Stephen M. Lichten

Stochastic estimation of tropospheric path delays in global positioning system geodetic measurements

Water vapor radiometric (WVR) and surface meteorological (SM) measurements taken during three Global Positioning System (GPS) geodetic experiments are used to calculate process noise levels for random walk and first-order Gauss-Markov temporal models of tropospheric path delays. Entire wet and combined wet and dry zenith delays at each network site then are estimated simultaneously with the geodetic parameters without prior calibration. The path delays and corresponding baseline estimates are compared to those obtained with calibrated data and stochastic residual delays. In this manner, the marginal utility of a priori tropospheric calibration is assessed given the ability to estimate the path delays directly using only theGPS data. Estimation of total zenith path delays with appropriate random walk or Gauss-Markov models yields baseline repeatabilities of a few parts in 108. This level of geodetic precision, and accuracy as suggested by analyses on collocated baselines estimated independently by very long baseline interferometry, is comparable to or better than that obtained after path delay calibration usingWVR and/orSM measurements. Results suggest thatGPS data alone have sufficient strength to resolve centimeter-level zenith path delay fluctuations over periods of a few minutes.

AN INNOVATIVE DEEP SPACE APPLICATION OF GPS TECHNOLOGY FOR FORMATION FLYING SPACECRAFT

This paper describes the Autonomous Formation Flying Sensor (AFF) concept for extremely precise autonomous relative position and attitude determination for formations of satellites or spacecraft where real-time or near-real-time knowledge of relative position and attitude are required. Its technology is built around commercially available receivers widely used in Global Positioning System (GPS) ground applications and recently flight-tested in space. This sensor, however, does not actually require any observations of GPS satellite data for precise spacecraft-to-spacecraft position and attitude measurements. Thus, the AFF could be used in deep space or in Earth orbit, with or without GPS satellite data. Studies indicated that the AFF can provide a relative range measurement accuracy of 1 cm, along with a relative attitude measurement accuracy of 1 arcminute over formation distances of 1 m to 1300 km.

Precise determination of Earth's center of mass using measurements from the global positioning system

Global Positioning System (GPS) data from a worldwide geodetic experiment were collected during a 3-week period early in 1991. Geocentric station coordinates were estimated using the GPS data, thus defining a dynamically determined reference frame origin which should coincide with the earth center of mass, or geocenter. The 3-week GPS average geocenter estimates agree to 7-13 cm with geocenter estimates determined from satellite laser ranging, a well-established technique. The RMS of daily GPS geocenter estimates were 4 cm for x and y, and 30 cm for z.

Sub‐daily resolution of Earth rotation variations wtth global positioning system measurements

Data from a worldwide Global Positioning System (GPS) tracking experiment have been used to determine variations in Earth rotation (UT1-UTC) over a time period of three weeks. Kalman filtering and smoothing enabled changes in UT1-UTC over intervals of 2 to 24 hrs to be detected with the GPS data. Internal consistency checks and comparisons with other solutions from very long baseline interferometry (VLBI) and satellite laser ranging (SLR) indicate that the GPS UT1-UTC estimates are accurate to about 2 cm. Comparison of GPS-estimated variations in UT1-UTC with 2-hr time resolution over 4 days with predicted variations computed from diurnal and semi-diurnal oceanic tidal contributions strongly suggests that the observed periodic sub-daily variations ∼0.1 msec (5 cm) are largely of tidal origin.

Demonstration of sub-meter GPS orbit determination and 1.5 parts in 108 three-dimensional baseline accuracy

Differential tracking of theGPS satellites in high-earth orbit provides a powerful relative positioning capability, even when a relatively small continental U.S. fiducial tracking network is used with less than one-third of the fullGPS constellation. To demonstrate this capability, we have determined baselines of up to2000 km in North America by estimating high-accuracyGPS orbits and ground receiver positions simultaneously. The2000 km baselines agree with very long baseline interferometry(VLBI) solutions at the level of1.5 parts in108 and showrms daily repeatability of0.3–2 parts in108. The orbits determined for the most thoroughly trackedGPS satellites are accurate to better than1 m. GPS orbit accuracy was assessed from orbit predictions, comparisons with independent data sets, and the accuracy of the continental baselines determined along with the orbits. The bestGPS orbit strategies included data arcs of at least one week, process noise models for tropospheric fluctuations, estimation ofGPS solar pressure coefficients, and combined processing ofGPS carrier phase and pseudorange data. For data arcs of two weeks, constrained process noise models forGPS dynamic parameters significantly improved the solutions.

Comparison of Kalman filter estimates of zenith atmospheric path delays using the Global Positioning System and very long baseline interferometry

Kalman filter estimates of zenith nondispersive atmospheric path delays at Westford, Massachusetts, Fort Davis, Texas, and Mojave, California, were obtained from independent analyses of data collected during January and February 1988 using the Global Positioning System (GPS) and very long baseline interferometry (VLBI). The apparent accuracy of the path delays is inferred by examining the estimates and covariances from both sets of data. The ability of the geodetic data to resolve zenith path delay fluctuations is determined by comparing further the GPS Kalman filter estimates with corresponding wet path delays derived from water vapor radiometric (WVR) data available at Mojave over two 8-hour data spans within the comparison period. GPS and VLBI zenith path delay estimates agree well within one standard deviation formal uncertainties (from 10–20 mm for GPS and 3–15 mm for VLBI) in four out of the five possible comparisons, with maximum differences of 5 and 21 mm over 8- to 12-hour data spans. For one comparison, the maximum difference between GPS and VLBI is 50 ± 20 mm and clearly shows an unexplained systematic difference which is probably related to poor elevation angle coverage in the VLBI data at the time, GPS fiducial network sensitivity, and poor azimuthal coverage. The root-mean-square differences between GPS estimates of the total path delays and WVR measurements added to the hydrostatic delay component determined from surface barometric pressure data are between 9 and 15 mm, however, with biases of 10–15 mm.

High Accuracy Global Positioning System Orbit Determination: Progress and Prospects

The Global Positioning System (GPS), by the mid-1990s, will include 21 navigation satellites launched by the U.S. Air Force. About one-third of the constellation is presently in orbit; however, most of these satellites are of a developmental Block I design and eventually will be replaced by the operational Block II satellites. The GPS constellation is designed to evenly distribute the satellites with circular orbits in three different planes at an altitude of about 20,000 km so that ground users can track typically six to eight GPS at any time from most locations.

Formation of a GPS-linked global ensemble of hydrogen masers, and comparison to JPL's linear ion trap

This paper describes the use of precision GPS time transfer to form an ensemble of hydrogen maser clocks. The performance of this ensemble, including the GPS time transfer system, was measured relative to a stable linear ion trap standard. Very high-precision techniques have recently been developed in support of efforts to achieve cm-level accuracy in the use of GPS for geodesy. A global network of tracking stations, equipped with precision dual-frequency GPS receivers, has been in operation for several years. The post-processing software developed for cm-level geodesy has been used to demonstrate sub-nanosecond (ns) time synchronization, as reported in the PTTI conferences of 1991 and 1993. The global network of GPS stations includes 25 which are run coherently from hydrogen masers. This paper explores the use of high precision GPS estimation techniques to transfer time globally in conjunction with ensembles of atomic time standards, which in principle should be superior in performance to a single clock. Together, these techniques have potential to provide a means of directly assessing performance of individual clocks for which statistical information about stability is desired.

A demonstration of sub meter GPS orbit determination and high precision user positioning

It was demonstrated that the submeter GPS (Global Positioning System) orbits can be determined using multiday arc solutions with the current GPS constellation subset visible for about 8 h each day from North America. Submeter orbit accuracy was shown through orbit repeatability and orbit prediction. North American baselines of 1000-2000 km length can be estimated simultaneously with the GPS orbits to an accuracy of better than 1.5 parts in 10/sup 8/ (3 cm over 2000 km distance) with a daily precision of two parts in 10/sup 8/ or better. The most reliable baseline solutions are obtained using the same type of receivers and antennas at each end of the baseline. Baselines greater than 1000 km distance from Florida to sites in the Caribbean region have also been determined with daily precision of 1-4 parts in 10/sup 8/. The Caribbean sites are located well outside the fiducial tracking network and the region of optimal GPS common visibility. Thus, these results further demonstrate the robustness of the multiday arc GPS orbit solutions.<<ETX>>High-accuracy orbits have been determined for satellites of the Global Positioning System (GPS), with submeter orbit accuracy demonstrated for two well-tracked satellites. Baselines of up to 2000 km in North America determined with the GPS orbits shows daily repeatability of 0.3-2 parts in 10/sup 8/ and agree with very long baseline interferometry (VLBI) solutions at the level of 1.5 parts in 10/sup 8/. Tests used to assess orbit accuracy include orbit repeatability from independent data sets, orbit prediction, ground baseline determination, and formal errors. One satellite tracked for eight hours each day shows RMS errors below 1 m even when predicted more than three days outside of a 1-week data arc. These results demonstrate the powerful relative positioning capability available from differential GPS tracking. Baselines have also been estimated between Florida and sites in the Caribbean region over 1000 km away, with daily repeatability of 1-4 parts in 10/sup 8/. The best orbit estimation strategies included data arcs of 1-2 weeks, process noise models for tropospheric fluctuations, combined processing of GPS carrier phase and pseudorange data, and estimation of GPS solar pressure coefficients.<<ETX>>

Sub-meter GPS orbit determination and high precision user positioning - A demonstration

High-accuracy orbit solutions have been obtained for GPS satellites, and submeter orbit accuracy is demonstrated for two well-tracked satellites. Orbit accuracy was tested based upon orbit repeatability from independent data sets, orbit prediction, ground baseline determination, and formal errors. Baselines of up to 2000 km in North America found with the GPS orbits show a daily repeatability of 0.3-1.5 parts in 10 to the 8th, and are found to agree well with VLBI solutions at the level of 0.3-3 parts in 10 to the 8th. Baselines were also determined between Florida and sites in the Caribbean region over 1000 km away, with a daily repeatability of 1-4 parts in 10 to the 8th.