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Global Positioning System for Accurate Time and Frequency Transfer and for Cost-Effective Civilian Navigation

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Global Positioning System for Accurate Time and Frequency Transfer and for Cost-Effective Civilian Navigation GLOBAL POSITIONING SYSTEM FOR ACCURATE TIME AND FREQUENCY TRANSFER AND FOR COST-EFFECTIVE CIVILIAN NAVIGATION James L. Jespersen, Marc Weiss, Dick D. Davis, and David W. Allan National Bureau of Standards, Boulder, Colorado ABSTRACT ephemerides are available for GPS and that GPS time is based on atomic clocks. It is assumed This paper described some alternative appli- cations of Global Positioning System (GPS) includ- that the civilian or clear-access code (C/A) is ing a method for very accurate time transfer and available during the observation periods. for civilian position location much less expen- sively than the designed Department of Defense method. The first part of the paper discusses FOUR METHODS FOR ACCURATE TIME TRANSFER several time transfer techniques with emphasis on what we call the "common-view" approach, and the second part considers the system for position There are four interesting methods to employ location. Both applications depend on the fact that accurate ephemerides are available for GPS GPS for accurate time transfer or for accurate and that GPS time is based on atomic clocks. It time and frequency comparisons (see Fig. 1). is assumed that the civilian or clear-access code (C/A) is available during the observation periods. First. Clock A and a GPS receiver are used to The accuracy of time transfer over a few thousand deduce from a GPS satellite's ephemeris, from km using the "common-view" approach is estimated at about 10 ns, and the accuracy of position clock A's location. and from received GPS time location at about 100 m. decoded from the same satellite, the time differ- ence (Clock A - GPS time) (1). This method is the simplest and least accurate (estimated to be INTRDDUCTIPN better than about 100 ns with respect to GPS time) (21, but has global coverage, is in the receive- This paper describes some altm-native appli- only mode, requires no other data, yields receiver cations of GPS including a mathoa for very accurate prices that could be competitive on a mass produc- time transfer and for civilian position location tion basis, and could service an unlimited audi- which could, In principle, be doni? much less ence. Also, GPS time will be referred to UTC(USN0) expensively less expensively than the designed DO0 and will be know with respect to UTC(BIH), method. The f5rst part of the paper discusses UTC(NBS), and other major timing centers. several time transfer techniques with emphasis on Second, Clock A and Clock B at different what we call the "common-view" approach, and the locations anywhere on earth can be compared by second part considers the system for position making successive observations of the same GPS location. The primary difference between the two satellite clock, at least one of which will appear appllcations is that the time transfer technique above their horizons with delayed view times of requires the use? to know his location, while the less than 12 hours. This is analogous to the position location technique allows the user to clock flyover mode reported by Besson (3) and determine his position (and clock offset from GPS J. others. The time prediction error for the satel- system time, if he desires) by making observations lite cesium clocks to be used in the GPS satel- of signals from several satellites; the usual lites will be about 5 ns over hours. Since the scheme for determining position from signals 12 same GPS satellite clock will be vtewed by both A emanating from several known locations. Both and 8, biases in the satellite ephemeris may tend applications depend on the fact that accurate 1 to cancel depending upon geometry, etc. Accura- It appears that as GPS becomes fully devel- cies of from 10 ns to 50 ns are anticfpated. This oped, GPS time may become operational world time. method requires communication of the data betwen Methods 1, 2, or 3 above would yield significant A and B, and hence the logistics may limit the improvements in national and international time customers. comparisons. If commerclal vendors take advantage Third (see figures 1 & 2). two users with of some of these methods, receiver costs could be- Clock A and Clock 8 at different locations, but in made reasonable. The same basic receiver could be simultaneous common-view of a single GPS satellite used in methods 1, 2, or 3; the main difference clock. can take advantage of common mode cancella- would be in the software supoort, modems, and tion of ephemeris errors in determining the time local clocks. This method has the most attractive difference (tA- tB), The satellite clock error accuracy/cost ratio and is being pursued by NBS. contributes nothing. Since the GPS satellites are The theoretical advantages and disadvantages are at about 4.2 earth radii (12 hour orbits). for reoorted herein. continental distances between A and B (z 3000 km) the angle L (A-Satellite-B) will be 2 loo, and the SYSTEM ERROR ANALYSIS effects of satellite ephemeris errors will be reduced by a factor of more than 10 over the first Errors Resulting from Satellite Ephemeris Location method. Using a fairly straightforward receiver Uncertainty system, an accuracy of about 10 ns in measuring the time difference (tA- tB) appears probable. The time transfer error is dependent upon the This again requires data communication between A ephemeris or position error of a satellite. and 8. With improved ephemerides and propagation Common-view tine transfer yields a great reduction delay characterization, the potential accuracy in the effect oc thesa errors between two stations, limit for this method appears to be about 1 ns. A and 0, as compared to transfer of Lime from the The receiver should be relatively inexpensive. and satellite to the ground. Common-vie* time transfer given the reasonable costs of data modems and the is accomplished as follows: potential accuracies achievable via this method. Stations A and 9 receibe a commoii signal it makes it very attractive and cost effective for from a satellite and each records the national, and in some instances, for international local time of arrival, t and tBrcspec- A time comparisons. tively. Fourth, a method being developed for Geodesy From a knohledge of station and satel- by JPL (Jet Propulsion Laboratory) (4) has baseline lite position in a common coordinate accuracy goals of about 2 cm over baselines of the system, the range between the satellite order of 100 km. This method can be inverted to and each of the stations is computed, rA do time comparisons with subnanosecond accuracies. and rB respectively. The two clocks A and B separated by abeut 100 kn The time of transmission of the cammon have two broadband receivers with tunable tracking signal according to each station, A and antennae such that sequentially. 4 satellites can 8, is computed by subtracting from the be tracked concurrently at A and 8. The data are times of arrival, the times of propaga- cross-correlated after the fact, the same as fn tion from the satellite to each of the long basel ine interferometry, to determine location respective stations, i.e. , the time to and time difference (tA- ta). The data density travel the distances, rA and rB, are T~ fs higher than with the other approaches and the and T~ (the range delays) and are given baselines are relatively short, but the accuracy by T~ = rA/c and T~ = r B /c where c is is excellent. the speed of light. This speed is subject to other corrections as are treated later. 2 4) Finally, the time difference, rAB, of that location. The current values of ephemeris station A's clock minus station 6's error for the GP5 satellites are estimated at clock at the times the signals arrived about 10 meters in-track, i.e., in the satellite's is: direction of motion; 7 meters cross-track, and 2 meters radial (5). This corresponds to 41.23 ns rms error (square root of the sum of squaredc). The projected values for 1985 are 7 m in-track, 3 in cross-track, and 0.6 m radial, corresponding If the ephemeris of the satellite is off, the to 25.46 ns rms error (5). computed ranges from the stations to the satellite Notice that the rms errors make an elongated will be off an amount dependent on the way the ellipsoid and are dependent on satellite direction. ephemeris is drong and the geometrical configura- Thus, to compute the range errors to a given pair tion of the satellite-station systems. The advan- of stations for a given satellite location, one tage of common-view time transfer is that the needs to know the satellite direction at that computed bias is affected not by range errors to location. The satellite moves in a fixed plane in individual stations, but by the difference of the space with the earth rotating under it. two range errors. Thus, much of the ephemeris The program which computed the figures used error cancels out. To see how this works in an orbital plane making an angle of 63' with the detail, suppose the ephemeris data implies range ecliptic with the satellite moving west to east in delays of 'A and rb, but the actual position of the plane. As an approximation, the orbit was the satellite, if known correctly, would give assumed circular at 4.2 earth radii (12 hour range delays of rA = TA - AT^ and rB= r; - ArB. period). At a given latitude, the satellite direc- Then the error in time transfer would be ArAB = tion in degrees east of north is determined by the ArB - OrA, where rAB= rAB -AxAB is the true time orbital plane and whether the direction is north- difference (clock A - clock B) and where riB is erly or southerly.
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