IPSRU (Inertial Pseudo Star Reference Unit) is an inertial line- of-sight pointing reference that has an ultralow jitter of less than 40 nrad rms from 0.4 to 312 Hz. IPSRU emits a collimated optical reference beam from a stabilized, sterrable two degree of freedom platform, and this reference beam is used to stabilize a payload's line-of-sight against vehicle vibrations. IPSRU's three critical performance parameters are isolation of host vehicle vibration from the reference beam, the intrinsic reference beam jitter due to self-generated platform noise, and open-loop inertial pointing accuracy of the reference beam. IPSRU, developed at Draper, is in final performance testing before delivery to Phillips Laboratory for the high-altitude balloon experiment (HABE). The challenges of measuring and validating IPSRU's performance, including a total beam jitter of less than 40 nrad, a base isolation transfer function that ranges from -93 dB at 1 Hz to -55 dB at 100 Hz, and an open-loop pointing accuracy of 21 ppm, are met with a specially designed pointing scoring facility that includes state of the art instrumentation and a quiet reference pier. Beam jitter and base motion isolation are measured with a Draper-enhanced quad-type detector with an rms noise of 6 nrad from 0.4 to 312 Hz and a field of view of 8 (mu) rad. Open-loop pointing maneuvers are measured with an enhanced-resolution laser interferometer (LI) (6.8 nrad per bit) with a 4 deg field of view. Additionally, IPSRU is scored in a thermal vacuum chamber that simulates the HABE environment or a satellite environment.

Date Published: 5 July 1994
PDF: 12 pages
Proc. SPIE 2221, Acquisition, Tracking, and Pointing VIII, (5 July 1994); doi: 10.1117/12.178969

Published in SPIE Proceedings Vol. 2221:
Acquisition, Tracking, and Pointing VIII
Michael K. Masten; Larry A. Stockum; Morris M. Birnbaum; George E. Sevaston, Editor(s)