NBS TECHNICAL NOTE 645

Time and Frequency Broadcast Experiments

from the ATS-3 Satellite
NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards 1 was established by an act of Congress March 3, 1901. The Bureau's overall goal is to strengthen and advance the Nation's science and technology and facilitate their effective application for public benefit. To this end, the Bureau conducts

research and provides: (1) a basis for the Nation's physical measurement system, (2) scientific and technological services for industry and government, (3) a technical basis for equity in trade, and (4) technical services to promote public safety. The Bureau consists of the Institute for Basic Standards, the Institute for Materials Research, the Institute for Applied Technology,

the Institute for Computer Sciences and Technology, and the Office for Information Programs. THE INSTITUTE FOR BASIC STANDARDS provides the central basis within the United

States of a complete and consistent system of physical measurement; coordinates that system

with measurement systems of other nations; and furnishes essential services leading to accurate and uniform physical measurements throughout the Nation's scientific community, industry, and commerce. The Institute consists of a Center for Radiation Research, an Office of Measurement Services and the following divisions: Applied Mathematics — Electricity — Mechanics — Heat — Optical Physics — Nuclear

Sciences " — Applied Radiation 2 — Quantum Electronics " — Electromagnetics 3 — Time

and Frequency 3 — Laboratory Astrophysics s — Cryogenics *. THE INSTITUTE FOR MATERIALS RESEARCH conducts materials research leading to improved methods of measurement, standards, and data on the properties of well-characterized

materials needed by industry, commerce, educational institutions, and Government; provides

advisory and research services to other Government agencies; and develops, produces, and

distributes standard reference materials. The Institute consists of the Office of Standard

Reference Materials and the following divisions: Analytical Chemistry — Polymers — Metallurgy — Inorganic Materials — Reactor

Radiation — Physical Chemistry.

THE INSTITUTE FOR APPLIED TECHNOLOGY provides technical services to promote

the use of available technology and to facilitate technological innovation in industry and

Government; cooperates with public and private organizations leading to the development of

technological standards (including mandatory safety standards), codes and methods of test; and provides technical advice and services to Government agencies upon request. The Institute consists of a Center for Building Technology and the following divisions and offices: Engineering and Product Standards — Weights and Measures — Invention and Innovation — Product Evaluation Technology — Electronic Technology — Technical Analysis — Measurement Engineering — Structures, Materials, and Life Safety ' — Building Environment * — Technical Evaluation and Application * — Fire Technology.

THE INSTITUTE FOR COMPUTER SCIENCES AND TECHNOLOGY conducts research and provides technical services designed to aid Government agencies in improving cost effec- tiveness in the conduct of their programs through the selection, acquisition, and effective

utilization of automatic data processing equipment; and serves as the principal focus within

the executive branch for the development of Federal standards for automatic data processing equipment, techniques, and computer languages. The Center consists of the following offices and divisions: Information Processing Standards — Computer Information — Computer Services —

Systems Development — Information Processing Technology.

THE OFFICE FOR INFORMATION PROGRAMS promotes optimum dissemination and

accessibility of scientific information generated within NBS and other agencies of the Federal Government; promotes the development of the National Standard Reference Data System and

a system of information analysis centers dealing with the broader aspects of the National Measurement System; provides appropriate services to ensure that the NBS staff has optimum

accessibility to the scientific information of the world. The Office consists of the following

organizational units: Office of Standard Reference Data — Office of Technical Information and Publications —

Library — Office of International Relations.

1 Headquarters and Laboratories at Gaithersburg, Maryland, unless otherwise noted; mailing address Washington, D.C. 20234.

2 Part of the Center for Radiation Research.

3 Located at Boulder, Colorado 80302.

4 Part of the Center for Builduig Technology.
TIME AND FREQUENCY BROADCAST EXPERIMENTS

) FROM THE ATS-3 SATELLITE

National Bureau of Sia.ndar&s

D. W. Hanson APR 2 9 1974 W. F. Hamilton

Time and Frequency Division

Institute for Basic Standards

National Bureau of Standards

Boulder, Colorado 80302

*»«

"*»tAU Of ^

U.S. DEPARTMENT OF COMMERCE, Frederick B. Dent, Secretary

NATIONAL BUREAU OF STANDARDS. Richard W Roberts. Director

Issued November 1973
National Bureau of Standards Technical Note 645

Nat. Bur. Sland. (U.S.), Tech. Note 645, 115 pages (November 1973)

CODEN: NBTNAE

For sale by the Superintendent of Documents, L.S. Government Printing Offie«, Washington, D.C. 20402

(Order by SD Catalog No. 03.46:645) $1.00
CONTENTS

Page

1. INTRODUCTION 1

2. BROADCAST EQUIPMENT AND FORMAT 6

3. RECEIVING EQUIPMENT 14

3. 1. ANTENNAS 18

3. 2. PREAMPLIFIERS 24

3. 3. RECEIVERS 24

4. SIGNAL PROCESSING 26

5. SIGNAL DELAY ' 34

5. 1. EARTH-SATELLITE-EARTH PATH DELAY;

SATELLITE MOTION 35

5.2. IONOSPHERIC AND TROPOSPHERIC DELAY .. 47

5. 3. EQUIPMENT DELAY 50

6. DOPPLER 53

7. DELAY COMPUTATION AND CLOCK SYNCHRONIZATION 59

8. RESULTS 64

9. SYSTEM PERFORMANCE COMPARISON 71

9.1. COVERAGE 72

9.2. RESOLUTION AND ACCURACY 72

10. CONCLUDING REMARKS 74

APPENDIX I. SLIDE RULE AND PERFORMANCE 86

APPENDIX II. ORBITAL ELEMENTS AND ORBIT

PREDICTION 95

APPENDIX in. COMPUTER PROGRAM FOR FREE SPACE

PROPAGATION DELAY 102

11. REFERENCES 105
LIST OF FIGURES

Page

Figure 1. Transmitting equipment 7 Figure 2. Time and frequency facilities providing reference

to the ATS -3 experiment 8 Figure 3. Transmitting antenna 9 Figure 4. ATS-3 Satellite 10

Figure 5. ATS-3 coverage at VHF 11

Figure 6. VHF transponder, block diagram 12

Figure 7. Time and frequency waveforms 15

Figure 8. Broadcast schedule from ATS-3 16

Figure 9. Equipment for ATS-3 time comparison including tick

phasing adjustment, block diagram 17

Figure 10. VHF receiving antennas: (a) crossed yagis; (b) 3 turn

helix; (c) crossed dipoles with reflector; (d) grounded

quarter wave dipole 21

Figure 11. Ground antenna pointing angles 23

Figure 12. VHF preamplifier 25

Figure 13. VHF receivers: (a) vacuum tube model; (b) solid

state model; (c) vacuum tube model; (d) solid

state model; (e) solid state model; (f) solid state

model; (g) receiver, active filter, and time code

reader 27

Figure 14. Digital delay generator 29

Figure 15. Received signals from ATS-3 on an oscilloscope:

(a) Sweep: 0. 1 s/cm; (b) Sweep: 1 ms/cm; (c) Sweep:

100 Us/cm; (d) Sweep: 100^s/cm. High vertical

gain 30

Figure 16. Received signals from ATS-3 at different sampling

rates: (a) Trigger rate: 1 sweep/s; (b) Trigger rate: 1000 sweeps/s 32

Figure 17. Geometry of an elliptical orbit 36

IV
Figure 18. Classical and perturbed orbits 38

Figure 19. Delays from NBS to five sites via ATS-3 39

Figure 20. Delays from NBS versus eccentricity 40

Figure 21. Delays from NBS versus inclination 41

Figure 22. Subsatellite point versus eccentricity 43

Figure 23. Subsatellite point versus inclination 44

Figure 24. Long-term, two-way delay between Boulder and

ATS-3 at 1700 GMT 45

Figure 25. Long-term, two-way delay between Boulder and

ATS-3 at 2330 GMT 46

Figure 26. One-way signal delay caused by the troposphere and

ionosphere 48

Figure 27. Total ionospheric electron content and associated

two-way delay for an average winter and summer

month 49

Figure 28. Closed loop system delay measurement, block

diagram 51

Figure 29. Doppler from Boulder to five sites 56

Figure 30. Doppler for signals from NBS via ATS-3 versus

eccentricity 57

Figure 31. Doppler for signals from NBS via ATS-3 versus

inclination 58

Figure 32. Propagation delay overlay at 2330 GMT 61

Figure 33. Slide rule and experiment brochure 63

Figure 34. Delay error at NBS-Boulder 67

Figure 35. Delay error at Arequipa, Peru 68

Figure 36. Delay error at Natal, Brazil 69

Figure 37. Delay error at AFCRL 70

Figure 38. Coverage showing reliability of reception (95% or

better) for WWV, WWVH, and ATS-3 73

Figure 39. Multiple oscilloscope sweeps of the WWV and ATS-3

ticks 75

Figure 40. Measured HF delay path from WWVH Maui, Hawaii,

to Greenbelt, Maryland 76

v
Figure 41. Acquiring the ticks from WWV and ATS-3

Figure 1-1. Sample slide rule top and cursor Figure 1-2. Sample slide rule base » Figure 1-3. Block diagram of the delay computation procedure. . Figure 1-4. Slide rule delay error. Theoretical delay minus

slide rule delay

Figure 1-5. Distribution of slide rule delay errors . . . Figure 1-6. Slide rule delay error with the longitudinal error removed. Theoretical delay minus slide rule delay.

Figure 1-7. Distribution of the slide rule delay errors with the

longitudinal error removed

Figure II-l. Classical orbital elements

LIST OF TABLES

Table 1. Down-link Calculations (f = 135. 625 MHz)

Table 2. Receiving Antenna Summary

Table 3. Signal Delay Calculations

Table 4. WWV and ATS-3 Performance Summary

Table 5. Comparison of Dissemination Systems

Table 1-1. Values of 6 for various values of cp and A. -A.

l , . | s s r showing o ~ A. -A. & i s r

i

78

86 a 86 b

87

89

90

91

96

19

22

65

77

79

93
TIME AND FREQUENCY BROADCAST EXPERIMENTS

FROM THE ATS -3 SATELLITE

D. W. Hanson and W. F. Hamilton

An experiment designed to reveal the advantages and

special problems associated with the broadcasting of time

and frequency information from geostationary satellites is

discussed. Included are discussions concerning satellite

motion, time delay variation, doppler shift due to the

motion, and calculation of delay. Receiver or ground station equipment requirements, time recovery techniques,

timing resolution and accuracy, and special advantages of

satellite broadcasts for time and frequency dissemination are also discussed. Specially equipped sites in North and

South America gathered data from the experimental satellite broadcast, which in turn were used to determine

the potential accuracy of satellite dissemination, the results of which are presented. Delay computation aids for

the user were designed to provide a simple and inexpensive

means of computing free space delays between the master

clock and the user via a geostationary satellite. The aids,

delay overlays on an earth map and a circular slide rule, are discussed with examples. Qualitative discussions of

the signals and broadcast format are given. Final comments are made concerning the results of the experiment

and how they might reflect upon a final system design for a permanent service using one or more geostationary

satellites.

Key words: Dissemination; frequency; satellite; synchronization; time.

1. INTRODUCTION

During the course of two years beginning in August 1971, the familiar WWV time and frequency signals, usually heard only on the National

Bureau of Standards high frequency radio stations, were transmitted

from the NBS laboratories in Boulder, Colorado, and relayed by a geostationary satellite to a major portion of the earth. Those signals cov- ered the North and South American continents, much of the Atlantic and
Pacific Oceans and parts of Europe and Africa, a total of about 40% of

the earth's surface.

A standard frequency tone, seconds ticks, a time code, voice an- nouncements of the time of day, and satellite position were transponded

by the satellite to the earth twice a day for 15-minute periods. The time

and frequency information was referenced to and derived from the NBS

Frequency Standard (NBSFS) and the NBS Coordinated Universal Time

scale, UTC (NBS), both of which are maintained at the Boulder NBS laboratories.

In general, two modes of satellite time transfers may be envisioned.

A one-way mode places a listen only requirement on the user. The twoway mode requires that the user communicate information about his

clock to the master clock, via the satellite, immediately after receiving

a transmission from the master clock. The present experiment may be

classified as being of the one-way mode.

Historically, the first satellite time experiments were conducted

in August 1962 using Telstar [l J. The purpose of those experiments was

to compare the clocks at the U. S. Naval Observatory (USNO) in

Washington, D. C. , and at the Royal Greenwich Observatory in England.

Signals were relayed between these locations by Telstar's microwave

transponder. A two-way exchange resolved the round trip path delay and

assumed that the paths were reciprocal. If the satellite motion was negligible, which is assumed to be the case for geostationary satellites over

a short period of time, the one-way path delay was one-half .the round trip

delay. The major advantage of the two-way exchange was that knowledge

of the location of the satellite and of the ground stations was not required.

The major disadvantage was that both ends of the path needed a transmitter and receiver and that only one user could be synchronized in any

one exchange. Similar experiments were carried out with the Relay
communications satellite in February 1965 [z] and again with ATS-1 and

ATS-3 in 1971 l3j. Those experiments reported accuracies ranging from

0.01 Ms to 1 \is. All of these experiments were conducted in the microwave radio region using wide signal bandwidths. Although great accuracy- can be obtained under these conditions, the equipment costs are too

great for many users. The need for a lower cost technique led NBS in

1967 to conduct two-way experiments using the National Aeronautics and

Space Administration (NASA) ATS-1 satellite VHF transponder [4]. Accuracies of about 5 |~is were achieved using inexpensive VHF receiving

and transmitting equipment. It was believed that accuracies better than

1 [is were possible with better equipment and higher signal to noise ratios.

An example of a satellite timing system operating in the one-way

mode is TRANSIT L5J, a low altitude polar orbit satellite designed primarily for navigation. The Z -minute TRANSIT frame includes time information and updated orbital elements. With this information and a measurement of the satellite's Doppler, a user, knowing his location, can determine the range to the satellite to correct the received time information. In principle, high accuracy time synchronization is possible

with TRANSIT, but the user cost is high since special equipment is re- quired to process the orbital elements and timing information. User cost can be reduced if high accuracy is not required.

GEOS [t>], a low orbit satellite, is another example of the one-way

technique and was used by NASA to synchronize tracking stations. Time

pulses are provided at intervals of 1 minute. Receiving station locations are accurately known with respect to satellite position and accuracies of

about ± Z5fJs are reported.

NBS, having had some experience in the two-way mode and motivated by an emphasis on low cost and simplicity, directed its efforts in
1967 to the one-way mode for time transfer. The first tests were con- ducted with geostationary satellites containing VHF transponders to relay signals from a master station to any user in common view of the satellite. Experiments were conducted with the NASA satellites ATS-1 and

ATS-3 [7] and with the U. S. military communication satellites, LES-6

and TACSAT [8], [9]. These satellites contained transponders which operated in the frequency range between 130 and 300 MHz. Accuracy was

limited primarily by errors in the path delay predictions since even geostationary satellites drifted in their position. Satellite time transfers

were made with accuracies ranging between 10 and 150 |~is. These ex- periments were conducted at sites in Alaska, Hawaii, Ohio, Massachusetts, Colorado, and a number of sites in South America. To check the

accuracy of the time transfer, the sites were equipped with commercial,

portable cesium standards which were independently synchronized to UTC to within a few microseconds.

All the experiments mentioned above fell short of providing completely unambiguous time and frequency information from a satellite.

The experiment described here transmitted time information unambiguous to 1 year and in a form suitable for all levels of users. It provided

the participants in the experiment with all the tools required to obtain

and maintain time without bilateral contact between NBS and the user.

The experiment was conducted in the following manner. Time and

frequency signals were relayed by the satellite twice a day. Four sites,

two in North America and two in South America, observed the signals

and recorded the apparent time delay by referencing the signals to their

on-site clock. Each site had receiving equipment which had been carefully calibrated by NBS prior to its use in the field. The time references

at each site consisted of, at a minimum, a commercial cesium clock

previously synchronized to the NBS master clock by portable atomic
clock carries. Some of the sites also monitored LORAN-C or VLF stations. The received satellite signals, referenced to on-site clocks,

provided the signal delay from the master clock at NBS via the satellite

to each of the sites as well as a means to test ways of computing the signal delays. Because the sites were situated at widely dispersed points

about the subsatellite point and at drastically different "look" angles to the satellite, the resultant data provided a comprehensive evaluation of

the system's ability to provide time information.

In addition to the NBS-sponsored sites, numerous other government

agencies, industries, and private individuals, used the broadcasts.

Through a special brochure offered to the public, equipment requirements, broadcast schedules, antenna pointing information, propagation

delays, and other pertinent data were offered. Over 9, 000 of the brochures were distributed in addition to many telephone inquiries.

For the higher accuracy user of time and frequency, a circular

slide rule was designed to compute propagation delays. This slide rule,

in addition to the voice announcements of satellite position relayed by

the satellite, enabled the user to compute the path delays with high accuracy. The satellite's position was computed from NASA generated orbital elements in advance of each broadcast, and provided the user with

the capability for immediate time recovery and synchronization.

The experiment very effectively demonstrated the potentials for

satellite dissemination of time and frequency information. It also provided many users a preview of the level of performance which could be

available using this space age technology.
2. BROADCAST EQUIPMENT AND FORMAT

Figure 1 shows the equipment that was used to broadcast the time

and frequency signals to the satellite. The time signals were referenced

as shown in figure 2 to the UTC (NBS) time scale and the NBS Frequency

Standard both of which are maintained at the NBS laboratories in Boulder,

Colorado. The timing format consisted of tones, ticks, a time code, and

voice announcements similar to the signals heard on the NBS high frequency stations, WWV and WWVH. The signals frequency modulated a carrier, were amplified, and transmitted to NASA's ATS-3 satellite.

The antenna shown in figure 3, a bifilar helix, was used to transmit the

time and frequency signals to the satellite.

The signals were received by the satellite at 149.245 MHz, frequency converted to 135. 625 MHz, amplified and retransmitted back to the earth. The deviation of the carrier was ±12 kHz occupying a 30 kHz

bandwidth. The ATS-3 satellite, shown in figure 4, was in geostationary

orbit and "station kept" at approximately 70 west longitude. The satellite's VHF antenna provided for an earth coverage as seen from synchronous altitudes and is illustrated in figure 5.

The satellite's VHF antennas consisted of eight dipole elements.

As the satellite spun on its axis, the dipole elements were electrically

despun and acted as a phased array always pointing at the earth. The

satellite's VHF transponder and antenna system is illustrated in the

block diagram of figure 6.

The ERP of the satellite was approximately 47. 6 dBm with both

transmit and receive antennas being linearly polarized. The corresponding field strength at the earth's surface was about 1. 2 microvolts

per meter or -145 dBW per square meter.
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12
Beginning with the first satellite relayed broadcasts of the time and

frequency signals in late 1971, the broadcast format underwent changes

in an effort to improve its performance as an experimental space to earth dissemination service.

The broadcast schedule was Monday through Friday except legal

holidays at the times of 1700 to 1715 and 2330 to 2345 Greenwich Mean

Time.''"" During those two 15-minute periods the format content generally

included the following:

Voice Announcements: Once a minute the time of day in hours

and minutes of Greenwich Mean Time was given. Once every five

minutes, during the second, seventh and twelfth minutes of the

quarter hour, the position of the satellite, in longitude, latitude

and radius (normalized) was given when available.

Ticks: At the beginning of every second a tick, consisting of

five cycles of 1 kHz, was transmitted to the satellite with the leading edge of the tick leaving NBS-Boulder on time.

Tones: During the first 45 seconds of every minute which had no voice announcements, a 1 kHz tone was transmitted. This tone

was coherent with the tick, i. e. , its zero crossings were on the

half millisecond intervals between the zero crossings of the sec- onds ticks. The beginning of the first second of each minute was

* Because of common usage of the name Greenwich Mean Time, the time

announcements from ATS-3 were referred to by this name. As noted in a resolution of Commission 31 of the International Astronomical Union,

August 1970: "The terms of 'GMT' and >Z' are accepted as the general

equivalents of UTC in navigation and communications. " More precisely

the actual reference time scale for the ATS-3 experiment was the

Coordinated Universal Time Scale, maintained by the National Bureau

of Standards, UTC (NBS).

13
identified by a 1500 Hz tone of 770 milliseconds duration beginning

30 milliseconds after the minute. This tone, being unique to the

format, was available to activate automatic equipment or reset

clocks.

Time Code: A 100 Hz subcarrier was added to the format.

This subcarrier was amplitude modulated 100% with a modified

IRIG-H time code. The code had a 1bps rate with a one minute

time frame, and was pulse duration modulated as shown in fig- ure 7.

The schedule for the above described format is shown in figure 8. The voice, tones, and ticks frequency modulated the carrier approximately ±12 kHz. The time code was added to the format, and amounted

to about 10% of the amplitude of the other components. To prevent distortion of the tick's waveform, the ticks were gated into the format and

all other components of the format were blanked during its presence.

Figure 7 also shows part of the time domain of the baseband as it existed

for the majority of the experiment.

3. RECEIVING EQUIPMENT

A block diagram of a receiving ground station used by NBS for the

ATS-3 time and frequency broadcast reception and time recovery is

shown in figure 9. This configuration was used extensively by NBS at

its laboratories in Boulder, Colorado, and at field sites. A number of

antenna and receiver combinations were used during the course of the

experiment to develop a familiarity with the basic requirements for the

equipment and the performance to be expected. Other equipment combinations not mentioned in this report worked well for most users' timing

requirements. For the majority of users, where only low accuracy time

was required (a few tenths of a second), a modest antenna and inexpensive

receiver provided a recognizable voice and tick reception. The high

14
TIME

TICK TICK

Figure 7. Time and frequency waveforms,

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accuracy user, of course, leaned toward the use of the highest gain

antenna and a receiver with high sensitivity and selectivity.

Because the signals were transmitted to the satellite at high power,

arriving at the satellite with high signal-to-noise ratios, the signals

arriving back at the ground were down-link limited; i. e. , the signal-tonoise ratios were determined solely by the characteristics of the receiving ground station. As an example of that determining down-link,

table 1 provides a calculation for the typical ground station used by NBS.

The satellite transmitter radiated 11.4 watts into a linearly polarized

array of antennas whose gain was 8 dB. A diplexer loss of 1. 8 dB re- duced the effective radiated power, ERP, from the satellite to 17. 6 dBW.

A free space loss of 167. 2 dB and a circularly polarized antenna with

10 dB gain resulted in a signal input to the receiver of -142. 6 dBW after

a 3 dB reduction for a linearly polarized signal incident upon a circularly

polarized antenna.

The antenna and receiver noise temperatures were calculated to be

700 K and 527 K, respectively. With a 30 kHz bandwidth the effective

noise input power was -152.9 dBW, resulting in a carrier-to-noise ratio

of 10.3 dB.

3.1. ANTENNAS

A number of antennas provided satisfactory reception quality. Because the ATS-3 satellite radiated a linearly polarized signal, these

signals, after passing through the earth's ionosphere,were incident upon

the earth's surface at an unpredictable polarization orientation due to the

Faraday rotation. A linearly polarized receiving antenna while optimized

in polarization to receive these signals required rotation about its axis

to maintain maximum coupling to the incoming signals which were slowly

rotating in their polarization orientation. In contrast, a circularly

18
Table 1

Down-link Calculations (f = 135. 625 MHz)

CHARACTERISTIC ATS-3

TRANSMITTED CARRIER POWER, dBW 11 .4 DIPLEXER LOSS, dB -1 .8 TRANSMIT ANTENNA GAIN, dB 8.0

SATELLITE EIRP, dBW 17.6

PATH LOSS, dB -167.2

POLARIZATION LOSS, dB -3.0

RECEIVER ANTENNA GAIN, dB 10.0

P dBw

rs -142.6

BOLTZMANN'S CONSTANT, dBW/K Hz -228.6

RECEIVER NOISE FIGURE, dB 4.5

RECEIVER NOISE TEMPERATURE*, dB K 30.9

NOISE POWER DENSITY, dBw/Hz -197.7

IF BANDWIDTH, dB Hz (30 kHz) 44.8

NOISE, dBW -152.9

(C/N), dB 10.3

BASED ON ANTENNA NOISE TEMPERATURE OF 700 K, AND RECEIVER

NOISE TEMPERATURE OF 527 K.

19
polarized antenna while rejecting one-half of the available incoming signal would always accept the other half without any rotation or reorientation. The circularly polarized antenna, though not the simplest antenna,

was preferred for this experiment.

Some of the antennas used by NBS are shown in figure 10. Fig- ure 10a is a circularly polarized antenna consisting of two linearly polarized yagi antennas mounted orthogonally on the same boom and phased

for circular reception. This antenna had a gain of approximately 10 dB

above a circularly polarized isotropic radiator and was available commercially for less than $50. 00 including impedance matched phasing

cables. Figure 10b shows a 3-turn helix with approximately 10 dB gain.

Figure 10c shows another circularly polarized antenna with 4-5 dB gain

and a wide beamwidth. In contrast to the two previous antennas this

antenna had a sufficiently broad beamwidth that it could be set upright

for satellite reception whereas the other two were required to be pointed in

the general direction of the satellite. Figure lOd shows a quarter-wave

dipole mounted on a receiver which was capable of receiving the signal

and providing good quality audio reception. The first two antennas were

considered to have the gain necessary for high-accuracy time recovery

using oscilloscope techniques whereas the last two provided for lowaccuracy timing using voice reception only. These antennas are summarized in table 2.

Considering the wide beamwidth of these VHF antennas, pointing

them at the satellite presented no particular problem. As information

for users, an antenna pointing angle overlay on an earth map was prepared like the one shown in figure 11. The cross in the center of the

overlay was the point on the earth directly below the satellite. An overlay such as this one was valid for the entire two-year duration of the

ATS-3 time and frequency experiment. The heavy line near the

20
Figure 10. VHF receiving antennas, (a) crossed yagis; (b) 3 turn helix;

(c) crossed dipoles with reflector; (d) grounded quarter wave dipole.

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23
periphery of the overlay was the five degree take off angle from the

ground to the satellite. For example, the antenna pointing from

Washington, D. C. , (77 west longitude and 39 north latitude) was 170

o azimuth and 42 in elevation.

3.2. PREAMPLIFIERS

In applications where a considerable amount of cable would exist

between the antenna and receiver, such as in a building, a preamplifier

was found necessary to overcome the cable loss. Additionally, the

preamplifier in some cases provided for a better signal-to-noise ratio

because of its lower noise figure. Commercial transistor preamplifiers

like the one shown in figure 12 were available for less than $20. 00.

3.3. RECEIVERS

A number of FM receivers were used in the experiment with good

results including both older vacuum tube models and more recent transistor units. Receiver choices varied from standard telemetry receivers,

communications receivers preceded if necessary with frequency converters, or the receivers could have been built from readily available parts.

The quality of the equipment used with success by NBS ranged all the way

from inexpensive receivers (less than $100) which had no manufacturer

specifications to accompany them, to higher priced equipment with specifications of 0. 5 l-iV for 20 dB quieting and selectivity of -80 dB.

In addition to using 30 kHz bandwidth receivers in the experiment,

the carrier deviation was reduced to ± 5 kHz on occasion to test the performance of the standard narrowband receivers. The narrowband re- ceivers performed very well with the ± 5 kHz transmission and were acceptable for voice and tick reception even when the full 30 kHz transmission was used. Conversely, the wideband units were also acceptable

when receiving the narrowband transmission.

24
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25
From the beginning of the experiment, it was decided to concentrate on using 30 kHz bandwidths rather than the more popular narrow

bandwidths. This decision was based on the desire to obtain the highest

timing resolution possible. It was also recognized, however, that a narrower bandwidth operation would generally reduce antenna requirements and allow use of the newer FM equipment being produced for general communications use.

Figure 13 shows a few of the receivers tested by NBS in this experiment. Quantitative measurements of the performance of each receiver

were not made. Satisfatory performance of each unit was consistent

with the desire to operate the experiment in a manner which would allow

the general public the best chance to participate without undue complication or expense.

NBS also interfaced an inexpensive solid state FM receiver into a commercial time code reader for automatic clock synchronization by the

satellite broadcasts. This device, shown in figure 13g, worked very

well. A 100 Hz active filter followed the output of the receiver and fed

into the time code reader. The reader usually was able to recognize

the code frame and reset itself within two or three minutes. Once the

time code reader had "locked" on to the satellite broadcast code, the

unit worked flawlessly. This illustrated the satellite's potential for

automatic, unattended timing applications.

4. SIGNAL PROCESSING

An antenna and FM receiver alone were sufficient for the recovery

of low resolution time information. Time-of-day announcements every

i minute, giving hours and minutes in Greenwich Mean Time, and ticks

every second could easily be heard. This method for clock synchronization was accurate to better than one-half second, remembering however,

26
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27
that the ticks left the NBS Boulder latoratories on time and, after an approximately 76, 000 km trip to the satellite and back to earth, were

delayed by about 1/4 second. For audio reception only, simple antennas

were used with good results. Whips and dipoles provided a generally

satisfactory performance dependent in part upon the level of man made

and natural noise and upon the receiver's selectivity.

Obtaining time information to better than a few tenths of a second

required additional equipment. An example of the equipment involved

for a time recovery technique is shown in figure 9. Here the receiver

was connected to the vertical input of an oscilloscope. The receiver

output was after the frequency discriminator and prior to the audio

section to avoid possible delay variations. The oscilloscope was triggered from a digital delay generator which had 1 pps and 1 kpps outputs,

both variable in their occurrences in time. A schematic of the digital

delay generator is shown in figure 14. Achieving submilli second timing

resolution using this technique with the ATS-3 time signals required only

a few simple steps. First the received tick had to be located on the

oscilloscope and positioned at the beginning of the sweep or at the left

side of the scope. This was done by setting the sweep rate to 0. 1 s/cm

and the trigger rate to 1 pps. The digital delay generator was then used

to position the received tick to the left side of the scope as shown in

figure 15a. This procedure was repeated four times. Each time the

sweep rate was increased until the sweep rate was at 100 Us/cm and the

tick was positioned as in figure 15c. It may be seen from the figures

that the zero crossing at the center of the tick rather than the beginning

of the tick served as time reference. The beginning of the tick was distorted and not easy to identify, due to the bandwidths of the transmitting

and receiving equipment. Movement of 2500 \ds into the center of the

tick itself was found to minimize the effects of this distortion. Because

28
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High vertical gain.

Figure 15. Received signals from ATS-3 on an oscilloscope.

IB- *° JU 'I USCOMM -

30
the receiver delay calibration, described in a later section, was measured in the same way using the same signals, the effects of distortion

were minimized. This assumed that the path added no further distortion.

Thus far, the described recovery procedures were nearly identical

to those used for WWV or WWVH. Taking advantage of the apparently

excellent short term path stability, a 1 kHz tone was added between the

ticks in such a manner that all its zero crossings were on the 500 |is

intervals between zero crossings of the ticks. Increasing the trigger

rate to 1 kHz allowed the observer to see more of these zero crossings

and enhanced his ability to average the crossings, yielding a correspondingly greater precision. The effect of the higher trigger or sampling

rate is shown in figure 16.

The unavoidable noise added to the received signal caused jitter

in the zero crossings and limited the precision of time delay measurements. Theoretically, assuming a Gaussian distribution to the noise,

the measurement of phase or time at which the waveform crosses the zero axis is given in LlOJ as

6t = S 1/2 • (1)

^(2-f-)

1 ' 2

where 6t was the rms error in measuring the time of the zero crossing,

T was the period of the sinewave, and S/N was the signal-to-noise power

ratio. For the conditions existing in this experiment a 6t was obtained

of 11.25 Ms for a S/N of 10 dB, and 3. 6 [is for a S/N of 20 dB. The increased triggering rate allowed for averaging of the zero crossings with

the improvement being proportional to the square root of the number of

crossings averaged. It was estimated that the effective number of samples available to the eye using the oscilloscope amounted to at least 50

sweeps at the 1 kHz rate or a 50 ms averaging time. These 50 samples

31
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(b) Trigger rate: 1000 sweeps/s

Figure 16. Received signals from ATS-3 at different sampling rates.

32
increased the resolution to 1. 6 |Js and 5 \is for the signal-to-noise ratios

of 20 dB and 10 dB respectively.

Prior to any measurements, the oscilloscope was calibrated and

adjusted to eliminate systematic errors in the measurements. The

sweep non-linearities of the oscilloscope were compensated for as follows. From the digital delay generator a 1 kHz squarewave was used

for vertical input and sweep trigger. With the sweep rate at 100 Us/cm,

the horizontal position of the squarewave from the delay generator was

placed so its zero crossing fell at the center of the oscilloscope as referenced by the graticules on the scope face. Triggering occurred 500 Us prior to this zero crossing and, therefore, looking at the center of the

oscilloscope with the 1 kHz satellite signal present insured a zero

crossing at the beginning of the sweep coincident to the trigger pulses.

Misinterpretation of the zero crossings was also possible due to a direct current bias on the post detected signals or from distorted waveforms. Careful balance and alignment of the band pass filters, limiter,

and discriminator circuits of the receiver eliminated large distortion

effects. The zero volt reference on the oscilloscope was set by momentarily grounding the input and centering the sweep in the vertical direction.

All of the above mentioned procedures insured that the trigger

pulse (lpps) was coincident with the center of the tick emerging from the

receiver. As shown in figure 9, this pulse was used to stop a time interval counter for which the local clock's 1 pps reference started the counter. The counter then displayed the apparent signal delay from the NBSBoulder reference clock to the output of the user's receiver. A difference

in this apparent delay and the computed signal delay, which included

equipment and path delays, indicated a clock error. The differences in

apparent and computed delays will be discussed in more detail in a later

section dealing with "Delay Computation and Clock Synchronization. " 33
5. SIGNAL DELAY

The propagation of the time signals between NBS and the user via

ATS-3 was, of course, not instantaneous. The free space propagation

Q velocity of electromagnetic waves was taken to be 2. 997925 x 10 m/s:

which, for the approximately 76, 000 km path from NBS to the user,

meant a delay in the arrival of the signal of about 1/4 second. The user,

to synchronize a clock to better than one second accuracy, had to know

this signal delay time. The total signal delay time included the time re- quired for the signal to pass through the intervening electronic equipment;

i. e. , the transmitting equipment at NBS, the satellite transponder, and

the user's receiving equipment; the delay in the ionosphere, and troposphere, and the free space path delay outside the earth's atmosphere between the earth and the satellite.

To avoid becoming entangled in definitions of phase and group velocities of electromagnetic waves, the net total delay in the link between

NBS-Boulder and the user was referred to as the signal delay. Signal

delay was defined in this case as the time required for an identifiable

point in the signal waveform to enter the transmitting equipment and re- appear at the output of a user's receiver. That definable, recognizable

point in the signal waveform was, in this experiment, the zero crossings

of the tick and/or the 1 kHz tone. This definition of signal delay applied '

to the equipment where it could be argued that the equipment was dispersive and group delay through it was not well defined. For delay calibration as well as signal reception, the emerging signal was never distorted sufficiently to make identification and measurements on any particular point

in the signal waveform difficult. In the earth's atmosphere the signal

delays coincided with group delay. Outside the earth's atmosphere the

8 . signal velocity was the velocity of light or 2. 997925 x 10 m/s.

34
5. 1. EARTH-SATELLITE-EARTH PATH DELAY;

SATELLITE MOTION

The ATS-3 satellite, like all so called "geostationary" satellites,

was moving about in a complicated manner. This motion was significant

when considering microsecond timing, since in one microsecond an

electromagnetic wave in free space will travel approximately 300 meters.

The causes for motion of a "geostationary" satellite are many. In the case of the ATS-3 satellite, its orbit plane was inclined to the earth's

equatorial plane by between 2 and 3 degrees. In a period of 24 hours, the

satellite moved north and south of the earth's equator by 2 to 3 degrees.

At a distance of about 42, 000 km from the earth's center, this meant a movement of nominally 5, 000 km peak to peak in a frame of reference

which was rotating with the earth. The satellite's orbit was also

slightly elliptical and in a period of 24 hours it moved in and out radially

with respect to the earth's center. For example, the typical eccentricity

for ATS-3 of 0. 0026 produced a diurnal variation in the distance to the

earth's center of about 220 km. The eccentricity, e, is defined as

e =yi-(b/a) , (2)

where b and a are the semi -minor and semi -major axes of the elliptical

orbit of which the earth was at one focus. The radius is given by

r = a d-e2

) (3)

1 + e cos 9

y '

The geometry of an elliptical orbit is shown in figure 17.

Finally there was a general east or west drift of typically 0. 05

degrees per day which moved the satellite up to 35 km/day in its mean

longitudinal position. The drift was caused primarily by the asymmetrical character of the earth's equator and imperfect orbit altitudes.

35
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The above three mentioned causes and effects are the most significant for ATS-3 or any geostationary satellite. Other effects important over a longer period of time were due to the fine structure to the

gravitational field of the earth, the sun's and moon's gravitational fields,

and solar radiation pressure. A complete understanding of the mechanics

of geostationary orbits is complicated and beyond the scope of this report.

The reader is referred to reference [l5j-[l6J for a complete treatment

and to the appendices where some of these facts are expanded upon.

As a substitute for a highly theoretical explanation, a series of

graphs is included in this report to provide the reader some insight into

the effect these satellite motions would have upon the user interested in

recovering time and frequency information.

Figure 18 shows, in exaggeration, how the effects perturb the

geosynchronous orbit. The satellite moves or deviates from a perfect

circular, non-inclined orbit in all directions. The resultant orbit might

be best related to the edges of a clam shell. Figure 19 shows these

effects on the ATS-3 satellite in terms of the total signal delay from the

NBS to selected sites in both hemispheres. (These curves were computed from the satellite's position on November 28, 1972). It can be seen that the delays, which do not include electronic equipment or atmospheric effects, are diurnal and are different in magnitude depending upon

the terminal site location.

Figures 20 and 21 show how these delays would be affected by

variations in the eccentricity and inclination of the orbit. The orbit for

November 28, 1972 was changed in its eccentricity and then in its inclination. The orbital elements for that date are given for reference.

37
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Figure 19. Delays from NBS to five sites via ATS-3.

39

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Date November 28, 1972 00H, 00M, 00. OS

Semi -major Axis 42169. 53 km

Eccentricity 0.002617

Inclination 3.0889 degrees

Argument of Perigee 23. 275 degrees

Right Ascension of

Ascending Node 74. 632 degrees

Mean Anomaly 258.98 degrees

It is interesting to observe how the subsatellite point moved about

the surface of the earth on November 28. Figures 22 and 23 show the

subsatellite trace for varying inclinations and eccentricities. These

curves are not directly relevant to users of satellite time and frequency

signals but are further indications of the satellite's complex motion.

The figures 20 and 21 show the short-term, 24-hour satellite motion.

The satellite position was nearly repeated every 24 hours. However, due

to effects which become significant in the long term, the satellite drifted

longitudinally and its orbit changed its shape and orientation with respect

to the earth. Figures 24 and 25 show the measured round trip delay from

NBS to NBS via ATS-3 at 1700 and 2330 GMT over a period of several

months. It can be seen that day-to-day delay repeated to within 10 or 20 (J-s but exhibited considerable "drift" over a period of weeks. These

two figures also represent the "view" afforded by two "narrow windows"

looking at only a very small segment of the total orbit.

There was another perturbation to the satellite's orbit which was

intentional. The satellite's drift in longitude, caused by natural perturbations and orbit bias required correction periodically to maintain the

satellite's relatively fixed position in the sky. This man made perturbation, called either an orbit or "Av" maneuver, was initiated by firing

small rockets on the satellite for a very brief period. Referring again

42
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Figure 23. Subsatellite point versus inclination.

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?ure 25. Long-term, two-way delay between Boulder and ATS-3 at 2330 GMT.

46

I ft- 108925 1
to figures 24 and 25 a "Av" maneuver was performed on February 2, 1972.

The results of that maneuver at 1700 and 2330 showed a pronounced

change at 1700 and a relatively small change at 2330. This apparent

discrepancy was the result of an incomplete picture due to the narrow

windows available to the NBS experiment. The obvious point here, however, was that orbit maneuvers are of very great significance to a user

of satellite disseminated high accuracy time and frequency information

because of the discontinuities in the path delay and orbit prediction.

5.2. IONOSPHERIC AND TROPOSPHERIC DELAY

A small portion of the total path between the earth and the ATS-3

satellite was not in free space. The earth's ionosphere and troposphere

reduced the signal velocity from that of free space, thereby adding additional time delay. The additional tropospheric delay was nearly frequency independent and is shown in figure 26. The ionospheric delay, also

shown in figure 26 was frequency dependent and nonisotropic. However,

for frequencies in the high VHF band and above, anisotropy could be

ignored.

The ionspheric delay also varied with time and was dependent upon

total ionospheric electron content which was greatest in sun light and decreased by almost a factor of 10 at night. The total ionospheric content

for a 24-hour period is shown in figure 27. The associated signal delay

increment for that same 24-hour period is also shown in figure 27 for a one-way path at 90 elevation angle at 135. 6 MHz. A peak value for

17 2 integrated electron content of 5 x 10 electrons /cm , typical of the

local time of 0900-1500 hours, was assumed in all the computations and

results in this report.

47
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5.3. EQUIPMENT DELAY

The time signals experienced added delay when passing through

electronic equipment. The ATS-3 transponder, or (more properly) frequency translator, introduced a signal delay of about 7 Us. The format

and transmitting equipment at NBS and the user's receiving equipment

introduced additional delay. For the carefully controlled receiving

equipment at the four NBS-sponsored sites, the total receiver and transmitter delay was measured as part of the calibration procedure. In this

way, a more accurate equipment delay value was obtained. The block

diagram of the measurement setup is shown in figure 28. The same time

and frequency signals used in the broadcast were used for equipment delay measurements and minimized possible effects of distorted waveforms

and varied receiver response to different signal waveforms.

Referring again to figure 28, the dashed lines enclose the equipment

whose delays were measured. The mixer was substituted in place of

the paths between the satellite and earth and the satellite itself. The

mixer was broadband and contributed an insignificant amount of delay.

The measurement of delay was made at approximately the same signal

levels as experienced when receiving the satellite signals; (however,

the receiver delay was not very dependent upon signal level).

The measurement's precision was discussed previously. The ac- curacy of the measurement was dependent upon delay variations in the

equipment. Prior to each measurement of the satellite signals at NBS,

the equipment delay was measured. These measurements showed the

equipment to have acceptably small variations in delay. Statistically,

the equipment delays had a 133 [is mean value with an rms deviation

from the mean of 2. 6 f-l s . Peak deviations from the mean were less than

10 [is in all cases. A total of 100 consecutive measurements, two a day,

were used to obtain the above statistical values.

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<c=>

z.

UJ

N to h- 3Z Q.

1— e: Q- <1 1 1

rsi I J= | J»£ en a: CL

UJ i— q: 1 O Q. a:

l— go o 1 ° UJ oS o

1— •=C 1— 1— i— i

O 1—

s: i—

u. 21CO

t_> <: ca: Q S Qi <c q; uj z^< 1 1 1 (_} 1 o: o _i ^ o o ^ UJ 1 •—i

<=C on u_ uj u_ C_> <_> C£ CQ o UJ oo

I— q: o u

A!oo

X)

COoo

i-H T)

0)COo

t—1 U

00

(MDU

GO

J 51
Careful attention to tuning and alignment of the receiver reduced

the scatter in the measurements of receiver-transmitter delay. The

receivers used in this experiment had their AGC removed and the video

inputs to the oscilloscope by-passed iron core transformers. This re- duced the receiver delay variation due to signal level changes. All bandpass filters were carefully aligned and the receivers were crystal controlled to reduce delay variations. This was a necessity since signals

off frequency were found to have different delays than those on frequency.

The limiter was adjusted for symmetrical limiting and the discriminator

was carefully aligned.

The receivers used by NBS were not designed to minimize delay

variations. Obviously, great improvement can be made in the receiver's

performance as related to timing if some relatively simple design details are emphasized.

To enhance the usefulness of any time and frequency broadcast

system, the user should be able to measure or compute in his laboratory

his receiver's signal delay without great expense or effort. Two methods

of achieving this goal were thought to have merit. The first was: If a large number of inexpensive transmitters could be built following one

design and if the scatter in the signal delays for these transmitters were

small, then they could be used as "standard" transmitters with a uniform

delay and used to calibrate the user's receiver delay. The second approach was: Given any FM signal generator and a wideband frequency

discriminator, a reference can be established with this discriminator

and the receiver delay measured relative to it. If the frequency discriminator was simple to construct and its delay properties were repeatable,

it could serve as a "standard" discriminator. Also if the discriminator

was broadband, its delay would be small and possibly it could be ignored.

Both philosophies were adopted with the understanding that the signals

52
from the "standard" transmitter or FM signal generator would be of the

same general character and level as that used for the time and frequencybroadcast. In the case of this experiment, the prescribed modulation

would be 1 kHz deviated ± 12 kHz.

NBS conducted limited tests on the standard transmitter and broadband discriminator approach with good results. The transmitter was

simple in design and no reason was apparent why it could not be specified

and built by users with low scatter in its resulting delay properties. Two

broadband discriminator designs were tried, also with encouraging results. The combined transmitter -broadband discriminator delays as measured by NBS, amounted to only a few microseconds and indicated

its applicability for the user's evaluation of his own equipment. Much

of what has been verified at NBS for wideband discriminators was substantiated by a parallel investigation at the Metrology Laboratory of the

Naval Avionics Facility at Indianapolis, Indiana.

Although there needs to be more work in this area the solutions do

not appear to be difficult.

6. DOPPLER

Because the satellite was in motion relative to points on the earth's

surface, frequencies of the satellite relayed signals were Doppler shifted.

The carrier frequency of 135. 625 MHz was not usable as a standard frequency because the instabilities of the transfer oscillator in the satellite

could not be compensated for and the Doppler shift of the carrier was

therefore indiscernible from the total frequency shift of the carrier. The

only standard frequency tone used in these experiments was the 1 kHz

modulation, which was also effected by the Doppler shift.

53
In general, the Doppler shift for electromagnetic waves in free

space may be written as

0) dr

where u^is the Doppler shift of the oscillator frequency uu. The oscil-

, , , dr

lator is moving towards the observer at the rate of ——, and c is the free

space velocity of light. This equation applied to the NBS experiment if the oscillator, located at NBS-Boulder, was thought to be moving relative

to the observer by virtue of the motion of the satellite. The influence of

the Doppler shift on the carrier was to produce frequency modulation.

For a frequency modulated signal, v(t), radiated from NBS,

v(t) = A cos (uu t +— sin w t). (5)

M

where (i) is the radian frequency of the carrier, Af is the maximum frequency deviation away from the carrier and is called the frequency deviation, and «J = 2TTf is the radian frequency of the modulating sinusoid.

The receiver "sees"

UU UU v'(t) = B cos [uu t +-^-r +4- (sinuu t +-^- r)l (6) C c i. , M c M

The instantaneous frequency or demodulated signal was

UU uu s(t) = uj +— — + 2nAf (cos uu t +—- r). (7) C c dt M c

x

'

After the carrier frequency was filtered out, the post detected signal was

uu s'(t) = Auu (cos uu, t +—— r). (8) M c

The Doppler shift of the modulated signal was then

uu M dr

DM c dt yy '

54
dr An expression for -— in the case of the ATS-3 satellite was possible

at but would not particularly contribute to the scope of this report. It was

obvious, however, that since the satellite's position could be predicted

with good results on a continuous basis, as will be shown later in the

report, the derivative could also be predicted everywhere except at discontinuities such as those which existed with orbit maneuvers.

Figure 29 shows the computed Doppler as seen by an observer at widely selected locations in view of the satellite. The Doppler values

7 were diurnal, as expected, amounting to a few parts in 10 in peak values,

a significant correction for most frequency measurements.

An alternative method for direct frequency calibrations using the

satellite signals was possible from clock synchronizations. As will be

shown in this report, time was available from the satellite accurate to about 10 |-ts. If the oscillator under examination drove a clock and that

clock was synchronized by the satellite at an interval of 24 hours, a frequency comparison was in effect, made. The resolution was 10 us out

of a total of . 864 x 10 |Js (24 hours) or ^=^i = ^ = 1.15xl0- 10

. f

* .864xl0%s < 10 >

Figures 30 and 31 provide further information on how variations in

eccentricity and inclination affect the Doppler values. All the Doppler

curves were again obtained from the ATS-3 orbit of November 28, 1972.

Some data was gathered from a direct frequency comparison of a standard frequency returned by the satellite at NBS. In special tests, a 10 kHz continuous tone, derived from a commercial cesium frequency

standard, frequency modulated the transmitted carrier at NBS-Boulder.

The frequency deviation of the 10 kHz tone was held low to avoid interfering with the other signals in the format. The satellite-returned 10 kHz

55
CO

<u 4J

•tri CO

cu>

•H

<-HO

+-> u

o

pq

o

v-l

!-i <D i—

Oh

OhoQ

•1-1

J-V idiHS udiddoa

56
5E cd.— cm n ^ in ic

UJ ooooooo <_> O O CD O O CD O

O O O O O CD O

O O O CD O O O

inH

<;

•H>m

IS

Ml

O

V

.-^ftoQ

o

CO

1)

d CSJ r— .— <\J PO

iV 1JIHS 831ddOQ

57
I L

rto

•rt •M

«ifl •H

.-Hy

•H

CO

COH

<u>

oni toH

<Ua

•H>w

ffl ZoHhomu

<B .—

o<oQu

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n cm f— cm oo «d-

*v 1JIHS a31ddOQ

58
at Boulder was fed into a phase comparator together with the 10 kHz from

the cesium frequency standard and recorded on a paper chart. Because

the broadcasts were only for 15-minute periods, the results were essentially linear on the chart representing apparent frequency differences of a few parts in ten million (1x10 ). The results however agreed with

predicted Doppler offsets and if a Z4-hour record was available, it would

no doubt look much like those in figure 29.

7. DELAY COMPUTATION AND CLOCK SYNCHRONIZATION

For high accuracy synchronization of the user's clock, the user had

to compute the total signal delay from the master clock at NBS through

the transmitting equipment to ATS-3, through its equipment, to his position, and through his equipment. As was shown in previous sections,

the propagation path delay was variable. With the satellite moving about

and dependent upon many complex forces, a complicated calculation of

path delay was necessary, assuming the user began with the fundamental

orbital elements. If the user was only interested in time to the nearest

second, no calculation was needed other than a mental note to remember

that the signals arrived at the earth's surface approximately 1/4 second

later.

Even for the higher accuracy, sophisticated, time user, dealing

with orbital elements to calculate path delay was deemed to be intolerable.

Consequently, NBS developed two computational aids and reduced this

burden to an acceptable level which would be available to the user at very

little or no cost.

The first of these computational aids was intended for the user who

needed intermediate accuracy time or no better than a few milliseconds.

This would be the same user who finds the services, in terms of accuracy,

from WWV or WWVH adequate. This aid consisted of a transparent delay

grid which was placed over an earth map as directed in the ATS-3 satellite time broadcast.

59
During the experiments, NBS broadcast in voice the satellite's position updated every five minutes, in terms of its longitude and latitude

of the subsatellite point and its distance from the earth's center in microseconds relative to a value 119, 300 |is. For example, if the radius were

39, 827. 54 km or 119, 400 \ds, the "radius correction" was given as + 100 l-is. Requiring time, or equivalently the path delay, to only a few milliseconds

meant the delay values could be taken from the delay overlay on the earth

map with its center superimposed at the subsatellite position. The

"radius correction" was not required in using the delay overlays. Fig- ure 32 shows a photograph of the overlay and earth map with the subsatellite point at 71 west longitude and 2. 8 south latitude. Because the

satellite's position repeated itself every 24 hours to within 10 or 20 |J.s

(see figs. 24 and 25), the positions of the overlays were valid for a particular time of day for several weeks provided only millisecond accuracy

was required. Two of these overlays, one for the 1700 GMT broadcast

and one for the 2330 GMT broadcast, were published monthly in the NBS

Time and Frequency Services Bulletin. Also published was the antenna

pointing information shown in figure 11, which provided the user with the

proper azimuth and elevation for pointing an antenna at the ATS -3 satellite.

These overlays were updated for each monthly publication and provided users a simple means for obtaining path delay values. As an example,

assume that the user was in Washington, D. C. From figure 32, it may

be seen that the delay from NBS-Boulder to ATS-3, represented by the cross at 71 west longitude and 3.2 south latitude, was 128.9 ms.

(NBS-Boulder is at 105° west longitude and 40 north latitude. ) The delay from ATS-3 to Washington, D. C., 77 west longitude and 39 north

latitude, was 126.1ms. The total delay from NBS-Boulder to Washington,

D. C. , was the sum of these two delays or 255. ms.

Because the overlays could not provide path delays accurate to a few microseconds, a circular slide rule was designed to fulfill that need.

60
Figure 32. Propagation delay overlay at 2330 GMT.

61
The slide rule along with the experiment brochure is shown in figure 33.

A cut-out version of the slide rule is provided in the appendix. A simple

device such as this one was possible because NBS had assumed the major

part of the computational burden by announcing the satellite's position.

Only a simple geometric calculation, designed into the slide rule, remained

to be performed by the user. The user, knowing his position, (easily

obtained from maps to the desired accuracy), the satellite's location, and

the location of the NBS transmitter, computed values of longitude and

latitude relative to the subsatellite point. He then computed the initial

path delay to which he added an oblateness and radius correction to obtain the free space path delay from NBS-Boulder to his location via the

ATS-3 satellite. Adding in the delays due to the earth's atmosphere,

satellite and receiving equipment yield the total signal delay.

The user computed his clock error or difference as,

Clock error = Apparent delay - Equipment delay

- Signal delay - Cycle correction. (11)

The apparent delay was the value of delay measured, for example,

with the time interval counter shown in figure 9 and explained in Section 4. As with the overlays, two computations were required with the slide rule, one for the up-link from NBS-Boulder to the satellite and the other the

down-link or the satellite to the user. To illustrate the use of the slide

rule and equation (11) the following example is included with an additional

example in Appendix I. The reader may cut out the slide rule in the

appendix to follow the examples.

The following are the locations of the transmitter, satellite, and

user. The transmitter location was fixed. The satellite position was

made known to the user via the satellite broadcast. The user found his

longitude and latitude from a map.

62
CDuAoou

x>

4->

0)

0)ft

<u

PIai

0)

i—I

CO

CO

a)u

=S

•i-f

63
Longitude Latitude

Radius

Correction

NBS -Boulder 105.26°W 40.00°N NA

Satellite 70. 37°W 2.25°N 135 |is

User 56. 11°W 47.85°N NA

The user during or adjacent to the satellite position announcement

measured the arrival of the ATS-3 transmitted tick relative to his clock

in the manner similar to that described in Section 4. From the time

interval counter he read an apparent delay of 257716 |-is.

The user then performed the free space path delay calculation

instructions on the slide rule as shown in table 3.

Substituting into eq (11) the previously determined transmitterreceiver delay of 133 (is and a cycle correction of 2500 M-s caused by measuring at the center of the tick the clock error was found to be 62 |Js, or

equivalently,

NBS (UTC) - User Clock = 62 [is.

The user's clock lagged or was late relative to the NBS (UTC)

master clock by 62 |_ls. The expected precision of this measurement

was dealt with in Section 4. The same calculation, including the slide

rule function, was developed into a simple computer program in the

BASIC language which could be used in small computer facilities. That

program is listed in Appendix III.

8. RESULTS

As a means of evaluating the performance of the ATS-3 time and

frequency dissemination experiment, NBS set up four observation sites.

These sites were selected to be as widely dispersed about the

64
Table 3

Signal Delay Calculations

Down-link Up -link

1. Satellite Longitude 70. 37°W

2. Earth Station Longitude 56.11°W

3. Relative Longitude (absolute value of 1 minus 2) 14.26

4. Corrected Satellite Latitude (step 1) 2. 32°N

5. Earth Station Latitude 47.85°N

6. Relative Latitude (line

5 minus line 4) 45. 53

7. Uncorrected Relay (step 2) 127030 |is

8. Oblateness Correction

(step 3) 22 |is

9. Radius Correction (step 4) 135 |is

10. Free Space Path Delay

(sum of lines 7, 8, and 9) 127187 |is

11. Total Free Space Path

Delay (sum of up -link and

down-link on line 10; step 5) 255007 Us

12. Ionosphere and Troposphere

Delay 14 [is

13. Total Path Delay (sums of

lines 11 and 12) 255021 |is

65

70. 37°W

105. 26°W

34. 89°

2. 75°N

40. 00°N

37. 25°

127670 |is

It) |JS

] .35 [is

127820 Us
subsatellite point as possible. These sites were NBS-Boulder (which

was also the location of the transmitter), Air Force Cambridge Research Laboratory (AFCRL) in Massachusetts, and the Smithsonian

Astrophysical Observatory (SAO) sites in Arequipa, Peru, and Natal,

Brazil. Each site was equipped with a receiving system similar to that

illustrated in figure 9. Each site had a high accuracy time reference to UTC. NBS generated UTC and acted as the master clock with all signals

sent to the satellite being derived from the NBS Frequency Standard and

UTC (NBS) time scale. See figure 2 for illustration of this relationship.

AFCRL maintained its time with a commercial cesium beam clock referenced to UTC through LORAN-C monitoring and portable clock carries

by standards laboratories of DoD. Arequipa, and Natal, also had cesium

clocks for reference and were synchronized by portable clock carries

from NBS with frequency "steering" from VLF monitoring of the Navy

station, NBA at Balboa, Canal Zone.

Each site measured the arrival of the time signals from ATS-3

relative to its local clock. Because each site's clock was previously

synchronized to the master clock at NBS, the measured arrival time

was equal to the total signal delay between the transmitter and the site.

These measured signal delays were compared to computed values derived

from orbital elements and complete descriptions of perturbative forces.

The results of these comparisons represented, in part, the merit of the

system or how well equipment delays could be measured and signal delays predicted. These results are shown in figures 34 through 37.

There are gaps in the data resulting from weekends, holidays, poor orbital elements caused by reduced tracking capabilities when the NASA

tracking sites were being modified, clock failures at the sites, and cancelled broadcasts to allow for satellite participation in APOLLO recovery

operations.

66
cc

LUo

oCOCO

GO

LUa

oGOCO

GO

M H0HH3 M aoaaa

67
Q_

DC

M U0UU3 M ao«d3

68
I— < <-»

(**) uoaa3 M uoa«3

69
eno

o

u

.—

<u Q

CO

(0u

60

(H uoaaa

70
The data shows a resolution of approximately ±10 (is. There were

also some apparent systematic errors which appeared as offsets to the

data from the zero error line. These systematic errors were thought to be due to errors in the orbital elements or in the program which includes

perturbative effects. An error in the values used for equipment delay in

the receiver or transmitter was also a possibility.

There was also seen a general slope to some of the results which

was also believed to be caused by the orbital elements. This belief was

substantiated by the fact that discontinuities existed in the results when

the orbital elements used for prediction were changed. The arrows at the top of the figures indicate the date of the orbital elements where new

predictions began. As an example, in figure 34, the orbital elements

for the later part of June were used for prediction until new elements

were generated in late August. It was natural not to see the same error magnitudes at all sites since the errors in the satellite's orbit were not

manifested in the same direction or magnitude at all points on the earth's

surface.

9. SYSTEM PERFORMANCE COMPARISON

It has already been indicated how the satellite experiment performed

in terms of accuracy. The purpose of this section is to inform the reader

how this experimental system compared with other systems which, at that

time, were carrying time and frequency information. Primarily the comparison was made with the NBS high frequency stations, WWV and WWVH.

It could be said that such a comparison was unfair or out of order because

of fundamental differences in the two systems. For instance, the WWV

broadcasts use high frequency propagation at 2. 5, 5. 0, 10. 0, 15. 0, 20. 0,

and 25. MHz with the earth and ionosphere acting as guiding or reflecting

boundaries, whereas the satellite operated at 135 MHz in line-of-sight

paths. There were similarities between the two systems, however.

71
Both used a similar format and WWV and WWVH were the only complete,

civilian oriented, time and frequency service in operation for what the

satellite experiment was attempting to duplicate by virtue of its wide

range of application and simplicity.

9.1. COVERAGE

Shown in figure 38 are the approximate coverages of the ATS-3

satellite, WWV and WWVH. WWV was located at Fort Collins, Colorado,

and WWVH was at Kauai, Hawaii. The satellite coverage had very definite

boundaries as shown. Inside the boundary the signal was always available

at about the same signal levels without noticeable signal fades and with

the same available accuracy. The boundaries shown for WWV and WWVH

were not as well defined. The signal strength and accuracy available

from these two stations were variable. Factors involved included the

sunspot cycle, the radio path and whether or not the path was in darkness

or daylight, the number of hops involved in the propagation of the signal,

the choice of frequency, and the accuracy for which the propagation path

length was known. The only really definitive comment which could be

made was that the signal was available within those boundaries most of

the time (95%) and at varying levels of accuracy dependent upon conditions.

9. 2. RESOLUTION AND ACCURACY

It can be estimated from the results that the accuracy and precision

for the satellite time signals were approximately 25 M-s and 1 Ms respectively. It has been generally accepted that WWV and WWVH can offer timing

accuracies and resolution of 1 ms and 0. 3 ms respectively. With all conditions ideal, those figures may be somewhat improved upon; however, a user would be faced with a difficult task recognizing when conditions were ideal.

72
u

d)

+-> +>VAuomo

PIo

•r< +»a,

<uo

a>uo

^3

ni •H

i-H<DMa

•H

•8

to 2COH <" w* M<* s >

rt o . O X>

<d

73
One may obtain from figure 39 some additional feeling for the

timing resolutions available from the two systems. Shown are 5 and 10

sweeps, of the WWV and ATS-3 ticks. The resolution for WWV appeared

to be poorer than 100 |is and was limited by noise and path delay variations.

The resolution for ATS-3 was estimated to be approximately 10 [is. A

major limiting factor to timing accuracy from WWV was path delay

variations and is shown in figure 40, see reference [ 1 3]. Here the delays for nearly nine consecutive months between WWVH and Greenbelt,

Maryland, are shown. Monthly means are shown, all different with peakto-peak variations during the nine-month period of more than 2 ms.

The comparisons between WWV or WWVH and ATS-3 are summarized in table 4. Some of the difficulties of time recovery can be appreciated from

the photos in figure 41. Shown are oscilloscope displays of the tick at different sweep speeds. The sequence of these photographs are in the

order usually taken to acquire and measure the time of arrival of ticks

with high time resolution. The ATS-3 tick was easier to distinguish

from the noise and the signals as well as being much less distorted.

Finally, a comparison of all existing and proposed dissemination

systems is shown in table 5. Some of these systems were operational,

some experimental, at the time this comparison was made.

10. CONCLUDING REMARKS

This report has described an experiment which revealed many of

the potentials of and problem areas associated with the broadcasting of

time and frequency information from geostationary satellites. A broadcast format very similar to the WWV format was used with effectiveness.

Assuming that this format was ideal and optimum for terrestrial HF

broadcasts, satellite-VHF broadcasts demand changes or modifications

74
wwv * ATS-3

five sweeps

riWiiEi

lAfffl

mrm mm

five sweeps

mmmM

ten sweeps

1"

H

ten sweeps

^Reproduced from Hewlett-Packard Applications Note 52

by permission.

Figure 39. Multiple oscilloscope sweeps of the WWV and ATS-3 ticks.

75
>

So

u

-ua

r-H T3h

ffi T) o

(1) T3 U a

'I nj

to

cd

i—

u

<L> Rt 2 s

# „ o* 0)

u3

01} ^

En a

saN0D3smiw

76
Table 4

WWV and ATS-3 Performance Summary

WWV or WWVH ATS-3

Coverage (continuous non -interrupted) 10-15% 40% of earth

Cost to user (receiver and antenna) $100-500 $100-500

Resolution 0.3 ms 10 (is

Accuracy 1 ms 25 |is

Path delay computations Complex Simple

Time recovery technique Medium Complexity Simple

77
wwv* ATS-3

r ~r

L. i ~±mdm i * f * *

sweep speed** 0.3 s/cra

-'I I I -'-'0 III

sweep speed 0.3 s/cm

"3

sweep speed 0. 1 s/cm

IMWWWfMli

(JflM^I idi.

t

sweep speed 0. 1 s/cm

MKummmmm

iUtUmilHlillill

* sweep speed 10 ms/cm

iem HlfflflffRlHi g^j !

sweep speed 10 ms/cm

i'l t

sweep speed 1 ms/cm sweep speed 1 ms/cm

Note: Arrow points to leading edge of tick.

*Reproduced from Hewlett-Packard Applications Note 52 -by permission.

Figure 41. Acquiring the ticks from WWV and ATS-3.

78
a

CD

+-) 01 mao

•r-t s

•rHa

cu

en

to Q

COCo

en HOU

fill | a ~ II = | S.TJ =

re £ » 2 >

s. J-s-i 5 - ~ = .s i

79
to that format to maintain an optimum design by virtue of one or more

of the satellite's fundamental differences with HF broadcasts. Those

differences were higher signal-to-noise ratios, excellent short-term

propagation path stability, wider available rf bandwidths, and freedom

from fading and other propagation anomalies. These were characteristics

observed in the experiment but not capitalized upon by changes in format

during the experiment. The experiment only noted and documented these

differences for consideration in later studies which may be directed

towards establishing a satellite time and frequency service.

Another fundamental difference with HF broadcasts noted in this

experiment was the long-term changing propagation path due to satellite

motion. This difference was found to be of real significance to the intermediate and high accuracy time and frequency user. It was demonstrated, however, that satellite movement could be dealt with in a simple

and inexpensive manner using special purpose user aids like the slide

rule and delay overlays.

Although the level of operation obtained in the experiment was not

fully satisfactory for a national service such as provided by WWV, it was indeed obvious that satellites are a very real candidate to meet the

growing needs for higher accuracy and more reliable time and frequency

signals. The experiment provided much of the insight necessary to specify and direct further work to develop the level of competence re- quired for such a service.

Specifically, the experiment demonstrated the superiority and

suitability of satellite broadcasts through the following areas:

1. Using a moderate receiving bandwidth and power, a potential

accuracy of 10 [Xs or better was indicated and, in some periods of

the experiment, achieved.

80
2. The signals were highly reliable in that they were available

to 40% of the earth's surface without interruption due to fading or other propagation anomalies. This implied a continuous

uninterrupted performance to the full coverage area of a permanent service.

3. The cost of user equipment was inexpensive and not substantially different from the equipment required for the reception

of the NBS stations WWV and WWVH.

4. To realize the high-accuracy timing potential, only very

simple delay calculations were required and were readily satisfied by the simple and inexpensive circular slide rule.

5. Considerable interest existed in the time and frequency

community for a satellite service as evidenced by the over 9000 inquiries about the experiment.

6. The time recovery techniques were basically identical to those

used with WWV or WWVH except that greater path predictability

and higher sampling rates enhanced the user's ability for time

recovery and reduced his involvement to obtain the highest

accuracy.

7. The signal quality was in every respect superior to the

WWV/WWVH broadcasts and resulted in a clearer voice channel.

Satellites could easily satisfy the growing needs of the nation for a higher

quality, more reliable, and more flexible time and frequency information

source. The availability of satellite-relayed time and frequency information would expand the applications for these signals. The high signalto-noise ratios and freedom from fading would lend themselves to automatic equipment uses. Applications can be projected in the power

industry where timing signals are required for fault monitoring and

81
power system regulation. Airports, environmental, and geophysical

monitoring sites also require automatic recording of time in parallel

with the recording of designated events. The time and frequency needs

in the transportation industry are growing beyond the capabilities of

present systems to serve them. Areas include vehicle location, vehicle

surveillance, traffic management, and collision avoidance. Digital

communications systems are moving to higher data rates requiring increased synchronization capability. All these mentioned applications

and many more can benefit directly from a reliable satellite time and

frequency service.

The experiment also revealed a number of obvious improvements

which could have been made to its operation. The improvements were

not implemented, however, due to their possible interaction to other lines

of investigation. For example, the tick arriving at the earth approximately

1/4 of a second late was not as convenient for the user as it could have

been if it had been advanced at the transmitter by 1/4 of a second, thereby causing it to arrive at the earth nearly on time. In fact, the signal in

the entire satellite coverage area would never have to be more than a few milliseconds off in the arrival of the tick. No correction for delay

would be needed by the low to intermediate accuracy user, thereby

greatly simplifying the system for 90% of its users. Also, the tick could

be adjusted in time so only the link between the satellite and ground

would need to be calculated for delay correction by the user. The transmitter to satellite link would "in effect" have a fixed delay.

A problem area in the ATS-3 experiments was evident whenever an

orbit maneuver was experienced. The maneuver created an entirely new

orbit for the satellite which required new orbital elements for satellite

position prediction. Operationally, NASA would track the satellite immediately after the maneuver. The tracking process required a day or two

82
after which an orbit was fitted to the data. Consequently, it was a week

or two before NBS had the information necessary to broadcast satellite

position to the user for delay calculations. For an operational service,

this delay in orbit determination would be unacceptable. NBS is now

investigating the feasibility of using the timing signals themselves as

continuous low resolution range measurements and computing orbits

from the resulting data. A small on-line computer could handle this

computation, thus eliminating the need for high quality orbital elements

which allowed for the prediction of satellite position weeks in advance.

The on-line computer need only consider the general shape of the orbit

and a few hours of data from the immediate past to predict into the near

future of, for example, the next 15 minutes. This would be, in effect,

continuous tracking at low resolution in place of high resolution short

duration tracking. Development of this continuous tracking concept with

somewhat higher resolution than what was available for ATS -3 should

also allow for more accurate prediction of satellite position, which is

believed capable of supporting 1 (is timing.

The time code was incorporated into the format in the easiest

manner possible for the existing generating equipment. The code format

and the method of modulation should definitely be reviewed for optimum

usefulness to customers and digital processing. The satellite position

information, not being of use to anyone but the high accuracy, sophisticated

user, should appear in a form much like the time code. In fact, it could

be interlaced into the time code in a way which would not produce interference to equipment designed for operation with the time code. Since

the satellite position data would require updating approximately every

5 minutes, interlacing seems very practical. If the code were slow

enough, it could be "read out" on a paper chart recorder for easy

reduction.

83
Fundamental to this entire topic on the potential of satellites for

time and frequency dissemination was the newly allocated channel for

space-to-earth time and frequency broadcasts. In June 1971, the Space

Telecommunications World Administration Radio Conference (WARC),

held in Geneva, allocated 400. 1 ± 0. 05 MHz to the purpose of time and

frequency dissemination from satellites. This allocation was exclusive,

international, and placed no practical limitations on the flux density

allowed on the face of the earth. Contained in this allocation were 25 kHz

guard bands making the effective information bandwidth 50 kHz. Such an

allocation opens the way for much-improved time and frequency systems

over those available today. The additional 20 kHz bandwidth available over the ATS -3 experiment could be used in a number of ways. The information useful to the low and intermediate accuracy time user could

be restrained to a sufficiently narrow bandwidth to be workable with conventional narrowband FM equipment which is readily available. Outside

of this bandwidth, not seen by the typical user, may be placed all information of interest to only the high accuracy user. A 10 kHz continuous

tone could serve for high accuracy timing just as the 1 kHz tone was used

with ATS-3. It would also serve users who wish to compare frequency

directly with the satellite broadcast. Doppler would still present some

problems but would be predicted simply as a frequency offset as shown

in figure 38. A wideband, 50 kHz bandwidth, receiver for this service

would be a more expensive special purpose receiver. It would be designed

to be delay stable and nondistorting. The wider bandwidth would make it more tolerant to temperature and aging effects which cause waveform

distortion.

Finally, a satellite system which offers a continuous time and frequency broadcast, as dependable as the local commercial AM or FM

stations, not subject to propagation anomalies, and available to 40% of

the earth's surface (assuming only one satellite; three would provide

84
global coverage) will satisfy many of today's requirements not being met

economically by present services. Additionally, these high quality

services will generate more uses of time and frequency technology, with

new products and markets being developed. Satellites will obviously

not satisfy everyone's needs--no one system ever will. Other systems

will be needed to complement this service such as the NBS TvTime System

which has been experiencing a parallel development to the ATS -3 experiment [l4j. A satellite service, however, should be a welcome advancement since its added advantages do not cost the user more than for

reception of WWV or WWVH.

85
APPENDIX I SLIDE RULE AND PERFORMANCE

A slide rule has been provided in this report which may be cut out

and assembled. Figure 1-1 is the top and figure 1-2 is the base. Assembly instructions are included in figure 1-1. The center of the slide rule

is marked and the two sections must be joined carefully to assure that

they are concentric. Positioning of either section off center will produce

errors in the computed delays.

The only real measure of slide rule performance comes from comparing its computed delay with the delay derived from orbital elements,

a digital computer, and complete descriptions of perturbating forces.

Figure 1-3 shows how the computations were obtained. NASA generated

tracking data on the satellite from their two tracking stations at Rosman,

North Carolina, and Mojave, California. This data then produced orbital

elements. Using a digital computer and a model of orbit perturbations,

the satellite's position during broadcast times was determined and was

relayed to the user via the satellite in terms of longitude, latitude, and

a radius correction. Inputing the user's position, two delay values were

derived. One value came from the slide rule and the other from the digital computer which went directly from orbit prediction to delays.

If the two delays were equal then the slide rule process would have

accomplished its intended task perfectly. However, this was not the case

as can be seen in figures 1-4 - 1-7. Figure 1-4 shows the difference

(digital computer minus slide rule) versus user longitude relative to the

subsatellite longitude for various user latitudes and selected subsatellite

points in both hemispheres. Figure 1-5 shows the distribution of the

points for each subsatellite point and for the total of all differences. The

obvious bias in the results was caused primarily by a longitudinal error

86

TOP PIECE

CURSOR

Z

Assembly Instructions (figs. 1-1 and 1-2).

1. Cut out top piece, cursor, and base of the slide rule.

2. Cut out the two windows on the top piece.

3. Using a straight pin or other suitable fastner, assemble the

cursor, top piece, and base at the central point, indicated with

a dot in the middle of a small circle.

Figure 1-1. Sample slide rule top and cursor.

86 a
Figure 1-2. Sample slide rule base.

86 b

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D. Total of A, B, and C.

Figure 1-5. Distribution of slide rule delay errors.

89

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ure 1-7. Distribution of the slide rule delay errors with the longitudinal error removed.

91
which is discussed fully in another report Ll5J. This longitudinal error became significant whenever the relative longitude and satellite latitude

fell into the shaded area in table 1-1 . As an additional example of slide

rule operation and removal of error, consider the following.

Longitude Latitude Radius Corr ection

NBS Boulder 105.26°W 40. 00°N NA

Satellite 74. 67°W 2. 50°N -176 Us

User (Tristan da Cunha) 12.30°W 37.15°S NA

Slide Rule Calculation

Down -link Up -link

1. Satellite Longitude 74. 67°W 74. 67°W

2. Earth Station Longitude 12.30°W 105.26°W

3. Relative Longitude (absolute

value of step 1 minus step 2) 62.37 30. 59

4. Corrected Satellite Latitude 5.40°N 2.90°N

5. Earth Station Latitude 37.15°S 40. 00°N

6. Relative Latitude (absolute

value of step 4 minus step 5) 42. 55 37. 10

7. Uncorrected Delay 134765 |Js 126880 \Js

8. Oblateness Correction 6 |Js 17 |_is

9. Radius Correction -176 |_ls -176 |as

10. Free Space Path Delay

(sum of steps 7, 8, and 9) 134595 ms 126721 (is

92
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93
Before determining the round-trip delay, the down-link should be

corrected for the longitudinal error since the relative longitude and

satellite latitude combinations fall into the shaded area of table 1-1. In- terpolating from this table we find that 6 should be 62. 26 . Using this

value in the table instead of 62. 37 , as in step 3 above, to compute the

down-link delay yields an uncorrected delay of 134740 [is, a one-way

delay of 134570 p.s, and a round-trip delay of 261291 |JL s

After this longitudinal error was removed the performance was improved as shown in figures 1-6 and 1-7. These results were obtained from

more than 500 manual computations with the slide rule and it was obvious

that this tool was capable of providing delay calculations for the user simply and inexpensively and accurate to 10 pis rms. This translated

into a 10 |J.s accuracy for timing provided the satellite position information

was perfect.

94
APPENDIX II ORBITAL ELEMENTS AND ORBIT PREDICTION

This appendix briefly describes factors relating to the description

of an earth satellite's orbit and prediction of that orbit. The discussion

progresses from a simple two-body orbit and its orbital elements, to an n-body orbit and concludes with perturbations to an orbit.

From the universal law of gravitation the force vector between two

bodies is given by

. 2 k mn m_

f

1 2 F

(E-i)

12 3 r i2'

r l2

where m and m are the masses of the two bodies, r is the vector

from the first body to the second, k is the gravitational constant. It is

assumed that the two bodies are spheres with homogeneous mass distribution and that the only force acting on either body is the gravitational

attraction of the other body. Dividing this force by the mass of the

second body yields the acceleration of the second body

d

2

r"2 k

2

nVl2 f (n _ 2)

, 2 3 dt r i2

Double integration of this equation gives the position vector.

Because the force vector has three component parts, six constants

of integration are required. These constants are called the satellite's

orbital elements. The choice of the reference coordinate system determines the form of the elements. The six "classical " elements, five of

which are illustrated in figure II- 1, are:

1) The semi-major axis, a, of the ellipse which the satellite traces

95
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96
out as a function of time.

2) The eccentricity, e, of this ellipse.

3) The inclination, i, or the angle of intersection between the

orbit plane and the reference plane (the equatorial plane is usually used

as the reference plane for an earth -centered system. )

4) The longitude of the ascending node, Q, or the angle between

the line pointing from the geocenter to the constellation Aries and the

line of intersection of the orbit plane and the reference plane.

5) The argument of perifocus, UU, the angle between the line of

nodes and the perifocal direction, measured in the orbit plane.

6) The time of perifocal passage, T.

It is interesting and instructive to examine the relationship of

these various parameters to a geostationary satellite. An "ideal" geostationary satellite would not appear to have any motion relative to points on the earth. This implies a circular orbit which lies in the

equatorial plane. Consequently, for this "ideal" orbit, three of the

classical elements, Cl, ou, and T, are degenerate. The position of the

semi-major axis, a, is also indeterminate, but one may talk about the

length of the axis which would be approximately 6. 61 earth radii. The

other two elements, e and i, are both zero. The effects of small deviations from the ideal value for these last three elements point out the

types of fluctuations one might expect from a real geostationary satellite.

Small changes in the length of the semi-major axis will cause the

satellite's period to differ slightly from equality with the earth's period

of rotation. Thus, the satellite would drift longitudinally from the desired subsatellite point. An increase of 0. 01 earth radii or 0. 15% will cause a westward drift rate of approximately 0. 8 degrees per day.

97
Small deviations from the ideal eccentricity will have three major

effects. First, an additional orbital element, the time of perifocal

passage T, will become defined, as will the major axis. Next, the distance from the earth will change as a function of time. Since the earth

is at one of the focii of the ellipse rather than at the center, this effect

is amplified above what one might expect. Finally, because of the

changing distance from the earth, the velocity of the satellite also fluctuates periodically and causes a daily east-west periodic motion. For

an otherwise ideal geostationary orbit, an eccentricity of 0. 001 will cause a peak-to-peak variation of distance from the earth of 85 km and

peak-to-peak longitudinal motion of approximately 0. 2 degrees.

Similarly, small deviations from the ideal zero degrees inclination

will have three major effects. An additional element, £1, the longitude

of the ascending node, becomes defined. Also, the satellite will experi- ence daily north-south movement in latitude. The maximum north or south latitude of the subsatellite point is equal to the inclination angle.

Finally, a non-zero inclination will cause longitudinal motion with a period of twice a day. The periodicity of longitude and latitude caused by

inclination combines in a manner similar to a lissajous pattern on an

oscilloscope to form a "figure eight" trace of the subsatellite point. An

inclination of 1 degree will cause peak-to-peak latitude and longitude

fluctuations of 2 degrees and 0. 08 degrees respectively.

When there are more than two attracting bodies and when there are

other perturbing forces, the equation of motion takes on a new form. For

n bodies

given by

bodies with perturbations then, the force vector on the i— body is

m

d

2-

n -

. _^- = k

2 V m. m.!ii_ + m.p. j#i . (H-3)

X dt2

J=i * J r

3

.

98
Here p. represents accelerations which cause deviation from an ideal n- body orbit. These might be caused by thrust, drag, non-uniform gravitational fields, solar radiation pressure or other forces.

The non -homogeneity of the earth's mass distribution may be

explicitly incorporated into eq (3) by substituting the gradient of the

gravitational potential for the term in the summation which deals with the

acceleration due to the earth. The gravitational potential of the earth may

be expressed as:

,2k m

** e {m) [sin6]

k

r

C, cos [mX ] + S, sin [mXl

k, m E k, m E

(II-4)

where m is the mass of the earth, p is the Leeendre function, 6 is

e

' *k s the declination of the satellite, C, and S, are the harmonic coeffik, m k, m

cients, and X is east longitude. The harmonic coefficients are related E

5

to the shape of the earth. For example, the term C is directly

related to the flattening of the earth. The term C represents the

"pear-shaped" nature of the earth with the stem at the north pole. The

terms C and S together may be interpreted as arising from an

elliptical equator. This ellipse has its major axis at about 18 west

longitude with a difference of approximately 70 meters between the

lengths of the semi-major and semi-minor axes. A synchronous satellite at the major axis will be in unstable equilibrium (likened to balancing a cone on its apex). A satellite at the minor axis will be in

stable equilibrium (a cone balanced on its base). At any other longitude a synchronous satellite will be slightly accelerated towards the nearest

99
minor axis. For a synchronous satellite, 6 and L of eq (4) are very

nearly constant. Consequently, the gravitational force will be relatively

constant even though it may be significantly different than for a uniform

sphere with homogeneous mass distribution.

The forces due to bodies other than the earth will cause significant

perturbations to the orbit of a "geosynchronous" satellite. The two

bodies which have the greatest perturbing effect on an artificial earth

satellite are the sun and the moon. The force vector acting on the satellite from such a body may be resolved into two component vectors, one

in the orbit plane and one normal to the orbit. The normal component

imparts a rotation of the orbit plane around the earth. Since this component is periodic, however, the net effect is a trend for the orbit plane to move towards containing the force vector. The coplanar component may

be further broken down into components parallel to and normal to the

major axis. This normal component will cause the axis to rotate towards

the coplanar component, while the coaxial component will cause a lengthening of the semi-major axis and increase the eccentricity of the orbit.

Fisher Ll6J gives the following estimation formula for the short term

effects of the moon on the semi -major axis of an artificial earth satellite: da~ 1. 1 x 10" ? a (II-5)

where a is the semi-major axis expressed in earth radius units. Evaluating this formula for a synchronous satellite yields a change of 1 . 4 km

or 0. 003%. This agrees with measurements shown in the same report

which gives a detailed analysis of sun and moon effects as perturbations

to a two -body orbit.

The problem of accurately determining a satellite's position as a function of time may be seen to be very complex. Herrick [ll] has summarized the processes necessary to the development of perturbation theory.

100
1) Initial conditions such as orbital elements for all of the included

bodies must be known.

2) A mathematical model of the force field must be developed.

3) Physical constants such as the harmonic coefficients of eq (4)

must be known.

4) A reference framework, i. e. , an origin and axes, must be

selected.

5) The integrands must be selected. Three choices exist. One

may integrate the total acceleration, or the perturbing acceleration, or selected functions of the perturbing acceleration.

6) The mode of integration, either numerical or analytical, must

be selected. Also, if numerical integration is chosen, there are various

formulas or processes from which one must choose.

For more detail the reader is referred to Brouwer and Clemence LI 2]

or Herrick 111].

101
APPENDIX in

COMPUTER PROGRAM FOR FREE SPACE PROPAGATION DELAY

The computer program listed below may be used to calculate the

free space propagation delay to a satellite. This program, written in

the computer language BASIC, uses essentially the same procedure as

the special purpose slide rule described earlier, although the approximations have been removed. The input parameters are the satellite's

position (latitude, longitude and radius correction) and the receiving

site location (latitude and longitude) along with an identifying date.

The up-link is computed assuming NBS Boulder to be the transmitting

site.

5 LET A = 6378.21

10 LET B = 6356.58

15 LET C = .2997925

20 LET D = 1.74533E-2

40 FOR I = 1 TO 5 STEP 1 41 PRINT

42 NEXT I 53 PRINT "INPUT OF TRANSMITTER SITE LOCATION"

55 PRINT "WEST LONGITUDE = "t 60 INPUT LI

61 PRINT

65 PRINT "NORTH LATITUDE = "I 70 INPUT L2

7 1 PRINT

75 PRINT "ELEVATION ABOVE SEA LEVEL IN METERS = "t 80 INPUT L3

81 PRINT

82 PRINT

83 PRINT

85 PRINT "INPUT OF RECEIVErt SITE LOCATION"

90 PRINT "WEST LONGITUDE = "J

9 5 INPUT Ml

96 PRINT

100 PRINT "NORTH LATITUDE = "i

105 INPUT M2

106 PRINT

110 PRINT "ELEVATION ABOVE SEA LEVEL IN METERS = "I

1 15 INPUT M3

116 PRINT

117 PRINT

118 PRINT

102
120 PRINT "INPUT OF SATELLITE POSITION AND DATE OF POSITION"

125 PRINT "MONTH, DAY, AND YEAR = "i

130 INPUT Yl, Y2, Y3

131 PRINT

135 PRINT "TIME = "i

140 INPUT T 141 PRINT

145 PRINT "WEST LONGITUDE = M J 150 INPUT Nl

151 PRINT

155 PRINT "NORTH LATITUDE = "i

160 INPUT N2

161 PRINT

165 PRINT "RADIUS CORRECTION = "I 170 INPUT N3

171 PRINT

200 LET L4 = L1*D

205 LET L5 = L2*D

210 LET M4 = M1*D

215 LET M5 = M2*D

220 LET N4 = N1*D

225 LET N5 = N2*D

230 LET Bl = <B*B)/(A*A>

235 LET B2 = B1*B1

240 LET HI = SQRCC 1 + B2*TAN(L5)*TAN(L5) )/C 1 + B1*TAN(L5)*TAN(L5) ))

245 LET HI = H1*A+.001*L3

250 LET H2 = SQRC ( 1+B2*TAN(M5) +TANCM5) ) / < 1+B 1*TAN(M5> *TAN(M5) )

)

255 LET H2 = H2*A+.001*M3

260 LET H3 = 42143. 4+N3*C

265 LET L5 = ATNC B 1 *TAN(L5)

>

270 LET M5 = ATN< B 1*TAN(M5>

)

275 LET CI = C0S(L5>*C0S<N5)*C0S( ABSCL4-N4)

)

280 LET CI = C1+SIN(L5)*SIN(N5)

285 LET C2 = COS(M5 ) +C0SCN5) *COS( ABS( M4-N4)

)

290 LET C2 = C2+SINCM5>*SIN(N5>

300 LET Rl = SuRCH3*H3+Hl*H 1 -2*H3*H 1*C 1 > 305 LET R2 = SQR(H3*H3+H2*H2-2*H3*H2*C2>

310 LET R3 = R1+R2

315 LET Tl = Rl/C

320 LET T2 = R2/C

325 LET T3 = R3/C

350 FOR I = 1 TO 5 STEP 1 355 PRINT

360 NEXT I 365 PRINT Y 1 J "/"J Y 2J "/"; Y

3

370 PRINT

375 PRINT T;"GMT"

380 PRINT

385 PRINT "UPLINK - RANGE = "JRU"KM"

390 PRINT " - DELAY = "i T 1 i "M I r.HD .SPTTINnS"

103
395 PRINT

400 PRINT "DOWNLINK - RANGE = "JR2J"KM"

405 PRINT " - DELAY = "i T2t "MI CROSECONDS"

410 PRINT

415 PRINT "ROUNDTRIP - RANGE = "i R3J "KM"

420 PRINT " - DELAY = "; T3i "MI CROSECONDS"

425 PRINT

430 PRINT "NOTE - EQUIPMENT DELAYS MUST BE COMPENSATED FOR"

435 PRINT " BEFORE COMPARING THEORETICAL AND EXPERIMENTAL"

440 PRINT " DELAYS."

104
11. REFERENCES

[l] Steele, J. McA. , Markowitz, W. , and Lidback, C. A., "Telstar

Time Synchronization, " IEEE Trans. Instrum. and Meas. , IM-13,

No. 4, pp. 164-170 (December 1964).

[Zl Markowitz, W. , Lidback, C. A., Uyeda, H. , and Muramatsu, K. , "Clock Synchronization Via Relay II Satellite, " IEEE Trans. Instrum.

and Meas., IM-15, No. 4, pp. 177-184 (December 1966).

L3J Ramasastry, J., Rosenbaum, B. , Michelini, R. D. , and Kuegler,

G. K. , "Clock Synchronization Experiments Performed Via the

ATS-1 and ATS-3 Satellites, " IEEE Trans. Instrum. and Meas.,

IM-22, No. 1, pp. 9-12 (March 1973).

L4J Jespersen, J. L., Kamas, G. , Gatterer, L. E. , and MacDoran,

P. F., "Satellite VHF Transponder Time Synchronization, " Proc. IEEE, 5_6, No. 7, pp. 1202-1206 (July 1968).

[5J Kershner, R. B. , "Transit Program Results, " Astronautics, 6, No. 5, pp. 30-38 (May 1961).

L6J Goddard Space Flight Center, "Tracking Intercomparison Tests

With GEOS-1, " Proc. of the GEOS Program Review Meeting,

12-14 December 1967, III , (February 1968).

[7] Gatterer, L. E. , Bottone, P. W. , and Morgan, A. H. , "Worldwide

Clock Synchronization Using a Synchronous Satellite, " IEEE Trans.

Instrum. and Meas. , IM-17, No. 4, pp. 372-378 (December 1968).

[8] Hanson, D. W. , and Hamilton, W. F. , "One -Way Time Synchronization Via Geostationary Satellites at UHF, " IEEE. Trans. Instrum.

and Meas., IM-20, No. 3, pp. 147-153 (August 1971).

[9] Hanson, D. W. , and Hamilton, W. F. , "Clock Synchronization

from Satellite Tracking, " IEEE Trans. Aerospace and Electronic

Systems, AES-7, No. 5, pp. 895-899 (September 1971).

105
LlOj Skolnik, M.J., Introduction to Radar Systems. New York:

McGraw Hill Book Co., Inc., 1962, p. 413.

LllJ Herrick, S. , Astrodynamics. London: Van Nostrand Reinhold Co. , 1971, p. 540.

Ll2] Brouwer, D. , and Clem. ence, G.M., Methods of Celestial Mechanics. New York: Academic Press, 1961, p. 598.

L13] Morgan, A. H. , "Precise Time Synchronization of Widely Separated

Clocks, " NBS Technical Note 22 (July 1959).

[l4j Howe, D.A., "Nationwide Precise Time and Frequency Distribution

Utilizing an Active Code Within Network Television Broadcasts, " IEEE Trans. Instrum. andMeas., IM-21, No. 3, pp. 263-276

(August 1972).

[l5] Hamilton, W. F. , and Hanson, D.W., "A Synchronous Satellite

Time Delay Computer, " NBS Technical Note 638 (July 1973).

[l6] Fisher, D. , "Analytic Short Period Lunar and Solar Perturbations

of Artificial Satellites, " Celest. Mech. , 6, No. 4, pp. 447-458

(December 1972).

ACKNOWLEDGMENTS

The National Bureau of Standard-s is deeply indebted to the National

Aeronautics and Space Administration for the use of their facilities and

for their generous cooperation. The authors also wish to recognize the

contributions of certain individuals to the success of the experiment.

Major contributors included:

National Aeronautics and Space Administration

Mr. R. B. Marsten

Mr. Russell Burke

Mr. Milton Baltas

Mr. William Davenport

Mr. Joe Gifford

Mr. James Meenan

106
Mr. Howard Pedolsky

Mr. Robert Youngblood

Smithsonian Astrophysical Observatory

Mr. Wyit Wright

Mr. Charles Tsiang

Mr. R. R. Pearlman

Mr. Jimmy Girigori

Mr. Gordon Newman

and staff at Arequipa, Peru

Mr. Jules Simms

and staff at Natal, Brazil

United States Air Force

Major John C. Herring, AFCRL

T/Sgt. Gerry Smith, AFCRL

Naval Avionics Center

Mr. Bill Long

Mr. George Miller

Special recognition is due to Mr. Alvin Clements who contributed

significantly to every phase of the experiment and whose help greatly

enhanced its value and completeness. His initiative and resourcefulness

were appreciated.

107
form NBS-U4A (1-71)

2. Gov t Accession No.

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

PUBLICATION OR REPORT NO. NBS TN-645

3. Recipient's Accession No.

4. TITLE AND SUBTITLE

Time and Frequency Broadcast Experiments from the ATS -3

Satellite

5. Publication Date November 1973

6. Performing Organization Code

7. AUTHOR(S)

D. W. Hanson and W. F. Hamilton

8. Performing Organization

10. Project/Taslc/Worlc Unit No. 2730114

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS, Boulder Labs,

DEPARTMENT OF COMMERCE

Boulder, Colorado 80302

11. Contract/Grant No.

12. Sponsoring Organization Name and Address Same as Item 9.13. Type of Report & Period Covered Final (8/1/71 - 8/1/73

14. Sponsoring Agency Code

IS. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant bibliography or literature survey, mention it here.) An experiment designed to reveal the advantages and special problems associated

with the broadcasting of time and frequency information from geostationary satellites

is discussed. Included are discussions concerning satellite motion, time delay

variation, doppler shift due to the motion, and calculation of delay. Receiver or ground station equipment requirements, time recovery techniques, timing resolution

and accuracy, and special advantages of satellite broadcasts for time and frequency

dissemination are also discussed. Specially equipped sites in North and South

America gathered data from the experimental satellite broadcast which in turn were

used to determine the potential accuracy of satellite dissemination, the results of

which are presented. Delay computation aids for the user were designed to provide a simple and inexpensive means of computing free space delays between the master

clock and the user via a geostationary satellite. The aids, delay overlays on an earth

map and a circular slide rule, are discussed with examples. Qualitative discussions

of the signals and broadcast format are given. Final comments are made concerning

the results of the experiment and how they might reflect upon a final system design

for a permanent service using one or more geostationary satellites.

17. KEY WORDS (Alphabetical order, separated by semicolons)

Dissemination; frequency; satellites; synchronization; time.

18. AVAILABILITY STATEMENT

[X] UNLIMITED.

I I FOR OFFICIAL DISTRIBUTION. DO NOT RELEASE

TO NTIS.

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115

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