| DATA_SET_DESCRIPTION |
Data Set Overview
=================
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 degrees North
latitude, and 33.21 degrees West 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 EDL phase and duration (Seconds) Data Vol
x Bits (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
Parameters
==========
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/s**2.
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/s**2).
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.
Processing
==========
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].
Data
====
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.
Coordinate System
=================
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.
Software
========
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.nasa.gov/. For more information or help in
obtaining the software, contact the PDS operator at the following
address:
Address: Planetary Data System, PDS Operator
Jet Propulsion Laboratory
4800 Oak Grove Drive
Pasadena, CA 91109
Phone: (818) 354-4321
Email: pds_operator@jpl.nasa.gov
WWW URL: http://pds.nasa.gov/
Media / Format
==============
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.
|
| CONFIDENCE_LEVEL_NOTE |
Confidence Level Overview
=========================
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.
Review
======
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
Data Coverage and Quality
=========================
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.
Limitations
===========
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.
|
.png)
.png)
