Formation flying radio frequency instrument: First flight results from the PRISMA mission

A satellite formation flying mission consists in a set of satellites flying in close configuration whose geometry is accurately measured and controlled. Ultimately this allows making space science using a so called “distributed instrument” on several spacecrafts. Several science missions are envisioned, for example PROBA-3 or Darwin from ESA. These missions require very accurate relative positioning and control of the platforms, to achieve the instrument pointing and quality of measurement. Optical metrology will be used for this purpose. Yet, a Formation Flying Radio Frequency (FFRF) metrology instrument will be needed to provide robust and autonomous measurements throughout all the mission phases: deployment, manoeuvres, transition to optical metrology, and even during nominal observations. Typically, the altitude of the formation will be high above the geostationary altitude or even at the Lagrange L2 point. Therefore, the use of GNSS signals will not be possible, and the FFRF equipment will thus have to provide its own signal to make cross-link measurements. Under CNES and CDTI contracts, Thales Alenia Space (TAS) has developed and delivered an S-Band FFRF instrument (2 flight models and 3 engineering models) for the PRISMA formation flying demonstration mission. This mission, under the responsibility of the Swedish Space Corporation, was launched in June 2010. CNES contributes to PRISMA with the FFIORD experiment, which aims at conducting open and closed loop formation flying using the data from the FFRF instrument. This paper first gives an overview of FFRF sensor. Then it details FFIORD experiments (timeline and activities). Finally it provides the first in-flight test results : FFRF performance (distance and Line of Sight) is assessed by comparison with differential GPS data. Results from first closed loop experiment are also presented.