Instrument Overview
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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. When in the Mapping Phase of the mission, MOLA provides measurements of the topography of Mars within approximately 120 m diameter footprints and a center-to-center along-track footprint spacing of 300 m along the MGS nadir ground-track. The elevation measurements are quantized with 1.5 m vertical resolution by the 100 MHz timing interval unit (TIU) and an interpolator, giving it effectively 0.375 m resolution. MOLA profiles are adjusted for orbit andpointing errors using 66 million altimetric crossover constraints.
MOLA profiles are assembled into global planetocentric grids referenced to Mars' center-of-mass with an absolute accuracy of approximately 1 m. Standard data products include NASA level 0 (CODMAC Level 2) corrected telemetry, NASA level 1a (CODMAC Level 3) profiles in engineering and geophysical units, and NASA level 2 (CODMAC Level 5) maps at various resolutions of planetary shape (radius) areoid (equipotential datum surface), topography (shape-equipotential), and maps of shot counts per bin. With roughly 10,000 usable orbital profiles, each with ascending and descending equator crossings, mapping resolution is limited mainly by the across-track spacing of individual orbits, and by the along-track spacing of MOLA footprints. At 1/32 degree by 1/32 degree per pixel, more than one half of all pixels contain at least one observation, while higher density occurs at the poles. Products at resolutions of up to 128 pixels per degree are available, in factor-of-two increments, interpolated where necessary byminimum-curvature surfaces under tension [SMITH&WESSEL1990].
Special products include images of topographic gradients, kilometer- and footprint-scale roughness, and a global 0.25 x 0.25 degree gridof 1.064 micrometer surface reflectivity.

All major components of MOLA except for the laser and telescope were designed, built and tested at NASA's Goddard Space Flight Center,Greenbelt, MD.

On June 31, 2001, after firing 670 million shots during 4.5 years of the primary and extended missions and undergoing several hundred power cycles, MOLA ceased operation as an altimeter. A 100 MHz timing oscillator signal, divided down to a 10 Hz interrupt, degraded rapidly and then failed. Laser firing, controlled in hardware by the 10 Hz signal, is no longer possible, but the receiver electronics are fully operational. A software patch was uplinked that records at high resolution the detector noise counts in place of laser data, providing a radiometric signal. The background noise counts from the MOLA receiver can be used as aradiometric measurement in the 1.064 micron band.

In radiometer mode, MOLA is clocked by 8 Hz interrupts from the spacecraft master clock, and the original MOLA 10 Hz interupt has been masked. Flight software sets the receiver threshold every 10 interrupts, to maintain an approximately constant rate of noise triggers. Noise counts are recorded at every interrupt on channels 1 and 2, and the totals for a half-frame of 10 interrupts arestored for all four channels in a compressed (pseudo-log) format.
The precision of the measurement is limited by the statistics of the approximately 1000 noise counts per shot. Summing channels 1 and 2 increases precision. Each half-frame constitutes a single record with a duration of 1.25 seconds and provides 10 radiometric measurements. The threshold settings and noise counts are interpreted radiometrically using an analytical model of the MOLA receiver characteristics. This model [SUNETAL1992, SUNETAL2001] has been calibrated with respect to preflight test data and in-flight experience, as well as similar measurements obtained by the Mars Global Surveyor Thermal Emission Spectrometer Bolometer and by the Hubble Space Telescope. Precision orbit data describing the instrument's position and target location has been added to each record. The precision orbit data is supplied by the MOLA ScienceTeam.


MOLA Science Objectives
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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 ofpossible future Mars landing sites.


Instrument Specifications
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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 40-30 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 2.2 nm bandpass Field of view ~0.85 mrad
Receiver Electronics Receiver type Match-filtered leading-edge trigger Time resolution 10 nsec Range resolution 1.5 m Pulse energy resolution 20%
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 617.14 bits sec^-1
* Nd:YAG is neodymium-doped yttrium aluminum garnet.
** FWHM is full width at half maximum.
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Operational Considerations
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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 singleautonomous mode, in which it produces ranging measurements.
Surface topography estimates can be derived from these data, given appropriate corrections for the position and attitude of thespacecraft.

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. The pulse energy was 45 mJ at the beginning of the Mapping Phase and 20 mJ at end of mission. Energy fluctuated somewhat as temperature varied aboard the spacecraft, and dropped several times when individual diode bars failed. The remaining diodes appear to have been operating at full output when the firing signalstopped. The laser consumed 13.7 W when operating.

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 opticalefficiency.