PDS_VERSION_ID = PDS3 RECORD_TYPE = STREAM OBJECT = TEXT PUBLICATION_DATE = 2007-08-21 NOTE = "Software Interface Specification for the Mars Odyssey Accelerometer Archive Volume." END_OBJECT = TEXT END Mars Odyssey Software Interface Specification prepared by Robert H. Tolson North Carolina State University Version 2.0 August 21, 2007 Software Interface Specification (SIS) for Mars Odyssey Accelerometer Data 1. Purpose and Scope of Document The purpose of this document is to provide a detailed description of the Accelerometer data products. This document describes the three levels of data products and the content, organization and format of the data for each level. Level 0 data includes 1 second y-axis acceleration. Level 2 data are reduced products including atmospheric density with areophysical parameters, and level 3 is the final data product including density, density scale height and other atmospheric properties at reference altitudes. 2. Applicable Documents PDS Standards and Data Dictionary documents Archive Volume SIS for the data product Tolson, R., et al., "Application of Accelerometer Data to Mars Odyssey Aerobraking and Atmospheric Modeling," Journal of Spacecraft and Rockets, Vol 42, No. 3, pp. 435-443, 2005." 3. Relationships with Other Interfaces none 4. Data Product Characteristics and Environment This section includes a description of the accelerometer as a scientific sensor, the areophysical parameters associated with the data, the details of the three levels of data products, data processing methods, data flow through the data processing system, and the labeling for each data file. 4.1 Instrument Overview An accelerometer is an instrument that measures the acceleration of the case of the sensor due to external forces. All accelerometers have a 'proof mass' and it is the tendency of the proof mass to move relative to the case that is a measure of the acceleration of the case. Early accelerometers produced output that was directly related to acceleration; but modern sensors integrate the internally measured signal to reduce noise and the output is proportional to the change in velocity over the integration time. In high precision accelerometers, like those on ODY, the proof mass is an electronically floating mass. The electromagnetic field is varied to keep the proof mass stationary relative to the case. The current required to accomplish this is proportional to the acceleration. The accelerometers on ODY are sensitive to acceleration of the center of mass (c.m.) of the s/c, pseudo-accelerations (i.e. centrifugal) due to rigid motion of the s/c about the c.m., and differences in gravitational force at the proof mass and the c.m. of the s/c (gravity gradient). The ODY Inertial Measurement Unit (IMU) contains three accelerometers. The principal accelerometer used in the aerobraking analysis is the y-axis accelerometer. This accelerometer is located at approximately (0.17, -0.52, 1.62)m relative to the center of mass. The accelerometers are Honeywell QA-2000 model Q-Flex and continuously integrate acceleration to obtain velocity data. The instrument is sampled every 0.01 seconds. The data are recorded in instrument counts or quantized velocity increments equivalent to 0.0753 mm/s per count. The QA-2000 bias has a specified temperature sensitivity of <30 ug/deg C or approximately 0.13 counts/deg C. The accelerometer bias was determined each pass by a line fit to data before and after periapse that was outside the atmosphere. The temperature sensitivity is <180 ppm/deg C and slight bias shifts can be seen during some orbits that correlate with IMU temperature. 4.2 Data Product Overview Data are packaged by periapsis number for each aerobraking orbit. The level 0 data, as defined in sections 1 and 4.3.1, are provided every second during an interval of time that generally assures that the initial and final data points are taken 60 seconds above 170 km above the surface of Mars. The level 2 data are obtained by converting each level 0 measurement to atmospheric density and providing simultaneous areo-location data as calculated from the orbit information. Level 3 data include atmospheric density, density scale height, and statistical information of the fitting processes at a number of reference altitudes. All data files are stored as ASCII. 4.3 Data Processing Version 2.0 of these products are based on processing the data during aerobraking operation. 4.3.1 Data Processing Level The ODY Accelerometer archival data consist of 3 levels as mentioned in section 1. Level 0 data include one set of global data which consists of spacecraft properties. These properties include the spacecraft mass, characteristic area, characteristic length, and aerodynamic properties. These data files are contained in a separate directory from the other level 0 data. Other level 0 data include all processed instrument data at 1 second resolution. These include accelerometer counts, quaternions, and rates. The thruster on times are shown whenever there is a thruster firing. The final level 0 data are the orbital parameters at periapsis which are used in numerous calculations. Level 2 data include density and areophysical parameters versus time from periapsis at 1 second resolution. The areophysical parameters include latitude, longitude, local solar time, solar zenith angle, and altitude. One sigma uncertainties are provided each density value. Level 3 data for each orbit consist of inbound and outbound density at constant altitude levels (i.e. 100, 110, ...) and corresponding derived quantities on each pass including density scale height, and standard deviation of the fit. For each altitude level, the areodetic latitude, longitude, local solar time, and Ls at Mars are given. Altitudes are determined above the reference areoid. Level 3 data also include, for each orbit, the 1.26 nanobar height and the latitude, longitude, local solar time and Ls at Mars. The standard deviation on derived quantities is also provided. 4.3.2 Data Product Generation The data products were generated by the ODY Accelerometer Team. Accelerometer counts, quaternions, and angular rates were received from Odyssey after each pass. Additionally, the orbital elements were supplied by the NAV team for each periapsis. Using this information along with the spacecraft physical and aerodynamic properties, the density of the atmosphere was calculated. The accelerometer counts are calibrated using the spacecraft properties to yield the acceleration along the y-axis of the spacecraft. The angular rates are then used to remove the acceleration caused by the rotation of the spacecraft. An iterative approach is finally used to solve the force-drag relationship for density. Once the density of the atmosphere is known, other atmospheric properties such as density scale height are computed. 4.3.3 Data Flow During operations, the data from the spacecraft are received by the ODY GDS. The accelerometer counts, rates, quaternions and thruster data are transferred from behind the Project firewall to the ODY Accelerometer Team operations computer at NASA Langley Research Center. Operational data products are then generated and sent to numerous operations teams. Many of these same products are used to generate Version 2 for the PDS. The size of the individual data products, consisting of one orbit worth of data, is around 160 kilobytes. The total size of the data products from ODY aerobraking is around 45 megabytes. 4.3.4 Labeling and Identification PDS labels are located at the beginning of each file. Due to similarity of the columns for each data set, the column by column description for each data set is listed only once for each type of data (i.e. acceleration, quaternions and rates, and the level 2 atmospheric profile.) Each data file, *.TAB, has a PDS label as a header, but references a format, *.FMT, file which gives a column by column description. 4.4 Standards Used in Generating Data Products 4.4.1 PDS standards 4.4.2 Time Standards Level 0: Acceleration, angular rates, quaternions, and thruster on times are all time stamped with the Coordinated Universal Time (UTC). The time standard associated with the orbital elements is also the Coordinated Universal Time (UTC). It has been converted to UTC using the Ephemeris to UTC given by NAV in the OPTG. Levels 2 and 3: The time standard used for all data in these two levels is time from periapsis, where the time of periapsis is that given by NAV on the reconstructed OPTG for each pass. 4.4.3 Coordinate Systems When applicable, all data are in the J2000 Mars centered, Mars equator coordinate system. 4.4.4 Data Storage Conventions The data are stored as ASCII text. A minus 1 (-1) (no decimal places) anywhere in the data files indicates that data was not available. For example, in the level 3 data where the atmospheric properties are given at constant altitude levels, there are times when the periapsis altitude is greater than the lower altitude levels. Hence there are no data for some altitude levels of some orbits. For these orbits and altitudes, all corresponding quantities in the data file are minus 1 (-1). Some passes did not yield accelerometer data. For example, no accelerometer data were collected during passes 001-009, and 327. There are no folders or files for these passes. 4.5 Data Validation Accelerometer data are validated by comparing the equivalent impulsive velocity change each aerobraking pass with the equivalent number from the navigation team. Comparisons are provided in reference TOLSONETAL2005. 5. Detailed Data Specifications 5.1 Data Product Structure and Organization Except for the spacecraft properties, the ODY accelerometer data are stored by periapsis number. The spacecraft properties are grouped together and saved in the directory 'CALIB'. The rest of the Level 0 data are in directories based on periapsis. The periapsis directories follow the naming convention of Pxxx where xxx represents the orbit number. These Level 0 directories contain 4 files. Acceleration, rates and quaternions, thruster on times, and orbital elements are stored as arrays in the files 'accel.tab', 'ratequat.tab', 'thruster.tab' and 'orbelem.tab' respectively. Within the Level 2 and 3 directories, are files for each periapsis using the naming convention Pxxx.tab which contains all of the data to describe the pass, where P is the orbit number and xxx the periapsis number. The Level 3 data files contain data for the constant altitude levels. These altitude levels are periapsis, 100, 110,...,140 km, those corresponding to a 1.26 nbar pressure level and the maximum density. Inbound and outbound data for each of these altitudes are considered separately except for maximum density and periapsis. 5.2 Data Format Descriptions Level 0: The array in 'accel.tab' is n-by-4 in size, where n is the number of seconds worth of data received during that aerobraking pass. Column 1 contains the time in UTC and follows the PDS format. The format is 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. Column 2-4 contains the 1 second acceleration for the second beginning at the time stamp in the body xyz frame. 'ratequat.tab' has an array which is n-by-8 in size. Column 1 is the time corresponding to the 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-4 array. The first column is time as described above. The next 4 are a running 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. Thruster locations can be found in refernece AACSDEFN. 'orbelem.tab' has a 1-by-6 array. Columns 1-6 are the 6 orbital elements as supplied by NAV on the OPTG. Level 2: 'Pxxx.tab' is a m-by-8 array, where m is the number of seconds of data within close proximity to the Martian atmosphere. The data in each of the 8 columns going from column 1 to 8 are seconds from periapsis, latitude, longitude, local solar time, solar zenith angle, altitude above the areodetic reference spheroid (a=3396 and f=5.206e-3), density and sigma. Level 3: 'Pxxx.tab' has 11 rows and 10 columns. The columns have inbound and outbound quantities for different constant altitude levels which are each in separate rows. The rows are separated as follows: periapsis, 1.26 nbar level inbound, 1.26 nbar level outbound, 100 km inbound, 100 km outbound, 110 km inbound, 110 km outbound, 120 km inbound, 120 km outbound, 30 km inbound, 130 km outbound, 140 km inbound and 140 km outbound. The columns from 1 through 10 are separated as follows: altitude, latitude, longitude, local solar time, longitude of the sun, density, density sigma, density scale height, and scale height sigma. 39 second averaged data are used to determine Level 3 values. 5.3 Label and Header Descriptions A label is located at the beginning of each data file. PDS3 is the PDS_VERSION_ID that was used to create the labels. 6. Applicable Software NASAView 7. Appendices 7.1 Acronyms ODY - Mars Odyssey Orbiter JPL - Jet Propulsion Laboratory Ls - Longitude of the Sun CG - Center of Gravity NAV - Navigation Team ET - Ephemeris Time UTC - Coordinated Universal Time NASA - National Aeronautics and Space Administration OPTG - Orbit Prediction and Trajectory Generation CM - Center of Mass GDS - Ground Data System PDS - Planetary Data System 7.2 Definitions of Data Processing Levels The definitions given below are from appendix iii of the Mars Global Surveyor Project Archive Generation, Validation, and Transfer Plan. Level 0 -- Instrument science packets (e.g., raw voltages, rates) at 1 second resolution, time ordered, with duplicates and transmission errors removed. Level 2 -- Areophysical parameters located in space and time commensurate with instrument location, pointing, and sampling. Level 3 -- Areophysical parameters mapped onto uniform space-time grids.