The first successful photon-counting airborne laser altimeter was demonstrated in 2001 under NASA's
Instrument Incubator Program (IIP). This "micro-altimeter" flew at altitudes up to 22,000 ft (6.7 km)
and, using single photon returns in daylight, successfully recorded high resolution images of the
underlying topography including soil, low-lying vegetation, tree canopies, water surfaces, man-made
structures, ocean waves, and moving vehicles. The lidar, which operated at a wavelength of 532 nm
near the peak of the solar irradiance curve, was also able to see the underlying terrain through trees
and thick atmospheric haze and performed shallow water bathymetry to depths of a few meters over
the Atlantic Ocean and Assawoman Bay off the Virginia coast.
Sigma Space Corporation has recently developed second generation systems suitable for use in a small
aircraft or mini UAV. A frequency-doubled Nd:YAG microchip laser generates few microjoule,
subnanosecond pulses at fire rates up to 22 kHz. A Diffractive Optical Element (DOE) breaks the
transmit beam into a 10x10 array of quasi-uniform spots which are imaged by the receive optics onto
individual anodes of a high efficiency 10x10 GaAsP segmented anode microchannel plate
photomultiplier. Each anode is input to one channel of a 100 channel, multistop timer demonstrated to
have a 100 picosecond timing (1.5 cm range) resolution and an event recovery time less than 2 nsec.
The pattern and frequency of a dual wedge optical scanner, synchronized to the laser fire rate, are
tailored to provide contiguous coverage of a ground scene in a single overflight.