Optimization of an airborne laser altimeter for remote sensing of vegetation and tree canopies

An existing airborne laser altimeter instrument (Bufton, Garvin, et al., 1991) was modified during 1992-93 to permit high-resolution recovery of backscattered laser pulse shapes for the purpose of measuring vertical vegetation structure and, in particular, tree heights. Most significantly, a digitization capability was added to the instrument in order to record the complex, time-varying return pulse energy, or waveform, that results from the reflection of a single laser pulse from multiple targets occurring at varying heights within the laser footprint. The return pulse is digitized at 650 Megasamples/sec for 742 nsec, yielding 22 cm sampling resolution over a vertical range of 110 m. In addition, a Nd:YAG Q-switched, cavity-dumped laser transmitter producing 4 nsec, 1.064 μm pulses was custom designed and constructed to provide a near "impulse" of laser energy. The 1.5 nsec leading edge of the transmitted pulse minimizes timewalk-induced range errors due to footprint albedo variations. System sensitivity is maximized by matching the bandwidth of the silicon avalanche photo-diode (Si:APD) detector to the laser output pulse width. Return pulse energy is collected and focused on the detector using a 38 cm diameter telescope with a 2 mrad field-of-view. The 1.5 mrad divergence of the transmitted laser pulse yields ground footprints that are 10 m in diameter from the nominal aircraft altitude above ground of 6.7 km. The instrument was integrated on the NASA T-39 aircraft at Wallops Flight Facility along with GPS receivers and an inertial navigation system. Aircraft position and pointing knowledge obtained from kinematic GPS trajectories and the 3-axis INS data are used with the laser range data to determine the elevation and geodetic position of the laser footprints.