Instrument Information
IDENTIFIER urn:nasa:pds:context:instrument:mla.mess::1.3
NAME MESSENGER Mercury Laser Altimeter
TYPE Altimeter
DESCRIPTION The MESSENGER Mercury Laser Altimeter (MLA) was designed to measure the topography of Mercury's surface. It used laser pulses to determine the distance between the spacecraft and the planet's surface, creating detailed maps of elevation and surface features. It 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. 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.