Relative Positioning Using Pseudolites in the Navigation Systems Testing Laboratory at NASA's Johnson Space Center

Status: Completed project, January 2000 -- December 2003
Type: GPS Receiver Algorithms Development

This research investigates the use of GPS receivers and pseudolites as tools for relative positioning in the Navigation Systems & Technology Laboratory (NSTL) at NASA's Johnson Space Center in Houston, Texas.

Pseudolites (false-GPS satellites) can be used to produce pseudorange and carrier phase measurements, which can be received and transformed into a position estimate in the software of a GPS receiver.

An application such as this could be used to mitigate the effects of signal loss in the receiver of a spacecraft that is in the proximity of a larger vehicle, such as the International Space Station. The smaller spacecraft could then rely solely on pseudolite measurements from the larger vehicle for navigation. Other applications might include indoor positioning and machinery automation. In order to apply this technology in space, it must be fully understood.

This investigation explores what has been learned about pseudolites and their receivers in the NSTL. The environment of the laboratory and its effect on the measurements is studied. Further, deterministic solutions that use pseudorange, carrier phase and carrier smoothed code measurements and Kalman filters that use carrier phase and carrier smoothed code are investigated.

It is shown that in the NSTL the errors on the pseudorange measurements limit any practical system to a carrier phase only solution. However, if multipath is overcome and the noise on the pseudorange is within normal levels, a carrier smoothed code algorithm could be incorporated to produce acceptable measurements for navigation. Additionally, if the system is in motion, an extended Kalman filter can be used to estimate position and velocity based solely on the carrier phase measurements.

For more details visit the project website.