A real-time data acquisition system for the NASA Airborne Laser Polarimeter Sensor (ALPS)

An ASYST-based real-time data-acquisition system for measurement of the depolarization properties of various materials on the Earth´s surface is described. The ALPS system is flown in a NASA P3 and uses an Nd:YAG laser pulsed at 20 Hz to irradiate the Earth´s surface. The ALPS data-acquisition system measures the multispectral reflected energy of the laser with 12 photomultiplier tubes, whose outputs are digitized with a charge-integrating A/D (analog/digital) converter. An IBM PC/AT configured with an Intel Inboard 386 and a Kinetic Systems 2926 PC interface with direct memory access card serves as the system controller. Between each pair of laser pulses (50 ms) the software must read the reflected laser signal, the dark current background level, and various items of housekeeping information and these store this data in a RAM disk. The data are displayed for analysis and permanently stored in the AT´s hard disk when the aircraft is turning for its next acquisition pass. The ALPS ASYST software is completely menu-driven and controls all data-acquisition, processing, analysis, and database functions