Title of Paper: Design and performance of a 3-D imaging, photon-counting microlaser altimeter operating from aircraft cruise altitudes under day or night conditions
Principal Author: Dr. John Degnan
Abstract: The present paper reports on the design and performance of a scanning, photon-counting laser altimeter, capable of daylight operations from aircraft cruise altitudes between 6 and 12 km. In test flights, the system successfully recorded high repetition rate returns from clouds, soils, man-made objects, vegetation, and water surfaces under full solar illumination. Following the flights, the signal was reliably extracted from the solar noise background using a Post-Detection Poisson Filtering technique. The passively Q-switched microchip Nd:YAG laser measures only 2.25 mm in length and is pumped by a single 1.2 Watt GaAs laser diode. The LBO nonlinear crystal converts the radiation to the 532 nm wavelength in order to take advantage of higher detector counting efficiencies and narrower spectral filters available in the visible regime. The transmitter produces several microjoules of green energy in a subnanosecond pulse at rates up to 10 kHz. The illuminated ground area is imaged by a 14 cm diameter, diffraction-limited, off-axis telescope onto a segmented anode photomultiplier. Each anode segment is input to one channel of "fine" range receiver (5 cm resolution), which records the times-of-flight of individual photons and provides crude 3-D imaging of the terrain within the nominal 50-100 cm laser ground spot. A parallel "coarse" receiver provides a lower resolution (>75 cm) histogram of all scatterers between the aircraft and ground and centers the "fine" receiver gate on the last set of returns. The instrumentation is currently being adapted to demonstrate kHz rate, 3-D imaging from the Space Shuttle in the June 2003 time frame.