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A. Caporali (CISAS, U. of Padova)
A prototype attitude sensor suitable for platforms in low Earth orbit has been developed using a pair of GPS receivers. The basic measurement consists in comparing the down-converted carrier phases from each receiver. The fringe phase for each visible satellite is further differenced between satellites, to remove the relative drift of one receiver clock relative to the other. The baseline joining a pair of antennas defines body-fixed angles, which are estimated in real time using a two step procedure: a coarse estimation is first made with the Ambiguity Resolution Function algorithm. The refined estimate is then made by least squares. This approach yields an estimate of a pair of body-fixed angles epoch-wise, i.e. regardless the value they had at previous epochs. Tests indicate that such unsmoothed, epoch-wise estimates of the angles are unbiased and refer to the true geographic pole. Assuming a short baseline of 0.600 meters, the r.m.s. (roor mean square) repeatibility at 1 Hz is 0.3° for the horizontal angle (e.g. azimuth, yaw), and a factor of two larger for the vertical angle (e.g. pitch, roll, depending of the baseline orientation). For platforms with low kinematics (e.g. marine or space applications), smoothing and /or complementary use of the GLONASS or GALILEO navigation satellites have the potential to bring down the r.m.s. figure to 0.1° or better, even with a short baseline. Alternatively, for greater accuracies longer baselines may be used. In such case, one or more intermediate antennas may be used in a bootstrap mode, as the epoch-wise solution may be unstable, especially with few satellites in view. The sensor has the capability to measure small( >0.003 m) changes in the baseline, simultaneously with the angles. As such, it can work as a strain gauge, e.g. to monitor large deformable structures in orbit. Having no moving parts, the sensor can withstand the shocks of the launch and is immune from thermal and mechanical drifts, but is very sensitive to the occultation of the navigation satellites produced by nearby obstacles or structures.
Acknowledgment: this research is supported by ASI Agenzia Spaziale Italiana.
The author(s) of this abstract have provided an email address for comments about the abstract: alex@geol.unipd.it