Data Set Information
DATA_SET_NAME MGS RS: IONOSPHERIC ELECTRON DENSITY PROFILES V1.0
DATA_SET_ID MGS-M-RSS-5-EDS-V1.0
NSSDC_DATA_SET_ID
DATA_SET_TERSE_DESCRIPTION
DATA_SET_DESCRIPTION Data Set Overview ================= MGS radio occultation experiments were conducted using a 3.6-cm wavelength radio signal transmitted by the spacecraft and received on Earth. In principle this 'downlink' configuration allows atmospheric structure to be sampled twice per orbit. Occultation entry and exit generally occurred in the northern and southern hemispheres, respectively, and the latitude of the measurements drifted gradually in response to changes in the observing geometry. However, far more experiments were performed at occultation entry than exit, owing to a problem with one of the gimbals used to point the spacecraft high-gain antenna (HGA). This resulted in particularly dense coverage at latitudes of 60-80 deg in the northern hemisphere. The coverage in longitude of the data set is fairly dense and uniform with few exceptions. EDS files are stored as ASCII tables, preceded by a header record which contains supplementary information as described below. The headers from each individual file have been collected in a single occultation summary file, which may be useful for scanning the data set, identifying profiles of particular interest, and reviewing the coverage in latitude, season, and solar zenith angle. David Hinson at Stanford University created all EDS files and the occultation summary file. For more information about these data, see [TYLERETAL1992], [HINSONETAL1999], [HINSONETAL2001], and [TYLERETAL2001]. Parameters ========== Each EDS file contains a table of electron number density versus radius from Mars' center of mass. The file also specifies the altitude of each sample relative to the equipotential surface, or areoid, whose mean equatorial radius is 3,396,000 m. Each profile extends in altitude from about 90 km to 200 km with a sample spacing of about 1 km. A header record prepended to each table provides information about observing times, experiment geometry, and files used in the data processing. The radius of the reference areoid at a given latitude and longitude is computed using a spherical harmonic model of the gravity field, with an additional centrifugal term that arises from the rotation of Mars. Note that different spherical harmonic models were used in defining the altitude scale for the experiments conducted in 1998 (GGM50A02.SHA) and those conducted in 1999-2006 (JGM75C01.SHA). See [KONOPLIV&SJOGREN1995] for a definition of how the coefficients in these models are normalized. The spherical harmonic models can be found in PDS data set MGS-M-RSS-5-SDP-V1.0. Processing ========== The MGS radio occultation experiments were optimized for sounding the neutral atmosphere, but electron density profiles were also obtained from a limited subset of observations. The retrieval algorithm involves no a priori assumptions about the shape of the electron density profile, the peak density, or the altitude of the peak, but the electron density is assumed to be zero at altitudes below about 90 km and above about 250 km. The typical uncertainty in the MGS electron density profiles is several thousand per cubic centimeter. At this level of sensitivity, reliable profiles could be obtained over an altitude range of about 90-200 km for experiments in which the solar zenith angle was less than about 90 degrees. Electron density profiles are derived from raw receiver output in several steps. First, deterministic sources of frequency change are identified and removed coherently. These included motion of the spacecraft, motion of the Earth receiving station, drift of the UltraStable Oscillator (USO), and relativistic effects associated with gravity fields of solar system bodies. This leaves the carrier signal at a known frequency except for phase drift associated with passage of the radio ray through Mars' atmosphere and/or ionosphere. Drift associated with passage through solar plasma or the Earth's atmosphere or ionosphere is assumed to be negligible over the time of a Mars occultation (approximately two minutes). Second, using an accurate reconstruction of the spacecraft trajectory, the phase of the signal versus time is converted into a measure of refraction angle versus impact parameter -- the perpendicular distance between the incoming raypath and the center of curvature of the atmosphere. Third, the index of refraction versus radius is obtained using an Abel inversion [FJELDBOETAL1971]. At ionospheric heights, the refractive index is determined predominantly by free electrons, and there is a simple linear relationship between refractive index and electron number density. An electron density profile is obtained by applying this linear transformation to the refractive index profile, using coefficients that depend on the measurement frequency (8423 MHz). Data ==== The following description of the data is based on the original, CD-WO form of the archive. The current form of the archive is an electronic archive volume containing all the data from the original CD-WO. In the current archive, the data are organized into subdirectories of the DATA directory, according to data type and (when possible) year and month of the data. The descriptions of the meanings and names of the various data types remain the same as those given in this file. Data are stored on a single volume in the EDS directory in subdirectories in the order they were collected. Twelve subdirectories have been defined with names of the form yyyy_ddd where yyyy is the year and ddd is the day-of-year of the first profile in each directory. Directory boundaries have been chosen so that collections have common characteristics, such as similar Ls values, or are easily separated by periods when there was little MGS RST activity, such as solar conjunction or spacecraft anomalies when no data were collected. When these criteria allowed more than 1000 profiles in a directory, arbitrary boundaries were added. The largest directory contains only 877 profiles. The directories and their boundaries are summarized below. Electron density profiles are stored with file names of the form ydddhmmC.EDS where y is the least significant digit of the year, ddd is the three-digit day number, h is a one-character string denoting the hour, and mm is a two-character string denoting the minute in which data acquisition began. h is 'A' if the hour was 00, 'B' if the hour was 01, ... and 'X' if the hour was 23. In most cases mm is the two-digit minute; but when data were collected from two sources and the start time was in the same minute, the second digit in mm was changed to a letter in the second file name (minute 00 becomes 0A, 01 becomes 0B, etc.). C is a single character indicating the version of the file, starting with 'A'. An occultation summary file is stored in the OCS directory. Its file name is 812506AA.OCS where 812 indicates that the first occultation was from December 1998 and 506 indicates that the last occultation was from June 2005. 'AA' is a two character string which indicates that this is the first version of the file. Ancillary Data ============== OCCLOG.TAB in the DOCUMENT directory provides single-line summaries for each observation opportunity. Parameters listed mostly relate to settings of and quick-look results from data acquisition. See detached PDS label OCCLOG.LBL for details. Coordinate System ================= MGS RST SDP files use a Mars centered body-fixed coordinate system with positive east longitude. Gravity models generally use the IAU 1991 [DAVIESETAL1992B] coordinate frame. Variations are noted in the labels of specific gravity products. Software ======== None. Media/Format ============ The following description pertains to the original, CD-WO form of the archive: The archival data set was written on CD-WO media using the Sun Ultra-60/PlexWriter system. The CD-WO volumes conform to ISO 9660 standards.
DATA_SET_RELEASE_DATE 2008-05-31T00:00:00.000Z
START_TIME 1998-12-24T03:47:00.000Z
STOP_TIME 2005-06-09T11:52:00.000Z
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 GRAVITY SCIENCE INSTRUMENT
RADIO SCIENCE SUBSYSTEM
INSTRUMENT_ID RSS
INSTRUMENT_TYPE RADIO SCIENCE
NODE_NAME planetary plasma interactions
ARCHIVE_STATUS ARCHIVED
CONFIDENCE_LEVEL_NOTE Overview ======== Data in this archive have been reduced as part of mission data analysis activities of the MGS Radio Science Team. Products of questionable validity have been flagged or omitted. Review ====== This archival data set was reviewed by the MGS Radio Science Team prior to submission to the Planetary Data System (PDS). All of the profiles had been in the public domain as elements of the MGS-M-RSS-5-SDP-V1.0 data set for as much as seven years before this reorganized data set was compiled. When the unified archive was created from the CD-WO volumes, many of the descriptive textual files were changed. However, their contents are essentially the same as in the original, reviewed files. Data Coverage ============== This section describes the data in the original CD-WO archive. The same data are preserved in the present form of the archive, but the directory structure has changed considerably. The following table describes the directories that contain the profiles. Each directory is represented by one row. The first two columns list the year and day-of-year of the first and last profiles in each directory, respectively. The first column is also the directory name. The third and fourth columns provide the same information in standard calendar format. The fifth and sixth columns give the number of the Martian year (MY, as defined by [CLANCYETAL2000]) and the approximate areocentric longitude of the Sun (Ls, in degrees) of the first and last profiles in each directory, respectively. The last column specifies the number of profiles in each directory. Blank rows denote major gaps between measurements in different Martian years. Start End Start End Start End yyyy_ddd yyyy_ddd yyyy-mm-dd yyyy-mm-dd MY/Ls MY/Ls Number -------- -------- ---------- ---------- ------ ------ ------ 1998_358 1998_365 1998-12-24 1998-12-31 24/074 24/077 32 1999_068 1999_086 1999-03-09 1999-03-27 24/108 24/116 43 1999_126 1999_149 1999-05-06 1999-05-29 24/135 24/146 220 2000_306 2001_031 2000-11-01 2001-01-31 25/070 25/111 732 2001_032 2001_157 2001-02-01 2001-06-06 25/111 25/174 840 2002_305 2002_365 2002-11-01 2002-12-31 26/089 26/116 526 2003_001 2003_080 2003-01-01 2003-03-21 26/116 26/156 650 2003_081 2003_155 2003-03-22 2003-06-04 26/156 26/197 630 2003_173 2003_183 2003-06-22 2003-07-02 26/208 26/214 76 2004_328 2004_357 2004-11-23 2004-12-22 27/119 27/133 270 2004_361 2005_090 2004-12-26 2005-03-31 27/135 27/185 877 2005_091 2005_160 2005-04-01 2005-06-09 27/185 27/227 704 MGS radio occultation experiments were capable of sounding the lower neutral atmosphere under a wide range of conditions, but reliable electron density profiles could be obtained only when the solar zenith angle was less than about 90 degrees at the location of the measurement. For this reason the MGS mission produced far fewer electron density profiles (5600) than profiles of the neutral atmosphere (21243). Coverage of the ionosphere was limited by other factors. This is particularly true during the pre-mapping phases of the mission, which extended into early 1999. Prior to the start of the mapping phase, the spacecraft frequently conducted special maneuvers for aerobraking and temperature control, and scientific observations were constrained by numerous other operational issues. The following discussion summarizes the most significant gaps in coverage that occurred during the mapping phase of the mission. Low Solar Zenith Angles ----------------------- Because the orbit of Mars is outside the orbit of Earth, occultations tend to take place near the terminator. Scientists interested in studying convection in the neutral atmosphere or peak electron densities in the ionosphere will find relatively few observations near local noon except where the occultation point is at polar latitudes. Solar Conjunction ------------------ The noise level of the radio occultation measurements increased dramatically near solar conjunction. This prevented retrieval of profiles in the following intervals: 2000_153 - 2000_213 2002_197 - 2002_250 2004_204 - 2004_283 2006_263 - 2006_277 Beta Supplement ---------------- The range of motion of one of the gimbals that controls pointing of the spacecraft HGA was severely restricted until late in the mission, when the obstruction vanished for reasons unknown. During the extended intervals when the failed gimbal prevented normal operations, the spacecraft was configured in a 'beta supplement' mode that restored most of its functionality. This had no effect on measurements at occultation entry (northern hemisphere) but experiments could not be conducted routinely at occultation exit (southern hemisphere) when the spacecraft was in the beta supplement mode: 2000_039 - 2001_171 2002_073 - 2003_253 2004_204 - 2005_251 Spacecraft Anomalies and Emergencies -------------------------------------- The spacecraft occasionally stopped collecting scientific data for periods of several days in response to problems with flight software or hardware. This prevented occultation experiments from being conducted in the following intervals: 2001_123 - 2001_129 2002_059 - 2002_065 2002_092 - 2002_100 2003_162 - 2003_168 2004_358 - 2004_360 2005_239 - 2005_251 2005_256 - 2005_261 Data Quality ============= Each profile includes uncertainties on the estimates of electron number density. This is the best source of information about data quality. Some obvious trends are apparent in the error bars, such as a general increase in the noise level when the spacecraft was close to solar conjunction. Data quality can vary significantly between successive experiments, and sometimes from one DSN complex to another, and these variations are reflected in the error bars. Some error sources are difficult to quantify, and their effects are therefore not included in the formal uncertainties. As a general rule, diametric occultations yield more reliable results than grazing occultations. (The header of each profile includes several parameters, such as the 'angle from diametric', that characterize the experiment geometry.) Limitations =========== The limitations in this data set follow from the quality of the execution, which is described above under Data Coverage and Quality.
CITATION_DESCRIPTION Hinson, D.P., Mars Global Surveyor Radio Occultation Profiles of the Ionosphere - Reorganized, MGS-M-RSS-5-EDS-V1.0, NASA Planetary Data System, 2008.
ABSTRACT_TEXT This data set contains 5600 ionospheric electron density profiles (EDS files) derived from Mars Global Surveyor (MGS) radio occultation data. The profiles were previously archived in the MGS-M-RSS-5-SDP-V1.0 data set along with other reduced data products from the MGS Radio Science Team (RST). Here they have been pulled from the original 38 volumes and reorganized in chronological order on a single volume. The profiles themselves have not been modified, and the labels have been edited only to conform with the requirements of the new data set. This set of profiles is accompanied by a single occultation summary file which lists key characteristics of each experiment.
PRODUCER_FULL_NAME DAVID P. HINSON
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