urn:nasa:pds:context:instrument:mla.mess 1.0 1.7.0.0 Product_Context 2016-10-01 1.0 extracted metadata from PDS3 catalog and modified to comply with PDS4 Information Model urn:nasa:pds:context:instrument_host:spacecraft.mess::1.0 instrument_to_instrument_host 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. reference.CAVANAUGHETAL2007 MERCURY LASER ALTIMETER Altimeter not applicable not applicable The Mercury Laser Altimeter (MLA) is one of the primary instruments on NASA's MErcury Surface, Space ENvironment, GEochemistry and Ranging (MESSENGER) mission, under NASA's Discovery Program. MESSENGER, the first spacecraft ever to orbit Mercury, reached its destination following one Earth flyby, two flybys of Venus and three of Mercury. It launched on 3 August 2004 and entered Mercury's orbit on 18 March 2011. Initial data collection began during the three flybys of Mercury, continued during the orbital phases and consist of global mapping and measurements of the surface, atmosphere and magnetosphere composition. MESSENGER completed its mission on 30 April 2015 when it impacted into the planet as planned. The nominal mission consisted of one year in orbit, but the mission was extended to span more than four years in orbit. Once in orbit around Mercury it began a series of observations using multiple instruments. 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. See the CDR_RDR_DS.CAT file for more information on the lower altitude campaign and the MLA data collected at those times. MESSENGER's observations provided data to answer questions about the nature and composition of the crust, tectonic history, the structure of the atmosphere and magnetosphere, interior structure, and the nature of the polar caps. The MLA is a bi-static system, meaning that it consists of separate transmitter and receiver systems. The transmitter uses a diode-pumped, Nd:YAG slab laser with passive Q-switching. The laser output is 20 mJ per pulse at 1064-nm wavelength. The beam pattern at the output of a 15X beam expander is roughly Gaussian, with 90% of its power lying within a divergence of 80 microradians. The laser electrical power consumption is 8.7 W, and its mass without the beam expander is 0.56 kg. The transmitter generates a 6-ns-wide laser pulse at 8 Hz intervals, and the instrument measures the time required for the light to reach the surface and return. The MLA data complements the visible and near-infrared imaging that will also be performed on Mercury. Unlike the imager, the MLA does not rely on solar illumination and can make measurements over the entire surface of Mercury including the night side. The MLA complements imaging because the direct range measurements enable unambiguous determinations of topography that will improve the interpretation of images. The MLA can also be operated as a laser transponder using the transmitter and receiver subsystems independently. The MLA range measurement unit (RMU) employs a unique APL-supplied 'Time-of-Flight' (TOF) ASIC which uses on-chip signal propagation time to gauge the time interval between pulses and the next clock signal. It has the unique advantages of subnanosecond (0.4 ns) timing but without the need of high speed digital electronics. The RMU can time up to 15 pulses with less than 2 microseconds recovery time, which enables the receiver to lower its detection threshold to register weak signals in the presence of several false alarm pulses. On-board software rejects most of the false alarms and downlinks only the most likely range signals to earth. The RMU measures the distance between the spacecraft and the surface of Mercury with 10-cm precision. This allowed the science team to take detailed measurements of Mercury's shape and surface structure. The laser output is sensed by a diode pickoff, and its energy is recorded. A leading and trailing edge start pulse time is recorded by comparators within the range measurement unit (RMU), as well as the time of the reflected pulse, as described below. Return echoes are collected by an array of four refractive telescopes that are fiber-optic coupled into a single silicon avalanche photodiode. To accommodate surfaces of greater slope and roughness, the received signal is passed through each of three matched filters into separate comparators within the RMU. These filter channels have response times of 10, 60, and 270 ns. A sufficiently strong return pulse will trigger comparators on one of the three lower threshold channels (channel 1, 2, 3), whose arrival is measured by a common pair (leading, trailing edge) of TOF chips. Pulses that trigger a separate high threshold on the 10-ns channel (Channel 1 high, also known as channel 0) are measured by an additional pair of TOFs. The combined detection of a single pulse at two thresholds will allow a solution for pulse energy and for the spread of an equivalent Gaussian pulse waveform. Thresholds are set independently for each channel by means of flight software (FSW) and programmable digital-to-analog converters, in response to changing numbers of noise counts issued by the comparators, so as to maintain a preset average rate of false alarms in the face of varying background illumination. One high-threshold return and up to 10 returns from low-threshold channels are returned by the FSW after signal processing. The FSW Science task tracks the surface so as to eliminate most noise returns, and outputs a variable-length packet. The RMU counts consist of coarse and fine times. Each of the six dedicated TOFs has a series of gates that generate a fine range count at the leading or trailing edge of a pulse. Thus each 200-ns interval generated by the 5 MHz clock is subdivided into approximately 500 subintervals of roughly 400 ps. Individual timing calibrations for each TOF are applied from a lookup table since each gate has a slightly different delay. Additionally, the MLA can perform active and passive radiometric measurements in a narrow spectral band centered at 1064 nm. The active measurement employs 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, provides the reflectivity of the target within a 0.08-mrad-diameter laser spot. Thus MLA may search for volatile deposits that exhibit high albedo within the permanently shadowed polar regions. Active measurements require sufficient signal to trigger the detector at both low and higher threshold settings on the lowest matched filter setting, 10 ms. The passive measurement employs the noise counters and threshold settings on the detector subsystem to infer radiance from a 0.4-mrad- diameter field of view. The passive mode is enabled in Science Mode, wherein the noise counts are summed over 0.5-s intervals. The Hardware Diagnostic packets may be enabled in Standby Mode to provide 8-Hz passive measurements. More details about the instrument can be found in [CAVANAUGHETAL2007].