Development of Laser Altimeter Sensors for Earth and Planetary Observations

Laser altimeter sensors are a class of devices, developed for airborne and space-based applications, with a primary role of measuring the topography of the Earth and other planetary surfaces. Laser altimeters are mounted for nadir observation on aircraft or spacecraft platforms and provide a high-resolution, high-accuracy measurement of the elevation and horizontal variability of planetary surfaces. The basis of the altimeter measurement is the timing of the round-trip propagation of short-duration pulses of laser radiation between the platform and the surface. Vertical (i.e. surface elevation) resolution of the altimetry measurement is determined primarily by laser pulsewidth, surface-induced spreading in time of the reflected pulse, and the timing precision of the altimeter electronics. With conventional (~10 nsec wide) gain-switched pulses from solid-state lasers and sub-nsec resolution electronics, sub-meter vertical range resolution is possible from aircraft altitudes (0.5 km - 20 km) to orbital altitudes of several hundred kilometers. Laser divergence angle and altimeter platform height above the surface determine the laser footprint size at the surface, while laser pulse repetition-rate, laser transmitter beam configuration, and altimeter platform velocity determine the spacing between successive laser pulses. Typical sensor footprint diameters for an aircraft altimeter application are 1-10 m and are ~ 100 m for a spacecraft altimeter. Ideally, the pulse-rate is adjusted to make a contiguous profile of topography so that horizontal resolution is equal to the sensor footprint size. Successful operation of the laser altimeter sensor requires additional information on the laser altimeter platform pointing attitude and position 1. Recent major advances in laser gyros, star cameras, and kinematic positioning with the Global Positioning System permit recovery of platform effects to a sufficient level (e.g. 5-500 μrad for pointing angle and 10 cm - 1 m for position) to enable sub-meter laser altimetry for the operating altitudes and velocities of both aircraft and spacecraft platforms.