Advanced Topographic Laser Altimeter System (ATLAS)

The Advanced Topographic Laser Altimeter System (ATLAS) is a photon-counting lidar altimeter aboard NASA's ICESat-2 spacecraft, launched September 2018 ^{[1] [2]}. ATLAS operates at 532 nm using a frequency-doubled Nd:YVO4 master-oscillator power-amplifier (MOPA) laser, firing at 10,000 pulses per second and detecting individual photon returns via photomultiplier tube (PMT) arrays ^{[1] [2]}.

The instrument fields six beams arranged in three strong-weak pairs. Within each pair the strong beam carries approximately 175 microjoules per pulse and the weak beam approximately 45 microjoules, a roughly 4:1 energy ratio. The pairs are separated 90 m cross-track, with shots landing 0.7 m apart along each track. Beam divergence is 0.02 mrad. Footprint diameter at the surface is 11 m according to NASA GSFC specifications ^[1]; other references cite values between 10 m and 17.4 m reflecting different energy-encirclement conventions ^{[3] [2]}. Timing precision of approximately 800 ps corresponds to a single-photon range precision near 12 cm, though the exact operational range accuracy depends on surface return density and atmospheric conditions ^{[1] [2]}.

The strong-weak beam-pair architecture serves dual purposes: it enables on-orbit calibration of detector performance and extends the useful signal range across surface types from bright sea ice to dark open ocean. Over vegetated land the pair asymmetry also allows simultaneous sampling of canopy and ground returns at two sensitivity levels within the same 90 m swath ^[4].

Supported methodologies