Deployment strategies for a formation of pico-satellites

A pico-satellite formation concept, aimed to gather spatially and temporally correlated data, is analyzed: A larger nano-satellite (the Chief) contains a limited number of smaller satellites (Deputies), it deploys them and receives the transmitted data. Severe constraints come from the limited resources of the involved spacecraft: In particular, a short relative distance between the Deputy and the Chief must be kept during the operational lifetime, in order to preserve the inter-satellite communication link. Moreover, pico-satellites usually lack of active orbital control: The only way to prevent the formation from rapidly getting apart too fast consists in a wise deployment strategy. The aim is to respect the constraints of maximum permitted distance during the mission lifetime, while avoiding collisions. The factors considered as most important for the relative dynamics are the orbital environment (mainly, gravity gradient and differential drag) and the errors on Chief's orientation at pico-satellites separation, that can be large for low-cost micro attitude control systems. The paper also introduces a simple solution for the relative navigation of the formation. A field strength meter onboard the Chief can provide the level of received signal power, which is an observable, even if poor and inaccurate, related 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, with reduced additional specific hardware.