Data Set Information
DATA_SET_NAME MGS ACCELEROMETER RAW DATA RECORDS V1.1
DATA_SET_ID MGS-M-ACCEL-2-EDR-V1.1
NSSDC_DATA_SET_ID
DATA_SET_TERSE_DESCRIPTION
DATA_SET_DESCRIPTION Data Set Overview : All level zero accelerometer data are packaged by periapsis number for each aerobraking orbit. Each orbit is identified by a folder with name Pyyyy where 'yyyy' is the four digit periapsis number. Level 0 z-axis accelerometer data are provided every 0.1 seconds during an interval of time that generally assures that the initial and final data points are taken at least 200 km above the surface of Mars. Additional data, required to reduce accelerometer counts to acceleration on the spacecraft, are provided at lower sampling rates. Parameters : Accelerometer counts: Units : counts (1 count : 0.332 mm/s change in velocity) Sampling Interval : 0.1 seconds Quaternions: Units : dimensionless Sampling Interval : 1 second Filtered body rates: Units : rad/s Sampling Interval : 1 second Thruster on-times: Units : sec (cumulative time thruster has fired) Sampling Interval : 8 seconds Orbital elements: Units : various Sampling Interval : once per orbit Data : For each orbit, level 0 data consist of four arrays in four files in folder Pyyyy. The array in 'counts.tab' is n-by-11 in size, where n is the number of seconds of data received during the aerobraking pass. Column 1 contains the time in UTC and follows the PDS format YYYY-DDDTHH:MM:SS.SSSZ where YYYY : four digit year, DDD : day of year, HH : hour, MM : minute, and SS.SSS : seconds. T is a separator for date and time and the Z is the UTC Z. Columns 2 through 11 contain the 0.1 second accelerometer counts for the second beginning at the time stamp. The array in 'ratequat.tab' is n-by-8 in size. Column 1 is the time corresponding to the filtered rates and quaternions in the same UTC format as described for the counts above. Columns 2-4 contain the angular rates about the x, y and z axes respectively. Columns 5-8 contain the quaternions. The third file, 'thruster.tab', is a k-by-13 array, where k is within 1 of n/8. The first column is time as described above. The next 12 columns are the cumulative reading of how long each thruster has been fired during the mission. Column two corresponds to thruster number one, column 3 with thruster 2, and so on. Column 13 corresponds with thruster 12. Thrusters 1-8 produce moments about the 'x-y' axes by forces along the s/c z-axis and thereby corrupt the accelerometer measurements. Thrusters 9-12 provide roll about the z-axis and no detectible corruption of z-acceleration has been found. The fourth file, 'orbelem.tab', is a 1-by-6 array of osculating elements at periapsis in the order semi-major axis (km), eccentricity, inclination (radians), longitude of the ascending node (radians), argument of periapsis (radians), and universal time of periapsis (seconds past J2000). Coordinate System : Spacecraft body coordinate system has the origin at the center of mass. The z-axis is normal to the main engine nozzle etiz plane and positive in the direction of the science instrument deck. Positive x-axis is in the direction of the high gain antenna. See DOCUMENT/JSR01_04.TIF or [CANCROETAL1998] for graphic. Acceleration and rates are given in the MGS body system with the z-axis along the centerline of the bus, y being along the solar array inner gimbal rotation axis, and positive x on the same side of the bus as the high gain antenna. The quaternions define the orientation of the body axes with respect to the IAU Mars Centered Mars Equatorial at Time of Jan 1, 2000 12:00. Orbital elements are given relative to the IAU system. The reference geoid used is a (4,4) representation of the Mars gravitational field. There is a negligible difference between current Mars gravitational models up to fourth order and degree. The objective is to determine the areo-potential altitude to within 100m. Timing : The times in the data files are UTC. For orbit calculations, ephemeris time was converted to UTC and the spacecraft data were already in UTC. During the early part of operations it was found that the time tags on the three fundamental data types - accelerometer counts, attitude rates, and thruster firing times - were not synchronized. Information on the telemetry maps for all these data were not available. Five of the 10 accelerometer channels came from each of the two onboard computers and special care was taken to assure that these data were properly ordered in time. Accelerometer data time tags were taken as the fundamental reference and all orbital velocities and positions are interpolated to this time. Between Phase 1 and Phase 2 of aerobraking, it was found that the accelerometer time tags were delayed 1.5 seconds on the s/c and subsequently all accelerometer data were shifted 1.5 seconds relative to UTC. As mentioned above, attitude rates have a frequency dependent delay in addition to any basic uncertainty in time tagging. Finally, there is uncertainty in the proper time tags for thruster firing. To synchronize the rates and thruster data with the accelerometer data, numerous firings of the x- and y- thrusters throughout the mission were studied. Such thruster firings should produce simultaneous effects in rates and acceleration. By these means it was found that for the purposes herein, the rate time tags required shifting by -2 seconds to be in sync with the already shifted accelerometer data. By studying thruster firings that occurred on either side of the 8 sec thruster data interval, it was concluded that the thruster time tag needed to be shifted by -0.5 seconds.
DATA_SET_RELEASE_DATE 1999-08-15T00:00:00.000Z
START_TIME 1997-11-13T12:58:24.680Z
STOP_TIME 1999-02-04T09:12:48.010Z
MISSION_NAME MARS GLOBAL SURVEYOR
MISSION_START_DATE 1994-10-12T12:00:00.000Z
MISSION_STOP_DATE 2007-09-30T12:00:00.000Z
TARGET_NAME MARS
TARGET_TYPE PLANET
INSTRUMENT_HOST_ID MGS
INSTRUMENT_NAME ACCELEROMETER
INSTRUMENT_ID ACCEL
INSTRUMENT_TYPE ACCELEROMETER
NODE_NAME Planetary Atmospheres
ARCHIVE_STATUS ARCHIVED
CONFIDENCE_LEVEL_NOTE Confidence Level Overview : After the effects of rate filtering are included (see Limitations below), the confidence level for the rate and quaternion data is sufficient for reducing the accelerometer data to density at the noise level of the accelerometer. Accelerometer data would most likely be corrupted by changes in temperature of the instrument. The temperature of the accelerometer environment is actively controlled, and short term (order 10 seconds) variations are expected to be less than 1/6 count. Variations in accelerometer bias due to temperature changes over a pass have been less than 0.1 counts. Except for the time of periapsis, errors in remaining orbital elements are of sufficient accuracy for data interpretation. During phase 1 of aerobraking, Sept. 1997 through March 1998, every orbit was reconstructed using DSN tracking data on each side of periapsis. This provided errors in the time of periapsis of less than 1 second. During phase 2, this was not the case and larger errors may result. Review : All of the data types included in the level 0 product are utilized by the MGS operations to monitor the health of the spacecraft. These data are reviewed in near real time to assure MGS performance. Data Coverage and Quality : Data coverage during an aerobraking pass has varied throughout the mission. Early in the mission, data were received for 500 seconds on either side of periapsis. Later, the data started between 200 and 250 seconds before periapsis. This change was made to reduce propulsion usage. Though this change somewhat reduced the accuracy of determining the accelerometer bias, it had the advantage of reducing the corruption of the data set with thruster firings while still inside the detectable atmosphere. The data for the early orbits of MGS have an eight second sample time. This does not agree with software devised to archive this data, therefore there is no plan, at this time, to archive the early orbit data. Limitations : The rate and quaternion data are calculated onboard the spacecraft. Raw rate information from the rate gyros are filtered to remove a potential interaction with a 2 Hz structural vibration mode. This produces an approximate 2 second delay in the output rates. Quarternions are obtained by onboard integration of rates, providing a delay of about 1 second.
CITATION_DESCRIPTION Citation TBD
ABSTRACT_TEXT All level zero accelerometer data are packaged by periapsis number for each aerobraking orbit. Each orbit is identified by a folder with name Pyyyy where 'yyyy' is the four digit periapsis number. Level 0 z-axis accelerometer data are provided every 0.1 seconds during an interval of time that generally assures that the initial and final data points are taken at least 200 km above the surface of Mars. Additional data, required to reduce accelerometer counts to acceleration on the spacecraft, are provided at lower sampling rates.
PRODUCER_FULL_NAME GERALD M. KEATING
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