Relative positioning of spacecraft in intense ionospheric conditions by GPS

Precise relative positioning by GPS is attempted traditionally by fixing the differential carrier phase cycle ambiguities to their integer values. This approach is of high interest for formation flying satellites in low Earth orbit since it can potentially deliver relative positions with (sub-)centimeter accuracy, but at the same time complicates the navigation system hardware and software. As an alternative to the fixed solution, different processing approaches making use of float carrier phase ambiguities can be proposed. The most simple of these solutions is analyzed in the present study and it is formed by differencing the float ambiguity-absolute navigation solutions obtained with navigation filters running independently on board each spacecraft. This paper presents a quantitative evaluation of the different accuracy achievable by the two approaches. Because in typical conditions ambiguity resolution should provide a markedly higher accuracy than the float solution approach under study, a particularly challenging condition for ambiguity resolution is considered. Specifically, a scenario where the inter-satellite separation is of hundreds kilometers and the ionospheric activity is severe is analyzed. The effect of un-modeled systematic errors is addressed by using real-world data from the Gravity Recovery And Climate Experiment (GRACE) mission. Results suggest that in such challenging conditions the two approaches have a similar decimeter-level accuracy.