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Overview
========
The resolution of the data is about 40 cm vertically, and about 330
m along-track, limited by the 10 Hz firing rate of the laser. The
absolute, long-wavelength radial orbit error is estimated to be
about 30 m. 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.
Review
======
MOLA AEDR and PEDR archive volumes are reviewed by MGS
mission scientists and by PDS.
Data Coverage/Quality
=====================
On May 26, 1998, the Mars Global Surveyor (MGS) spacecraft entered
into Phase 2 of the Science Phasing Orbit (SPO-2). SPO is a
near-polar (92.869 degrees) inclination orbit with a period of 11.6
hours and a periapsis altitude of about 170 km. During SPO-2 MOLA
will collect observations of Mars' northern hemisphere, with
emphasis on detailed mapping of the north polar ice cap. Late June
and early July 1998 is expected to be the period of maximum ice
loading for the northern cap for the current Martian year and thus
represents an especially exciting and crucial time for MOLA
observations. We anticipate that the observations collected during
this period will contribute significantly towards understanding the
present-day Martian volatile budget. We have just completed a
two-week period where the MGS spacecraft was tilted on alternating
orbits so that MOLA could fill in the 2 degree coverage gap at the
north pole that occurred because the spacecraft orbital inclination
is not exactly 90 degrees.
MOLA collected 61 topographic profiles of Mars' northern hemisphere
during the first phase of the MGS Science Phasing Orbit (SPO-1) that
spanned the period from March 26, 1998 until April 28, 1998. All of
the MOLA data collected during SPO-1 were presented in thirteen
talks and posters during the week of May 26, 1998 at the Spring
Meeting of the American Geophysical Union in Boston.
MOLA's SPO-1 observations were collected during orbital passes in
which targeted imaging of surface features was not being attempted.
Collection of images of target sites (Viking 1 & 2 and Pathfinder
landing sites and Cydonia) resulted in a loss of about 25% of the
data that MOLA could have been collected during that period.
SPO-1 ended in mid-May, just before solar conjunction. During
conjunction the sun is in the line of sight of the spacecraft, which
interferes with communication, so commanding of the spacecraft is
minimized.
The Science Phasing Orbit represents a hiatus from aerobraking that
is needed so that the spacecraft will achieve the desired local time
for the mapping orbit that will be entered next spring. SPO will
last until September 11, 1998, after which time MGS will resume
aerobraking to circularize its current elliptical orbit. During
aerobraking passes, the MOLA instrument does not collect data
because the instrument is not pointed at the surface during the
period of time when the spacecraft is within ranging distance.
Previous MOLA data was collected during the capture orbit phase of
the MGS mission shortly after orbit insertion on September 15,
1997. A further 17 passes were collected between October 14 and
November 6, 1997, during a hiatus in the aerobraking phase
necessitated by a study of the integrity of a solar panel that was
slightly damaged after launch.
Limitations
===========
Our current understanding of the Martian environment, the
capabilities of MGS, and its suite of instruments is changing
rapidly. MOLA has met or exceeded its design expectations. It has
demonstrated a measurement precision of 30 centimeters over flat
terrain. While designed for nadir-looking operation in a circular,
365- to 445-km- high orbit, MOLA has ranged successfully to Mars at
distances from 170 to 786 km, and to surface slopes up to 60
degrees. MOLA has ranged to the surfaces of clouds lying at
elevations of a few hundred meters above the surface, to over 15 km
high, and returned measurements of atmospheric opacity greater than
2 during dust storms. MOLA returned 628 ranges to the moon Phobos in
an orbital fast-flyby. The planetary range detection rate in clear
atmosphere has exceeded 99% over smooth and rough terrain.
The MOLA ranges and precision orbit data are preliminary, and will
be revised as our knowledge of the spacecraft and the Martian
gravity field improves. Important details of the instrument design
and the progress of the mission are found in the files INST.CAT and
MISSION.CAT. The orbital, atmospheric and thermal environment of the
Orbit Insertion phase has introduced uncertainties in the data
quality. The eccentric orbits and frequent off-nadir pointing during
ranging cause a greater sensitivity to errors in spacecraft timing
and attitude knowledge than expected in mapping orbit.
Orbital location is derived from radio observations and a host of
dynamic variables, most important of which is the gravitational
attraction of Mars. Improvements in the gravity field are best
obtained from tracking at low elevations, now being obtained from
MGS. The gravity model used to calculate the orbits is an interim
solution, internally designated mgm0827e, derived from Goddard Mars
Model 1. This model is given in the software directory as GMM1.2
for the purpose of defining an equipotential topographic reference
surface. GMM1.2 is necessarily constrained and lacks detailed
resolution of the polar regions, so that unmodeled orbital
perturbations accumulate. At the same time, the areoid reference
surface may vary by tens of meters depending on the choice of
gravity model. The altimetric error budget is currently dominated by
orbital uncertainty, and does not yet meet our goal of 30 m
accuracy.
The spacecraft radial distance from Mars may change up to 1.6 meters
in a millisecond due to orbital eccentricity, and up to 8 meters
between the time the pulse is fired and it is received. Altimetric
processing therefore depends strongly on timing accuracy and
knowledge of the direction in which the laser is fired. MOLA data
are time-tagged once per packet with a spacecraft time code,
calibrated to ground time. An instrument clock synchronized to the
Payload Data System provides 1/256 second resolution timing. The
PEDRs contain interpolated laser transmit time to a precision of a
tenth of a millisecond.
Altimetric crossovers are being used to assess the accuracy of the
data. It has been determined that the observations have a systematic
timing bias, further, that the attitude knowledge of the spacecraft
is offset. The range observations have been registered with orbital
position by assuming that the time tag of the MOLA range, as derived
from the spacecraft clock, is 113 milliseconds earlier than the
actual transmit time. In addition it is assumed that the time tag of
the attitude kernel provided by the MGS Project is one second later
than the time of the spacecraft attitude sensor readings, due to a
software filter delay. The precise causes and amounts of offset are
under investigation.
Range measurements are affected by the counting frequency standard,
electronic delays, and spreading of the returned pulse due to ground
slope and detector characteristics. The MOLA timing interval unit
has a an accuracy of ~2.5 nanoseconds, its precision being extended
from the 10 ns clock rate by two interpolator bits. However, 'range
walk' due to variable threshold settings, pulse amplitude and shape,
can be many times greater than measurement precision, especially
over rough terrain. The MOLA instrument records the pulse width and
amplitude during the time that the signal exceeds a
software-controlled threshold. Shot ranges are corrected in
processing via a mathematical receiver model [ABSHIREETAL2000],
assuming linear instrument behavior. Flat and highly reflective
terrain, short ranges, and abnormal atmospheric conditions can drive
the electronics into saturation, increasing detected pulse width and
invalidating the instrument model. The range corrections for
saturated returns are limited to their equivalents for terrain with
a slope of one in sixteen. Meter-level changes in topography must be
interpreted in the context of the range correction values in the
PEDR files.
The returned-optical-pulse-width and energy measurements must also
be interpreted with caution, in view of the above-mentioned effects.
Moreover, the detectors were not calibrated for the unusually cold
conditions experienced during Orbit Insertion. Energy values are
slightly higher than measured by test equipment under optimal
conditions. The unsaturated return energy and reflectivity
measurements were only designed for 5% accuracy in any case.
Lastly, the presence of highly reflective clouds, and a level of
noise returns consistent with instrument tradeoffs, has necessitated
an empirical classification of shots as to their origin. The first
shot of every 140 is likely to be triggered by an internal test
source, but may be a valid ground return, while 0.5% of the shots
result from detector noise exceeding the triggering threshold. The
probable ground returns have been flagged based on a combination of
measurements and a stochastic model of topographic variability. An
unambiguous classification is often impossible, given clouds that
often follow the surface, and the dramatic variability of Martian
terrain. The classification should be used only as a guide.
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