Instrument Information
IDENTIFIER urn:nasa:pds:context:instrument:mla.mess::1.2
NAME MERCURY LASER ALTIMETER
TYPE ALTIMETER
DESCRIPTION The Mercury Laser Altimeter (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.
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REFERENCES Cavanaugh, J.F., J.C. Smith, X. Sun, A.E. Bartels, L. Ramos- Izquierdo, D.J. Krebs, A.M. Novo-Gradac, J.F. McGarry, R. Trunzo, J.L. Britt, J. Karsh, R.B. Katz, A. Lukemire, R. Szymkiewicz, D.L. Berry, J.P. Swinski, G.A. Neumann, M.T. Zuber, and D.E. Smith, The Mercury Laser Altimeter instrument for the MESSENGER mission, Space Science Reviews, 131, 451-479, 2007.