A lidar apparatus called a microaltimeter has been proposed for use aboard a spacecraft in orbit around the Earth for mapping land and sea surfaces, including such features of special interest as ice, tree canopies, and flood plains. The microaltimeter is short for "microlaser altimeter" and is so named because it uses a very compact, low-energy, subnanosecond pulse, solid-state microlaser as its source and relatively small (typically 10 to 20 cm in diameter) telescopes, resulting in a factor of 100 reduction in telescope weight and volume, as compared to conventional spaceborne laser altimeters. Operating at thousands of pulses per second, the surface sampling rate is approximately 100 times higher than that of prior spaceborne laser altimeters having the same transmitter power-aperture product.
Many of the design concepts to be embodied in the microaltimeter, and the components to be used to implement the concepts, were derived from an eye-safe satellite laser ranging station called SLR2000. The laser in the microaltimeter would operate at a wavelength of 532 nm, a pulse energy of the order of a millijoule or less, and a pulse-repetition frequency of the order of several kilohertz. The receiver in the microaltimeter would operate in a photon-counting mode, with a mean signal level on the order of one photoelectron per laser pulse. With an ability to measure the times of flight of individual photons and to determine their origin within the receiver field of view through the use of pixellated or imaging detectors, the receiver can provide ranging unambiguous registration of range (and thus height) data with surface locations.
The theoretically predicted performance of the microaltimeter has been tentatively verified in simulations of the operation of the microaltimeter from Earth orbit, performed by use of software developed previously for simulating the operation of the Mars Orbiter Laser Altimeter (MOLA) and the Geoscience Laser Altimeter System (GLAS). In addition to its potential utility for Earth science, the microaltimeter could likely be used to rapidly generate nearly contiguous maps of other planets, moons, comets, and asteroids.
This work was done by John Degnan of Goddard Space Flight Center.
This invention is owned by NASA, and a patent application has been filed. Inquiries concerning nonexclusive or exclusive license for its commercial development should be addressed to
the Patent Counsel
Goddard Space Flight Center; (301) 286-7351.
Refer to GSC-14098.