PDS_VERSION_ID = PDS3 LABEL_REVISION_NOTE = "S. SLAVNEY, 1998-09-10; G. NEUMANN, 2003-02-26" RECORD_TYPE = STREAM OBJECT = INSTRUMENT INSTRUMENT_HOST_ID = MGS INSTRUMENT_ID = MOLA OBJECT = INSTRUMENT_INFORMATION INSTRUMENT_NAME = "MARS ORBITER LASER ALTIMETER" INSTRUMENT_TYPE = "LASER ALTIMETER" INSTRUMENT_DESC = " Instrument Overview =================== The principal components of MOLA are a diode-pumped, Nd:YAG laser transmitter that emits 1.064 micrometer wavelength laser pulses, a 0.5 m diameter telescope, a silicon avalanche photodiode detector, and a time interval unit with 10 nsec resolution. Additional delay fibers increase the effective resolution to 2.5 ns. When in the Mapping Phase of the mission, MOLA provides measurements of the topography of Mars within approximately 160 m footprints and a center-to-center along-track footprint spacing of 300 m along the MGS nadir ground-track. Range measurements, with an effective resolution of 37 cm, are converted to profiles of planetary radius and topographic height after correction for orbit and pointing errors. Radial accuracy of individual profiles is approximately 1 m RMS, as determined by altimetric crossovers, and shot locations are determined to within 100 m in the along-track and across-track directions. MOLA profiles have been assembled into global grids referenced to Mars' center-of-mass, with resolutions of up to 1/128 degree per pixel, although at this density some interpolation is required across-track. Other standard data products will include near-global grids of footprint-scale roughness and 1.064 micrometer surface reflectivity. The background solar illumination noise level provides seasonal maps of narrow-band Lambert albedo. All components of MOLA except for the laser and telescope have been designed, built and tested at NASA's Goddard Space Flight Center, Greenbelt, MD. MOLA Science Objectives ======================= The primary MOLA objective is to determine globally the topography of Mars at a level suitable for addressing problems in geology and geophysics [ZUBERETAL1992, SMITHETAL1998]. Secondary objectives include characterizing the 1.064 micrometer wavelength surface reflectivity of Mars to contribute to analyses of global surface mineralogy and seasonal albedo changes. Other objectives include addressing problems in atmospheric circulation and providing geodetic control and topographic context for the assessment of possible future Mars landing sites. Instrument Specifications ========================= The following table summarizes MOLA characteristics. Parameter Value Unit ---------------------------------------------------------------- Physical Characteristics Volume 0.15 m^3 Mass 26.18 kg Power (TOTAL) 28.74 W Heater Power 10.00 W Laser Transmitter Laser type Q-switched, diode-pumped Nd:YAG* Wavelength 1.064 micrometer Laser energy 20-48 mJ pulse^-1 Laser power consumption 13.7 W Pulse width ~8.5 ns (FWHM**) Pulse repetition rate 10 sec^-1 Beam cross-section 25x25 mm^2 Beam divergence 0.25 mrad Altimeter Receiver Telescope type Cassegrain Mirror composition Gold-coated beryllium Telescope diameter 0.5 m Focal length 0.74 m Detector type Silicon avalanche photodiode (Si APD) Sensitivity 1 nW Optical filter 2. nm bandpass Field of view ~0.85 mrad Receiver Electronics Receiver type Match-filtered leading-edge trigger Time resolution 2.5 nsec Range resolution 0.4 m Pulse energy resolution 5% Measurements Footprint size (@ 400 km) 120 m Footprint spacing (@ velocity = 3 km/sec) (center-to-center, along-track) 300 m Computer Type 80C86 Data rate 617 bits sec^-1 * Nd:YAG is neodymium-doped yttrium aluminum garnet. ** FWHM is full width at half maximum. ---------------------------------------------------------------- Operational Considerations ========================== The MOLA instrument measures the round-trip time of flight of infrared laser pulses transmitted from the MGS spacecraft to the martian surface. The instrument normally operates in a single autonomous mode, in which it will produce ranging measurements. Surface topography estimates can be derived from these data, given appropriate corrections for the position and attitude of the spacecraft. MOLA's transmitter is a Q-switched, Nd:YAG laser oscillator which is pumped by a 44 bar laser array. Each bar contains ~1000 AlGaAs (Aluminum, Gallium Arsenide) laser diodes. The Q-switch controls the emission of the laser, and Nd:YAG refers to the composition of the material that is optically excited to produce laser action: Neodymium-doped Yttrium Aluminum Garnet. The laser emits 8.5-ns-wide (full width at half the maximum pulse amplitude, FWHM) pulses at 1.064 micrometers. The pulse repetition rate is 10 Hz, and the pulse energy was 48 mJ at the beginning of the Mapping Phase and 20 mJ at the end of its operation. The laser consumed 13.7 W when operating, and its on-orbit lifetime was 6.7x10^8 laser pulses (~2 years) when the laser firing signal ceased. The failure was traced to loss of gain in a transistor circuit driving the 100 MHz ovenized crystal oscillator, probably a result of aging and radiation damage. The development of a space-qualified, long-lifetime laser represents one of the primary engineering challenges associated with MOLA. For comparison, the ruby flashlamp laser altimeters flown on the Apollo 15, 16 and 17 missions [KAULAETAL1972, KAULAETAL1973, KAULAETAL1974] each operated for less than 10^4 laser pulses. High pulse-repetition-rate lasers with lifetimes on the order of 10^9 shots have been made possible due to breakthroughs in solid-state laser technology, resulting in improvements in the peak power, brightness, and availability of semiconductor diodes and arrays [CROSSETAL1987, BYERETAL1988]. The key technological advance has been the replacement of the flashlamp, which is the device that has traditionally been used to pump optical energy into the laser rod, with a highly efficient array of laser diodes. While flashlamp lasers fail catastrophically, diode-pumped lasers such as MOLA's instead undergo a gradual degradation in energy output as individual pump diodes fail. Laser diodes also produce the required pump energy only in a narrow region near the laser rod's absorption band, which dramatically improves the laser's electrical to optical efficiency." END_OBJECT = INSTRUMENT_INFORMATION OBJECT = INSTRUMENT_REFERENCE_INFO REFERENCE_KEY_ID = "BYERETAL1988" END_OBJECT = INSTRUMENT_REFERENCE_INFO OBJECT = INSTRUMENT_REFERENCE_INFO REFERENCE_KEY_ID = "CROSSETAL1987" END_OBJECT = INSTRUMENT_REFERENCE_INFO OBJECT = INSTRUMENT_REFERENCE_INFO REFERENCE_KEY_ID = "KAULAETAL1972" END_OBJECT = INSTRUMENT_REFERENCE_INFO OBJECT = INSTRUMENT_REFERENCE_INFO REFERENCE_KEY_ID = "KAULAETAL1973" END_OBJECT = INSTRUMENT_REFERENCE_INFO OBJECT = INSTRUMENT_REFERENCE_INFO REFERENCE_KEY_ID = "KAULAETAL1974" END_OBJECT = INSTRUMENT_REFERENCE_INFO OBJECT = INSTRUMENT_REFERENCE_INFO REFERENCE_KEY_ID = "SMITHETAL1998" END_OBJECT = INSTRUMENT_REFERENCE_INFO OBJECT = INSTRUMENT_REFERENCE_INFO REFERENCE_KEY_ID = "ZUBERETAL1992" END_OBJECT = INSTRUMENT_REFERENCE_INFO END_OBJECT = INSTRUMENT END