Design, navigation and control of relative motion for INSAR and BSAR formations

Paper number

IAC-08.C1.6.10

Author

Mr. Marco Sabatini, University of Rome "La Sapienza", Italy

Coauthor

Mr. Giancarmine Fasano, University of Naples "Federico II", Italy

Coauthor

Dr. Giovanni B. Palmerini, Universita' di Roma 'La Sapienza', Italy

Coauthor

Prof. Marco D'Errico, Seconda Universita' di Napoli, Italy

Year

2008

Abstract

Remote sensing is one of the most relevant applications for formation flying missions. Synthetic Aperture Radar interferometric (INSAR) and bistatic (BSAR) techniques, which allow for a multiple exploitation of the transmitted signal, are especially apt to be carried out by means of a formation. A common requirement is given by the maintenance, within a defined tolerance, of a relative trajectory among the spacecraft belonging to the formation

In this paper the nominal formation’s dynamics are accurately described and justified, according to the different remote sensing modes (INSAR and BSAR). To this end, relative trajectories are designed including perturbations; in fact control efforts could be minimized by exploiting as much as possible natural perturbations in nominal trajectory design. Since non-modeled perturbations and inaccuracies during orbit acquisition phase would anyway remove the formation from the optimal state, a control strategy is implemented, based on a modification of the standard linear quadratic regulator. This techniques, a hybrid continuous-discrete strategy, is proved to enable a large ?V saving with respect both to classical impulsive and continuous thrust. For given system performance, more observations are therefore possible and more geometries can be explored, thus significantly enriching the scientific content of these missions.

Bistatic and interferometric phases are characterized by different baselines, from few hundreds of meters to some tens of kilometers. The navigation system, needed to provide an as accurate as possible estimate of the relative state, is deeply affected by the magnitude of the inter-satellite distance: considering the GPS – or GNSS – as the basic tool, different way to exploit the signal (code absolute, code differential, phase differential) should be considered. The controller should be accordingly tuned, especially with regard to the maximum magnitude of the actions and on the dead zone accepted between two following maneuvers.

The existence of a critical length of the baseline, which makes separate exploitation of absolute position measurement more convenient than relative position measurements, is studied. Different extended Kalman filters estimating the relative or the absolute position using relative or absolute measurements are implemented at this purpose. The performance of the closed GNC loop is analyzed, showing as interferometric and bistatic phases may require not only a different tuning of the navigation and control algorithms, but even completely different GNC strategies.

Abstract document

IAC-08.C1.6.10.pdf

Manuscript document

IAC-08.C1.6.10.pdf (🔒 authorized access only).

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