DATA_SET_DESCRIPTION |
Data Set Overview
=================
The Mars Global Surveyor spacecraft includes a laser altimeter
instrument. The primary objective of the Mars Orbiter Laser
Altimeter (MOLA) is to determine globally the topography of Mars at
a level suitable for addressing problems in geology and geophysics.
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.
The ability of MOLA to determine the surface albedo actively is
limited by the precision and dynamic range of the energy measurement
of returned pulse echoes, and by the changing characteristics of the
atmosphere. There is a passive measurement capability as well, that
depends on the background light received by the detector from solar
illumination reflected off the surface. This capability has been
enhanced in resolution by replacing the altimetry in the MOLA
telemetry packet with high-rate background counts. The detector
images an area on the surface approximately 300 m by 300 m. The
detector was calibrated prior to launch while in thermal vacuum
testing with a series of noise vs. threshold tests, as well as for
noise response vs.temperature. Packet data include detector
threshold voltages and average noise counts once per second, as well
as temperature readings from an interface plate on which the
detector is mounted. The noise counts on channels 1 and 2 are now
recorded 8 times per second, during which time MOLA travels
approximately 375 m, for an along-track resolution of approximately
1/2 km. The measurements have been made continuously throughout the
mission at 3-km resolution along-track, with a lower average
background count rate and therefore lower overall precision.
A Precision Radiometry Data Record (PRDR) contains MOLA science mode
telemetry data that has been converted to engineering and physical
units. Each PRDR contains a 1.25 second span of data, called a
half-frame, that is retrieved from the MOLA science mode telemetry
packet. Therefore, 14 PRDRs are generated from each MOLA packet
during a 17.5 second interval. Additional packet information, e.g.,
time, thresholds, temperature etc, are stored in the PRDR. These
values are used to process the telemetry data into the PRDR. The
raw telemetry data continue to be stored in the AEDR data set.
Contained in a PRDR are range value, target position, and the
planetary radius at the half-frame mid-point, interpolated from
precision orbit data, spacecraft attitude data, and a MOLA shape
model of Mars at 1/16 degree per pixel resolution. Also included are
solar illumination data and season. These data are for convenience
in interpretation and do not constitute independent planetometric
data. There are 10 radiometric values or 'shots' in each
half-frame. The location of individual shots may be obtained by
interpolation for a pair of half-frames via the generic formula:
shot_location = x * mid_pt_location1 + (1.-x) * mid_pt_location2,
where x = (shot-5.5)/10. for half-frame 1, and x =
(shot-15.5)/10. for half-frame 2.
The range and precision orbit data are given with respect to the
Mars Global Surveyor center of mass. The planetary radius values are
computed with respect to the center of mass of Mars. Ground
locations are given in the IAU 2000 coordinate system
[SEIDELMANNETAL2002].
A complete listing of all parameters contained in a PRDR can be
found in the accompanying detached label.
Data
====
The Precision Radiometry Data Record (PRDR) was not defined prior to
the end of the altimetry mission. The files are in ASCII text format
with detached PDS labels. The PRDRS consist of 38 columns of
instrument science and engineering data, sub-spacecraft location,
and ancillary illumination data. For further information see
SUNETAL2006 in the DOCUMENT directory.
Parameters
==========
The MOLA instrument measures the spectral radiant flux from the
Martian surface at 1064 nm wavelength, in a 2-nm-wide waveband.
Such measurements have been normalized to a constant mean solar
distance for convenience. They may be interpreted as a spectral
albedo using the solar constant of roughly 290 milliwatts per meter
squared per nanometer at 1064 nm wavelength, given appropriate
corrections for the illumination angle of the sun and for surface
scattering.
Processing
==========
The PRDRs are processed using the threshold calibrations in the
receiver model of [ABSHIREETAL2000]. The thresholds and noise counts
are inverted for radiant power at the detector using an analytic
model [SUNETAL1992, SUNETAL2001] corrected for measurement
biases. The model assumes a Gaussian noise characteristic convolved
with the response of a 5-pole Bessel lowpass filter, using detector
response and noise characteristics. A further correction for changes
in the detector sensitivity with respect to temperature is applied,
allowing for phase shifts between the temperature of the detector
and the measurement point. Caution must be exercised when
interpreting these measurements pending inter-instrument
comparisons.
Time tags are given in ET seconds of MOLA fire time. Timing of the
shots is synchronized to the spacecraft 8-Hz real-time interrupt
(RTI) signal, as well as the spacecraft one-second time marks. The
latest spacecraft SCLK timing corrections have been applied. In the
software version RTIB, the RTI signal counter was stored for each
shot in the storage once allocated for laser transmit energy.
Following instrument reset on 19 January 2002, after current
anomalies suggested that the laser might occasionally be firing, the
laser energy record was restored and the RTI counter was stored once
per frame in an unused location. This packet format change was
implemented in the processing software version RTIB2.
The ground location and planetary radius is calculated in inertial
(J2000) coordinates by projecting the MOLA boresight vector to the
surface of Mars, using project-supplied spacecraft attitude kernels
and the boresight calibration of the instrument with respect to the
spacecraft. An iterative solution is obtained for the planetary
range using a 1/16 degree per pixel MOLA shape model. The ground
point position vector is transformed into planetocentric body-fixed
coordinates, using the IAU 2000 planetary rotation model
[SEIDELMANNETAL2002].
Ancillary Data
==============
N/A
Coordinate System
=================
The areocentric coordinate system is used to describe data products
on this volume. The areocentric coordinate system, more
generally described as planetocentric, is body-centered, using the
center-of-mass as the origin. Areocentric latitude is defined by
the angle between the equatorial plane and a vector extending from
the origin of the coordinate system to the relevant point on the
surface. Latitude is measured from -90 degrees at the south pole to
+90 degrees at the north pole. Longitude extends from 0 to 360
degrees, with values increasing eastward (i.e., it is a right-handed
coordinate system) from the prime meridian [DAVIESETAL1994B]. This
coordinate system is preferred for use in geophysical studies in
which, for example, estimates of elevation or gravitational
potential are generated mathematically.
Software
=======
N/A
Media/Format
============
The MGS MOLA PRDR dataset will be available on DVD media and
electronically via the PDS Geosciences Node web site at
http://wwwpds.wustl.edu and the MOLA Science Team web site at
http://ltpwww.gsfc.nasa.gov/tharsis/mola.html. Formats will be
based on standards established by the Planetary Data System (PDS).
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CONFIDENCE_LEVEL_NOTE |
Overview
========
The resolution of the data is about 300 m across-track and 500 m
along-track. The uncertainty in absolute ground spot location is
limited by the attitude knowledge of the spacecraft, and is
estimated to be about 400 m at a nominal range of 400 km.
The radiometry calibrations are accurate to approximately one
milliwatt per meter squared per steradian per nanometer, subject
to further calibration.
Review
======
The volume containing the MOLA PRDR radiometry dataset was reviewed
by MOLA science team members and the PDS.
Data Coverage/Quality
=====================
On June 30, 2001, the Mars Orbiter Laser Altimeter (MOLA) ceased
altimetry operations. Operation in radiometry mode commenced on
October 10, 2001, and have continued with some interruptions due
to spacecraft anomalies and investigation of a current anomaly
on the MOLA instrument on January 19, 2002. A total of 134 days
of data were collected up to MGS entry into contingency mode
on February 27, 2002. Operations resumed and are ongoing as of
July 2002.
Limitations
===========
The use of MOLA as a radiometer was not part of the initial instrument
requirements. Such calibrations were part of routine checkout.
The long-term stability of radiometric measurements has not been
quantified.
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