Stellar alignment of the High Resolution Doppler Imager

Abstract

A method is described for determining the angular misalignment between the High Resolution Doppler Imager instrument and its spacecraft, the Upper Atmospheric Research Satellite, using observations of stars by the instrument. Roll, pitch, and yaw misalignments have been measured to within 0.0025 deg (l<r), which is an order of magnitude better than the largest tolerable alignment error. The misalignment results are validated using observations of the nighttime 5577-A O^S) green line emission. In addition, the long-term behavior of the derived alignment offsets, which shows a clear dependence of the alignment on the solar (3 angle, is presented. Nomenclature px = vector in the instrument's viewing direction expressed in the x coordinate frame Tx/y = transformation matrix from the y to the x coordinate frame t = measured time of the star crossing, s to = calculated star crossing time for the known star position and spacecraft position and attitude, s rjo = known angular misalignment vector, deg ?7i = angular misalignment between the instrument and spacecraft frame about the roll axis of the spacecraft, deg 772 = angular misalignment between the instrument and spacecraft frame about the pitch axis of the spacecraft, deg 773 = angular misalignment between the instrument and spacecraft frame about the yaw axis of the spacecraft, deg 3rj/31 = derivative of the misalignment with respect to crossing time expressed as a three-element column vector (in roll, pitch, and yaw), deg/s