Moderate accuracy relative navigation in formation flying by filtered radio measurements

Formation flying missions involve relative navigation, or the determination of the relative kinematic state of the spacecraft involved. Different options do exist, both in visible and conventional radio-frequency operations, with passive and active architectures. All the solutions need to take into account the strict requirements in terms of mass, volume and power typical to spacecraft, not to forget that they are not directly part of the payload, so that the overall impact of this avionic subsystem needs to be minimized. This paper aims to discuss a simple solution for the relative navigation of a formation, especially suitable for moderate cost, moderate performance missions (like the ones exploited by cubesats). A field strength meter onboard the chief-satellite can provide the level of received signal power, which is an observable related - even if with a poor quality - to the inter-satellite distance. An extended Kalman filter including a suitable dynamical model can exploit these measurements in order to estimate the formation kinematic state at a very limited cost. The combination of the two elements makes up for a technique which is accurate enough even in the phases immediately following the release from the launcher, where the spacecraft are in close proximity. The presented numerical simulation proves the valuable performance of such a simple system, also useful as a possible back-up solution at a limited cost in terms of spacecraft requirements.