SGPS RELATIVE POSITIONING

The main application of relative spaceborne GPS positioning is for real-time spacecraft rendezvous and docking. Automated GPS-based systems are seen as a promising technique to be used for this evolving and expanding application.

Benefits of GPS:

GPS can provide highly accurate post-processed or real-time relative positioning between space vehicles.

Methodology:

The methodologies are analogues to terrestrial differenced and relative GPS positioning. The former involves orbit determination being performed on both spacecraft, and the "target" spacecraft (e.g. a space station) transmitting its solutions to the "chaser" spacecraft (e.g. a space shuttle) for differencing. The latter involves processing the single-difference observations (observations differenced between pairs of pseudorange measurements of a common GPS satellite) or double-difference observations (observations differenced between pairs of single differences for the same epoch) and transmitting the raw measurements from the target to the chaser for computation of relative position and velocity.

Mission examples:

- In a trial, orbits were determined independently with two receivers, one on the Wake Shield Facility-02 and one on the Space Shuttle. These one hour arcs were formed from double difference observations between the on-orbit receivers and International GPS Service for geodynamics (IGS) network receivers. Pseudoranges were observed with one receiver and carrier phase with the other. The distance between the two receivers was computed by Shuttle instruments. 10 metre relative positioning accuracy was attained in this post-processed experiment.

References:

Due to the rapid developments in the field of spaceborne GPS and the aerospace industry in general, any comments, information or corrections pertaining to information on this site are welcome and encouraged.