GPS used for satellite orbit determination

Experiments were performed on both low-earth orbiting satellites (TOPEX) and geosynchronous satellites (TDRS) to evaluate the ability to determine these spacecraft orbits using GPS positioning. For TDRS, measurements were recorded in a region around White Sands, NM, to test the feasibility of replacing traditional TDRS orbit determination with a small ConUS-only network.

For more details, see Using GPS for Satellite Tracking

Early use of GPS for accurate ground location and coupled orbit determination

This was some of the first work done using GPS for positioning, and was compared to previous VLBI geodetic station information. To achieve these results, a complete software system for atmospheric (troposphere, ionosphere) calibration and Kalman filter state estimation of ground, orbit, and oscillator states was required. More details are presented here: GPS-based geodesy and orbit determination

Simple GPS simulations: elevation cutoff, and tree blockage

Often, custom tools were required for sensor fusion evaluation and visualization. Here are some outputs that show satellite motion and GDoP...

<< click to animate

... and position error growth and number of satellites visible are shown while the sky is partially blocked by nearby terrestrial objects...

<< click to animate

Static position averaging with GPS

For satellite communication systems utilizing multi-user shared channels, such as with TDMA/CDMA systems, time synchronization can be achieved by accurate ground position and time knowledge derived from GPS. For the Teledesic system, the example below shows the required number of hours of user ground station position averaging with selective availability on (note: no longer an issue) to achieve the ground position accuracy needed for unlikely time desync collision of the first uplink transmission.

Satellite CDMA time sync and geolocation

TRW's Odyssey satellite communication system design used CDMA for it's multi-user access, which required complex time synchronization of user handheld devices to realize synchronous CDMA capacity advantages. We designed the handset time synchronization measurement/loops as well as how time was handled across the entire satellite system (needed since satellite handoffs require sync even during re-acquisition).

A by-product of time synchronization was the ability to measure range and range-rate from each communicating satellite, allowing geolocation to 1km or better on the ground. Even with one satellite, range-rate could be used to roughly geolocate. The principle is shown in the next figure

Note that knowing in which beam communication is happening breaks any ambiguity that would otherwise occur with the intersection of 2 circles.

Finally, the entire geolocation process, with ancillary time sync info, is shown in the next diagram