Carrier Phase Based Relative Positioning above the GNSS Constellations

Abstract

Carrier phase based GNSS relative positioning is explored for formation flying spacecraft in highly elliptical orbits. A relative positioning filter is presented which is capable of handling the challenges unique to above -the-constellation positioning, in particular sparse measurements and poor positioning geometry. For the first time, the achievable performance is tested using a variety of different receiver types, and the immediate and significant impact of the receiver choice on the relative positioning problem is demonstrated. It is found that while a 10 cm level relative positioning solution can be demonstrated thro ughout the orbit, this is only possible under ideal conditions when sufficient measurements are available for a position fix at nearly every epoch, and when no maneuvers are conducted during the apogee arc. With maneuvers the relative positioning accuracy decreases to meter level. When a less ideal receiver is used, capable of constant tracking of at least one GNSS satellite but with infrequent position fixes, the relative position accuracy in the presence of maneuvers is degraded to the 10 meter level, and when a standard receiver is used subject to frequent long data gaps, the solution is unstable and the relative positioning solution in the presence of maneuvers is only accurate to the 100 meter level. It is further demonstrated that maneuvers handling and outlier detection both have a dramatic impact on the achievable relative positioning performance. Maneuver uncertainties increase the covariance of the relative position and make the filtered solution susceptible to measurement and model errors even when sufficient measurements are available to recover quickly. As the number of measurements decreases, outlier and cycle slip detection also present a substantial challenge.