Star Tracker (STR)
The Star Tracker (STR) is comprised of three Camera Head Units (CHUs) mounted on the innermost end of the optical bench. Nominally, the attitudes of all three heads are provided simultaneously at 1 Hz rate, however one head is regularly blinded by the Sun leaving the attitudes of just two heads. The attitudes of the 2-3 CHU are combined into one attitude, the attitude of the STR Common Reference Frame (CRF). The combination uses the method described in [RD-2]. The attitudes of CRF are then interpolated to obtain the CRF and S/C attitudes at required time instants: 50 Hz VFM measurements, 2 Hz EFI measurements and UTC seconds.
STR Algorithm Overview
First, the time-stamps are corrected for any delays: tout,STR = t0,STR - CCDB.STR.Delay. Then, the aberrational correction is verified and if necessary applied. The valid attitudes of the STR are combined into common attitude solutions, qCRF←ICRF, providing the CRF → ICRF transformations. This is the Level 1bInst.STR product. Then these are interpolated to the time instants to be used (UTC seconds, tout,VFM, etc.) using a smoothing, cubic B-spline method. Finally, the various required output transformations are generated by combining series of transformations.