Principal Investigator: John J. Degnan III, NASA Goddard Space Flight Center
Proposal Title: Multikilohertz Microlaser Altimeters ("Microaltimeters")
Prior art spaceborne laser altimeters in Earth orbit use high energy (`100 mJ) solid state lasers, large telescopes (50 to 100 cm), and high detection thresholds to achieve unambiguous surface returns with little or no noise background. As a result, spacecraft prime power and weight constraints typically restrict operations to modest repetition rates on the order of a few tens of Hz which in turn limits along-track spatial sampling on the ground to 180 meters or more. There is great scientific interest in obtaining higher along-track resolution and better cross-track coverage which can only be achieved through higher system repetition rates. Our analysis demonstrates that the conventional high SNR approach to laser altimetry does not make efficient use of the available laser photons. The surface return rate of an Earth-orbiting altimeter can be increased up to two orders of magnitude for a given laser output power by emitting the light in a high frequency (few KHz) train of low energy (~1 mJ) pulses as produced by recently developed solid state "microlasers". Furthermore, new photon-counting detectors capable of determining the angular source of the single photon event remove the range ambiguity inherent in current altimeters, and the visibility of the underlying terrain against the solar-induced noise background is actually enhanced through the use of small receive telescopes on the order of 10 cm, resulting in huge reductions in telescope weight and cost. Relatively simple onboard software algorithms can be employed to identify and extract the surface data from the background prior to transmission. It is the intent of our proposal to develop and flight test an engineering model microlaser altimeter or "microaltimeter" on an aircraft and demonstrate its advantages in developing high spatial resolution digital topographic databases, monitoring biomass and vegetation canopy heights, sea and lake levels, ice sheet thickness, etc.