The Mercury Laser Altimeter (MLA) was one of the primary instruments
          on NASA's MErcury Surface, Space ENvironment, GEochemistry and
          Ranging (MESSENGER) mission, under NASA's Discovery Program. MLA
          ranged to the surface only during the periapsis of the 12-hour
          orbit, limited by its 1800-km maximum range. During the final
          months of the mission, the spacecraft altitude was reduced and MLA
          took measurements at altitudes as low as 25 km. 
          
          The MLA was a bi-static system, meaning that it consisted of separate
          transmitter and receiver systems. The transmitter used a diode-
          pumped, Nd:YAG slab laser. The laser output was 20 mJ per pulse at
          1064-nm wavelength. The instrument measured the time required for
          the light to reach the surface and return. 
          
          MLA performed active and passive radiometric measurements in a
          narrow spectral band centered at 1064 nm. The active measurement
          employed a dual-threshold measurement of pulse width to infer the
          area and width of the return pulse. The pulse area, together with a
          transmit energy monitor, provided the reflectivity of the target
          within a 0.08-mrad-diameter laser spot. The passive measurement
          employed the noise counters and threshold settings on the detector
          subsystem to infer radiance from a 0.4-mrad-diameter field of view.