Data Set Overview : The Mars Odyssey Gamma-Ray Spectrometer (GRS) is a suite of three instruments working together to collect data that will permit the mapping of elemental concentrations on the surface of Mars. The suite of three instruments, the gamma sensor head (GS), the neutron spectrometer (NS) and the high-energy neutron detector (HEND), are a complimentary set of instruments in that the neutron instruments have better counting statistics and sample to a greater depth than the GS, but the GS determines the abundance of many more elements. A full description of the Mars Odyssey Gamma-Ray Spectrometer instrument can be found in [BOYNTONETAL2002].  The ODY MARS GAMMA RAY SPECTROMETER 2 CGS (CGS) data set is a time series collection of corrected gamma spectra measurements from the Mars Odyssey spacecraft. The CGS are corrected for gain, offset and linearity. The end result of the correction is that all the corrected spectra have a common energy scale, which allows the direct comparison of individual spectra.  The GRS collects a new spectrum (pixel) approximately every 20 seconds, 360 times per orbit. Approximately 4200 spectra are expected to be received every day. The data (both science and engineering) are downloaded from the spacecraft by the Jet Propulsion Laboratory (JPL) into the Telemetry Data System (TDS). The TDS sends data to a process that translates data packets and examines instrument health via messages. Data are output to a spooler that passes it to the University of Arizona (UA) database ingest process. The ingest process inputs raw data into the UA database. Gamma data are processed through a number of programs to yield corrected gamma spactra. The corrected gamma spectra are retrieved from the UA database to build the CGS data.  The CGS is intended to be the first intermediate data product available for the gamma portion of the GRS. These data should be useful to those scientists who are experienced in gamma spectroscopy.   Parameters : The CGS data set is composed of a single data type (CGS). The objective of compiling the CGS is to create a series of gamma data that have been processed to a common energy scale that allows for direct comparison to each other. Because there are relatively few counts in each individual spectrum, it is necessary to sum spectra in order to accumulate enough counts to separate the signal peaks from the background. In order to perform a summation of spectra, they must all have a common energy scale, which is why CGS are created. Each CGS product data file will contain a time series of corrected spectra collected over a 2-hour time period. The 2-hour data files will be grouped by Earth days, i.e., 12 data files per day. The reason CGS data files are broken into 2-hour intervals is that the file sizes are large.   Processing : A full description of the gamma data processing can be found in the Mars Odyssey Gamma-ray Spectrometer Gamma Data Processing document located in the document folder that accompanies this release. The following paragraphs are a summary of how the data are processed from data receipt through corrections.  GRS gamma data are downloaded from the Mars Odyssey spacecraft by JPL into the Telemetry Data System (TDS). The TDS sends the data to a process called GRS_tl, which translates GRS packets from any source to any destination and examines instrument health via messages. The data is checked for packet types, header information, messages, errors, engineering and channelized data values and is output to a spooler, which passes data to the UA database ingestion process. The ingestion process inputs raw data into the database.  All data packets are processed for timing and spatial information. Once time data is extracted from the data packet, UTC time is calculated from the spacecraft event time (sc_ev_time) by a SPICE function. The UTC time is then inserted into the database. The spatial portion of the data packet is calculated using other SPICE routines. The returned values are all the spatial elements of an observation, including latitude and longitude at the mid-point of the observation.  Engineering values have an additional step of processing. Raw engineering values (digital) are converted to engineering unit values by scaling the raw value with a stored polynomial. The polynomials were derived through hardware calibrations, and are calculated by a series of database functions.  Once raw gamma data, with associated timing, spatial and engineering data is entered into the database, a series of processing steps are started to prepare the data for correction.  The processing begins with Engineering Smoothing. The Engineering Smoother uses a gaussian-weighted smoothing algorithm to remove noise from engineering data, and inserts the results into the engineering database tables. The next process calculates the temperature for the 170k board, an electrical component used for signal amplification in the GS. This temperature must be modeled as the temperature of the board is not directly measured. The modeled temperature is then inserted into the database. The third process extracts and deciphers digital housekeeping information that identifies the state of various components on the spacecraft. The digital housekeeping data is needed for the corrections. The engineering interpolator queries the database for uncorrected gamma spectra records where the associated engineering data fields are NULL. The program queries the engineering tables for one record on each side of the record of interest and does a linear interpolation from the two engineering records to fill in the engineering fields in the record of interest. The next process, fill_apps_gain, uses the digital housekeeping information to insert the shaping amp gain values into each spectrum record. The gain values are then used by the corrector.  The correction process calculates the gain, offset, and linearity of the uncorrected spectra based on measured temperatures of various spacecraft subsystems. Because there are not enough counts in any given collection interval to establish a calibration, the correction process uses a spectrum shifting algorithm to re-bin the counts in each spectrum, aligning the channels in all of the spectra to a common energy scale. The corrected spectra can then be directly compared and are a scientifically useful data product.   Data : The CGS data set is composed of a series of time and date stamped files that contain 2-hours worth (2 hours, 00:00:00 UTC to 02:00:00 UTC, etc) of data.  Corrected Gamma Spectra ------------- Corrected Gamma Spectra data are composed of corrected gamma spectra and the associated timing, spatial and engineering information. The CGS consists of the cumulative counts of gamma rays at the detector over one collection interval binned into 16384 channels based on energy. The collection interval is approximately 19.7 seconds, but may vary over the course of mapping. The counts in Channel 0 are all the counts that would have been shifted to 0 or less than 0 by the spectrum shifting algorithm. The counts in Channel 16383 are all the counts with energies greater than 10MeV, and counts in Channel 16382 are all the counts that belong in either 16382 or would have been shifted to 16383. The timing and spatial data provided with the spectra includes spacecraft clock values and spacecraft geometry data. The sc_ev_time, utc time and spatial fields are all recorded at the center of the collection interval.   Ancillary Data 	: Ancillary data needed to understand the gamma data processing, can be found in the Mars Odyssey Gamma-ray Spectrometer Gamma Data Processing document located in the document folder that accompanies this release.   Coordinate System : The coordinate system used for all GRS data is a Mars aerocentric system following the IAU convention [SEIDELMANNETAL2002], with east longitudes from 0 to 360.   Software : A library of source code to parse the CGS data product files is included in the software directory. This library allows a programmer to build applications that display or manipulate CGS data. This source is written in the Java language, and requires version 1.3 of the Java Runtime Environment (JRE) or Java Software Development Kit (SDK).  Documentation for the code is located in the software directory in the file GRS_DR_CODE_DOC.ZIP. The contents of this file are described in the label GRS_DR_CODE_DOC.LBL and the source and the binary classes that make up the library are in the file DR_CODE.JAR.   Media/Format : The CGS will be delivered using DVD media. Formats will be based on standards for such products established by the Planetary Data System (PDS) [PDSSR2001].

Confidence Level Overview : The data presented in the CGS is intended to be the first intermediate data set released for the GRS. Data presented here are a highly processed representation of the GRS data as received from the spacecraft. It is possible that changes will be made in the processing and correction procedure if errors are found. If errors are found, the data will have to be regenerated from the raw unprocessed data set. The usefulness of this data is limited by a significant factor; the large volume of data with a small signal-to-noise ratio.   Review : The CGS was reviewed internally by the GRS team prior to release to the PDS. PDS will also perform an external review of the CGS.   Data Coverage and Quality : Data reported are the processed gamma data received from the spacecraft during the mapping mission stage. Data coverage and quality are addressed in the following sections.   Primary Mapping --------------- The Mapping portion of the mission began February 19, 2002, and is expected to last until August, 2004. Primary mapping is broken into two phases: stowed mapping and deployed mapping.   Stowed Mapping -------------- The stowed mapping configuration began on February 19, 2002 and lasted until June 1, 2002. The stowed mapping configuration is with the Odyssey spacecraft in a mapping orbit, with the GRS in data collection configuration without the 6-meter boom deployed. The reason for data collection in the stowed configuration is to measure the background signal from the spacecraft.  During the stowed mapping portion of the mission, two anneals were performed on the gamma detector. The anneals were performed between March 8, 2002 to March 22, 2002 and May 6, 2002 to May 20, 2002. No science data was collected during these two periods. On June 1, 2002 the GRS was shutdown to prepare for boom deployment, which occurred on June 4, 2002. There may also be short periods of missing data due to missing telemetry that cannot be recovered.   Deployed Mapping ---------------- Deployed Mapping began on June 5, 2002 and is expected to continue until at least August, 2004. Gamma data collected on June 19, 2002 is suspect due a number of temperature fluctuations in the gamma subsystem. Data collected from July 16, 2002 to July 24, 2002 are suspect due to a large solar particle event. There are short data gaps due to missing telemetry that cannot be recovered.   Limitations : The major limitation of this data set is that it is derived from the raw minimally processed data. The data is received from spacecraft telemetry, and ingested into a database. If gaps exist in the telemetry, data is lost. Timing and spatial components of the data set rely on the accuracy of the NAIF SPICE kernels. The validity of the corrected gamma spectra is based on the 'correctness' of each step in the processing. It is possible that changes will be made in the processing and correction procedure if errors are found. If errors are found, the data will have to be regenerated from the raw unprocessed data set.   Data Compression : No compression is used on the CGS data set.