Differential Ionospheric Effects in GPS Based Navigation of Formation Flying Spacecraft

For the Tandem-X and TerraSAR-X forma- tion to achieve its scientific goals, a rela- tive positioning accuracy is required of 1 mm. This formation is characterised by rel- atively small baselines (less than 1 km). Sin- gle differenced ionospheric delays in the GPS observations are therefore expected to be small, which can be exploited in a filter con- struction. This paper investigates the ionospheric de- lays that might be expected on small base- lines. As a test case for Tandem-X and TerraSAR-X the GRACE switch manoeuvre is taken. From the investigation some results are derived on the influence of spacecraft separation on a relative navigation Kalman filter. The relative navigation algorithms gener- ally make use of the observation data on both GPS carrier frequencies to eliminate ionospheric interference. Using the iono- sphere free combination of these frequencies comes at the cost of introducing some addi- tional noise to the observables. This noise affects the ambiguity resolution process. It is shown that high precision relative naviga- tion with only one GPS frequency is feasible for short baselines and might benefit the in- teger ambiguity resolution process