An objective of the Wideband Angular Vibration Experiment (WAVE) is to measure optical bench vibrations of the Relay Mirror Experiment (RME) laser beacon tracker. The WAVE sensor package measures six degrees of freedom of optical bench base motion over a frequency band of 1-1,000 Hz. The WAVE package is comprised of 16 sensors: three angular displacement sensors, six magnetohydrodynamic (MHD) angular rate sensors, and seven linear accelerometers. Redundant angular measurements allow the qualifying of the MHD sensor for space operation. The noise floor of the angular measurements, integrated over the full frequency band, is less than 0.1 (mu) rad in each axis. Measurements taken while tracking an internal self-check laser diode indicate that vibrations from attitude control system horizon scanners, reaction wheel, and excited structural modes in the beacon tracker optical assembly dominate the base motion environment. Most of the vibrations are narrow band with amplitudes on the order of 10.0 nanoradians. Coherence analysis between WAVE sensors and unrejected quad cell errors indicates that track performance during self-check is limited by base motion.
In the Relay Mirror Experiment (RME), three laser beams are propagated from ground-to- space and retroreflected; two argon-ion laser beams at 488 and 514 nm and a Nd:YAG laser beam at 1.06 micrometers wavelength. The full-angle 1/e2 divergences of the argon beams are operated in the 60-90 (mu) rad range while the Nd:YAG beam is in the 30-60 (mu) rad range. Uplink irradiances at the RME spacecraft (s/c) and retroreflected irradiances at the ground have been measured. The high correlation (>=0.8) between the s/c signals and retroreflected signals indicate that uplink scintillation dominates the retroreflected signal. Likewise high correlation between ground sensors separated by distances much greater than the atmospheric coherence length rO confirms that uplink scintillation is dominant over downlink scintillation. The temporal power spectrum of the scintillation shows a roll-off above 700 Hz which is steeper than theoretical predictions. The profile of the beam mean irradiance and the normalized standard deviation ((sigma) I) of the scintillation were measured by stepping the beam across the s/c in a square 7 X 7 array. These data show moderate scintillation on-axis ((sigma) I equals 0.5) and increases dramatically for off-axis pointing exceeding 0.6 1/e2 radius ((sigma) I >= 1). These data will be compared to predictions from analytic models for gaussian beams developed by R. Sasiella and J. Shelton of MIT/LL.
The Relay Mirror Experiment is a space experiment in which an IR laser beam is propagated from one ground station, the Laser Source Site (LSS), to an orbiting relay mirror and back to another ground station, the Target Scoring System (TSS). A sparse array of 37 telescopes senses the position of the relayed beam at the second ground station for purposes of scoring the pointing capability of the relay mirror. Data from these telescopes is processed to determine the position of the beam as a function of time. These signals contain the effects of atmospheric turbulence on both the uplink and downlink of the IR beam. Spectral and correlation analysis is used on the telescope data to minimize the effects of atmospheric turbulence, as well as other environmental effects.