Autonomous Formation Flying RF Sensor Development for the PRISMA Mission

L. Lestarquit, J. Harr, T. Grelier, E. Peragin, N. Wilhelm, C. Mehlen, C. Peyrotte

Abstract:	The Formation Flying (FF) concept allows to open new horizon for space science mission. A FF mission consist in a set of satellite flying in a close formation whose geometry is accurately measured and controlled. Ultimately, this allows to make space science using a so called “distributed instrument”, that is an instrument distributed on several spacecraft. Several science Mission are envisioned, such as Simbol-X from CNES, or Darwin from ESA. These missions require very accurate relative positioning and control of the platforms, to achieve the instrument pointing and quality of measurements. Laser metrology will be used for this purpose. Yet a FF Radio Frequency (FFRF) metrology will be needed to provide robust andautonomous measurements throughout all the mission phases : deployment, manoeuvres, transition to laser 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 GPS signal won’t be possible, the FFRF equipment will have to provide its own signal to make cross-link measurement using dual frequency Sband navigation signal. Each satellite will be able to make ranging and line of sight measurement with every other satellites. Coarse measurements will be made using the ranging from the C/A code, and fine measurement with sub-centimetre accuracy using carrier phase measurement. The dual frequency will allow to have carrier ambiguity resolution using a wide lane. 2-ways measurements have to be used to account for the relative clock drift of the platforms. Line of sight measurement are made possible using a 3- antenna base, with one master antenna and 2 slaves by measuring the carrier phase difference between the master and slave antenna. Attitude, though, has to come from external attitude sensor such as star trackers. There is also a centralised PVT algorithm that will be detailed. For formations with more than 2 satellites, it uses all the measurements made by all the satellites to give the relative position of the satellites. Alcatel Alenia Space is currently developing the S-Band FFRF instrument for PRISMA under a CNES contract. The PRISMA mission, scheduled for 2008 is a demonstration FF mission, under the responsibility of SSC (Swedish Space Corporation). It will have 2 satellites, one called “Target” will have an inertial motion, while the other, called “Main” will make motions ranging from 3 meters to 10 km around Target. Unlike the other FF project PRISMA will have a LEO orbit at 700 km and the satellites will have GPS receivers allowing differential GPS measurement so as to produce a reference to validate the FFRF performances. Multipaths created by satellite structure surrounding FFRF antennas will be the major source of error on FFRF line of sight (LOS) and distance measurements. To mitigate Multipath, a method of calibration of multipath errors in anechoic chamber has been set up, tested with Lband mock-up already developed in 2002-2004 under ESA and CNES contracts [5], is being used. Example of Multipath calibration maps will be presented. This experimentation is a preparation of the calibration that will be done again using the S-band electrical model that will be available in 2007, so as to produce the actual multipath calibration tables for the PRISMA mission.
Published in:	Proceedings of the 19th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2006) September 26 - 29, 2006 Fort Worth Convention Center Fort Worth, TX
Pages:	2571 - 2578
Cite this article:	Lestarquit, L., Harr, J., Grelier, T., Peragin, E., Wilhelm, N., Mehlen, C., Peyrotte, C., "Autonomous Formation Flying RF Sensor Development for the PRISMA Mission," Proceedings of the 19th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2006), Fort Worth, TX, September 2006, pp. 2571-2578.
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