GPS Sensing for Spacecraft Formation Flying

Carrier-phase Di erential GPS (CDGPS) for orbit and attitude determination is emerging as a very promising low cost alternative to more conventional methods, such as sun sensors, magnetometers, and star trackers. Relative spacecraft position and attitude determination are important for missions involving formation ying, such as those proposed for stellar interferometry under NASA's New Millennium Program, and LEO missions for coordinated Earth observing. This paper describes research on the development of a GPS based relative position and attitude sensing system in a laboratory environment for future application on spacecraft formations. Previous papers have discussed the demonstration of spacecraft rendezvous and capture in 2D, in a fully functional indoor GPS environment using simulated spacecraft on an airbearing table. The work presented in this paper extends Presented at Institute of Navigation GPS-97, Kansas City, Missouri, September 1997 the previous GPS based sensing to a formation of three prototype spacecraft, and experimentally demonstrates very accurate real-time solutions ( 2 cm error) for the three relative vehicle positions. A trade study of estimation algorithms and architectures is presented. Search based techniques for the carrier phase integer ambiguity resolution problem for vehicle formations are also demonstrated on simulation data.