Clementine Laser Rangefinder (LIDAR)

The LIDAR unit shares the telescope of the HIRES camera, splitting the 1064 nm return signal from the NdYag source off to an avalanche photodiode (APD) detector with dichroic filter. The optics are non-imaging, providing an exit pupil through simple relay optics at the APD. The APD electronics includes a temperature compensation feature for the APD bias voltage and programmable thresholding of the output signal. The APD current is amplified and inverted to a voltage by a transimpedance amplifier with a gain of 230X, a low frequency cutoff of 3 MHz, and a high frequency cutoff of 23 MHz. The APD current is amplified, then discriminated for changes (increase) through 14 MHz discriminator. Voltage changes exceeding the programmed threshold are flagged as returns.

Range value is determined by the clock cycles since a laser output sampled start pulse is received. The clock counter has only 14 bits owing to the hardware availability limitations. In order to allow returns up to the 640 km maximum range required in the lunar mission, returns from the discriminator are binned 4 to a clock count, turning the 23 MHz response into a 40 meter height bin. Internal memory in the LIDAR unit saves up to 6 "returns" per laser firing, with up to 4 saved between programmable search range minimum/maximum values. Threshold is set for the best compromise between missed detection and false alarms.