Prelaunch Testing of the Laser Geodynamic Satellite (LAGEOS)

摘要

Principal emphasis was placed on pulse spreading characteristics, range correction for center of mass tracking, and pulse distortion due to coherent effects. A mode-locked frequency doubled Nd:YAG laser with a pulse width of about 60 ps [full width at half-maximum (FWHM)] was used as the ranging transmitter and a crossfield photomultiplier was used in the receiver. High-speed sampling electronics were employed to increase receiver bandwidth. LAGEOS-reflected pulses typically had a width of 250 ps (FWHM) with a variability in the range correction of less than 2 mm rms. Pulse distortion due to coherent effects was inferred from average waveforms and appears to introduce less than ± 5 0 ps jitter in the location of the pulse peak. Analytic results on this effect based on computer simulations are also presented. Theoretical and experimental data on the lidar cross section were developed in order to predict the strength of lidar echoes from the satellite. Cross section was measured using a large-aperture laser collimating system to illuminate the LAGEOS. Reflected radiation far-field patterns were measured using the collimator in an auto collimating mode. Data were collected with an optical data digitizer and displayed as a three-dimensional plot of intensity versus the two farfield coordinates. Measurements were made at several wavelengths, for several types of polarizations, and as a function of satellite orientation. Theoretical predictions of the corresponding far-field patterns were computed and are shown to be in close agreement with experimental results. Several unusual polarization effects caused by the use of total internal reflection cube corners were noted and confirmed by computer analysis. Velocity aberration compensation methods used for LAGEOS are discussed. The array was found to have slightly lower cross section than expected and possible causes for this difference are suggested.