First developments on laser metrology technologies and drag-free/formation control techniques

Stefano Cesare (1), Sergio Mottini (1), Fabio Musso (1), Manlio Parisch (1), Gianfranco Sechi (1), Enrico Canuto (2), Marco Pisani (3), Miguel Aguirre (4), Bruno Leone (4), Luca Massotti (4), Pierluigi Silvestrin (4)
(1) Thales Alenia Space Italia, Strada Antica di Collegno 253, 10146 Turin, Italy
(2) Politecnico di Torino, Corso Duca degli Abruzzi 24, 10129 Turin, Italy
(3) Istituto Nazionale di Ricerca Metrologica, Strada delle Cacce 73, 10135 Turin, Italy
(4) ESA-ESTEC, Keplerlaan 1, 2201 AZ Noordwijk ZH, The Netherlands

Since 2003, the European Space Agency has initiated assessment studies and technology developments in preparation of a future space gravity mission based on the Satellite-to-Satellite Tracking technique with laser metrology. Thales Alenia Space Italia has led three of these studies in which, in particular, a first concept of the laser metrology system has been defined and the drag-free, formation and attitude control issues has been investigated.

The distance between the satellites is measured using a Michelson-type heterodyne interferometer fed by a Nd:YAG laser source with 1064 nm wavelength, suitably designed to work at long distances (10 km and larger). A breadboard of the interferometer has been built and tested in laboratory: a measurement noise <1 nm/Hz down to a frequency of 10 mHz has been achieved, in agreement with the specified requirement. The laser metrology system is completed by a device for measuring the orientation and the lateral displacement of the laser beam in the reference frames of the satellites and by a device for the precise pointing of the laser. These devices have been also breaboarded and verified in laboratory, including a closed-loop test of the laser beam pointing mechanism driven by the measurements of the lateral displacement metrology.

The challenge of the formation control design for this mission consists in keeping the satellite relative motion bounded without interfering with the scientific measurements (the satellites must be “free” to move under the effect of the gravity field to be measured), without degrading the drag-free environment (each satellite must be kept in drag-free condition due to the presence of ultra-precise accelerometers on-board) and minimizing the thrusters use (in terms of dynamic range, propellant consumption). A formation controller capable to fulfill these requirements has been preliminarily defined. Its performances have been verified by means of a high-fidelity numerical simulator implemented by TAS-I and derived from the GOCE End-to-End System Simulator. The possibility of realizing the formation, drag-free, attitude control system needed for such a mission requires the availability of ion thrusters with large dynamic range (~40), in particular if the measurement duration is such to encompass periods of maximum solar activity.