Mars Pathfinder EDL Raw and Calibrated Data
Dataset Description
Table of Contents
|
 |
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 Atmospheric Structure Instrument and Meteorology Package collected
data during six sequential, non-overlapping, time intervals while the
spacecraft was descending to the surface. These intervals are
described below:
- Free-fall Data Capture
- Commenced as a timed event exactly 15 minutes after Cruise Stage
separation, at an altitude in excess of 160 kilometers.
- Entry Data Capture
- Commenced as a timed event exactly 29 minutes and 30 seconds
after Cruise Stage separation (hence 30 seconds before the defined
time of atmospheric entry) coincident with the start of the
accelerometer-based algorithm to determine the parachute mortar firing
time.
- Descent Data Capture
- Commenced as a timed event exactly 20 seconds after parachute
mortar firing, coincident with the firing of the group 2 heatshield
separation nuts pyros.
- Terminal Data Capture
- Commenced as a timed event exactly 48 seconds after parachute
mortar firing, coincident with the enabling of the radar altimeter
transmitter.
- Landing Data Capture
- Commenced as a timed event exactly one half second after bridle
cut pyro firing, coincident with powering off the radar altimeter.
- Deployment Data Capture
- Commenced as a timed event exactly 60 seconds after bridle cut
pyro firing, coincident with the start-roll-stop-algorithm event, and
terminated with the end-of-surface-deployment event.
This table shows the timing of many of the major entry events (table
taken from [GOLOMBEKETAL1997B]):
| Event |
Time |
Altitude |
Velocity |
| Cruise stage separation |
L - 35 min |
|
|
| Entry |
L - 5 min |
130 km |
7470 m/s |
| Parachute deployment |
L - 134 s |
9.4 km |
370 m/s, 16g |
| Heatshield separation |
L - 114 s |
|
|
| Lander separation |
L - 94 s |
|
|
| Radar ground acquisition |
L - 28.7 s |
1.6 km |
68 m/s |
| Airbag inflation |
L - 10.1 s |
355 m |
|
| Rocket ignition |
L - 6.1 s |
98 m |
61.2 m/s |
| Bridle cut |
L - 3.8 s |
21.5 m |
|
| Landing |
2:58 a.m. |
0 |
14 m/s, 19g |
| Roll stop |
L + 2 min |
|
|
| Deflation |
L + 20 min |
|
|
| Airbag retracted |
L + 74 min |
|
|
| Petals opened |
L + 87 min |
|
|
The sampling rate of ASI/MET data during EDL was determined
automatically by the Attitude Information Management system and was
linked to the EDL phase.
Data were directed to RAM, science EEPROM & critical EEPROM, each of
which had different sampling rates. The parameters directed to each
also varied. No real-time data were transmitted during EDL.
| Data type |
Parameters
x Bits |
EDL phase and duration (Seconds) |
Data Vol.
(kBytes) |
| |
Free-fall |
Entry |
Descent |
Terminal |
Landing |
|
| |
870 |
210 |
27 |
92 |
60 |
|
| |
Samples per second |
|
| RAM |
|
| Accel, 3-axes |
3x16 |
1 |
32 |
32 |
32 |
32 |
79.9 |
| Accel., Hsk. |
13x16 |
0.125 |
0.125 |
0.125 |
0.125 |
0.125 |
4.3 |
| MET, Sci. |
12x16 |
0.125 |
0.125 |
2 |
2 |
1 |
11.5 |
| MET, Hsk. |
12x16 |
0 |
0 |
0.25 |
0.25 |
0.125 |
1.0 |
| |
96.7 |
| Science EEPROM |
|
| Accel.3-axes |
3x16 |
0.5 |
8 |
2 |
2 |
8 |
17.4 |
| Accel., Hsk. |
13x16 |
0.063 |
0.063 |
0.063 |
0.063 |
0.063 |
2.1 |
| MET, Sci. |
4x16 |
0.125 |
0.125 |
1 |
1 |
0.125 |
2.3 |
| MET, Hsk. |
12x16 |
0 |
0 |
0.125 |
0.125 |
0.125 |
0.6 |
| |
22.4 |
| Critical EEPROM |
|
| Accel. Z-axis |
1x16 |
0.031 |
1 |
0.125 |
0.125 |
0.25 |
0.5 |
| Accel., Hsk. |
2x16 |
0.031 |
0.031 |
0 |
0 |
0 |
0.1 |
| |
0.6 |
For details on the format of the EDL telemetry packets, please see
[MEYER1996].
Three ASI/MET instruments produced data during EDL. These were the
science and engineering accelerometers and the MET instrument. The
accelerometers each produced identical sets of science and
housekeeping parameters and the MET instrument produced science
parameters, housekeeping parameters, and Aeroshell Instrumentation
Package (AIP) data.
All data records are tagged with a spacecraft clock start count and an
elapsed time.
- Spacecraft Clock Start Count
- The value of the spacecraft clock at the time the related data
were acquired.
- Elapsed Time
- The elapsed time, in seconds, since a set time. In each case,
this T=0 time is specified as an SCLK and a UTC time.
Accelerometer Data
The accelerometer data files contain measurements of acceleration and
the gain states of the relevant accelerometer at the time the
measurement was taken. While all six accelerometer values were stored
in ROM and Science EEPROM, only the engineering +YZ and science Z axis
accelerometer values were stored in Critical EEPROM.
- Acceleration
- There are three engineering and three science accelerometers.
Each set of three accelerometers is orthogonal. The engineering
accelerometers measure acceleration along the +YZ, -YZ, and X axes of
the spacecraft. (+YZ indicates halfway between the +Y and +Z axes.)
The science accelerometers measure acceleration along the X, Y, and Z
axes. (See section on Coordinate Systems below.) The raw
acceleration values are in raw counts, while the calibrated values are
in units of Earth gravity, 9.795433 m/s2.
- Accelerometer Gain Range
- The accelerometers can each be set to one of three different gain
ranges, 0.016g, 0.800g, or 40.0g (where g = 1 Earth g, 9.795433
m/s2).
The following accelerometer housekeeping values were also recorded:
- Accelerometer First Amplifier Temperature
- The first amplifier temperature sensor values are provided for
each of the six accelerometers. The calibrated measurements are in
units of degrees Kelvin.
- Accelerometer Head Temperature
- These values provide the temperature of the accelerometer head
sensor for each of the six accelerometers. The calibrated values are
in units of degrees Kelvin.
- Accelerometer Multiplexer Temperature
- The engineering and science accelerometer multiplexer temperature
sensor values were recorded. The calibrated values are in units of
degrees Kelvin.
- Accelerometer Volt Reference
- These are the values of the 0 Volt, -2.5 Volt, and 2.5 Volt
references for both the engineering and science accelerometers. The
calibrated values are in units of Volts.
- Accelerometer Volt Reference Temperature
- These are the temperatures of the +2.5 Volt reference for both
the engineering and science accelerometers. The calibrated values are
in units of degrees Kelvin.
- Analog to Digital Converter Temperature
- These are the values of the engineering and science accelerometer
analog-to-digital converter temperature sensors. The calibrated
values are in units of degrees Kelvin.
- Hygrometer Voltage
- These are the values of the hygrometer sensors for both the
science and engineering accelerometers. The calibrated values are in
units of Volts.
MET Data
The MET data files contain primarily measurements of atmospheric
pressure, temperature, and wind. The following science parameters
were recorded.
- Pressure
- The air pressure was measured in two ranges, zero to twelve
millibars, and six to ten millibars. Raw values are reported as raw
counts, and calibrated values as millibars.
- Temperature
- This was measured by four thermocouples, labeled 'top', 'middle',
'bottom', and 'descent'. The top thermocouple is located 103.8 cm
above the plane of the lander petal; the middle and bottom
thermocouples are 50.8 and 27.3 cm above this plane. The descent
thermocouple is located just below the wind sensor at the top of the
mast. Calibrated units for the temperatures are degrees Kelvin.
- Wind Sensor Element Temperature
- These are the measured temperatures of each of the six wind
sensor segments (or elements). Calibrated values are shown in degrees
Kelvin.
The following MET housekeeping values were also recorded:
- +/-12 Volt Power Supply Voltage
- The voltage from the twelve volt power supply is provided in
units of Volts.
- +5 Volt Reference Voltage
- The calibrated units for this reference voltage are provided in
Volts.
- +5 Volt ADC
- The voltage of the positive five Volt analog to digital converter
supply, measured in Volts.
- -5 Volt ADC
- The voltage of the negative five Volt analog to digital converter
supply, measured in Volts.
- Circuit Board Temperature
- The temperature of the MET circuit board is reported in units of
degrees Kelvin.
- Wind Sensor Thermocouple Temperature
- This is the measured temperature of the wind sensor thermocouple.
The calibrated values are shown in degrees Kelvin.
- Wind Sensor Current
- The calibrated current from the wind sensor is measured in units
of milliamps.
- PRT4 Temperature, Drive Current Voltage, and Sensor
Voltage
- The temperature of the mast base isothermal platinum resistance
thermometer (PRT4) is reported in units of degrees Kelvin. Its drive
current voltage and sensor voltage are measured in Volts.
- PRT5 Temperature and Drive Current Voltage
- The temperature of the pressure sensor platinum resistance
thermometer (PRT5) is reported in degrees Kelvin, and the drive
current voltage in units of Volts.
The raw data values in this data set have been converted from 14-bit
binary numbers to ASCII.
The calibrated values have been converted from the raw counts from
each sensor to engineering units. The steps used to produce the
engineering units vary with the type of data, but are all described in
the 'Calibration' section of the ASI/MET instrument description. More
information is also available in these references: [SCHOFIELD1996A, SCHOFIELD1996B, and SCHOFIELD1997A].
All of the data in this data set are contained in ASCII tabular files,
(file extension '.TAB') with detached PDS labels (file extension
'.LBL'). The files are named as follows:
| Critical EEPROM Engineering Accelerometer Science Data |
C_EACC_S.TAB |
| Critical EEPROM Engineering Accelerometer Housekeeping |
C_EACC_H.TAB |
| Critical EEPROM Science Accelerometer Science Data |
C_SACC_S.TAB |
| Critical EEPROM Science Accelerometer Housekeeping |
C_SACC_H.TAB |
| |
| Science EEPROM Engineering Accelerometer Science Data |
S_EACC_S.TAB |
| Science EEPROM Engineering Accelerometer Housekeeping |
S_EACC_H.TAB |
| Science EEPROM Science Accelerometer Science Data |
S_SACC_S.TAB |
| Science EEPROM Science Accelerometer Housekeeping |
S_SACC_H.TAB |
| Science EEPROM FE & DTL MET Science Data |
S_MET_S.TAB |
| Science EEPROM Descent/Term/Landing MET Housekeeping |
SD_MET_H.TAB |
| |
| RAM Engineering Accelerometer Science Data |
R_EACC_S.TAB |
| RAM Engineering Accelerometer Housekeeping |
R_EACC_H.TAB |
| RAM Science Accelerometer Science Data |
R_SACC_S.TAB |
| RAM Science Accelerometer Housekeeping |
R_SACC_H.TAB |
| RAM Free-fall/Entry MET Science Data |
RF_MET_S.TAB |
| RAM Descent/Terminal/Landing MET Science Data |
RD_MET_S.TAB |
| RAM Descent/Terminal/Landing MET Housekeeping |
RD_MET_H.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. All fields in the tables are separated by commas,
and are right justified. The 'start byte' and 'bytes' values listed
in the PDS labels do not include the commas between fields. The
records are of fixed length, and 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 TABLE) 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. For
example:
^TABLE = 'R_SACC_S.TAB'
indicates that the TABLE object is in the file R_SACC_S.TAB, in the
same directory as the detached label file.
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 X, Y, and Z axes of the accelerometers are those of the Mars
Pathfinder Entry Vehicle Coordinate System. This coordinate system is
described by [MELLSTROM&LAU1996]. The
coordinate system is right handed, orthogonal, and defined by axes
Xe, Ye, and Ze. The
Xe/Ye plane is defined as the plane of the entry
vehicle interface pads located at the top of the backshell interface
plate.
Xe lies in the entry vehicle separation plane
(Xe/Ye plane), positively directed outward in
the entry vehicle separation plane from the Ze axis and
orthogonal to Ye and Ze.
Ye lies in the Xe/Ye plane and
positively directed outward in the entry vehicle separation plane
outward from the Ze axis toward the Star Scanner Assembly
(SSA) when the entry vehicle and the cruise stage are attached to each
other (-Ye passes through the interface bushing #1).
Ze is coincident with the nominal spacecraft spin axis,
through the geometric center of the three interface bushing holes for
connecting the entry vehicle to the cruise stage, and positively
directed outward from the spacecraft center of mass towards the
heatshield.
The MET EDR/RDR tables can be displayed on UNIX, Macintosh, and PC
platforms as simple ASCII files, or using the PDS developed program,
NASAView. This software is freely available from the PDS Central Node
and may be obtained from their web site at http://pds.jpl.nasa.gov/. For
more information or help in obtaining the software, contact the PDS
operator at the following address:
The ASI/MET EDL raw and calibrated data will be stored on compact
disc-read only memory (CD-ROM) media. The CD will be formatted
according to ISO-9660 and PDS standards. The data files will not
include extended attribute records (XARs), and will therefore not be
readable on some older VMS operating systems.
The quality of the raw data from the accelerometer and MET instruments
is good throughout EDL. There are no gaps, and all the planned data
were returned to Earth.
The reduced/calibrated data are also complete. In general, the
accuracy of calibration is best for the accelerometer and MET science
parameters, and is less accurate for the housekeeping parameters and
AIP temperatures.
The contents of this CD have been peer reviewed by the following
people:
| 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 |
| John Wilson |
- |
Non-MPF scientist, Geophysical Fluid Dynamics Laboratory/NOAA,
Princeton University |
The accelerometer data are of high quality throughout EDL. However
both raw and calibrated acceleration measurements for a particular
accelerometer should not be used for a period of 1 second following a
gain change. Gain changes produce an acceleration pulse which is an
artifact of the electronic time constant of the sensor. Gain changes
only occur in the science accelerometer measurements.
MET science and housekeeping data are also of good quality throughout
EDL. Pressure, temperature and wind data accurately describe the
properties of the appropriate sensors. However, during free-fall and
entry, the heatshield is in place and these properties are determined
less by the atmosphere than by the internal lander environment. After
heatshield separation sensor properties are influenced both by the
atmosphere and the lander environment.
Dynamic pressure measurements are most reliable. The descent
temperature sensor is exposed for EDL but the exposure is not good
enough to prevent serious contamination from the lander environment so
that its results can not readily be converted to atmospheric
temperature during the parachute descent, terminal or landing phases
of EDL. Measurements by the other temperature sensors and the wind
sensor are essentially meaningless during all phases of EDL, as these
sensors are designed for operation after landing.
The accelerometer data set has been used to reconstruct the entry
vehicle trajectory and derive the vertical density, pressure, and
temperature structure of the atmosphere above 10 km. In order to
repeat this analysis various supplementary data, not included in this
data set, are required. Such supplementary data include spacecraft
mass, cross-sectional area and aerodynamic drag coefficient, as well
as navigation solutions for entry vehicle position and velocity
relative to Mars immediately before entry.
Although the calibrated MET data are of good quality, only the
pressure measurement gives good information on the vertical profile of
the atmosphere during parachute descent for the reasons discussed
above. As the pressure is a dynamic measurement made through a Pitot
tube, it must be corrected for atmospheric flow velocity around the
lander and orientation relative to this flow before it can be
converted to static atmospheric pressure.
Mission
Mars Pathfinder
Instrument Host
Mars Pathfinder Lander
Instrument
Atmospheric Structure Instrument /
Meteorology Package
Target
PDS Welcome to the Planets:
Mars
PDS High Level Catalog:
Mars
References
GOLOMBEKETAL1997B
MELLSTROM&LAU1996
MEYER1996
SCHOFIELD1996A
SCHOFIELD1996B
SCHOFIELD1997A
SCHOFIELDETAL1997
SEIFFETAL1997
VAUGHAN1995