Mars Pathfinder bounced down and rolled to a stop on the surface of Mars on July 4, 1997. It landed in an ancient floodplain in the Ares Vallis region of Chryse Planitia at 19.17°N latitude, and 33.21°W longitude.
The total duration of the mission was 83 sols. Surface meteorology data were collected by the MET instrument on 76 of these sols (sols 17, 31, 43, 45, 46, 48, and 51 contain no meteorology data).
MET data are organized into science and housekeeping data records. Each record is composed of 12 parameters, which for all intents and purposes are measured simultaneously. The science data record consists of surface pressure (two instrument sensitivity ranges), atmospheric temperature at three vertical locations on the MET mast (referred to as TOP, MIDDLE, and BOTTOM), atmospheric temperature as measured by the Descent Thermocouple located near the top of the MET mast, and wind speed and direction at the top of the MET mast. See [SEIFFETAL1997] for a complete description of the instruments. The housekeeping data record consists of various instrument voltages, currents, and reference temperatures.
At the time of writing (Sept. 1998), we have not yet derived reliable wind speeds and directions from the data received from the Martian surface. We continue to re-calibrate the wind sensor, and hope to have the wind results released soon.
The parameters contained in the science data record are the measured atmospheric pressure obtained with the two instrument sensitivity ranges (6-10 and 0-12 millibars). Also included are the three temperatures measured along the MET mast, as well as the temperature measured by the Descent Thermocouple. We have also included in this data set the measured temperatures of each of the six wire segments which comprise the wind sensor. These temperatures, and their spatial variation around the sensor's central cylinder, are the data from which wind speed and direction will be derived. The housekeeping data consist of several reference voltage measurements from which the stability of instrument power supplies and sensor constant current sources can be determined. Additionally, thermocouple reference junction temperature, the internal temperature of the wind-sensor cylinder, the temperature of the pressure sensor, and the temperature of the MET circuit board are measured. All science and housekeeping parameters are derived from sampled digitized voltages.
The time intervals during which data were collected are referred to as sessions. Each session is characterized by a session number, a duration, and science and housekeeping data record sampling rates. Session numbers during the mission ranged from 39 (~ 7 AM LST on Sol 1) to 1430 (Sol 83). Session durations varied in duration from 184 seconds to 89052 seconds. This latter duration, used on 5 occasions, produced what we have referred to as 'Presidential MET Sessions', during which data were continually collected at a 0.25 Hz rate for an entire sol. The science data record sampling rate used during the mission varied per the specifications of the ASI/MET science team. It ranged from 0.25 Hz (a single point measurement of each variable once every four seconds) to 2 Hz (2 point measurements per second).
For more info, see [SEIFFETAL1997, SCHOFIELDETAL1997].
There are some time issues involved with this data reduction as follows:
CHRONOS was not used on each data point (there were several hundred thousand during the course of the mission), but rather the interpolation routine above was used to generate a systematic time step on any given sol. Mars orbital characteristics result in differing day lengths (in terms of Earth seconds) with advancing season. Since the most useful form for meteorology data is knowing what the local time was, the data have been put into that form.
For those who desire to know the precise time separation between samples in Earth seconds, these values can be obtained from the SCLK values and the time tools available at the Navigation and Ancillary Information Facility at JPL.
A version 2 of this data set may be generated in which the local times are the precise conversion from SCLK to Local Time with the SCLK values having the digits to the right of the decimal point correctly indicating the number of 256ths of a second.
The data as they are now are perfectly suitable for analysis. Direct comparisons with other data sets should be based upon SCLK values rather than the precise local times, as other groups/teams have used different conversions.
A random sampling of local times in this data set with precise conversions between SCLK and Local Time indicates variations much less than one second in magnitude.
There were some instances within sessions in which the indicated time stamp (SCLK value) for the first science data record of a packet would indicate the same time as the final record of the preceding packet. (Data were packaged into packets, consisting of one header record and 85 data records, within each session prior to transmission from the lander). This error does not propagate through to the next packet. It has been corrected in this data set by identifying packets with this time problem, and adding an increment of time equal to the science sampling interval to all the science data record SCLK values in the packet. Housekeeping data record SCLK values are not affected by this problem.
The Ls (solar longitude) value associated with the START_TIME of each data collecting session was obtained by using the CHRONOS time tool. The SCLK value at the session START_TIME was introduced into CHRONOS, which produced the corresponding Ls value. Thus, the Ls values included in each *.TAB file header are precisely determined for the associated SCLK time at which the session began.In order to be able to determine the Ls value for any particular data point (SCLK time) during the mission, an analytic expression relating SCLK to Ls has been derived. At each 1000 SCLK count interval during the mission, the CHRONOS tool was used to determine the corresponding Ls value. A third-order curve was then fit to these Ls values, with the independent variable being the difference between the SCLK count at the time of interest and the SCLK value at midnight of the sol on which Pathfinder landed (approximately hours prior to landing). This midnight SCLK value is 1246715602.143.
The 3rd-order curve and its coefficients are:
- Ls = 142.66 +
- (5.7837E-06 * (SCLK - 1246715602.143)) +
- (5.6779E-14 * (SCLK - 1246715602.143)2) +
- (4.5717E-22 * (SCLK - 1246715602.143)3)
With this curve-fit, Ls values for each SCLK count can be determined. Note that Ls values derived with this curve-fit WILL NOT exactly correspond to the Ls values contained in the *.TAB file headers, since these header Ls values were each precisely determined with the CHRONOS tool.
The data downlinked from the spacecraft are digital numbers derived from measured voltages. Digitization is to 14 bits, compared with the 10 bit resolution available to the Viking meteorology instruments. These numbers are stored in the Experiment Data Record (EDR) files, whereas the Reduced Data Records (RDR), contain results in scientific units (millibars, Kelvin, etc.). EDR values are converted to volts and subsequently scientific units using calibration information.
During the nominal mission (sols 1-30), the nominal MET observation strategy was 51 equally spaced 3 minute measurement sessions per Sol. For each session science data records were sampled at a rate of 0.25 Hz. Interspersed with these sessions were longer (15 minute, one hour, entire sol) sessions often sampled at a higher rate (1 Hz). The purpose of this strategy was to characterize the full diurnal cycle throughout the sol, as well as higher frequency fluctuations at a variety of times during the sol. These times were spread over a number of sols to remain within data downlink constraints. Thus, we planned to characterize the diurnal cycle at 'low-frequency' (51 times each sol), and then over the period of ~10 sols to accumulate information at higher sampling frequencies so that every hour of the day would have such sampling. Continuous sampling for a complete sol at 0.25 Hz (a 'Presidential MET Session') was first conducted on Sol 25 and subsequently on sols 32,38,55, and 68. These are the only sols during the extended mission (sols 31-83) when MET data were collected outside the hours of 0900 - 1500 LST.
Data are stored on a session-by-session basis. Thus, data for a particular sol are included in multiple files, for sols 1-49. On later sols, single sessions were specified to cover the duration of spacecraft activity on that sol (nominally 9 AM to 2:30 PM). Thus, from sol 50 (Session 1398) through sol 83 (Session 1430), data for each sol are contained in a single data file (session).
Session numbers ranged from 39-1430, but no data for sessions 1330, 1338, 1340, 1341, 1362, 1370, 1376, 1379, 1384, 1385, 1386, 1387, 1389, 1390 were returned. Thus, there are no files for these sessions.
All of the data in this data set are contained in ASCII tabular files with detached PDS labels. Science and housekeeping data have been split into separate files, stored in the directories SURF_RDR/SCIDATA and SURF_RDR/HKPDATA, respectively. Beneath that, the data files are separated by session number into fourteen directories labeled SR00XXS through SR14XXS (for the science data) and SR00XXH through SR14XXH (for the housekeeping data).
Individual filenames are constructed as follows:
SR0559S.TAB1234567.8901: The first character will always be an 'S', representing Surface data. 2: The second character will always be an 'R', signifying the calibrated data (Reduced Data Record). The corresponding raw data (ie., Experiment Data Record) can be found in the SURF_EDR directory, in a file with the same name, except that the 'R' will be an 'E'. 3-6: The next four characters provide the session number of the data file. 7: The seventh character will be either an 'S' or an 'H'. 'S' signifies science data; 'H' signifies housekeeping data. 8-0: The file extension indicates which type of file it is. The only two options are 'LBL' (the PDS label file), or 'TAB' the ASCII table containing the data.
As an example, the data from session 559 is stored in the following locations on the ASI/MET CD:
SURF_EDR/SCIDATA/SE05XXS/SE0559S.TAB- raw data fileSURF_EDR/SCIDATA/SE05XXS/SE0559S.LBL- PDS label for SE0559S.TAB
SURF_EDR/HKPDATA/SE05XXH/SE0559H.TAB- raw housekeeping fileSURF_EDR/HKPDATA/SE05XXH/SE0559H.LBL- PDS label for SE0559H.TAB
SURF_RDR/SCIDATA/SR05XXS/SR0559S.TAB- calibrated data fileSURF_RDR/SCIDATA/SR05XXS/SR0559S.LBL- PDS label for SR0559S.TAB
SURF_RDR/HKPDATA/SR05XXH/SR0559H.TAB- calibrated housekeeping fileSURF_RDR/HKPDATA/SR05XXH/SR0559H.LBL- PDS label for SR0559H.TAB
The tabular files are formatted so that they may be read directly into many database management systems (DBMS) or spreadsheet programs on various computers. Each of the files contains two tables. The first is the header table, and is only a single record in length. The second table contains all of the data records for a session and varies in length.
All fields in the tables are stored in columns of fixed width and are right justified. The records are of fixed length; since the header records are shorter than the data records, they have been padded with blank spaces at the end of the record. The last two bytes of each record contain the ASCII carriage return and line feed characters. This allows the tables to be treated as fixed length record files on computers that support this file type and as normal text files on other computers.
The PDS labels are object-oriented. The object to which the labels refer (the tables) is denoted by a statement of the form:
^object = location
in which the carat character ('^', also called a pointer in this context) indicates that the object starts at the given location. For an object located outside the label file (as in this case), the location denotes the name of the file containing the object, along with the starting record. For example:
^DATA_TABLE = ('SR0559S.TAB', 2)
indicates that the DATA_TABLE object begins at record 2 of the file SR0559S.TAB, in the same directory as the detached label file. (Records are counted starting at 1, not 0.)
The detached label files are stream format files, with a carriage return (ASCII 13) and a line feed character (ASCII 10) at the end of each record. This allows the files to be read by the MacOS, DOS, UNIX, and VMS operating systems.
The software employed to generate data in scientific/engineering units from the downlinked digital data was developed in IDL and FORTRAN, and the software used to create these data tables was FORTRAN. The software took account of pre-flight and in-flight calibration of the sensors to convert the downlinked digital numbers to voltages, and ultimately to the desired units. The software algorithms are available on the ASI/MET CD.
Questions should be addressed to:
Dr. Jim Murphy
Dept. of Astronomy, MSC 4500
New Mexico State University
Box 30001
Las Cruces, NM 88003
Ph: (505) 646-5333
FAX: (505) 646-1602
email: murphy@nmsu.edu
The ASI/MET SRFEDR and SRFRDR data will be stored and distributed on compact disc-read only memory (CD-ROM) media. The CDs are formatted according to ISO-9660 and PDS standards.
| Lyle Huber | - | PDS Atmospheres Node, New Mexico State University |
| Julio Magalhaes | - | MPF ASI/MET Team, NASA Ames Research Center |
| Jim Murphy | - | MPF ASI/MET Team & PDS Atmospheres Node, New Mexico State University |
| Tim Schofield | - | MPF ASI/MET Team Lead, Jet Propulsion Laboratory |
| Rob Sullivan | - | MPF Participating Scientist, Cornell University |
| Betty Sword | - | PDS Central Node Data Engineer, Jet Propulsion Laboratory |
| Joel Wilf | - | PDS Central Node Data Engineer, Jet Propulsion Laboratory |
| Elizabeth Duxbury | - | PDS Imaging Node, Jet Propulsion Laboratory |
| John Wilson | - | Non-MPF scientist, Geophysical Fluid Dynamics Laboratory/NOAA, Princeton University |
During pre-flight testing, the pressure sensor was tested at temperatures (190 K) that were significantly lower than its design limits (220 K), and much colder than it experienced within the spacecraft on the Martian surface. This extreme stressing of the sensor cast doubt upon the calibration which had been obtained prior to that point, and the schedule thereafter did not permit recalibration of the sensor in a controlled environment. During cruise to Mars, the ASI/MET system was periodically powered up and the pressure sensor signal, as well as its temperature, were measured. These measurements, together with measurements obtained during free fall and entry, provided a data set from which the variation of sensor zero-offset as a function of the temperatures experienced during cruise (270-280K) could be determined. It became clear that thermal stressing had changed the offset and increased its variation with temperature by a factor of 3. It was not possible to verify gains in flight, but laboratory testing on the flight spare sensor suggested that sensor stressing would not produce significant gain changes. The flight data, in addition to the pre-flight calibration data, have been used to determine pressure from the down-linked pressure sensor signals.
The thermocouples appeared to work very well, and there is no reason to question the results from them.
As stated above, the determination of wind speed and direction is ongoing, and a new wind data set will be issued following analyses of recent sensor recalibration work, and application of its results to wind-sensor data returned from the Martian surface.
The following links provide some additional information about topics related to this dataset. You must be connected to the Internet for most of these links to work, since they are located on the web site of the Central Node of the Planetary Data System.
Atmospheric Structure Instrument /
Meteorology Package
Mars Pathfinder IMP Windsocks
PDS Welcome to the Planets:
Mars
PDS High Level Catalog:
Mars