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.  The MOLA Experiment Gridded Data Record (EGDR) is a topographic map of Mars based on altimetry data acquired by the MOLA instrument and accumulated over the course of the mission. Two types of EGDR products are produced: the Initial Experiment Gridded Data Record (IEGDR), consisting of data accumulated through at least the first 30 days of the mapping mission, and the Mission Experiment Gridded Data Record (MEGDR), consisting of data accumulated over the whole primary mission. Different resolutions of the IEGDR and MEGDR may be released, and multiple versions of each product may be released. See the MOLA EGDR Software Interface Specification [MOLAEGDRSIS1999] for details.  The MOLA Precision Experiment Data Records (PEDRs) are the source for the EGDRs. See the MOLA PEDR Software Interface Specification [MOLAPEDRSIS1998] and the PDS Catalog entry for the PEDR data set (MGS-M-MOLA-3-PEDR-L1A-V1.0) for a description of the PEDRs.   Data : The MEGDR product is a global map of planetary radius, areoid, topography, and number of observations, derived from MOLA PEDR products and aggregated into latitude-longitude bins. The binned data include all MOLA nadir observations from the Mapping Phase through the Primary and Extended missions, from the end of aerobraking in February 1999 through June 2001. Additionally, off-nadir observations of the north pole are included from 87 N latitude and northward, taken during the spring of 1998, and of both poles taken during Mapping from 87 N and S to the poles. Data are adjusted using a first-order crossover solution for radial, along-track, and across-track position. Parts of orbits are excluded where solutions for these orbits are deemed to be poor. Also excluded are shots more than 1.2 degree off-nadir (except as noted above), channel 4 returns, and any returns not classified as ground returns, e.g. clouds or noise, according to the SHOT_CLASSIFICATION_CODE. A total of nearly 600,000,000 observations are represented.  Each image is a binary array of 8- or 16-bit integers in most-significant-byte-first storage order. The image file name is in the form MEGkxxdyyyrv.IMG, where  k : A for areoid, C for counts, R for radius, T for topography xx : latitude of pixel in upper left corner of the image d : N for north latitude, S for south yyy : longitude of the pixel in the upper left corner of the image r : map resolution in pixels per degree, e.g. C : 4 pix/deg E : 16 pix/deg F : 32 pix/deg G : 64 pix/deg H : 128 pix/deg v : version letter.   Parameters : N/A   Processing : The PEDRs incorporate the best multi-arc orbital solutions derived from the Goddard Mars potential model GMM1.6, and the available tracking. The latest spacecraft SCLK timing corrections have been applied. The ranges account for instrument delays and the leading edge timing biases, estimated by the receiver model of [ABSHIREETAL2000]. This model assumes a Gaussian shape for the transmitted and surface-scattered pulse waveforms, using the detector threshold settings and the observed pulse width and energy measurements between the threshold crossings to infer the true pulse centroid, width, and amplitude. The eccentric orbit brought MOLA much closer to the surface of Mars than the design called for, thus the pulse width and energy measurements were saturated for much of each pass. Caution must be exercised when interpreting these measurements. Laser energies are calculated according to the transmitter model of [AFZALETAL1997]. A post-launch calibration to the MOLA oscillator frequency has been applied, based on the difference between the spacecraft high-resolution timer and the MOLA clock, resulting in an estimated frequency of f:99,996,232 +/- 5 Hz. This frequency is given in the PEDR and may change due to clock drift. The interval between shots, as well as the shot time-of-flight, is controlled by this frequency. The shot interval in seconds, delta_t : 10,000,000 / f.  Time tags are given in ET seconds of MOLA fire time. Timing of the shots is interpolated to ~100 microseconds. This step is essential in the highly elliptical orbit insertion geometry because the spacecraft may change its radial distance by as much as 1600 meters per second.  The spacecraft time, from which the shot time is derived, is subject to further timing corrections. The range observations have been registered with orbital position by assuming that the actual time of observation is 117 milliseconds later than the time tag of the MOLA range as derived from the spacecraft clock.  The ground location and planetary radius is calculated in inertial (J2000) coordinates as the difference between the spacecraft position vector and the MOLA one-way range vector. The direction of the MOLA vector is obtained from project-supplied spacecraft attitude kernels and the boresight calibration of the instrument with respect to the spacecraft. The one-way range of the laser shot to the planet is obtained from the two-way range by correcting for the change in spacecraft position during laser shot time-of-flight.  Due to the inverse-square-law energy return in the link equation [ZUBERETAL1992], the instrument detector was saturated during a part of the periapsis approach. Received pulse energy and pulse width are resolved during the portion of the pass when the detector is not saturated. The absolute accuracy of these quantities is about 5%.  There is a table entry for each non-zero shot range detection for all in-range packets in the data stream. Occasional corrupted range values occur due to transmission errors, and some packets are lost entirely. A packet sequence number is generated by MOLA. The sequence number was initialized to 0 just before the planet came within range during the SPO-1 and 2 data passes via a restart command, while during the Hiatus subphase the restart occurred earlier.  Some MOLA ranges are either clouds or false detections due to the intrinsic noise characteristics of the receiver. The MOLA ranges that are true ground hits are flagged with a positive number in the tables.   Ancillary Data : N/A   Coordinate System : The MEGDR products use the areocentric coordinate system [DAVIESETAL1994B], more generally described as planetocentric. The areocentric system 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.  The MEGDR coordinate system is based on the IAU2000 cartographic standard, which differs from the IAU1991 standard used for previous MOLA products [SEIDELMANNETAL2002, DUXBURYETAL2001].   Software : N/A   Media/Format : The MGS MOLA MEGDR dataset is available 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 are based on standards established by the Planetary Data System (PDS).

Confidence Level Overview : The resolution of the altimetry data is about 40 cm vertically, and about 300 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.   Data Coverage/Quality : The MEGDR product is based on altimetry measurements acquired by the MOLA instrument from Mars orbit insertion on September 15, 1997, through the aerobraking phase, two sets of Science Phasing Orbits, the regular Mapping Mission, and the Extended Mission, up to June 30, 2001 when the MOLA laser stopped operating.