NBSIR 73-348

AN ENGINEERING FEASIBILITY STUDY

FOR ONE-WAY TIME TRANSFER

USING THE GOES SATELLITE RANGING SYSTEM

J.B. Milton

W.F. Hamilton

Time and Frequency Division

Institute for Basic Standards

National Bureau of Standards

Boulder, Colorado 80302

Final Report

December 1973

Prepared for

Federal Aviation Administration

Systems Research and Development Service

Washington, D.C. 20591

NBSIR 73-348

AN ENGINEERING FEASIBILITY STUDY

FOR ONE-WAY TIME TRANSFER

USING THE GOES SATELLITE RANGING SYSTEM

J.B. Milton

W.F. Hamilton

Time and Frequency Division

Institute for Basic Standards

National Bureau of Standards

Boulder, Colorado 80302

Final Report

December 1973

Prepared for

Federal Aviation Administration

Systems Research and Development Service

Washington, D.C. 20591

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

NATIONAL BUREAU OF STANDARDS. Richard W Roberts Director

CONTENTS

1. INTRODUCTION 1 2. SATELLITE RANGING TECHNIQUES 3 2.1. PRESENTLY USED SIDETONE RANGING TECHNIQUES 4 2.2. TRILATERATION SIDETONE RANGING 4 2.3. THEORY OF SIDETONE RANGING 4 3. ONE-WAY TIME TRANSFER METHOD 5 4. GEODETIC CONSIDERATIONS FOR SATELLITE TRILATERATION TIMING 11

4.1. CDA AND TARS SITE LOCATION 11

4.2. RECEIVING SITE LOCATION 13

5. ADVERSE EFFECTS UPON THE TIME TRANSFER 13

5.1. PROPAGATION EFFECTS 13

5.2. SATELLITE MOTION 17

5.3. SATELLITE LOCATION ERRORS 17

5.4. EQUIPMENT DELAYS AND INSTABILITIES 2 0 5.5. EFFECTS ON NOISE ON TIME TRANSFER 20

6. GENERAL LOGISTICS 20

6.1. SYSTEM ENVIRONMENTAL CONSIDERATIONS 21

6.2. RECEIVING SITE EQUIPMENT COMPLEMENT 21

6.3. PERSONNEL REQUIREMENTS 23

6.4. COMPONENT RELIABILITY AND MEAN-TIME-BEFORE-FAILURE 23

6.5. POWER REQUIREMENTS 25

6.6. SPARES COMPLEMENT 25

7. SUMMARY OF TIMING BIASES 25

8. CONCLUSION 26

9. REFERENCES , 30

APPENDIX A 31

APPENDIX B 33

iii
LIST OF FIGURES

Figure 1. Geocentric coordinate system used for trilateration ranging 2 Figure 2. Interrogation channel format 8 Figure 3. One-way time transfer geometry 9 Figure 4. Time transfer receiving systems 10

Figure 5. A simple triangulation network 12

Figure 6. Range change due to tropospheric effects 15

Figure 7. Range (time) change due to ionospheric effects 16

Figure 8. Predicted range, range rate, and range acceleration from CDA station,

Wallops Is., Virginia, satellite longitude 95°W 18

Figure 9. Frequency standard intercomparisons from station WWV - 3 units 22

Figure 10. TARS components failure rates 24

LIST OF TABLES

Table 1. Peak range (time errors) due to noise in the GOES ranging system... 6 Table 2. Range errors in meters due to errors in location of the trilateration stations 19

Table 3. Trilateration system systematic errors for all sidetones - expressed

as tim.e 27

Table 4. Trilateration system random errors for all tones - expressed as time 28

Table 5. Summation of timing biases and sources 29

iv
AN ENGINEERING FEASIBILITY STUDY FOR ONE-WAY TIME TRANSFER

USING THE GOES SATELLITE RANGING SYSTEM

J. B. Milton and W. F. Hamilton

The Time and Frequency Division of the National Bureau of Standards

has conducted an engineering study to determine the feasibility of using

the GOES satellite ranging system for precise (0.1 microsecond, one-sigma)

time transfer to a receiving-only timing site. The GOES satellite ranging system, termed a trilateration system, will accurately locate this

satellite within some coordinate structure. The sources of time transfer

errors have been studied in some detail. These errors can be caused by

satellite location errors, ground station location errors, unknown delays

caused by the troposphere, the ionosphere, and the various equipments.

Simplified designs for an automatic and a manually operated timing site are presented. Some technical problems found in the associated equipment

are discussed. The study indicates that a secondary, or slave site, clock

could be synchronized to within 0.1 microsecond, one-sigma, of some master

clock utilizing a one-way, or receiving-only system.

Key words: Clock synchronization; one-way time transfer; satellite timing;

synchronous satellite.

1. INTRODUCTION

Sometime in early 1974, NASA plans to place the first of a series of new environmental satellites in synchronous orbit. These satellites will be called Synchronous

Meteorological Satellites (SMS). When NOAA, the operating agency, begins using these

satellites, they will be redesignated as Geostationary Operational Environmental Satellites (GOES) . Part of the mission for GOES will be to take pictures of storm patterns, frontal systems and the like. In order to accurately locate these pictures with

respect to earth longitude and latitude precise information on the location of the satellite must be obtained.

Toward this end, a new ranging system for these satellites has been developed for

NOAA that has a theoretical precision of about one meter and an accuracy limited basically by the real time knowledge of the effects of the ionosphere and the troposphere

[1] . This ranging system, termed trilateration, uses only time delay data obtained

from three widely separated ground stations. No azimuth measurements are necessary.

The primary tracking station, referred to as the Control and Data Acquisition Station

(CDA) , is located at Wallops Station, Virginia. The two unmanned secondary stations,

called Turn-Around Ranging Stations (TARS) will be located at Santiago, Chile and

either Honolulu, Hawaii, or Ascension Island, in the mid-Atlantic Ocean. This system

of three locations will provide three slant ranges to the satellite. The location of

the satellite is then determined geometrically (see fig. 1).

Another major mission of the GOES satellite will be to gather environmental data

from many sensors located in rivers, tidal areas, and other strategic locations.

The sensor data will be returned to the CDA via the GOES satellite. The ground equipment that will perform this function is referred to as a Data Collection Platform Radio

Set (DCPRS) [2] . Some sensor data will be transmitted on satellite command, and some data will be transmitted at a prearranged time. The former is called an interrogated

platform, the latter a self-timed platform. The interrogating signal is sent from the

satellite to the platforms in the interrogation channel, 468.825 MHz. Time-of-day

information, and other special data, will also be transmitted to users via this

channel. The GOES frequency plan will be found in Appendix A.
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Time transfer via satellite has been performed in the past using two-way and one- way systems [3] -[7]. In a two-way system, a timing signal is sent from a reference

clock to a secondary clock via the satellite, and then, depending upon the method used, some timing signal is sent from the secondary clock back to the reference clock.

These timing signals usually take the form of some type of modulation on a carrier signal.

Electronic time "ticks" have been used as well as continuous tone modulation. The

timing signal from the secondary clock to the reference clock can be either a signal

generated by the secondary clock system or an actual retransmission of the timing signal

received from the reference clock. Regardless of the method used, the desired result of

the timing signal transfer procedure is to obtain sufficient data to accurately calculate

the propagation delay time from the reference clock to the secondary clock. Once this

delay is obtained, the secondary clock corrections are easily made. Notice that no reference has been made to satellite location. In a one-way system the reference clock

signal is sent to a secondary clock site, but no return transmission is made. In this

method, there is insufficient data to make an accurate determination of signal delay.

The delay has to be computed by making assumptions as to satellite location, tropospheric effects and ionospheric effects. A two-way time transfer system has yielded

precise (<_ 1 microsecond, 1 sigma) time transfer while a one-way system has yielded medium precision (>^ 10 microseconds, 1 sigma ) time transfer. During the early precise

time transfer experiments, a two-way system was necessary because adequate information

on the satellite position was not available. A disadvantage of a two-way system is

that a full transmitting and receiving facility is required at the secondary clock

site. Also, only one secondary timing site may be serviced at any one time.

With the advent of the accurate trilateration ranging system for GOES, precise

one-way timing systems become feasible. The basic requirements are accurate knowledge

of transmitting location, receiving location, and the tropospheric and ionospheric

conditions

The Time and Frequency Division has performed a study to determine the practicality

of precise (0.1 microsecond, 1 sigma) one-way time transfer systems. Specific attention has been paid to the errors caused by variations in the propagation medium and errors due to noise on the various radio links. Errors caused by faulty information

concerning the locations of the CDA and TARS stations and the various receiving facilities that might be involved in the system are also discussed. Representative errors

from these and other sources will be listed. Details of the trilateration method and

the theory of one-way timing systems will be given. Technical factors involved in a one-way timing system are tabulated and discussed. The general logistics of a manned

and unmanned receiving site is gone into in some detail. The GOES link analyses will

be found in Appendix B.

2. SATELLITE RANGING TECHNIQUES

In order to determine the orbit of any satellite, certain parameters must be obtained. These parameters, called orbital elements, describe the position and velocity

of the satellite at some specific time. These elements are determined by computer after

examining a quantity of data. These data consist of measurements of signal delay and

rate of change of signal delay, from one or more satellite tracking stations.

3
2.1. PRESENTLY USED SIDETONE RANGING TECHNIQUES

The Goddard Range and Range Rate System (GRRR) [8] uses a series of tones, called

sidetones, to modulate a microwave carrier. After transmission to the satellite, the

carrier with sidebands is returned to the tracking station through the satellite ranging

transponder. After demodulation, the phase and rate of change of phase of the returning

sidetone is continuously measured with respect to the phase of the same sidetone transmitted to the satellite. These phase data are converted to range and range-rate data.

If this information is available in sufficient quantity an accurate determination of

the orbital elements may be made.

2.2. TRILATERATION SIDETONE PJ^NGING

In a trilateration system, the computation of the orbital elements is somewhat

simplified. There are always three tracking stations involved. Each station measures

the phase of a sidetone such that three slant ranges may be computed. The three slant

ranges may be geometrically resolved into a satellite range-to-earth center. This

purely geometrical solution implies simultaneous slant range measurements, i.e., there

will be a time, t^, associated with the range calculation. Now if a new range is

computed from new slant range data at time t^^, sufficient information is available to

compute the orbital elements. The drawback to this system is that for accurate determination of range, the three tracking locations should be widely separated. These

large separations make the task of synchronizing the tracking stations' clocks, (and

therefore the slant range data) much more difficult.

In the GOES tracking system, there is no time synchronization problem. The master

station (CDA) transmits the sidetone to the satellite. The satellite transmits the

sidetone back to the CDA and simultaneously to each TARS station. The TARS stations

return the sidetone transmission on their own individual carrier frequencies, via the

satellite, to the CDA. This phase information is sufficient to allow a range determination to be made at the CDA station. Synchronization is no problem since all

transmissions occur simultaneously.

2.3. THEORY OF SIDETONE RANGING

Because of the ambiguity associated with the determination of range based upon

the phase measurements of a continuous tone, a multiple tone system is employed.

While there are no constraints on the actual frequency of the tones, there are requirements on the relationships of the tones to each other and requirements on the high and

low tones. The highest frequency tone determines the resolution and accuracy of the

ranging system while the lower tones resolve the ambiguity of the range measurement.

The number and relationship of the intermediate tones is determined basically by the

link signal-to-noise ratio. The GOES ranging system's lowest tone has a frequency of

about 35.4 Hz. The period is exact at 28,224 microseconds.

To begin with, the range to the satellite must be estimated to better than 1/2

wavelength of the lowest tone. The wavelength, X, according to C = X/t, is approximately

8,467 km. C is the velocity of light and x is the period of the tone. The estimate

of synchronous altitude must therefore be accurate to about ± 2,100 km. If a synchronous satellite has a reasonably circular orbit and low inclination, the satellite altitude is about 36,000 km. Dividing the round trip distance of 72,000 km, by the wave- length of the 35.4 Hz tone, yields a total of 8.5 wavelengths round trip delay for that

tone. The actual proceedure is to measure the phase difference, expressed as a time

4
interval, between the transmitted 3 5.4 Hz tone and the received 35.4 Hz tone from the

satellite. The measurement is of the fractional or residual wavelength only. If the

original estimate of round trip delay was accurate, it would be clear that 8 wavelengths should be added to the residual, rather than 7 or 9. If there were no noise

in the system, the range measurement could stop there, but of course, this is not the case. Therefore, there is an error estimate attached to the phase measurement that

translates to a range error, AR. In the case of GOES, this is about 30 km for this

lowest tone. From Skolnik [9]

where f is the side tone frequency, S/N is the signal-to-noise power ratio and C is the

free space velocity of light.

The requirement that determines the first tone frequency above 35.4 is that 1/2

its wavelength be greater than the AR of the 3 5.4 Hz. If this requirement is met for

each higher tone in turn, then progressive measurements may be made that will allow

unambiguous range determination using the highest frequency tone. The range error,

AR, is inversely proportional to the tone frequency, and so the range accuracy requirements and the signal-to-noise ratio determine the highest tone frequency. Table 1 lists the GOES ranging system tones and their respective range errors due to systematic

bias and random noise. These are computed values [1].

There are, of course, restrictions on the use of the GOES satellites. The GOES

satellites are part of a future, world-wide group of environmental satellites that

will be operational rather than experimental in nature. Consequently, except for a few strictly agreed upon and minimal services that can be performed by NOAA, all other use of these satellites by other agencies must be strictly passive. That is, any transmissions from the satellite may be received and utilized, but no transmission to the

satellite will be allowed that is not in accordance with the GOES mission. This discussion will take place within the framework of these restrictions.

As can be seen from the GOES frequency plan (Appendix A) , the S-band circuits in

the satellite are used primarily for services other than ranging. Consequently, it may

not be possible to schedule the time signal as desired. Hopefully, once the satellite

is fully operational, a schedule of ranging times would be made available to time transfer users. In any case, the time transfer should take place during the 200 kHz tone

transmission. The lower frequency tones could be utilized for time transfer, but their

use presents certain problems. These tones are for ambiguity resolution only, and it

doesn't matter to the ranging system that none have frequencies that are multiples of

1 Hz. To a user wishing to transfer time, however, the use of these tones will greatly

increase the system complexity and bookkeeping requirements.

The data available at the CDA, after the ranging sequence, consists of a series

of phase difference measurements (expressed as a time interval) , along with a time-of-day

label for each measurement. For the 200 kHz tone sequence, a measurement consists of

a one second average of the phase difference values between the transmitted and the

received tone. The average phase difference value is, of course, some interval between

(1)

3. ONE-WAY TIME TRANSFER METHOD

5

0 and 5 microseconds, the period of the 200 kHz tone. The complete 200 kHz tone sequence

consists of sixty such average phase values from each of the three range links: TARS I,

TARS II and the CDA.

After the ranging sequence is completed all the phase data thus accumulated is

transferred via telephone line, to the NOAA computer at Suitland, Maryland. This com- puter determines the range, the satellite coordinates, range-rate, and the time of

ranging. The time of ranging and satellite coordinates are, via telephone line,

returned to the CDA for transmission to the receive-only sites via the 468 MHz interrogation channel time code, (see fig. 2)

At the receive-only site the time recovery procedure would be as follows. Let

the subscripts c, s and r refer to the CDA, satellite and receive-only site, respectively.

The satellite coordinates X , Y , Z , are received from the 468 MHz transmission. The

s s s coordinates X^, Y^ , , have been obtained previously from maps or surveys. The distance between the satellite and the receiving site is simply

R, > = [(X -X )^ + (Y -Y )^ + (Z -Z )^\ (2) (s-r) s r s r s r J

1/2

(see fig . (3 ) . Also, the coordinates of the CDA, X^, Y^, Z^, must have been obtained by the

receiving site. The distance between the satellite and the CDA, R, _ > , is computed as

\ c X J was R , V . (s-r)

The propagation time, t, of an electromagnetic wave in a vacuum, over a distance

R, is t=R/C where C is the velocity of light. Then the total delay time from the

CDA through the satellite to the receiving site will be

t, , = t, , + t, , = ""l^ + ""i^ (3) (c-r) (c-s) (s-r) C C

where the two distances were previously computed.

The theoretical propagation delay must be corrected for the following:

1. The difference between the CDA master clock pulse and the phase of the

200 kHz range tone at the range tone generator, if any.

2. The time delay from the range tone generator to the CDA transmitting

antenna.

3. The propagation delay in the timing signal due to the troposphere

and the ionosphere.

4. The equipment time delay in the receiving system.

5. The delay through the satellite transponder.

The receive site operator compares his received phase data with a value computed

from summation of all the delays. Any difference can be attributed to receive site

clock error. This sequence presupposes that the receive-only site clock tick was within

1/2 period of the 200 kHz tone, (2.5 microseconds), of the CDA master clock. See figure

4 for simple diagrams of receiving site systems.

The ambiguity resolution at the receive-only site could take various forms depending upon the circumstances. One time honored method is to carry a portable

7
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cesium clock from the CDA to the receiving site. If the receiving site were vithin good

ground-wave distance of a LORAN-C station, the LORAN signal might be used.

4. GEODETIC CONSIDERATIONS FOR SATELLITE TRILATERATION TIMING

The problems associated with determining ground station locations have two implications for the trilateration timing scheme of this report. Because of the method of

solution for the satellite's position, the location of the TARS stations and of the

CDA must be known very accurately. The location of the time recovery sites need only

be known to an accuracy commensurate with the accuracy of timing desired at that particular site.

4.1. CDA AND TARS SITE LOCATION

The location of the CDA and TARS sites must be known to the highest possible

accuracy. Knowledge of the location of these sites to a large degree determines the

accuracy of the calculated satellite position and thus can greatly effect the overall

accuracy of the proposed timing system. Two methods capable of delivering the desired

accuracy are described below - triangulation surveys and doppler receiver surveys. Other

surveying methods also may need to be considered, however, these two appear to be the

most probable solutions to the location problems for the three trilateration stations.

Triangulation is a method of surveying which primarily involves the measurement

of the angles in a triangulation network as illustrated in figure 5. In this figure

A and B are known locations and AB is a known distance. The angles to be measured

are shown by the arrows and the positions to be determined are indicated by the dots

inside the small circles. Ultimately the location of position C is calculated.

Criteria exist to statistically determine the accuracy of a position determined

in such a manner. These criteria include the probable error in the measurement of each

angle, the number of angles measured and the size of those angles, the length of the

sides of the triangles, and the closure of the angles within each triangle and the

whole figure. On the basis of such criteria, a survey is assigned an order which denotes

the accuracy of that survey and of the locations determined by that survey. A first

order triangulation is the most rigorous and would be acceptable for determining the

location of the CDA and TARS sites.

A process of adjustment is used to make the overall accuracy of a given survey

as high as possible. This involves adjustment of the locations determined, such

that certain statistical properties are improved. The result, however, is that the

determined locations are mathematically dependent, thus increasing the probability of

systematic bias.

The coordinate system for a triangulation survey is determined by the reference

spheroid. This spheroid is chosen such that for a given survey area, the surface of the

spheroid closely approximates the surface of the earth in that area. The center of the

spheroid, the geometric center of the earth, the center of mass of the earth, are distinct points which coincide only by chance.

The accuracy of a first order triangulation of intercontinental extent is estimated to be one part in one million [10]

An alternative method of locating the positions of the CDA and TARS is based

upon data from a special radio receiver. This instrument measures the Doppler

11
KNOWN DATA:

LENGTH OF BASE LINE AB.

LATITUDE AND LONGITUDE OF POINTS A AND B.

AZIMUTH OF LINE AB.

MEASURED DATA:

ANGLES TO NEW CONTROL POINTS.

COMPUTED DATA:

LATITUDE AND LONGITUDE OF POINT C, AND OTHER

NEW POINTS.

LENGTH AND AZIMUTH OF LINE AC.

LENGTH AND AZIMUTH OF ALL OTHER LINES.

A SIMPLE TRIANGULATION NETWORK

Figure 5.

12

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shift of the signals transmitted from the TRANSIT [11] satellites. The orbits of these

satellites are determined by measurements made at special tracking sites. From the

satellite orbits and the Doppler measurements the position of the satellite receiver

may be calculated.

This method of position determination yields a location that is essentially independent of other survey points, the dependency is only upon a limited number of tracking

locations from which the satellite position was determined. This mathematical independence means that systematic errors will not accumulate, but that biases may exist.

The accuracy of this method is estimated to be one meter, one sigma, in each coordinate relative to the calculated orbit of the satellite.

The coordinate system for locations based on the satellite receiver has its

origin at the center of mass of the earth. This is because the satellite travels in an orbit around the center of mass of the earth and the position fix is relative to this

orbit. The accuracy of the receiver location relative to the center of mass is estimated to be 10 meters, one sigma, in each coordinate.

The timing implications of the inaccuracies of the satellite receiver and triangulation surveys will be delineated in section 5.3. of this report.

4.2. RECEIVING SITE LOCATION

The position location accuracy requirements for the receiving site are much less

stringent than for the CDA and TARS sites. The requirement is a location accuracy

which will support the timing needs of the site. Within the continental United States

this translates roughly to an allowable location error of a few hundredths of a degree

to support tenth microsecond timing. For instance, if the satellite were due south of

Washington, D. C. , a receiver located at Washington, D. C, would need to be located

accurately to three hundredths of a degree if tenth microsecond timing were required.

Any of the surveying methods commonly in use will provide this accuracy. In fact,

for most sites it is probably sufficient to determine the site location from a high

quality 15 minute or 7.5 minute quadrangle map.

In order to compute the delay between the satellite and the receiving site it is

necessary to ensure that the satellite's location is given in the same coordinate system as the site's location.

5. ADVERSE EFFECTS UPON THE TIME TRANSFER

5.1. PROPAGATION EFFECTS

The Troposphere

An electromagnetic wave or ray propagating through the troposphere will experience

a bending and a slowing due to the fact that the refractive index of the medium is other

than unity [12]. The index of refraction, n, is generally an inconvenient number with

which to work; therefore, the ref ractivity , N, is used for convenience in the various

equations. The ref ractivity is defined as (n-l) x 10 and is in the range of about

200 to 400 at the earth's surface. For the troposphere, N is dependent upon water

vapor content of the air, the absolute temperature, and the barometric pressure. A

widely used equation for N for the troposphere can be written as

N =ZZ^[p .181^] (4)

13
where T is the absolute temperature in K, P is the total pressure, and e is the partial

pressure of water vapor, both in millibars [13, 14].

Figure 6 shows the path length change for two extreme values of N as a function of

antenna elevation angle. Elevation angles of above 15° should be used unless some accurate measure of N is available on a real time basis. Eauation 4 or an equivalent

form could be used in these calculations.

Since the parameters that determine N are altitude dependent, N, for the troposphere

varies in some fashion from its sea level value to essentially 0 to about 60 km [13]

As radio rays pass through the atmosphere from a satellite to the earth, these rays are continuously bent due to the gradient of N. This bending again is most severe at low

antenna angles. Unless very large antennas with very small beam angles are used, the

bending can usually be ignored. The path length change and ray bending in the troposphere are essentially independent of frequencies below about 3 0 GHz.

The Ionosphere

The slowing and bending of radio waves passing through the ionosphere is due to the

electron content of that region. The change in group path length, A£, due to the ionosphere is given by

M = y* d£, (5)

where N is the electron density, b is a function of the charge and mass of an electron

^ 3 as well as the permeativity of free space and has a value of 1.63 x 10 (mks) [15]

The integration is along the ray path to the satellite. It might be noted that this

path length change, unlike that caused by the troposphere, is inversely proportional

to the square of the angular frequency, oj. Path length changes for 90° antenna angles

are of the order of 1 meter at 2 GHz. This will increase to about 25 meters at 0°

antenna elevation. These values are for typical total electron content and can vary

an order of magnitude in either direction. Figure 7 shows the path change with respect

to antenna angle, for one representative value for total electron content, N^^, expressed

as time.

There will be certain times of the year or day, and certain times of the sunspot

cycle where the correction to the group delay caused by the ionosphere must be adjusted.

The total electron content may be written as

j^l + (B^/A^) {R^-40)J X = 10^"^, A^ I 1 + (B,/AJ {R_-40)| X 10"", (6)

where A^ and are variables that depend upon the number of days elapsed from the

beginning of the year, and is the Zurich sunspot number [16]. When the total electron content has been determined from eq (6) , A£, the change in group path length, may

be computed using the following equation rather than eq (5)

A£ = fQN„f, f^l meters (7) (Q^T^L^t)

14
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compensating factor and f^ is a time-of-day compensating factor [16, 17],

5.2. SATELLITE MOTION

Any geostationary satellite is in constant, although minor, motion with respect

to any point on the earth's surface. Figure 8* shows the curves of satellite distance,

velocity and acceleration versus elapsed time from the ascending node with respect

to the CDA station at Wallops Island, Virginia. The maximum relative velocity is about

27 meters/second. Ranging and time transfer should take place within no more than

about 10 milliseconds of each other to keep the error due to satellite range rate to

an acceptable minimum. Actually, for the system under study the difference in time

between ranging and time transfer will be on the order of 1 millisecond.

5.3. SATELLITE LOCATION ERRORS

In order to evaluate the magnitude of timing errors caused by inaccurate knowledge

of ground station locations, it is first necessary to estimate the values of the errors

in ground position. The doppler receiver locations are known with an estimated one- sigma position error of less than 17 meters (root sum square of the 10 meter error in

each coordinate) . To calculate the errors for the first order trilateration , one m.ust know the distance between each of the trilateration stations. This may be estimated as 8000 km (Wallops-Hawaii, 8030 km; Wallops-Santiago, 7960 km; Wallops-Ascension, 8070 km;

Hawaii-Santiago, 9080 km; Santiago-Ascension, 6370 km) . The maximum error in this distance is between one part in 10^ and one part in 10^ as specified for intercontinental

surveys; i.e., between 8 and 80 meters [10]. The 17 meter error for the doppler receiver

is probably a workable figure for both types of surveying.

The assumed one sigma error of 17 meters will cause a proportionate error in the

position of the satellite which may be caluclated from the trilateration formula [18]

For the particular ground stations of GOES trilateration and the assumed satellite position, the maximum displacement of the satellite due to ground station position errors

is approximately 400 meters. It should be emphasized that whatever the error is in

satellite position, it will not vary significantly and the net effect will be to introduce a constant bias to the calculated delays.

The amount of bias introduced in this manner is, of course, a function of re- ceiving site as well as of the error in satellite position. In order to evaluate the

timing potential of this system within the specified area of interest, 6 6 sites were chosen to comprehensively cover the longitudinal and latitudinal extend of the continental United States. The range betv;een the satellite and each of these ground stations

V7as determined from two satellite locations: (1) the assumed "true" location and (2)

the "erroneous" satellite location caused by errors in knowledge of the location of the

three trilateration stations. The erroneous satellite location was the one of maximum.

(400 m) displacement in the satellite's position. The difference between these two

range calculations is shown in table 2 to the nearest m.eter. The 64 meter maximum

range error implies an absolute time synchronization of 0.2 microsecond one sigma,

within this limited geographic area. The average range error was approximately 40

*The initial position of the first GOES satellite (GOES-A) will probably be 100° west

longitude. The range values shown in figure 8 for 95 will change by about 1.5% due

to this 5 shift is satellite longitude.

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meters with a standard deviation of 13 meters. This implies a relative time synchronization within the continental United States of approximately 40 nanoseconds one

sigma, and an average offset from an arbitrary time scale of 120 nanoseconds. It should be once more emphasized that these figures represent outside maximum values for errors. The actual system may have significantly less positioning error and bias than

have been estimated here.

5.4. EQUIPMENT DELAYS AND INSTABILITIES

Another source of error in time transfer is equipment delay instability. For the

ranging system, great care has been taken to insure short-term stability of the TARS

stations. The signal through the TARS is down—converted to 70 MHz and filtered with

a bandwidth of 1 MHz. This filter is the only VHF narrowband device in the TARS and

consequently much attention has been given to its design. Long-term drift of IF stages,

amplifiers, and the like is in the order of 10% of one cycle at the particular operating

frequency. At 70 MHz, this drift would be in the order of 2 nanoseconds. At the higher

frequencies, the drift is even less.

The delay through the CDA equipment, both transmitting and receiving, is measured

prior to each range measurement; therefore, it is automatically taken care of in the

ranging program.

The delays in the receiving system for a time transfer station must be taken into

account with the same care as was applied to the TARS stations. Also the transmitting

delay at the CDA station must be monitored separately for time transfer. The delay

at the CDA that is measured for the range calculation does not separate the receive

delay from the transmit delay.

5,5. EFFECTS OF NOISE ON TIME TRANSFER

In section 2, the theoretical considerations of noise were presented for the

sidetone system,

AR = C/^27rf V2S/N ^

(8)

where f is the frequency of a particular sidetone. Note how this range error is effected

by the signal-to-noise power ratio (S/N) . If the range error due to noise is unacceptable, averaging techniques may be applied to reduce that error provided the noise is

uncorrelated. The range error may also be reduced by increasing S/N. This can be

done by narrowing the effective bandwidth using a suitable filter. Both techniques may

be applied using some caution. Equation 8 gives the range error, AR, for a single phase

measurement. AR will be reduced by a factor M, if M is the number of independent phase

measurements taken during the averaging process. The independence of the samples is of

prime importance when considering an averaging process. The number of independent

samples or measurements taken is related to the tone frequency, f , only if the

effective bandwidth is greater than 2f. If the bandwidth is less than 2f , the maximum

number of independent samples available is related to the bandwidth of the filter.

6. GENERAL LOGISTICS

The term general logistics is taken to include the details of specifying components

and spares, of operational and maintenance personnel, and of overall annual costs.

Environment, auxiliary power, and geographical location for the timing site will be

mentioned
6.1. SYSTEM ENVIRONMENTAL CONSIDERATIONS

Good phase stabilities in a timing system depends on a carefully controlled environment. In the GOES ranging system, the TARS station's electronics are housed in special foam insulated rooms whose heating and cooling systems maintain the ambient inside

temperature at 21°C (70°F) in an outside temperature range of -57°C (-70°F) to 49°C

{120°F). The phase shift stability through dividers, clocks, multipliers and other

components of a timing system is no less critically dependent upon temperature.

Probably the most environmentally sensitive component in the timing system is the

cesium standard. The phase stability of a standard of this type, or any other type,

is also dependent to some extent upon line voltage regulation. All time and frequency

equipment at the NBS high frequency radio stations is powered by regulated voltage

supplies. When on occasion, a regulator has failed, the phase stability of the affected

equipment v;as obviously decreased.

Figure 9 is a representative 24-hour sample of the phase intecomparisons of the

three cesium standards at station WWV in Ft. Collins, Colorado. It should not be

inferred from figure 9 that any three cesium standards at widely separated locations

could be placed in service and achieve this degree of frequency synchronization without

any expenditure of time and effort.

The absolute frequency output of a commercial cesium standard will be specified

plus or minus some value, e.g., 5 MHz ± 10 '^'^

. The adjustments that must be made to

bring the frequencies of a sample of units into synchronism are time consuming because

-12 -13

as one works with frequency differences of between 1 x 10 and 1 x 10 , nothing

much happens very fast. It might take days of measurements before a systematic frequency

offset is even detected. Therefore, the initial effort required at each receiving site

is considerably greater than the ongoing effort after frequency synchronization is

achieved.

Also, it is not possible to synchronize all units at a central location and then

deploy them. Some of the adjustments pertain to the particular site involved. This

includes geographical location, the building and room housing the standard, presence of

local mineral deposits, local electric lines, etc.

The clocks that are deployed at the receiving site rate special attention also.

An electronic clock is nothing more than a set of dividers that are arranged in a fashion such that, for N input cycles, pulses, or electrical events, the clock delivers one output pulse. These output pulses are then added, and the sum displayed or recorded.

It is important that no extraneous pulse is allowed to get into the system. Line

voltage transients, surges, static electricity, lightning discharge, and poorly

grounded circuits, are all potential problem areas. These problems can be minimized

by operating all on-line equipment from batteries. This of course, adds to the maintenance problem, but for the timing system, it is essential. The microwave and UHF

receiving systems are not as susceptible to this type of problem, but backup power

systems should still be provided.

6.2. RECEIVING SITE EQUIPMENT COMPLEMENT

The equipment at a typical basic timing site might consist of:

1. A cesium standard.

2. A UHF receiving system that is tuned to the GOES interrogation

channel, (see Appendix A)

21
FREQUENCY STANDARD INTERCOMPARISONS

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3. A decoder used in conjunction with the UHF system to provide

the ranging information.

4. The microwave system that receives the timing signals from the

satellite via the ranging channel, (see Appendix A)

5. The time interval counter that makes the actual time difference

measurement

6. A set of local clocks.

To this basic list must be added the battery backup systems, shielded enclosures (if

necessary) , rack space, and spares. If the site is to be fully automatic, a small com- puter and a digitally controlled phase shifter must be added. The hardware interfacing

and the software requirements make a fully automatic system exceedingly expensive, and

added cost should be weighted very carefully against the cost of a manual operation.

6.3. PERSONNEL REQUIREMENTS

In discussing personnel requirements, we refer to the experience gained by NBS in

operating the various NBS field sites as well as in maintaining the NBS frequency standard and time scale. The control room at radio systion VW\7 contains a triply redundant

set of timing systems. The time signal originates at a commercial cesium, beam standard and progresses through sets of dividers and a manual phase shifter to the format

generator. The format generator controls the broadcast format and generates the various

tones, codes and time ticks. Also in the control room is all the measuring equipment

required to monitor the three systems: phase detectors used for intercomparisons , chart

recorders, time interval counters, and the like.

During the period April 1970 through December 1972, forty entries were made in the

WWV maintenance logs. Many related to simple adjustment and some to replacements of

peripheral equipment. But most of the items dealt with erratic clock behavior. This

experience supports our contention that shielding the clock systems from outside disturbances is most important. We estimate the total time to make these adjustments,

change components, and make repairs, to be twenty manhours for about 1,000 days, or

a few minutes per day. In addition routine measurement adjustments consume about another

twenty minutes per day. Therefore, after initial shake down, modern timing equipment

is remarkably reliable, and normal maintenance personnel should be able to absorb the

added duties.

6.4. COMPONENT RELIABILITY AND MEAN-TIME-BEFORE-FAILURE

There will be some significant maintenance tasks from time to time that have not

yet been mentioned. Nickel-cadmium batteries should be cycled every few months and

should be replaced every one to two years. The beam tubes in the cesium standards

have a guaranteed life of three years and some models in laboratory environments exhibit mean time before failure (MTBF) of something in the order of 20,000 hours. The failure

rate for the satellite position decoder and the mini-computer will probably be the highest

of all components simply because of the complixity of these items. A conservative estimate of the MTBF for these units might be 5,000 to 10,000 hours. There is evidence that

suggests that continuous operation should be discouraged. For example, the TARS units

have a total system MTBF estimate of about 7,000 hours which is not very long until one considers that many of the TARS components are powered only during the ranging sequence.

Figure 10 lists the failure rates for the various TARS components.

23
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6.5. POWER REQUIREMENTS

The power requirement for the timing system and computer is about 900 watts. The

computer consumes about 300 watts, the most power consuming component in the system.

This power consumption, could dictate that the system operation include standby mode for

some items that are used only a few minutes per day.

6.6. SPARES COMPLEMENT

The spares complement for each receiving site should be considered on an individual

basis. For example, the overall spares complement of a Central Processing Unit would

be determined by the failure rate of that component in that particular computer. We

feel that MTBF information should be required from the manufacturer. If necessary,

the failure rates can be computed from basic information on the number of components

on each plug in card. The various cards in the UHF decoder, the time interval counter,

and other units should be treated the same way. Failure of main frames on items like

computer counters and decoders is somewhat more serious than card failures. This is

because the main frame contains the power supply which is more time consuming to fix,

takes more tools, and will take the entire machine out of service. One solution is to

require modular power supplies; another is to have an entire main frame spare. All that

would be required then would be to move the plug-in cards from one unit to another.

Probably the best solution would be to reqiure the total system designer to determine the failure rates of all the assemblies and sub-assemblies, main frames, and even components like power transformers. This should determine the kinds and numbers of spares

to be procured for the total timing systems. The deployment of these spares should be

determined on a geographical and logistical basis by the actual site locations.

7. SUMMARY OF TIMING BIASES

The Time and Frequency Division of NBS has completed a study to determine the

feasibility of one-way time transfer utilizing the trilateration ranging system of the

GOES satellites. The effects of noise in the channel, the propagation medium, the

site locations, and the ranging system equipment were examined in some detail. The errors associated with these aspects are summarized below.

The Ranging System

The final design plan for the GOES ranging system lists the following range errors

for the 200 kHz tone: total random errors, 2.2 nanoseconds (RMS); total systematic

errors, 3.8 nanoseconds (RMS). The random and systematic error sources are such that

they may be applied to an independent receiving site also. The range system systematic,

random, and peak errors will be found in tables 3, 4, 1, respectively.

The Troposphere

With an antenna elevation angle of zero degrees (worst case) , the predicted change

in radio path length to the satellite due to tropospheric effects is in the order of

100 meters with a possible variation of ± 50 meters due to extremes in refractivity

But with the GOES satellites stationed at 65°W and 135°W, the range error due to the

troposphere goes down considerably for all locations in the United States. For midAtlantic seaboard stations tracking the eastern satellite, the antenna elevation angles

will be in the order of 40 to 50 degrees. For antenna elevation angles greater than

15° above the horizon, the radio path length change is predicted to be in the order of

25
10 meters. A value of 15° antenna elevation angle has been used when considering the error contributed to the total by the troposphere.

The Ionosphere

The predicted path length change due to the effects of the ionosphere can be calculated fairly accurately if the electron density or total electron content is known

at the time and place of ranging. Lawrence et al. , [15] have indicated that the path

length change for zenith angles and a frequency of 2 GHz is in the order of one meter

with a possible variation of an order of magnitude in either direction. Tables produced

by Hughes Aircraft Co. [17] conducting studies on the atmospheric effects on the

NAVSAT ranging signals indicate a range (time) change in the order of 82 nanoseconds

with an RMS variation of 24 nanoseconds for an antenna elevation angle of zero degrees.

The errors along the mid-Atlantic seaboard (angle -45°) would be in the order of 33

nanoseconds with an RMS error of 9 nanoseconds. For stations operating at low antenna

angles, these corrections along with additional references like sunspot number, should

be applied.

Site Location

From our own studies of geodesy, and discussions with the geodetic squadron personnel of Warren AFB, Cheyenne, Wyoming, we have arrived at a value of 17 meters as a probable maximum satellite range error due to site location errors. Experiments have

indicated that satellite doppler receiver data would locate an earth point with reference to some satellite orbit with an error of 1 meter in each coordinate. It has been

estimated that this would allow determination of an earth point location with respect

to the mass center of the earth to about 10 meters in each coordinate [19] . In this

paper we have used a value of 30 meters as a safe estimate of the RMS range error to a receiving site caused by geodetic considerations.

Receiving Site Equipment

The random and systematic timing errors or biases, found in tables 3 and 4, that

would apply to a receiving site have been entered into the bias summation. The re- ceiving equipment as with the CDA and TARS equipments contributes to some fixed time delay

These delays must be measured and removed from the time transfer computations.

8. CONCLUSION

Table 5 lists the various sources of timing biases, their uncertainties, and the

summation. The uncertainties of the various contributing factors are with respect to

the representative antenna elevation angles that were previously mentioned. The rootsum-square (RSS) of all bias uncertainties indicates that a time synchronization between

a receiving site and the CDA master clock could be achieved with an accuracy of ~ 0.3

microseconds (3 sigma)

.

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9. REFERENCES

[I] . Wooldridge, J. J., "Final Design Plan for the Trilateration Range and Range

Rate System," Hughes Aircraft Co., NASA Contract NAS 5-21629 (September 1971).

[2]. Griniger, G., "Data Collection Platform Radio Set Design Plan," NOAA Contract

1-3 5287, The Magnavox Co., Ft. Wayne, Indiana (December 1971).

[3]. Steele, J. McA. , Markowitz, W. , and Lidback, C. A., "Telstar Time Synchronization," IEEE Trans, on Inst, and Meas . , IM-13 , pp. 164-170 (1964).

[4]. Markowitz, W. , Lidback, C. A., Uydea, H. , and Muramatsu, K. , "Clock Synchronization via Relay II Satellite," IEEE Trans, on Inst, and Meas., IM-15 , pp. 177-184 (1966) .

[5]. Jespersen, J. L. , Kamas, G. , Gatterer, L. E., and MacDoran, P. F., "Satellite

VHF Transponder Time Synchronization," Proc. IEEE, 56_, pp. 1202-1206 (1968).

[6]. Grubb, R. N. , and Gerrish, S. D., "Results of Preliminary Tests of a Time

Dissemination System Using the VHF Transponder on the ATS-1 Satellite,"

ESSA Research Lab., Tech. Memo ERL-TM-SDL-9 (March 1968).

[7], Gatterer, L. E., Bottone, P. W. , and Morgan, A. H. , "Worldwide Clock Synchronization Using a Synchronous Satellite," IEEE Trans, on Inst, and Meas., IM-17 , pp. 372-378 (1968)

[8]. Habib, E. J., Kronmiller Jr., G. C, Engels, P. D. , and Franks Jr., H. J.,

"Development of a Range and Range Rate Spacecraft Tracking System," GSFC,

NASA Technical Note D-2093 (June 1964)

[9]. Skolnik, M. I., Introduction to Radar Systems , New York: McGraw-Hill (1962).

p. 648.

[10]. Gossett, F. R., "Manual of Geodetic Triangulation , " U. S. Dept. of Commerce,

Coast and Geodetic Survey Spec. Publ. 247 (1965).

[II] . Kershner, R. B, "Transit Program Results," Astronautics, 6^, No. 5, pp. 30-38

(May 1961)

[12]. Weisbrod, S., and Anderson, L. J., "Simple Methods for Computing Tropospheric

and Ionospheric Refractive Effects on Radio Waves," Proc. IRE, 47_, No. 10,

pp. 1770-1777 (1959).

[13]. Bean, B. R. , and Dutton, E. J., Radio Meteorology , NBS Monograph No. 92

(March 1966) , p. 435.

[14]. Winkler, G. M. R., "Path Delay, Its Variations, and Some Implications for the

Field Use of Precise Frequency Standards," Proc. IEEE, 60_, No. 5, pp. 522-529

(1972)

[15]. Lawrence, R. S., Little, C. A., and Chivers, J. J. A., "A Survey of Ionospheric

Effects Upon Earth-Space Radio Propagation," Proc. IEEE, 5_2/ pp. 4-27 (1964).

[16]. Escobol, P. R. , "Trilateration Measurement and Position prediction,"

(preliminary draft), Hughes Aircraft Co., NASA Contract NAS 5-21554 (June 1970)

[17]. Mayfield, W. W. , Hughes Aircraft Co. (private communication).

[18]. Escobol, P. R. , Methods of Orbit Determination , New York: John Wiley and Sons,

Inc. , (1965) , p. 463.

[19]. Harris, H. G., 1st Geodetic Survey Squadron, Warren AFB (private communication)

30
APPENDIX A

SMS/GOES FREQUENCY PLAN

Receiving:

401.7 MHz to 401.85 MHz

50 self -timed 3 kHz channels for Data Collection Platform Radio sets

(DCPRS) reports.

401.85 MHz to 402.0 MHz

100 interrogated 1.5 kHz channels for DCPRS reports.

402.0 MHz to 402.1 MHz

Spare channels

2025.0 MHz to 2033.2 MHz

Ranging or stretched Visible or Infrared Spin-Scan Radiometer (VISSR)

also WEFAX.

2034.2 MHz ± 30 kHz

Command from CDA

2034.9 MHz + 5 kHz

DCPRS interrogation from CDA, also special time code in time sharing mode.

Transmitting:

468.825 MHz ± 5 kHz

DCPRS interrogation and special time code in time sharing mode.

1687.6 MHz to 1695.6 MHz

Wideband VISSR

1685.0 MHz to 1693.2 MHz

Ranging or stretched VISSR

1690.1 MHz ± 25 kHz

WEFAX

1694. 0 MHz ± 100 kHz

Telemetry beacon

1694.5 MHz ± 200 kHz

DCPRS reports

Note: Wideband VISSR, stretched VISSR, ranging and WEFAX are time-shared function.

31
Appendix A continued.

LINK FREQUENCY PLAN

SMS/GOES

RANGING SYSTEM

CDA Link:

Up link

Down link

2026.0 MHz

1684.0 MHz

CDA to SMS/GOES

SMS/GOES to CDA

TARS I Link:

Up link

Down link

Up link

Down link

2026.0 MHz

1684.0 MHz

2030.2 MHz

1688.2 MHz

CDA to SMS/GOES

SMS/GOES to TARS I TARS I to SMS/GOES

SMS/GOES to CDA

TARS II Link:

Up link

Down link

Up link

Down link

2026.0 MHz

1684.0 MHz

2032.2 MHz

1690.2 MHz

CDA to SMS/GOES

SMS/GOES to TARS II TARS II to SMS/GOES

SMS/GOES to CDA

Receive Site Link:

Up link

Down link

2026.0 MHz

1684.0 MHz

CDA to SMS/GOES

SMS/GOES to Site

#

32
APPENDIX B POWER BUDGET:

LINK 1 CDA TO SMS/GOES

Transmitter Power (dBm) 48.00

Transmitter Antenna Gain (dB) 48.00

Transmitter Line Loss (dB) -1.60

Transmitter Off Beam Center Loss (dB) -1.00

(Angle in Degrees) 0.25

Free Space Loss (dB) 190.20

Polarization Loss -0.20

Receiver Antenna Gain (dB) 13. 40

Receiver Off Beam Center Loss (dB) -1.40

(Angle in Degrees) 7.00

Receiver Line Loss (dB) -4.50

Receiver Input Power Level (dBm) 89 .50

System Noise Temperature (dB-K) 32.12

Boltzman's Constant (dBm/Hz-K) -198.60

Receiver Input No (dBm/Hz) -166.48

Receiver Input C/No (dB-Hz) 76.98

Overall C/No (dB-Hz) 76.98

Receiver Bandwidth (MHz

LJ. . UU Receiver Bandwidth (dB) 70.79

Receiver Output C/N (dB) 6 . 19

Limiter Improvement (dB) 2.31

Transmitter Output C/N (dB) 8 . 50

LINK 2 SMS/GOES TO CDA

Transmitter Power (dBm) 43.00

Transponder Power Sharing Loss (dB) -0.57

Transmitter Line Loss (dB) -3.40

Transmitter Antenna Gain (dB) 19 .10

Transmitter Off Beam Center Loss (dB) -1.60

(Angle in Degrees) 7.00

Free Space Loss (dB) -188.60

Polarization Loss (dB) -0.2

Receiver Antenna Gain (dB) 48.00

Receiver Off Beam Center Loss (dB) -0.70

(Angle in Degrees) 0 .25

Receiver Line Loss (dB) -0.40

Receiver Input Power Level (dBm) -85.37

System Noise Temperature (dB-K) 20.00

Boltzman's Constant (dB/Hz-K) -198.60

Receiver Input No (dBm/Hz-) -178.60

Receiver Input C/No (dB-Hz) 93.23

Overall C/No (dB-Hz) 79.12

Modulation Loss (dB) -2.20

S/N Output (600 Hz BW) (dB) 49.10

33
Appendix B continued.

Two Way Range Timing Error (S)

(For One Second Averaging Time)

LINK 3 SMS/GOES TO TYPICAL RECEIVING SITE

Transmitter Power (dBm)

Transmitter Power Sharing Loss (dB)

Transmitter Line Loss (dB)

Transmitter Antenna Gain (dB)

Transmitter Off Beam Center Loss (dB)

(Angle in Degrees)

Free Space Loss (dB)

Polarization Loss (dB)

Receiver Antenna Gain (dB)

Receiver Off Beam Center Loss (dB)

(Angle in Degrees)

Receiver Line Loss (dB)

Receiver Input Power Level (dBm)

System Noise Temperature (dB-K)

Boltzman's Constant (dBm/K-Hz)

Receiver Input No (dBm/Hz)

Receiver Input C/No (dB-Hz)

Overall C/No (dB-Hz)

Receiver Bandwidth (MHz)

Receiver Bandwidth (dB)

Receiver Output C/N (dB)

Modulation Loss (dB)

S/N Output ' (dB)

Two Way Range Timing Error (S)

(For One Second Averaging Time)

,06 X 10

-9

43.00

-0 .57 -3.40

19 .10

-2.5

9 .00

-188 .60

-0.20

30.40

-2. 40

2.20

-0.70

105.87

28. 50

-198 .60

170.10

64.23

64.10

1.00

60.00

4.10

-2.20

61.90

0.72 X

10"

34
FORM NBS-114A (1-71) U.S. DEPT. OF COMM. K PUBLICATION OR REPORT NO. 2. Gov't Accession

BIBLIOGRAPHIC DATA NBSIR 73-348

5n c c T

3. Recipient's Accession No.

4. TITLE AND SUBTITLE

AN ENGINEERING FEASIBILITY STUDY FOR ONE-WAY TIME

TRANSFER USING Trirj GUbb bAiJiijlji iii' KAlNtaXiNHa sioisnAn

5. Publication Date December 1973

6. Performing Organization Code

7. AUTHOR(S)

J. B. Milton and W. F. Hamilton

8. Performing Organization

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS , Boulder Laboratories

DEPARTMENT OF COMMERCE

Boulder. Colorado 80302

10. Project/Task/Work Unit No. 2730410

11. Contract/Grant No. DOT/FA73WAI-324

12. Sponsoring Organization Name and Address Federal Aviation Administration

800 Independence Avenue SW

Washington, D. C. 20591

13. Type of Report & Period Covered

Final IR

14. Sponsoring Agency Code

15. 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.) The Time and Frequency Division of the National Bureau of Standards

has conducted an engineering study to determine the feasibility of using

the GOES satellite ranging system for precise (0.1 microsecond, one-sigma)

time transfer to a receiving-only timing site. The GOES satellite ranging system, termed a tri later ation system, will accurately locate this

satellite within some coordinate structure. The sources of time transfeji:

errors have been studied in some detail. These errors can be caused by

satellite location errors, ground station location errors, unknown delay|s

caused by the troposphere, the ionosphere, and the various equipments.

Simplified designs for an automatic and a manually operated timing site

are presented. Some technical problems found in the associated equipment

are discussed. The study indicates that a secondary, or slave site, clojck

could be synchronized to within 0.1 microsecond, one-sigma, of some master

clock utilizing a one-way, or receiving-only system.

17. KEY WORDS (Alphabetical order, separated by semicolons)

Clock synchronization; one-way time transfer; satellite timing;

synchronous satellite.

18. AVAILABILITY STATEMENT

fxl UNLIMITED.

1 1 FOR OFFICIAL DISTRIBUTION. DO NOT RELEASE

TO NTIS.

19. SECURITY CLASS

(THIS REPORT)

UNCL ASSIFIED

21. NO. OF PAGES

20. SECURITY CLASS

(THIS PAGE)

UNCLASSIFIED

22. Price

USCOMM-DC 66244-P71

USCOMM ' ERL
/