and benefits.

ii
CQNTENTS

Page

SUMMARY 2_ Part A. USER ACCURACY REQUIREMENTS FOR SATELLITE COMMUNICATION

SYSTEMS 3 Part B. VERIFICATION OF OSP MEASUREMENTS 7 I. The OSP System 7 II. Measurement System Configuration 11

III. Earth Station Checking and Recalibration 17

Part C. OSP TRADEOFFS 18

I. Tradeoffs among Various Methods of Measuring System

Performance 18

II. Orbit Tradeoffs 21

III. Users To Be Serviced 23

IV. Hardware Tradeoffs 23

Part D. ECONOMIC IMPACTS OF PRECISION MEASUREMENT CAPABILITY UPON

SATELLITE COMMUNICATION SYSTEMS 25

I. Introduction 25

II. Impact Upon System Investment and Operating Costs 25

III. Payment of Incentives 30

IV. Effective Utilization of Spectrum and Orbit Resources 31

REFERENCES 35

APPENDIX I. THE NBS "SIX-PORT" POWER MEASURING SYSTEM 37

iii
LIST OF ILLUSTRATIONS

Page

Figure 1. Satellite to OSP orientation error 10

Figure 2 . OSP system interactions with user system ]_2

Figure 3. Configuration of OSP-- ' 14

Figure 4 . Measurement module • 15

Figure 5. Costs of large aperture parabolic antennas • — 27

Figure 5. Costs of low noise front ends -- 29

Figure 7. Geo-Stationary satellite locations as seen from

Boulder, Colorado - —-— 34

Figure 8. Six-Port progenitors 39

Figure 9o 6-Port concept - 40

Figure 10. A typical six-port, with redundancy- 41

LIST OF TABLES

TABLE I. SIGNAL RADIATION, TRANSMISSION, AND RECEPTION—- 5

TABLE II. SPECTRUM MEASUREMENTS • — 6

iv
FEASIBILITY STUDY OF ORBITING STANDARDS PLATFORM (OSP)

A. J. Estin

and

R- C. Baird

SUMMARY

Under the joint sponsorship of NASA Goddard Space Flight Center, Office

of Telecommunications Institute for Telecommunications Sciences, COMSAT

Laboratories, and National Bureau of Standards Electromagnetic Fields Division,

a preliminary investigation has been undertaken into the technical and eco- nomic feasibility of using a satellite system to make accurate measurements

on the many satellite communications systems and sub-systems in operation and

being prepared for use. This report represents the contribution of the

National Bureau of Standards to this investigation. It consists of four parts,

as called for in the Statement of Work attached to NASA/GSFC PR No. 952-45635,

dated February 1, 1977. These four parts are designated in the Statement of

Work as "Deliverables": (5.2) Identification of accuracies of measurement

required; (5.3) Definition of the method that will be used to verify Orbiting

Standard Platform (OSP) measurements; (5.4) OSP tradeoff study; and (5.1) Benefits and cost analysis of an OSP. These parts appear in this report in the

order listed.

Early in the course of this feasibility study, a survey was taken under

the leadership of Dr. H. T. Dougherty of OT/ITS among potential users of the

OSP system to ascertain the nature of their requirements and to serve as a basis for preliminary design of OSP. The results of this survey indicated

overwhelmingly that, (a) there is a significant need for this kind of measurement capability, and (b) that there is support for this approach. In Part A

of this report, we have summarized in some detail, and with as much quantitative specifics as were warranted, the types and accuracies of measurements to

be made. These requirements are sufficiently within the capability of present

day state-of-the-art measurement technology that they constitute a set of

realistic goals for this project. Nevertheless, the technical challenge of

performing such measurements rapidly, and automatically, in an unattended

space environment, while insuring traceability to fundamental laboratory

standards will itself be a significant advance in measurement science.

Part B of this report discusses the major aspects of this technical

challenge, and outlines a proposed approach. Recent developments in microwave measurement techniques at the National Bureau of Standards, including

principally the Six-Port, the Type IV power meter, and antenna near-field

measurements and transformations, have provided an adequate basis on which

this program can rest. Part C examines the various alternatives to OSP and

within OSP that should be addressed before major aspects of the design are
frozen. Many of these considerations dovetail into the studies performed by

Mr. Walter L. Morgan of COMSAT Laboratories, who has examined state-of-theart space electronics and vehicle technology in order to assess such requirements as vehicle size, weight, and power requirements and consequent cost

considerations. The last part of this report. Part D, discusses several

important aspects of the economic impacts of the OSP system. It can be con- cluded that the system, which might cost in the neighborhood of several

million dollars a year (including development, design, and deployment of a satellite, procurement of the necessary supporting earth station, and operation of the entire system) will return to the nation many times over this

amount in terms of benefits to reduced capital and operating costs of satellite communication systems, resolution of procurement disputes between vendor

and operator of these systems, supporting and aiding U.S. export of satellites

and earth stations, and—most important of all--in extending the all too

limited commodity of spectrum and orbit space available to us. We wish to acknowledge with greatest pleasure the support and cooperation

of our colleagues W. L. Morgan of COMSAT Laboratories, T. Keating Jr. and

J. J. Woodruff of NASA Goddard Space Flight Center, and H. T. Dougherty of

Office of Telecommunications Institute of Telecommunications Sciences. We are grateful to other persons (too numerous to list) from our respective

organizations, as well as those from the community of satellite communications who have assisted and guided us.

2
Part A. USER ACCURACY REQUIREMENTS FOR SATELLITE COMMUNICATIONS SYSTEMS

The purposes of this part of the report are to define the various

parameters which are useful to the deployment and operation of a satellite

communications systems and, insofar as possible, to define the accuracies

required. Such a tabulation is needed as a basis for later trade-off decisions as to exactly which quantities OSP should be designed to measure and

in structuring the uncertainty limitations upon the measurement and re- calibration procedures. The importance of obtaining careful and exact

uncertainty statements is that the overall resulting error budget goes far

toward demonstrating a cost/benefit relationship for the entire project.

More succinctly, the specific uncertainty required in a measurement places

that measurement's difficulty somewhere between trivial and beyond the

achievable state-of-the-art, and reflects accordingly in its cost.

In this report, we have attempted, on the basis of "The OSP Survey

Response" [1] and other literature [2,3] to identify all possible parameters

of interest and to deduce whatever user accuracy requirements have been set

forth

.

We do not in any sense imply that all of these param.eters are to be

evaluated by means of OSP, but only that these are quantities that are of

interest to a satellite communications system operator, manufacturer, or regulator. The question of specifically which parameters OSP will measure

will be addressed in a preliminary way in Part B on "Verification Methods,"

and more completely in the subsequent cost/benefit study.

We can generally divide parameters associated with these systems into

three categories of system operation and testing: I. Signal radiation,

transmission, and reception; II. Frequency and spectral characteristics of

sources, signals, and receivers; and III. Mechanical properties of systems.

These are listed at the end of Part A in Tables I, II, and III, respectively.

While there is some room for discretion in such a classification (for example,

is phase noise a signal-reception or a spectral parameter?), it is useful in

making a first order cut on what OSP should not do and as a completeness and

procedural check list on what it should do. These parameters may be useful

and significant to either satellite or earth station (or both) and to either

transmission or reception (or both) . We have indicated this by checking

appropriate columns in the tables.

The "accuracy" re^quirements given in the list are obtained, as is

mentioned above, from a variety of user sources. Thv;s, they represent a broad range of reliability and thoughtfulness . The question of definition

of accuracy/uncertainty of a measurement is quite complicated, when one attempts detailed quantification, and must be the subject of a very careful

and complete analysis.

3
A further factor complicating this error analysis is the selection of

how component errors are combined. Random independent errors are usually

assumed normally distributed. Systematic errors, however, are not at all so straightforward, as they can have many different distributions (uniform, binary, Gaussian, etc. ) and the method of propagation of errors may be difficult

to establish. Moreover, it is not always even clear whether a given source of

error is random or systematic and whether correlations exist between different

errors in the budget. [4]

This discussion on errors and uncertainties does not directly apply to

the results reported herein, but is included in order to place the longer

term goals of the project in perspective. For the present, the "user accuracy"

is stated in the tables under whatever form seems convenient, with explanatory

remarks as appropriate. In view of the preliminary nature of this series of

reports, a more definitive statement of accuracies seems neither warranted

nor feasible.

4
TABLE I. SIGNAL RADIATION, TRANSMISSION, AND RECEPTION

<u

ca

COC

' ca a

U 0) H 1^ X X

X X

X X

X X

c

o

M CO

CO 4J W CO

COc • CO tJ H

X X

X X

X X

X X

Parameter

Antenna Characteristics

1. Pattern (Field Magnitude)

a) On-axis gain

b) Off-axis structure of main lobe

c) First and second sidelobes

d) Beamwidth; specific contours,

e.g., 3, 4, 10, 20 dB

e) Footprint; beam isolation of

multiple antennas.

f) Gain/ System temperature

g) Sidelobe envelope

Accuracies & Remarks

All quantities strongly

frequency dependent.

0.1 dB

0.05 dB (Sat); 0.1 dB (E/S)

0.5 dB (Sat); 0.2 dB (E/S)

Measure of interference

and coverage

Frequency reuse

Accuracy strongly depends

on freq. & antenna gain.

ITU Requirements

Polarization. (Axial ratio, tilt angle, ellipticity angle, etc.)

XX XX

a) On-axis properties

b) Off-axis structure of main lobe

c) First and second sidelobe peaks

d) Sidelobe crosspolarization

e) Mainlobe crosspolarization

0.5 dB for axial ratio < 2dB

Same

Same

To 35 dB below main lobe

co-polarization

Below 30 dB down (Sat)

" 32 dB down (E/S)

Signal Characteristics

X X

XX XX

X

X X

X X

X X

6.

7.

EIRP

Field at antenna Magnitude and polarization

a) Flux density to saturate transponder 0.5 dB

b) Power input to antenna

c) Backoff

Antenna power handling capacity

Gain stability

Gain/Operating temperature

Operating temperature and system

temperature

Carrier power /operating noise density

Propagation effects (atmos and ionsph)

a) Atmospheric attenuation

b) Cross polarization

c) Phase jitter

d) Group delay

e) Group delay distribution

f) Phase jitter and phase noise

g) Faraday rotation

0.2 dB

0.3 dB

0.1 dB

See (A.l.f) above;

for 4 GHz and G/T > 35 dB

0.2 dB for E/S;

0.5 dB (Sat.)

20K (Sat) IK (E/S)

0.5 dB

0.2

0.2

5"0.1

dB

dB

100 Hz

5° BW

*N0TE: An "X" in one or more of the columns on the left means the parameter in question

is relevant to that function.

5
0) 4J H a

rH o

iH x: •H

0) 4J U

u ta nJ to

CO M

to CO c : C

M <J nj O

J-( cu H H D4 X X

X X

X X

X

X

X X

X X

X X

X X

X X

TABLE 11. SPECTRUM MEASUREMENTS

Parameter

A. Measures of Performance

1. Frequency stability

2. Modulation characteristics

3. Spectral purity

4. Dynamic range

5. Gain-transfer characteristic

6. In-band freq. response

7. In-band group delay

8. In-band group delay distortion

9. Turn-on transients

-10. Error rate

Accuracies & Remarks

0.1 dB (Sat)

B. Measures of Interference

XX 1. Sideband rejection

2. Spurious bandpasses

X X X X 3. Filter skirt shape

X X X X A. Adjacent channel interference XX 5. IM and other spurious signals XX 6. Out-of-band emission XX 7. Crosstalk

X 8. Adjacent transponder interference XX 9. Coding problems

TABLE III. MECHANICAL MEASUREMENTS

A. Position

X X X X 1. Relative to earth

XX 2. Ranging

XX 3. Slew rate

XX 4. Tracking rate and accuracy

B. Orientation

XX 1. Setting of attitude (roll, pitch, and yaw)

XX 2. Measurement of existing attitude (R, P, and Y)

X X X X 3. Boresight

X X X X 4. Pointing accuracy

6
Part B. VERIFICATION OF OSP MEASUREMENTS

I. ' The OSP System

In the previous part we have shown, in three tables, an essentially

complete list of performance measurements useful in a Satellite Communications

System. In this part, we shall select from that list those measurements

which are suitable for measurement by OSP and we shall discuss the general

means by which the validity of such measurements can be confirmed. In order

to facilitate this selection, we shall first eliminate those measurements

which clearly are inappropriate to OSP for at least one of the following

four reasons:

1. Those parameters which the User* can efficiently and adequately

determine for himself. (Examples: channel crosstalk, turn-on

transients, error rates)

.

2. Those parameters which require radiation from OSP on frequencies

that would interfere with assigned communications channels.

(Examples: filter skirt shapes, bandpasses, and other broadband

spectral properties of receivers.)

3. Those parameters which would require excessive space maneuvering

on the part of OSP or offset of a User Earth Station antenna from

its satellite thereby interrupting all service through that E/S

.

(Example: slew and tracking rates of an E/S.)

4. Those parameters which require maneuvering and precise roll,

pitch, and yaw adjustment on the part of the user satellite.

(Example: footprint measurement).

In general, the dividing line between those parameters whicn should and

should not be measured by OSP finds practically all of the former in Table 1

and most of the latter in Tables II and III of Part A. It will be seen that

almost all of those parameters most usefully measured by OSP can be determined from physical measurements (some absolute, somLe relative) of vector

field strengths and power levels and phases of signals received and transmitted by the OSP system. We shall, therefore, proceed to outline the

recommended measurement processes and discuss methods of verification for

adequate support of these processes.

*The term "User" refers to the operator of the Satellite Communication System

or other organization which is obtaining data from the OSP system.

7
The starting point for examining performance of a Satellite Communication System is the "Basic Link Equation, " given below, which is easily

derived from the well-known radar range equation for power transfer between

two widely-spaced antennas:

C/kT (dB-Hz) = EIRP -10 log(4TrR^) + 10 log G/T . -10 log k

T

+ 10 log + 20 log r -10 log L (1)

op

where

C = Total received carrier power in the channel in question

'^syst ".^^"^^i^i^^ system effective temperature under non-operating

conditions. This includes antenna, receiver, background, sky,

and atmospheric contributions, and is the temperature seen with

a radio star measurement.

T^p = Receiving system effective temperature under operating conditions. This includes T , plus the noise from the link transsyst *^ mitter and all noncorrelated interference.

EIRP = Effective isotropic radiated power; the pov/er radiated by the

transmitter times the gain of its antenna in the direction of

the receiver, expressed in dBw.

G = Gain in the receiving antenna in the direction of the transmitter . R = Distance between antennas.

r = Polarization coupling factor, 0_<r£l. = Atmospheric losses,

k = Boltzman's constant (-10 log k = 228.6).

All logarithms are to base 10.

The left side of (1) , the ratio of carrier to noise density power, is a figure of merit for the link. Based upon fundamental information theory,

this figure of merit sets an upper limit for the achievable data transmission

rate, given an error rate. The carrier to noise density power ratio is

normally measured at the User's Earth Station and is frequently measured online, v;ithout interrupting service. If all channels at the User's E/S show

an acceptable value, the Satellite Communication System is operating satisfactorily, and no further action is normally required. On the other hand,

if some channels show a degraded C/kT^^, the operator knows that system

deficiencies are present, but in the absence of further testing cannot

identify the causes. OSP would measure the various terras on the right side

8
of (1) , and thus would be a reliable and accurate means of monitoring and

diagnosing cause of system deterioration. Because OSP can act as an extremely

well characterized sending and receiving terminal of a link between itself

and the User's E/S, measurement of the relevant parameters of the E/S can be

made directly (and also without interrupting service of the communications

system) . Although it might be feasible to also evaluate User satellite

parameters in a direct way with OSP, several reasons indicate that this

would be less efficient:

1. The determination of satellite parameters involves less measurement uncertainty if it can be made through a calibrated earth

station rather than through a second satellite, even OSP. Ideally

such an earth station should be as well calibrated as possible,

which would mean, in descending order of quality of characterization: a) an NBS earth station designed for this purpose, b) another

earth station to which NBS is able to obtain access and at least

partially characterize, and c) the User's own earth station which

would be characterized with the aid of OSP.

2. A link between OSP and a User Satellite would necessitate doubling

the facilities carried on OSP. For example, OSP would normally

carry receiving equipment for a Satellite Communications System

uplink at 6 GHz and transmitting equipment for the downlink at

4 GHz. In order to work the User Satellite directly, it would be

necessary to also include transmitting equipment at 6 GHz and

receiving equipment at 4 GHz. Such an arrangement would nearly

double the space and power requirements on OSP, would increase the

control complexity significantly, and would greatly increase the

probability of self -interference in OSP thus reducing its overall

accuracy of measurement and its usefulness. In addition, spectrum

allocation difficulties would become significantly increased.

3 . The additional pointing requirements on OSP imposed by the need

for reorientation to work a User Satellite would require severe orbit constraints as well as considerable fuel reserves for frequent

large attitude changes. The near-synchronous orbit proposed for

OSP, which is designed to meet earth-station calibration requirements, places OSP relatively close to its customer satellites,

perhaps within 40 kilometers at point of closest approach. If we assume that the position box permits variation from assigned

position of up to 0.1° in longitude or latitude, it is easily

shown that satellite-to-OSP orientation errors could amount to

over 60° if one of the two were precisely on station and the other

were at the box limit in latitude. (See angle 0 in Figure 1.)

9
10
This would necessitate considerable roll, pitch, and yaw adjustment

and control on the part of both the User satellite and OSP which

in turn put unacceptable design constraints on both satellites.

Moreover, a major attitude change on the part of the User satellite

would be likely to interrupt his service.

Thus, we conclude that the initial OSP should be restricted to making

transmit and receive mode measurements on the User E/S only, and not on his

satellite. We will show subsequently how necessary measurements on the

satellite can be made.

The method of verification of OSP measurements is basically two-fold.

First, we will design into the OSP receivers and transmitters a variety of

crosscheck and self-check procedures that are based on present NBS measurement developments and capability. Second, an earth station which has been

independently evaluated and calibrated and which has excellent tracking and

pointing capability as well as access to NBS measurement standards will be

used to work OSP directly at predetermined intervals and can be used to

confirm (or correct) operational characteristics of OSP after orbiting.

This earth station must have its receive and transmit characteristics,

especially on-axis gain and polarization, well enough measured that fields

transmitted and received by it can be absolutely determined to high accuracy.

The goal for on-axis determination is 0.1 dB . Polarization (circular and

linear) must be pure enough and well enough established so that measurements

made on OSP are not degraded by E/S uncertainties. An added dividend which

will be considered later in more detail is that having such extremely well

characterized satellite and earth terminals of a single link will enable

resolution of propagation uncertainties such as atmospheric cross polarization,

phase scintillation, and other anomalous effects in all frequency bands of

interest. Figure 2 shov/s schen;atically the interactions of the components

of the User Satellite Communications System and the OSP system.

II . Measurement System Configuration

A typical receiving/measuring/calibration/transmitting unit on OSP will

be designed in modular form, where each module will be used at a certain

number of discrete frequencies in one or perhaps two adjacent frequency

bands. Such a "measurement module" will then be replicated in as many

frequency bands as is desired to supply OSP service. Tradeoffs in the

design of specific components of the measurement module will determine the

number of specific frequencies and bands that can usefully be covered with a single module, and therefore how many are required. For example, mismatch

magnitudes in the polarizing network of an antenna feed section ultimately

11
USER SATELLITE
determine its useful band limits, within which a computer-stored calibration

and correction procedure can be used to reduce the residual error to an acceptable amount at each desired spot frequency. Certain common elements,

such as computer control and interfacing, will, of course, be external to

the measurement module. These are designated as the "command, control, and

data relay module," the "central processing module," etc. Their relationship

to the measurement module is shown in Figure 3. Their functions, which are not critical to the subject of this report, will be addressed and discussed

only as necessary to the understanding of the actual measurement process and

its verification. We show in Figure 4 a block diagram of a typical measurement module. We might note that, although this diagram shows a duplexed

(transmit and receive) signal antenna, the choice between having that

arrangement and separate transmit and receive antennas is a matter for the

equipment designer to decide, based on the design limitations of available

components and weight/volume/space tradeoffs on the vehicle. The sections

shown in boxes represent specialized measurement sub-systems and are discussed in some detail following:

(A) Signal Antenna and Polarizing Network . The signal antenna is a stable horn having approximately 20 dB gain, which with the aid of

the polarizing networks receives or transmits a pure circularly or linearly polarized wave, as called for by the central processing

module. Circular polarization can be either right or left hand,

and linear polarization can be at any desired angle. In this way,

either desired or undesired polarization components can be sent or received. Gain, axial ratio, and tilt angle of the antenna will

be predetermined for all required angles and directions off boresight and for each required frequency by means of accurate nearfield techniques developed by NBS . These data and suitable interpolation routines can be stored in computer memory so that magnitude

and configuration of any field received by this antenna from an E/S

can be accurately determined within the error of non-calculatable

atmospheric effects from measurements made by the Measurement

Module. Similarly, the magnitude of any signal transmitted from

OSP can be predicted at the aperture of the E/S being worked.

Calibration factors on the polarization network will be measured

and stored so as to be applied as corrections on measurements.

(B) Six-port Networks for measuring transmitted and received pov7er . The six-port network is a new method developed at NBS for measuring

complex microwave power flow (phase and amplitude of incident and

reflected waves) in a transmission line without significantly

affecting that power flow. A brief explanation of the properties

of the six-port is given in Appendix I. It has a number of advantag

13
MEASUREMENT

MODULE

(S-BAND)

MEASUREMENT

MODULE

(C-BAND)

MEASUREMENT

MODULE

(K-BAND)

CPU, PROGRAM MEMORY,

DATA STORAGE, VARIOUS

CORRECTION FACTORS, ETC.

CONTROL/RCV

& DATA XMT ANTENNA

CONTROL & DATA RELAY MODULE

Figure 3. Configuration of OSP.

CENTRAL

PROCESSING

MODULE
SIGNAL ANTENNA

(A)

POLARIZATION

NETWORK

NBS

TYPE

IV

(C)

TO ENCODE, ETC

NBS

TYPE

IV

DOWN

PROGRAMMABLE ATOMIC

FREQUENCY CLOCK

SYNTHESIZER (E)

Figure 4. Measurement module.

15
that make it ideally suited for application in OSP:

1. It is accurate and has considerable redundancy in the measuring

system. With the aid of computer control, self-test and

self -calibration techniques can be included in the measurement. Partial system failure can be tolerated with little or

no loss of information or accuracy because of the intrinsic

redundancy

.

2. It can measure over a dynamic range of 30 dB with accuracies

of .05 dB or better, and over wider range at reduced accuracy.

3. Using sample-and-hold techniques, measurements can be made in

one to ten milliseconds, at a rate of ten to one hundred

measurements per second.

4. Frequency and temperature compensation are straightforward

with computer control.

The six-port networks would be used in OSP both to measure received

signal levels and to monitor transmitted signals and thus insure

that no shift in level had occurred in the signal source or power

amplifier of OSP.

(C) NBS Type IV Power Meter . This power meter is a self -balancing dc

substitution device capable of making absolute microwave power

measurements to an accuracy of better than 0.1%. It is intended

to serve for periodic absolute calibration of both received and

transmitted signals. Since normal measurement time required is of

the order of several tenths of a second, it will not be used for

routine measurements, but only with test signals for recalibration

of the six-ports.

(D) Noise-Add Source . The noise-add is a solid-state diode which is

triggered periodically in the course of a received power measurement. The known amount of noise power added to the measurement

enables the effects of system gain changes to be removed, thus

increasing accuracy ten to one hundred fold. It also is used for

a front end temperature measurement and thus provides an additional

cross-check on absolute received power level measurements.

(E) Atomic Clock . This is an optional component which might be used

as a phase lock for the programmable frequency synthesizer so that

phase jitter, group delay, and other measurements of propagation

phenomena that require a common phase reference at both ends of

the path can be made.

16
All other components of the receiving and transmitting systems are not

intrinsic parts of the precision measurement process, but serve to select or reject, amplify, convert, encode and decode, and otherwise process measurement, control, and data signals.

III. Earth Station Checking and Recalibration . The National Bureau of Standards is proposing a program initiative to

the Department of Commerce for establishing an Earth Station at its Boulder,

Colorado, site which will provide a means for recalibrating OSP. The NBS

E/S will consist of a parabolic dish (approximately 10 meters) of extremely

precise surface with advanced tracking and pointing capability. It is

anticipated that receive and transmit capability from 2 to 30 GHz will

eventually be acquired. Direct communications with OSP may be maintained (as

one of several possible means) of exercising command, control, and data

acquisition functions. The dish will be thoroughly evaluated by means of

spherical and cylindrical near-field scanning techniques developed by NBS.

These measurements will provide a complete farfield pattern at each required

frequency, including gain and polarization structure for of f -boresight conditions, as well as radiation characteristics deep into the sidelobe structure. With such a unique and meticulous evaluation, this Earth Station will

be capable of measuring fields and noise at its aperture and generating test

fields at OSP v;ith an accuracy and precision never before accomplished. With

this Earth Station, test signals to and from OSP can be periodically measured

and thus confirm and update the accuracy being maintained by OSP.

In the same way that OSP transmits standard signals to User earth

stations and performs measurements on signals transmitted from his earth

stations, the NBS Earth Station will be used to work the User satellites

directly.

Additional benefits can be derived from this combination of a wellcharacterized Earth Station and OSP. It is anticipated that one of the

problems of extending satellite communications links to higher frequency

bands, such as 20 and 30 GHz, is that atmospheric effects will become more

pronounced and probably will in fact be the limitation to our ability to

predict the performance of these links. Such phenomena as rain and snow depolarization, water vapor dispersion, atmospheric scintillation, and

possibly ionospheric Faraday rotation need to be examined. Cross-correlation

of these effects and measures of their correlation across the entire frequency

spectrum will be needed in the future v/hen millimeter waves become more

fully utilized for communication links.

17
Part C. OSP TRADEOFFS

Many decisions and compromises must be made in the complete design and

development of a system like OSP which involves new engineering concepts

.

This system is intended for calibrating and measuring performance parameters

on satellite communications systems and subsystems, and the tradeoffs includes

(1) Choices among possible methods of conducting these measurements, of which

OSP is one possibility; (2) Prioritization of requirements and schedules among

various classes of potential users; (3) Orbit considerations; (4) Nature of

services to be offered; (5) Frequency and band coverages; (6) Hardware design;

(7) Payload allocations to equipment, fuel, etc., and (8) Methods of processing data and disseminating results.

The purpose of this part of the report is to identify as many trade-offs

as possible and to indicate the factors involved in resolving the necessary

decisions. In a few cases, when the resolution of the trade-offs is already

apparent we have so indicated and merely listed the considerations to complete

the record. In most cases, however, the decisions are engineering, require

quantitative design data, and must await a more advanced state of development

of the project. In these cases, we indicate the preliminary nature of the

decision required.

I . Tradeoffs among Various Methods of Measuring System Performance

(A) Radio Stars . Several possible alternatives exist in present and

' anticipated methods of evaluating gain, temperature, and other

related parameters of earth stations. One of the main methods

now in use utilizes the radio flux of one of several radio stars,

Cassiopeia A, Taurus A, Cygnus A, and possibly our own sun and

moon to measure G/T of an earth-station [5] . The advantages of

using radio stars to determine G/T are:

1. The sources are of generally world-wide availability

Standards need not be carried to remote ground stations and

used under conditions which degrade their accuracy.

2. Only moderate technical ability on the part of the user is

required. "Cookbook" techniques are available and a massive

measurement capability of hardware and supporting software is

not required.

3. These stars are so-called "natural" standards which once

i evaluated need not be recalibrated, although steady changes

in flux density will need to be monitored. The analogy is

! that of determining a very accurate length measurement based

upon the appropriate spectral emission of Krypton 86 in con- trast to obtaining it by comparison with the standard meter

maintained in Paris, France.

18
4. The possibility exists of obtaining polarization data from

the flux of stars.

The disadvantages are:

1. The "signal" level is low, and not suitable for sidelobe

evaluation. If G/T of the earth station is too low, measurement accuracy becomes badly degraded. On the other hand, if

the gain of the antenna is too high, the narrow beamv/idth will

resolve the fine structure of the star and thus also reduce

accuracy.

2. The Starflux intensity is frequency-sensitive and, for most

earth stations, becomes unusable above about 10 GHz. While

it is true that discrete extraterrestrial sources, such as water molecules and various free radicals have been identified,

their usability for this purpose remains to be determined.

3. A good pointing capability is required of the ground station.

The ability to intercept a star is somewhat more demanding

than locking onto a geostationary satellite.

4. The method requires total service interruption for periods of

hours, and is usable only to obtain receive characteristics of

the antenna.

(B) Comparison With Standard Gain Antennas . A second method of obtaining ground station antenna parameters is by direct comparison vrith a calibrated reference antenna. The advantages of this method are:

1. Accurate on-axis gain can be obtained. If the reference

antenna has good polarization characteristics (very low axial

ratio if used with circular polarization and very high axial

ratio if used with linear polarization) , then the copolarization

of the antenna under test can also be obtained.

2. The method is economical and does not require high expertise

on the part of the user.

3. It is available on a roninterrupt basis for on-axis measurements.

The limitations are:

1. It requires a steady and stable satellite beacon transmitter

on the desired frequency.

2. Sidelobe data requires service interruption.

3. The measurement is available in the receive mode only and is

subject to significant interference which cannot be corrected

for

.

4. Linear cross polarization is not available unless the antenna

under test has a mount of unusual flexibility.

19
5. The attainable measurement accuracy degrades significantly

if the reference antenna has significantly lower gain than the

antenna under test. Typically, if the former has a gain of

30 dB and the latter over 50 dB, the three-sigma error can be

almost 1.2 dB, whereas if the gain of both were 5 0 dB, it

would be less than 0.1 dB for normal signal levels [6].

Attaining this low error, however, is not feasible, for it

would mean having a calibrated reference antenna which is as large as the earth station itself.

(C) Near-Field Scanning . The National Bureau of Standards has been

developing techniques of determining complete antenna properties

by making thorough scans of the near field over a surface very

close to the aperture plane. The characteristics of the antenna,

including amplitude and polarization of the entire pattern, can then be calculated for any other distance from the antenna,

including the usually-desired far field. The advantages of this

technique are:

1. It gives complete information about the antenna, including

mainlobe, sidelobe, and null structure down to the order of

60 dB below the boresight gain.

2. Data can be obtained at any frequency desired, transmit or receive, without fear of interfering or being interfered with.

3. The accuracy on a relative basis is the highest available by

any known technique. Tying the on-axis gain to an absolute

reference then enables one to achieve high accuracy on an absolute basis for the entire pattern.

The limitations are that expensive and sophisticated instrumentation, including hardware, computation facility, and software, must be used. It requires state-of-the-art measurement

expertise. Extensive service interruption is needed, because

of the large amount of data that must be taken and processed.

As an example, if an antenna having a diameter to wavelength

ratio of 500 were being measured (an 85-foot dish at 6 GHz)

,

2.5 million data points would be needed with a typical total

measurement time of 20 hours. In addition, data processing

in a CDC 6600 class computer would take about 30 hours.

(D) Calibrated Satellite and Earth Station . The OSP system with its

supporting calibrated earth station answers most of the problems

raised by examining the alternative approaches to measuring the

receive and transmit properties of communications system satellites.

20
This system can measure all of the required parameters accurately

and economically without requiring extensive skills of the User,

without major interruption of his operation, and without converting every satellite and ground station into a standards laboratory.

The investment in the OSP measuring system, its support, recalibration, and operation, of course, is major, but the purpose of

this entire feasibility study is to evaluate the benefits derived

against the costs.

II. Orbit Tradeoffs

Several types of orbits are under consideration for an operational OSP,

most of which are near-geosynchronous. Although a low, or spacelab, type of

orbit is important and can be useful for test and demonstration of the OSP

concept and for obtaining vital design and operational data, it does not seem

feasible for a fully operational system designed to interact with various

user satellite coramunications systems. Two major reasons why this is so are:

1. Position control in a greatly sub-synchronous orbit would be exceedingly critical and might require almost continuous fuel expenditure

to maintain precise passage relative to each individual user's geostationary satellite.

2. Even with satisfactory maintenance of passing position, a lov; orbit

OSP would pass through a typical earth station half-power beamwidth

in less than 10 seconds. This is far too rapid to conduct an extensive set of measurements with an acceptable degree of accuracy

and definiition on any antenna except one which has extraordinary

tracking, positioning, and offsetting capability. Furthermore, it

is clear that making such a series of measurements on an operating

link v;ould necessitate complete interruption.

Accordingly, since most of the functions of OSP would require a near- synchronous orbit, we list only the tradeoffs for various near-synchronous

orbits.

(A) Constant Drif t. This is a circular equatorial orbit whose radius

is slightly sub- or super-synchronous. It is fuel-conservative,

but in order to transverse slowly enough to be useful, will require

long unproductive periods during transit between satellites being

serviced. Some fuel V7ould be required to reverse the direction of

drift at the ends of the sector occupied by users. The monotonic

direction of drift (between reversals) would preclude opportunities

to repeat measurements should the need arise.

(B) Visitation Approach . This is a modification of the constant drift

orbit, in which thrusters are used to increase the drift rate

during the unproductive periods and then decrease the drift rate

21
for the measurement series. It is essentially similar to (A) in

other respects, except that the fuel requirements would be

increased significantly.

(C) Gravitational Well . This orbit is near geostationary, and makes

use of the slight geopotential well which exists over the western

hemisphere. The net effect is a self -reversal of drift over the

Atlantic and the Pacific potential barriers. This is probably

not feasible, because the drift rate (required to use these low

potential barriers effectively) is so slow that an unacceptably

long time would elapse between productive periods.

(D) Tilted Near-Synchronous Orbit . Tilting the constant drift orbit

with respect to the equatorial plane would have the effect of making

OSP oscillate in a north-south direction relative to geostationary

satellites as it drifted. This orbit configuration would assist OSP

to intercept links in which the user satellites were slightly out of

the zero latitude position. Disadvantages are that unless the drift

rate were exceedingly slow, such a path probably decreases, rather

than increases, probability of boresight interception of a user

link. Also, this path tends to be unstable, with the N-S oscillation tending to increase with time.

(E) Elliptical Orbit . With a near-synchronous elliptical orbit, OSP

would appear to have an oscillatory drift across the orbital arc which would be superimposed upon easterly or westerly drift, depending upon whether the period was sub- or super-synchronous. The

amplitude. of the oscillatory motion is determined by the ellipticity

of the orbit and the rate of drift by the departure of mean elevation

from synchronous level. Although this orbit conceivably could be

combined with equatorial tilt, the same objections as stated in (D)

would hold. The problem of nonalignment with user satellites is

probably not serious, and is likely to diminish still further in

the future, as station-keeping box size is tending to decrease to

and perhaps below 0.1 degrees [2]. One major advantage of the

elliptical orbit is that multiple measurements can be taken on a given system without waiting for the return of OSP many months

later. It would facilitate sidelobe measurements in the equatorial

direction, which is the direction that is significant with respect

to sidelobe interference with nearby satellites. One disadvantage

is that fuel estimates and difficulties of the orbital mechanics

have not been investigated as yet for this orbit. Another disadvantage is that of having a continually varying distance between OSP and

the user earth station. This will, of course, affect the signal

strength received at both places. The complexity introduced, however.

22
should be readily handled through softv/are in the onboard computer

and, if needed, by continuous control of the signal output level.

III. Users To Be Serviced

It became clear at an early stage of this study that the only Users who can be reasonably served are those v/ith satellites in near geo-stationary

orbits and those who are able to aim their earth-station antennas into this

orbit. Fuel consumption required to take OSP in its equatorial near- synchronous orbit to and from other orbits v/ould be enormous. It is also

clear that the preponderance of satellite communication system operations

would be served, even with this restriction. A second point that seems clear

is that OSP will not perform measurements directly upon other satellites.

Requirements of pointing control upon both OSP and the User satellite would

be major--at least 60 degree changes in pitch and yav/ and quite possibly

360 degree changes would be required. In addition to requiring major com- mitments to attitude controlling and determining capability on each satellite,

fuel expenditures v;ould increase markedly. Such drastic attitude changes on

the part of the User satellite v/ould take it out of service while being

measured, an unacceptable consequence. Finally, transmit and receive capabilities on OSP would have to be doubled. It should be noted, hov/ever, that

this does not mean the satellite end of the User link v/ill not be served;

only that it will not be served by OSP, but by OSP's companion earth station.

Within hardware restrictions, military and civilian users alike can be

served, save only that the OSP system must remain com.pletely free of security

classification. Imposing secrecy of operating frequencies, methods of

measurement, processing procedures, etc., would unacceptably limit the general

usefulness of OSP, although NBS policy is that the results of a reimbursed

calibration are the sole property of the purchaser of that calibration.

A design goal is to permit the User to maintain service of this system

during the measurement, dedicating the equivalent of only one or a few audio

channels to the measurement process.

An alternative which can be considered is to schedule measurements during

periods when communication loads are light, as for example, past midnight.

This, however, constitutes fine-tuning of the system and its feasibility

must await resolution of many of the operational decisions.

IV. Hardware Tradeoffs

A great many engineering and operational tradeoffs must be resolved as the engineering design of the system proceeds. To some degree, these can be

stated here, but since most of these involve quantitative evaluation of weight,

pov/er , volume, complexity, redundancy, interference, accuracy, economy.

23
realizability , and many other qualities, we cannot expect at this early state

of the project to resolve or even fully identify them. Mr. W. L. Morgan has

in his reports shown the scope of interdependence of these decisions and has

begun a serious analysis. We shall not repeat his work here, but only suggest a few of the items that must be on a final checklist.

(A) Frequencies and Band Coverage . The general lower and upper operational limits of OSP are approximately 2 and 30 GHz, respectively.

OSP antenna size, NBS Earth Station size and surface precision,

anticipated User system deployment, space available on OSP, and

response to the User questionnaire [1] , all comJDine to dictate

these outer limits. The number of transmit and receive frequencies

in each band, the number of bands, the spectral location of these,

the amount of duplexing and diplexing, all are questions to be.

resolved. Some of the tradeoffs involved include obtaining designcenter performance as opposed to accepting poorer hardware performance and compensating by means of software corrections.

(B) Antennas and Polarization Networks . One of the most important

components in the measuring system is the antenna and its polarization network. The earth coverage pattern and the polarization over

that pattern must not only be well calibrated, but must maintain

high purity. In addition, the internal circuit parameters must be

such that mismatches are low and loss is minimized. Design of the

antennas (probably microwave horns having gains of about 20 dB) and

temperature stability must be carefully optimized, and this requirement must be balanced against the system economy of diplexing and

duplexing.

(C) Other Factors . Some of the questions that are especially i.mportant

to a "satellite standards laboratory" as contrasted with a "satellite

communications terminal" are very briefly suggested here. The

tradeoff between cost and state-of-the-art feasibility on the one hand and desirable accuracy on the other hand represents the focal

point of success of the project. As has been mentioned earlier,

a tradeoff is available between quality hardware design and software improvement of performance. It is just this factor which has

been a current subject of interest in many government and industrial

standards laboratories and quality control facilities. The entire

software problem of instrument control, data acquisition, data

processing, customer queuing, and many other aspects must be

engineered and allocated to on-board or ground computer facilities.

24
Part D. ECONOMIC IMPACTS OF PRECISION MEASUREMENT CAPABILITY

UPON SATELLITE COMMUNICATION SYSTEMS

I. Introduction

This part identifies economic and social benefits resulting from accurate

measurement of satellite communication systems. As a general axiom, areas

of science and technology benefiting most from an accurate, precise measurement technology are those already at an advanced, demanding state of technology. At early or rudimentary states of development, major technological

concerns are primarily qualitative: that something exists is frequently

adequate; that its quantitative nature be known is not necessary.

Satellite communication is an advanced technology in which there is

little room for imprecise adjustments because of engineering and economic

constraints imposed upon its systems. In an analogous sense, the performance

of an advanced racing car is much more critically dependent upon precise

tuning than that of an old, neglected jalopy. A small maladjustment in the

former will cause a much more noticeable degradation in performance than in

the latter. This part of the report describes various specific ways in

which exact measurement capability acts to the benefit of planners, designers,

operators, and suppliers of communication systems.

For the planner and regulator, an important concern is that of interference as determined by undesired interactions of systems with each other.

From the operator's standpoint, efficiency of utilization depends upon

obtaining adequate management data on performance and capital costs. Between

the supplier and operator, questions may arise as to v/hether or not a major

system component meets its specified performance, and therefore whether or not it is fulfilling incentive and delivery requirements. Measurement

techniques which are accurate, credible (traceable), convenient, relatively

inexpensive, rapid, and operationally noninterfering become valuable resources

for all of these parties. It is our aim in this report to develop the basis

for quantitative estimates of those values.

II . Impact Upon System Investment and Operating Costs

The Orbiting Standards Platform (OSP) and its supporting NBS Earth

Station (NBSES) will be designed to make accurate measurements on several

parameters, (listed in Table I) that are relevant to the performance of both

earth stations and satellites. We will first discuss only those parameters

concerned with improving the operation of the user's own system. These

primarily concern measurements for evaluating signal and noise levels,

especially gain and system temperature determinations of earth stations. We

can arrive at an estimate of the worth of these measurements by assuming that

uncertainty in the measurement must be compensated by an equivalent over- design of the system.

25
If a certain communication system has a target data rate and associated

probability of error requiring a corresponding G/T of an earth station, then

the uncertainties in achieving gain and temperature dictate a design goal

G/T which must exceed the required operating values by at least these uncertainties. The cost of this over-design can be approximately quantified by

evaluating the incremental costs of each individual parameter per unit of

measurement uncertainty. A complete analysis would require that all relevant

parameters be considered, that all alternative methods of measurement and

their uncertainties be analyzed, and that these data be summed over all the

existing and planned systems. Recognizing the impracticality of such a complete analysis under the constraints of available time and resources, we must select the most significant elements and only indicate the magnitude of

the total effect.

One of the most important link parameters to be considered is the G/T

of an earth station, that is, its figure of merit. We will examine separately

consequences of gain and system temperature measurements of electrically

and physically large antennas.

Three studies performed in the late 1960 's attempted to make empirical

analyses of the costs of large earth station antennas [7,8,9]. Each study

obtained cost formulas which have been re-examined and enlarged upon recently

[10] and updated to 1974 [11] . Although it is necessary to account for inflation, advances in manufacturing technology, and increased competition, in

order to extrapolate these costs to the present and future, the consensus is

that the effects of these perturbing factors on incremental cost dependence

upon size are probably second order. Accordingly, with the aid of figure 5, one can obtain a reasonably consistent picture of incremental costs. There

is generally good agreement among these estimates of cost, especially for

costs of antennas above approximately 50 feet in diameter. A straight-line

(log-log) composite of these curves, from which none of the individual curves

departs by more than 10%, indicates a cost in hundreds of thousands of

dollars equal to one-fourth of the square of the antenna diameter (in feet)

.

To a good approximation, a gain-change of 1 dB corresponds to an increase in

diameter of 11.5%. One can therefore conclude that the cost of antennas

over 50 feet in diameter rises about 23% per dB. This corresponds to a cost-effectiveness of measurement in which reduction of uncertainty is worth

between $150,000 and $2,500,000 per dB , for 50- and 150-foot antennas

respectively. (These are probably the lower and upper limits of validity of

this model.) One must bear in mind, of course, that an antenna is not

actually purchased because it has a gain of so many dB, but rather because

it is in a certain size class. As a statistical averaging, however, this

approach to costing seems reasonable and valid.

26

Present typical accuracies of measurement of antenna gain are about

1 dB, but errors of several dB are not uncommon [ 5 ] . The estimated uncertainty to be achieved by OSP and its support NBSES will be in the vicinity

of 0.1 to 0.2 dB. The increased accuracy, using this measurement alone, may

well reduce the cost of purchasing hundreds, of earth stations by the amounts

stated above, for a cost savings on the order of $100 million.

The system temperature is the other factor that affects G/T measurements.

The principal contributors to system temperature are antenna factors

(efficiency, sidelobes, and feed losses) over which relatively little control

is available; and noise temperature of the low-noise receiving front end,

(LNR) , which can be reduced by purchasing more expensive front-end electronics.

In general, for large antennas the LNR costs are small compared with other

capital costs but are still important to performance. As the antenna diameter

is reduced, the cost of lowering the noise temperature of the LNR becomes an increasingly large part of the station's cost [10], The difficulty of measurement increases and the LNR costs rise abruptly as the noise temperature

decreases and as the frequency increases above 10 GHz. Finally, we note

that the use of radio stars for measurement of G/T becomes increasingly

inaccurate and difficult for these cases of small terminals and higher

frequencies [12]

Putting these factors together in a qualitative way, we conclude that a typical noise measurement would be made on an 8 GHz system with an LNR

temperature of 3 0 K. For a typical total front-end temperature of 70 K

(antenna plus LNR) , the measurement uncertainty would be about 10 K, which

amounts to a fractional error of 30% on the LNR, which is the only controllable

part.

The steeply rising costs of cooled paramps with lower noise figures are shown in figure 6, which is reprinted from reference [13]. Very recent data

[14] on cooled paramps agree with that of figure 6. This figure covers a sufficiently wide range so we may conclude that the cost in the range of

noise temperature from 10 to 100 K is inversely proportional to the temperature, or that the fractional cost increment is equal to the fractional

uncertainty of the temperature measurement. Combining this result with that

of the previous paragraph, we can see that the worth of a f actor-of -f ive

improved temperature measurement is $7000 per receiver, with the number of

LNR's in use measured in the hundreds. Thus, although OSP can decrease the

cost of over-design on both temperature and gain of earth stations, the

dominant benefit is through the gain component of the G/T ratio.

Accurate measurement capability can also favorably affect satellite

communication system investment (capital) costs when replacing major equipment, especially satellites. The principal system operators, such as INTELSAT,

have replacement satellites in orbit in anticipation of the failure of

28
Noise Figure (dB)

0.2 0.5 1.0 1 3 4 5 6

10 20 50 100 200 500 1000

Noise Temperature (K)

Figure 6. Costs of low noise front ends (Courtesy Akima [13])

29
operating satellites. Such failures occur for one of three reasons:

(1) exhaustion of fuel used for precise station-keeping; (2) sudden, death of

a sufficient number of on-board transponders so that the satellite cannot

maintain adequate data rates; and (3) slow degradation in the satellite EIRP

so that error rates increase to unacceptable levels because of poor signalto-noise ratios. All of these factors are presently estimated on the basis

of statistical predictions, and satellite replacement schedules are generated

accordingly. Especially with respect to the last failure category, the

ability to make measurements with negligible interruption of service provides

a method of continuous quantitative determination of the state of health of

the various transponders in the satellite. Such a record of performance vs. time (which is analogous to the Control Charts in common use in standards

laboratories) enables the system operator to maintain continuing surveillance on the -performance of the system and thereby to accurately schedule the

launching of replacement satellites and upgrading of earth station equipment.

The advantage of such system management information is that capital and

replacement costs are reduced in several ways: (a) satellites are launched

at a more accurately defined time of need and thus their costs are postponed

until actually needed; (b) too early launch requires that the satellite be

idle for a longer period prior to use. During this time, degradation,

failure, and use of fuel all continue to exact their toll; (c) delaying the

launch permits taking advantage of late advances in state-of-the-art in

equipment needed.

Assuming a launch plus equipment cost of approximately $10 million per

satellite, the ability to delay deployment of these replacement units

for an additional year every seven years means a savings of more than $1

million per satellite. In the same way, the general condition of the system

can be more closely monitored so that maintenance and repairs can be anticipated before major indicators arise, such as: breakdown, complaints from

subscribers about quality of service, and actual loss of operating channels.

III. Payment of Incentives

Usually, procurement of major satellite communication components is

contingent upon the vendor's fulfilling a set of incentives. Payment for

these incentives, significant increments to the basic purchase price,

frequently make the difference between the vendor's profit and loss on a satellite. As an example, the satellites comprising the Global Positioning

System (GPS) will carry a total of approximately $20 million in incentives.

The conditions upon which incentives are based are usually stated in

terms of satisfactory operation of a specified number of channels over a certain period of time. Complete breakdown on the one hand and completely

satisfactory service on the other are conditions which raise no controversy

30
between purchaser and vendor. Marginal performance, however, can lead to

considerable difficulty and expense in resolving whether the incentives have

been met. It is possible that apparently defective performance of a satellite

(such as low EIRP) is actually indicative of a faulty measurement from an earth station. The combined role of OSP and NBSES can be that of a neutral

third party in resolving such difficulties [12]

.

IV. Effective Utilization of Spectrum and Orbit Resources

In the previous sections, we have emphasized those measurem.ents considered

most important by a system operator because parameters of signal and noise

directly affect the performance of his own system. With the present and

future trend of dense packing of systems into the frequency spectrum and the

geostationary orbit, the problem of interference between separate systems

becomes an important matter of regulatory, and therefore of public, interest.

Even with current trends toward frequency re-use by means of orthogonal

polarization, spot coverage, and extension to the higher frequency bands, it

is clear that available frequency channels and orbital positions are being

filled rapidly.

The satellite communication industry has an exploding market. Today,

some sixty systems either are in operation or are being planned. The

economic and societal consequences of this kind of growth are both far

reaching and difficult to predict adequately. Joseph N. Pelton, of INTELSAT,

recently wrote [15], "When people seek to predict the future, they frequently

are guilty of two errors. They underestimate how quickly a new technology

can take off, and they overestimate the ability of institutions and policy

makers to cope with the resulting societal impact." By way of analogy, he

points out that in 1950, "fearless forecasters" suggested that there was no real market for computers, since twelve computers, with the speeds then

possible, could perform all necessary calculations for the United States,

and two could satisfy the needs of the United Kingdom. W. L. Pritchard [16]

seconded Pelton' s viewpoint, saying that we tend to be terribly conservative

in our long-range predictions. He adds, "It is even conservative to predict

that by the end of the century (based upon population growth, increase of

per capita GNP, and concomitant increase in demand for communications services)

there will be three times the present number of satellites in orbit."

In the future, there will be a tremendously increased load upon available

resources of frequency spectrum and geostationary locations. Attempts to

stretch these limited resources are taking a number of forms, including

refined coding, use of time-division-multiple-access, multiple-beam satellites,

orthogonal polarizations, and extension into the higher frequency bands.

Information theory and atmospheric effects define intrinsic limitations to some of these techniques. Accurate and rapid measurements, however, can

31
open doorways o For example, present minimum spacing in the geostationary

orbit is four degrees for common frequency satellites c At present, the

limitations of effective utilization of the orbit and spectrum depend solely

upon the state-of-the-art of antennas, not upon the space transportation

system. Sarkar [17] wrote: "The effective use of the geostationary orbit

and the frequency bands have a direct bearing on these problems. The Radio

Regulations and the studies made by the International Radio Consultative

Committee (CCIR) , although not yet conclusive, provide certain guidelines.

The studies state that no simple comprehensive criteria have yet been developed to indicate whether or not satellite systems make efficient use of the

geostationary orbit or the frequency spectrum." He goes on to say that

antenna radiation pattern and cross-polarization performance are the two

main parameters. At least two authors closely involved in the regulatory

process point out that a reduction in sidelobe content of earth stations

would increase the utilization of the geostationary orbit [17,18],

Present design techniques of large aperture antennas have gone through

several generations of development over the past ten to fifteen years,

beginning with early shaping techniques [19] and carrying through diffraction

theory to more fully account for the effects of subreflector configuration

and truncation [20]- We are now in a fourth generation of beam shaping,

which has led to antennas approaching 80% aperture efficiency in the 4 and

6 GHz bands, including effects of spillover, depolarization, etc. The main

purpose of these design techniques, however, has been to increase antenna

efficiency, with the aim of optimizing G/T by obtaining maximum gain performance for a given physical size and decreasing far sidelobes to reduce the

amount of high temperature earth seen by an antenna whose main lobe is

looking at a cool sky. With the exception of the COMSAT "Torus" antenna

(which is a section of a paraboloidal torus with multiple offset feeds)

[21], however, little attention has been paid to suppression of sidelobe

radiation lying specifically in the equatorial belt. With the burgeoning of

small, inexpensive earth terminals, the sidelobe problem is likely to grow

more, rather than less, severe. The difficulty of this problem is impressive.

Sidelobe shaping requires extremely detailed attention to a very minor

portion of the total energy radiated from the antenna. Care must be taken

not to adversely affect the major radiation properties by causing boresight

gain reduction, increased cross-polarization, defocussing, phase errors,

squinting, and a variety of degradations. Although it is well known that a Gaussian illumination produces no sidelobes, this distribution makes such

poor utilization of a given reflector diameter that it is not an acceptable

solution to the sidelobe problem. In general, theoretical design techniques

must be confirmed by measurements. Variations among individual units, even when standardized in production, suggest the need for testing after delivery

32
and installation at the customer's site. Several measurement techniques are

in use [22] but accurate determination of sidelobe structure, including

polarization characteristics, is presently limited to either near-field

probing or measurements made on a true far-field range. Each of these has

severe practical limitations. Near-field probing (i.e., measurements made

on a plane, cylindrical, or spherical surface very close to the antenna and

then mathematically transformed to obtain far-field characteristics) requires

sophisticated instrumentation and advanced skills. Even with such capability,

the measurement is slow and expensive for large antennas. For example,

measurements on a 12-meter dish at 6 GHz would require some ten hours of

actual measurement time (at 25 milliseconds per measured point) and five

hours of processing time on a CDC 6600 computer [23]. Outdoor ranges, which

attempt direct far-field measurements, involve expensive towers and considerable real estate. In order to be at least several Rayleigh distances away

from the antenna (in the above example, a Rayleigh distance is 1.5 Km),

examination of the sidelobes would require tall towers to eliminate multipath

reflections. Other techniques, such as the "compact range" [22] are useful

for boresight and main lobe examination but introduce sufficient aberration

that sidelobe structure and cross-polarization errors become unacceptably

large.

The advantages of earth station measurement by means of OSP immediately

become apparent. Since it is a true far-field measurement, no extensive

computer processing or specialized expertise is required. Such measurements

can be done from either the manufacturer's plant or the user's earth station

site, and include all effects introduced by the operating surroundings.

With a drifting orbit of OSP, the antenna need not be taken offline for the

measurement. One merely lets OSP move across the equatorial belt of the

antenna, making measurements at programmed intervals over the period of a 12-hour half -oscillation of its synchronous, slightly elliptical, equatorial

orbit. With such a readily available, inexpensive, accurate measurement

technique yielding detailed sidelobe amplitude and polarization structure,

several major benefits can be realized. Antenna design innovations and

modifications can easily be tested and evaluated. This would advance the

capability of equatorial sidelobe suppression such that more dense packing

of geostationary satellites could be accomplished. The need for this is

indicated by the density of present and planned satellites, as shown in

figure 7. Specifically, in the fifty degrees of longitude encompassing the

United States, eleven satellites are now deployed using the 6/4 GHz band

(2 SATCOM's, 3 COMSTAR' s, 3 ANIK's, 2 WESTAR's, and an ATS). One immediate

benefit of being able to develop and regulate earth stations with improved

sidelobe characteristics v/ould be an increased orbit capacity of perhaps 20%

to 50%. As a more far-reaching benefit, the development and production

33
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34
advantages provided to American industry would help maintain its present

lead in the world equipment market. One of the United States' strongest

principal exports is this production obtained from our electronics and

communications high technology. This market is being penetrated by other

nations [24], and the retention of the United States' advantage can be

realized, by intensive efforts needed to keep us on the leading edge of

technology development. The measurement capability of OSP will provide

leverage when hammering out agreements in such international areas as the

World Administrative Radio Conferences, and will strengthen our position

of world technology leadership.

REFERENCES

[I] Dougherty, H. T. , The OSP survey response, U.S. Dept. of Commerce/Office

of Telecommunications OT Tech. Memo 78-249.

[2] Boston University Seminar Notes "Communication Satellite Antenna

Technology" (unpublished)

.

[3] Kreutel, R. , Difonzo, D., and Mahle, C. , Satellite system measurements,

to be published Proceedings of the IEEE, April, 1978.

[4] Mandel, J., The Statistical Analysis of Experimental Data ( Interscience

,

Second Printing, April, 1967). Also see: J. J. Ku, Precision Measurement and Calibration; Statistical Concepts and Procedures (NBS Special

Publication 300, Vol. 1, 1969), and M. G. Natrella, Experimental

Statistics (NBS Handbook 91, 1963).

[5] Wait, D. F., Daywitt, W. C. , Kanda, M. , and Miller, C. K. S., A study

of the measurement of G/T using Casseopeia A, National Bureau of

Standards Report, NBSIR 74-382.

[6] Kanda, M. , Accuracy considerations in the measurement of the power gain

of a large antenna, IEEE Trans. Antennas and Propagation AP-2 3 , 407-411

(May 1975)

.

[7] Potter, P., Merrick, W. , and Ludwig, A., Big antenna systems for deep

space communications. Astronautics and Aeronautics A_, 85-95 (Oct. 1966).

[8] Stack, B., An approximate expression for cost gain relationship in large

parabolic antennas, Stanford Research Institute Report (Dec. 1967).

[9] Pope, D. , Parametric representatives of ground antennas for communication

studies, BSTJ 46, 2145-2168 (Dec. 1967).

[10] Cuccia, C. L. and Teicher, S., The economics of antenna receiving

systems. Microwaves 8^, 86-95 (June 1969).

[II] Juroshek, J. R. , McManamon, P. M. , Nesenbergs, M. , and Hill, D. A.,

Study of satellite frequency requirements for the U.S. Postal service

electronic mail system, Vol. Ill Technology Studies, Office of Telecommunications Report 74-27 (Feb. 1974).

[12] Wait, D. F., Satellite earth terminal G/T measurements. Microwave J.

20, 49 (April 1977)

.

35
[13] Akima, H. , Minimum antenna dimensions for satellite communication earth

stations, Office of Telecommunications Report (in press)

.

[14] Humphries, T. , LNA noise/cost trade-offs in small aperture earth

stations. Satellite Communications 1,58-60 (Oct. 1977).

[15] Pelton, J. N. , Communication satellite proliferation: Will we have too

much of a good thing?. Satellite Communications 2_, 22-26 (Jan. 1978).

[16] Pritchard, W. L. , Is satellite communications a viable market?.

Telecommunications 1_2, 14 (March 1978).

[17] Sarkar, S. K. , Space station orbit and frequency spectrum in radiocommunications services. Telecommunications Journal 4_2, 4 01-412

(July 1975)

.

[18] Jansky, D. , Factors affecting orbit utilization, Communications Satellite

Systems : An Overview of the Technology (IEEE Press, New York,

pp. 103-107, 1975)

.

[19] Galindo, V., Design of dual reflector antennas with arbitrary phase and

amplitude distributions, IEEE Transactions Antennas and Propagation

AP-12 , 403-408 (July 1964).

[20] See eg., Clarricoats, P. J. B., and Poulton, G. T., High efficiency

microwave reflector antennas—A review, Proc. IEEE 65_, 1470-1504

(Oct. 1977).

[21] Private Communication, Morgan, W. L., (COMSAT Laboratories).

[22] Kummer, W. , and Gillespie, E. S., Antenna measurements--1978 , to be

published Proc. IEEE (April 1978).

[23] Private Communication, Stubenrauch, C. F.. (National Bureau of

Standards)

.

[24] Edelson, B. I., Remarks at a panel discussion. Is the U.S. losing its

technical leadership, EASCON Symposium, Washington, D.C. (1976)

.

36
APPENDIX I

THE NBS "SIX-PORT" POWER MEASURING SYSTEM

The "Six-Port" is a new concept in measuring microwave circuit parameters

such as power flow, impedance, reflection coefficient, and scattering parameters in general. In the applications in OSP, we are expecting to use it

for measuring net power flow (of the power transmitted from OSP) and incident

power flow (of received power) with speed, accuracy, and high reliability.

In the accompanying illustrations, a simplified logical development of

the six-port is shown.

In figure 8a, we have the problem statement. Power flows from the source

on the left to the mismatched load on the right. The incident power, P^., is

that flowing down the line, but the net pov/er delivered to the load is Pj^^^rp'

which is the incident power reduced by the power reflected from the generally

imperfect matched load. In OSP, when we are looking at pov;er received by

OSP, we V/ish to know the total received power irrespective of a mismatched

down-converter; v/hen we are monitoring a test signal transmitted by OSP, we wish to know only Pj^^rp^ the power delivered to the antenna.

An obvious method of measuring the power is to insert a calibrated

directional coupler in the transmission line, as shown in figure 8b. The

limitations of such a measurement are major: It measures P_, and not P.,^„

I NET

and it has noncalibratable errors indicated by e^r caused by mismatches at

the load and source. A refinement upon the preceding arrangement is to place two directional

couplers back- to-back, as in figure 8c. This constitutes a "four-port" and

permits direct measurement of the incident and reflected power, and therefore

of P^,„rn- However, it still suffers from the introduction of noncalibratable

errors indicated by the terms involving and because of load mismatch

magnitude and angle.

A simple six-port is shovm schematically in figure 9, and is completely

calibratable for imperfect components in terms of the coefficients of each

P. and may be used to measure both P^ and P,TM„. Figure 10 shows a realization

1 I NET ^ of the six-port using power dividers, quadrature hydrids, and simple diode

detectors, all of which can be built into a single microwave integrated

circuit. It has the following additional advantages in this application:

1. High redundancy. Failure of any one of diodes P^ through Pg causes

no reduction in performance. Failure of any tv/o causes slight

reduction in accuracy of phase measurement and essentially none in

amplitude measurement. Failure of any three causes loss of phase

measurement capability and some degradation in amplitude measurement.

Failure of all four still permits a reduced accuracy measurement of

amplitude which is the equivalent of figure 8b. (Note that P^ is

37
necessary in all the above cases, and that a system for our purpose

should also provide specific redundancy in this part of the six-port)

.

2. Phase measurements are obtainable without the need for phasesensitive detectors.

3. Measurement speed is commensurate with the time required for

moderate accuracy (12-bit) digitizing of the diode outputs, that is

to say, the order of one to ten milliseconds.

38
b

39
Z 6. p.

i 1 1

T. a. P.

i 1 1

T. z.P.

i ^ ^

Z 6. P. i 1 1

i = 3. .,6

Figure 9. 6-Port concept.

40
D= Pcvfer Divider

0= Ouodroture Hybrid

P= Diode Dete ctor

10. A typical six-port, with redundancy.

41
NBS.t14A (RCV. 7-73)

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO. NBSIR 78-886

2. Gov't Accession

No.

3. Recipient's Accession No.

4. TITLE AND SUBTITLE

Feasibility Study of Orbiting Stcindards Platform

5. Publication Date June 19 7 8

6. Performing Organization Code 723.05

7. AUTHOR(S) A.J. Estin and R.C. Baird

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No. 7237276

11. Contract/Grant No.

12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP) 13. Type of Report & Period Covered

14. Sponsoring Agency Code

IS. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or less factual suaunary of most sigpificant information. If document includes a significant bibliography or literature survey, mention it here.) This report consists of four components of a feasibility study for a satellitebased measurement system for determining important operational parameters of

satellite communications systensand its major sub-systems. We have addressed the

questions of required accuracy, methods of attaining and maintaining measurement

accuracy and traceability , system tradeoffs, and economic impacts and benefits.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons) Antenna gain; antenna measurements; antenna pattern;

antenna sidelobes; G/T; EIRP; metrology for satellite communications.

18. AVAILABILITY O Unlimited

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