A pulsed laser altimeter

Abstract

The paper discusses the development of an equipment which determines the height of an aircraft by measurement of the transit time of a pulse of laser light travelling from the transmitter to the ground, and back to the receiver. The transmitter source is a GaAs laser diode mounted in a Dewar, and cooled to 80° K by a self-regulating Joule–Thomson cooler. A cold cathode modulator is employed to drive the laser, the emitted energy being collimated by a multi-element transmitter lens. The receiver is mounted coaxially with the transmitter and takes the form of a Cassegrainian telescope. Detection is provided by a silicon photodiode which drives a high-gain video amplifier via a low-noise f.e.t. pre-amplifier. Transit time measurements are made with a modified high speed counter, and ancillary circuitry is incorporated to allow height information to be fed to an aircraft tape recorder system.The system design parameters are presented together with a discussion of the major component design features and problems. This includes design of the Dewar, laser modulator, transmitter and receiver optics, and the receiver detector/amplifier assembly. The Dewar design problems result from the requirement for a low inductance feed into the Dewar for the laser drive pulse, and the necessity of retaining a vacuum over long periods of time under adverse environmental conditions. The laser modulator design presents the problem of driving the laser with fast high current pulses (40 ns pulse width, 200 A). The detection system design requires the optimization of the detector/pre-amplifier combination for fast, sensitive, low noise performance. The engineering, environmental testing, and functional testing of this equipment is mentioned, and the performance figures given.