Abstract

Diode lasers have been chosen for space missions in view of their long lifetime, low weight and volume, high electrical-to-optical conversion efficiency, and direct modulation capability. A pulsed diode laser range sensor is being developed for ranging between co-located geostationary spacecraft (INSAT series) separated on the average by distances from 35 to 75 km with a minimum separation of 5 km. The laser range sensor consists of a transmitter, a single axis scanner, a retroreflector on the target spacecraft, and an avalanche photodiode receiver. The pulsed GaAs / GaAlAs diode laser at 850-nm wavelength has a pulse optical power output of 1000 W, 30-ns pulse width, 1-kHz pulse repetition rate, and 1-mrad beam divergence. The scan field is 360° X 120°. The retroreflector has a 200-mm aperture. The receiver optics has a 200-mm diam clear aperture. The silicon avalanche photodiode detector has 10-nW noise equivalent power. The signal-to-noise ratio at 100-km range is 4. The time interval between the start and stop pulses is measured by a coarse counter with a 50-MHz clock. The fractions of clock pulses are determined by a time-to-time conversion circuit. This provides an accuracy range of 2 cm from 6 m to 1 km and 3 m at low signal-to-noise ratios, up to 100 km.

© 1989 Optical Society of America