satellites in formation flying missions. It generates relative positions, speeds and lines of sight (LOS) for the Guidance, Navigation and Control (GNC) system and provides rough metrology data. As the first element in the RF metrology system line, the FFRF sensor ensures a high precision of the initial relative navigation for the following optical metrology subsystems (rough lateral optical metrology, precise optical metrology and precise longitudinal metrology).
The subsystem was composed of an FFRF terminal and a set of up to 4 antenna arrays on each satellite in formation. An antenna array was either triple (1 master Rx/Tx antenna and 2 slave Rx antennas) or single (1 Rx/Tx antenna). The terminal worked in dual frequency S-band, and each terminal transmitted and received signals to and from other satellites in the constellation using a TDMA configuration. Distance and orientation data were extracted from the incoming signals and used to calculate relative position, speed, and line of sight. Aside from relative navigation data, the FFRF subsystem also provided an inter-satellite link (ISL) as an auxiliary function (12 or 4 kbps).
The FFRF terminal comprised the following components: RF front end (RFE), RF transmitter module, RF receiver modules, digital processing unit (DPU), and DC/DC and OXCO unit. All modules were assembled, except for the RF filters (Rx and Tx), which were included to ensure RF compatibility for S-band TTC and could be separate.
Within the Prisma mission, the FFRF had 2 possible configurations: one for the main satellite, and another for the target satellite. On board the main satellite, the FFRF terminal was connected to a single triple-antenna array. The only differences with the general case were in the head. On the target satellite, the FFRF terminal was connected to 3 distinct single-antenna Rx/Tx arrays. Therefore, there were differences for the modules on several aspects:
- in the front end;
- in the RF Rx module, where there was only one receiving line (Rx master signal);
- in the digital processing unit, where only one digital ASIC was needed to process the single Rx signal.