Instrument Information
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.  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 and pointing 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
  by minimum-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 grid
  of 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 a radiometric 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 are stored 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
  Science Team.

  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                       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%

  Footprint size (@ 400 km)          120              m
  Footprint spacing (@ velocity = 3 km/sec)
 (center-to-center, along-track)   300                m

  Type                               80C86
  Data rate                          617.14           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 produces ranging measurements.
  Surface topography estimates can be derived from these data, given
  appropriate corrections for the position and attitude of the

  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 signal stopped.
  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 optical