| 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 19.1 degrees North
Areocentric latitude, and 326.48 degrees East longitude.
As the spacecraft traveled through the atmosphere to its landing
site, the Atmospheric Structure Investigation/Meteorology package
(ASIMET) measured aerodynamic accelerations on the lander
throughout the entry using onboard accelerometers. In addition,
sensors directly measured pressures and temperatures during the
parachute descent phase, which began at about 8.5 km above the
landing site. This data set contains the profiles of atmospheric
density, pressure, and temperature derived from the accelerations
measured prior to parachute release.
A first description of the ASIMET results can be found in
[SCHOFIELDETAL1997]. The ASI experiment is described in the same
reference.
Data
====
All of the data in this data set are contained in an ASCII
tabular file, ('EDL_DDR.TAB') with a detached PDS label
('EDL_DDR.LBL').
The tabular file is formatted so that it may be read directly
into many database management systems (DBMS) or spreadsheet
programs on various computers. All fields in the table are
separated by commas, and are right justified. The 'start byte'
and 'bytes' values listed in the PDS label 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 table to be treated as
a fixed length record file on computers that support this file
type and as a normal text file on other computers.
The PDS label is object-oriented. The object to which the label
refers (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 = 'EDL_DDR.TAB'
indicates that the TABLE object is in the file EDL_DDR.TAB, in
the same directory as the detached label file.
The detached label file is a stream format file, with a carriage
return (ASCII 13) and a line feed character (ASCII 10) at the end
of each record. This allows the file to be read by the MacOS,
DOS, UNIX, and VMS operating systems.
Parameters
==========
The first column in the data table contains the Spacecraft Clock
Start Count (SCLK) at the time the data sample was acquired. The
SCLK is formatted as decimal fractions of a second.
The second column shows the elapsed time, in seconds, since the
spacecraft passed the altitude of 3,597.2 km. The first data
point in the file is at time = 34.156 seconds, with a
corresponding SCLK of 1246726312.75. At that time, the
equivalent UTC time was 16:51:46.440, July 4, 1997.
The third column shows the altitude of the spacecraft above the
landing site, in kilometers. Note that the radius of Mars at the
landing site is 3389.72 km [FOLKNERETAL1997B].
Column four shows the planetocentric latitude, in degrees, of the
spacecraft at the time the sample was acquired.
The fifth column contains the east longitude, in degrees, of the
spacecraft.
The sixth column contains the calculated atmospheric density, in
kilograms per meter cubed.
The seventh column contains the calculated atmospheric pressure,
in millibars.
The eighth column contains the calculated atmospheric
temperature, in Kelvin.
Column nine contains the mean molecular weights (amu) that were
used to evaluate the atmospheric temperatures. These mean
molecular weights were derived from molecular weight as a
function of pressure measured by the Viking 1 Upper Atmosphere
Mass Spectrometer [SEIFF&KIRK1977].
The tenth column shows the width of the uncertainty envelope in
the positive direction, of the atmospheric density. It is
measured in kilograms per meter cubed. The value in the sixth
column plus the value in this column gives the upper boundary of
the density uncertainty envelope.
The eleventh column shows the width of the uncertainty envelope
in the negative direction, of the atmospheric density. It is
measured in kilograms per meter cubed. The value in the sixth
column plus the value in this column gives the lower boundary of
the density uncertainty envelope.
The twelfth column shows the width of the uncertainty envelope in
the positive direction, of the atmospheric pressure. It is
measured in millibars. The value in the seventh column plus the
value in this column gives the upper boundary of the pressure
uncertainty envelope.
The thirteenth column shows the width of the uncertainty envelope
in the negative direction, of the atmospheric pressure. It is
measured in millibars. The value in the seventh column plus the
value in this column gives the lower boundary of the pressure
uncertainty envelope.
The fourteenth column shows the width of the uncertainty envelope
in the positive direction, of the atmospheric temperature. It is
measured in Kelvin. The value in the eighth column plus the
value in this column gives the upper boundary of the temperature
uncertainty envelope.
The fifteenth column shows the width of the uncertainty envelope
in the negative direction, of the atmospheric temperature. It is
measured in Kelvin. The value in the eighth column plus the
value in this column gives the lower boundary of the temperature
uncertainty envelope.
Processing
==========
Measured accelerations from the three science accelerometers on
the Mars Pathfinder lander have been processed to reconstruct the
trajectory and profiles of atmospheric density, pressure, and
temperature. The measured accelerations used as input for the
reconstruction can be found in the
'MPFL-M-ASIMET-2/3-EDR/RDR-EDL-V1.0' data set.
A full description of the data and the procedure used to generate
the atmospheric structure profiles on this CD can be found in
[MAGALHAESETAL1999]. The procedure for the reconstruction of
planetary atmospheric structure from accelerometer data was first
described by [SEIFFETAL1973], and a more detailed description of
the procedure appeared in [SEIFF&KIRK1977]. The analysis of the
Mars Pathfinder ASI data has utilized the same basic
deterministic approach, but the analysis algorithm and software
were independently developed to refine and extend the atmospheric
structure reconstruction procedure used previously. The
reconstruction procedure used to derive the profiles in this file
can be divided into four general steps. First, the trajectory of
the lander was reconstructed using the measured accelerations, a
model for the gravity field of the planet, the equations of
motion of the lander, and an initial condition provided by the
Navigation Team. Second, the atmospheric density profile was
derived from the measured aerodynamic decelerations and the
aerodynamic characteristics of the Pathfinder lander. Third, an
atmospheric pressure profile was derived by integrating the
equation of hydrostatic equilibrium using the density profile.
Fourth, an atmospheric temperature profile was derived from the
ideal gas law using the density profile, pressure profile, and a
model for the variation of mean molecular weight with pressure.
With knowledge of the external forces acting on the spacecraft,
the velocity and position of the probe were determined by
integrating the equations of motion starting with an initial
condition provided by the Navigation Team. The equations of
motion written in a planet-centered spherical coordinate system
rotating with the planet were used for the computations. The
acceleration due to gravity was not directly measured by the
accelerometers during passage through the atmosphere. To include
the effects of gravity, we have used a gravity model that
includes the first zonal harmonic coefficient J2 in the
normalized spherical harmonic expansion of the gravity field (cf.
[HUBBARD1984]). For Mars J2 = 8.759 x 10**-4 [SMITHETAL1993].
Higher order harmonics are of much smaller magnitude
[SMITHETAL1993] and are not important over the short time
interval of EDL. The aerodynamic drag on the vehicle was
directly obtained from the three-axis science accelerometer
measurements. Due to the small angle of attack alpha (= angle
between the symmetry axis of the entry vehicle and the direction
of the relative velocity vector) during the Pathfinder entry (cf.
[BRAUNETAL1995]) and due to the close proximity of the z-axis
accelerometer to the axis of symmetry, the measured z-axis
accelerations during entry were very close (generally within
0.02%) to the true deceleration along the flight path.
Initial conditions for the integration were provided by the Mars
Pathfinder Navigation Team in the form of an entry velocity and
entry location at a prescribed time. This so-called 'entry
state' is based on observations of range and rate of motion along
the line-of-sight derived from radiometric tracking of the
spacecraft combined with trajectory solutions based on high-order
models of the gravitational forces on the vehicle from solar
system bodies (cf. [KALLEMEYNETAL1996]). The 'entry state' for
4-July-1997 16:51:12.28 UTC with 1-sigma uncertainties listed is
r = 3597.2 +/- 1.7 km, theta = 23 +/- 0.04 degrees N Areocentric
latitude, phi = 343.67 +/- 0.01 degrees East Longitude, VR =
7444.7 +/- 0.7 m/s, gamma = 16.85 +/- 0.02 degrees, psi = 255.41
+/- 0.02 degrees. r is the radial distance from the center of
mass of the planet. VR is the entry speed, gamma is the flight
path angle below horizontal, and psi is the flight path azimuth
measured clockwise from North (all in a Mars-fixed, ie rotating,
coordinate system). This initial state was marched forward in
time using a fourth and fifth order Runge-Kutta-Fehlberg
integrator with automatic step-size control.
Atmospheric density rho is related to the aerodynamic
deceleration of a spacecraft through the aerodynamic drag
equation:
-2m Av
rho = ----- ------ (1)
C_D A V_R**2
where Av is the acceleration along the flight path relative to
the atmosphere, V_R is the probe velocity relative to the
atmosphere, m is the probe mass, C_D is the drag coefficient of
the probe, and A is the probe's cross-sectional area. The drag
coefficient C_D varies during the entry, and this variation can
be accounted for iteratively using aerodynamic databases compiled
from pre-flight experiments and numerical simulations, as
discussed further below. V_R was set equal to the velocity
relative to the surface of the planet (V), which was derived from
the trajectory reconstruction, since the Pathfinder lander's
velocity was very much greater than expected wind magnitudes
during the time interval covered by this file. During the entry
phase, the total mass of the Mars Pathfinder entry vehicle was m
= 585.3 kg and it's area was A = 5.526 m**2. During the
parachute descent phase, the total mass was initially decreased
by the mortar firing (0.5 kg) and later by the separation of the
heatshield (74 kg). The total area of the parachute was
approximately 121 m**2.
The aerodynamic drag coefficient C_D of the lander is needed in
order to evaluate atmospheric densities from the observed
decelerations and the reconstructed trajectory, as shown in eq.
1. The aerodynamic properties of the Mars Pathfinder entry
vehicle were determined from experimental data and computational
fluid dynamic simulations, and these data have been used to
create an aerodynamic database [BRAUNETAL1995]. C_D varies
during the entry and depends on the velocity of the vehicle, the
angle of attack alpha, the Knudsen number (Kn), and Mach number
(Mach). Since Kn depends on the atmospheric density and Mach
depends on the atmospheric temperature, an iterative procedure
was required to accurately determine the atmospheric density and
aerodynamic coefficients. Initial density, pressure, and
temperature profiles were evaluated by assuming a constant value
of C_D. From these profiles, Kn and Mach as a function of time
were determined and used to evaluate improved values of C_D from
the aerodynamic database. The resulting new density, pressure,
and temperature profiles were then used as a basis for the next
iteration. This procedure was continued until successive
iterations yielded minor change in the atmospheric structure.
Two iterations beyond the initial profiles were found to yield
excellent convergence.
Atmospheric pressure (p) was derived from the density by
integrating the equation of hydrostatic equilibrium from an
initial altitude z0 down to each value of altitude z. z0 = 140
km was used since at this altitude the density measurements are
well above the instrument's threshold value, which is determined
by the digital resolution. The initial value p(z0) for the
integration is estimated by using rho(z0) and d[rho]/dz at z0 to
derive a density scale height and hence temperature, which can be
used with the ideal gas law and an estimate of the mean molecular
weight to derive a starting pressure. The temperature profile
was derived from the density and pressure profiles by using the
ideal gas law and a model for the variation with pressure of the
mean molecular weight.
We have assumed that the atmosphere is well mixed below ~100 km
altitude with a mean molecular weight of 43.49, which is based on
Viking Lander mass spectrometer measurements [OWEN&BIEMANN1976].
At higher altitudes photodissociation and diffusive separation
lead to a gradient with altitude of the mean molecular weight.
We have created a model for the variation of molecular weight
with pressure based on the Viking 1 Upper Atmosphere Mass
Spectrometer results [NIER&MCELROY1977, SEIFF&KIRK1977]. The
mean molecular weights in column 9 are derived from this model.
Software
========
The MET RDR table can be displayed on UNIX, Macintosh, and PC
platforms as a simple ASCII file, 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 derived data will be stored and distributed on
compact disc-read only memory (CD-ROM) media. The CD will be
formatted according to ISO-9660 and PDS standards.
|
| CONFIDENCE_LEVEL_NOTE |
Confidence Level Overview
=========================
Uncertainty envelopes based on the 3-sigma uncertainties in the
entry velocity and the effects of digitization are included in
columns 10-15. The uncertainty envelopes have been
conservatively chosen. The effects of uncertainties in C_D can
be estimated from the iteration procedure for the entry profiles
(described in the 'Processing' section). We found the
uncertainties in the profiles introduced by C_D are well within
the uncertainty envelopes we have included in the file.
The temperatures have been evaluated using a molecular weight
model based on the Viking 1 Upper Atmosphere Mass Spectrometer
results [NIER&MCELROY1977, SEIFF&KIRK1977], as discussed above.
Although the season, landing site coordinates, and solar cycle
phase were very similar for the Pathfinder and Viking 1 landings,
the Local Solar Time of the two landings were quite different.
The NCAR Mars Thermospheric General Circulation (MTGCM) models of
the upper atmosphere (cf. [BOUGHERETAL1990]) indicate that
substantial diurnal variation in the molecular weight profile can
occur (Bougher, personal communication). To estimate the
uncertainties in the temperatures due to possible diurnal
variations in the molecular weights, we have evaluated
temperatures using molecular weight profiles from the MTGCM
(Bougher, personal communication) runs for local times around the
Pathfinder landing. To the extent that the MTGCM and Viking 1
profiles bound the possible range of molecular weights, the
uncertainty ranges from ~4 Kelvin near 140 km altitude down to
zero at altitudes near and below 100 km. Note that these
uncertainties are much smaller than the error envelopes included
in the file.
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 calibration and quality of the accelerometer data has been
checked in several ways. Measurements by the accelerometers were
acquired during the cruise phase and known accelerations were
compared to the accelerometer readings. At the surface of Mars,
surface acceleration measurements of 3.716 m/s**2 agreed well
with the value of 3.717 m/s**2 calculated for the lander location
and height (cf. [FOLKNERETAL1997B]). The observed deceleration
profiles during atmospheric entry were also compared to entry
simulations. Finally, the landing site position computed from
our trajectory reconstruction was compared to the accurate
determination of landing site location based on measurement of
the radio signal from the lander on the surface as reported in
[FOLKNERETAL1997B].
After accelerometer gain state changes, very brief transients
were produced in the accelerometer data as expected based on
tests of the accelerometers and associated electronics prior to
launch. The extent of these transients were defined by careful
inspection of the data near the time of the gain change and by
extrapolating data acquired immediately prior to the gain change
to define the end of the transient. The time constant for the
transients was far less than one second. Inspection of the data
using the procedure mentioned above showed that the effects of
the transients were generally undetectable within about 1 second
of the gain change. The values in the transient were then
replaced by interpolation between the values acquired before the
gain change and the values acquired about 1 second after the gain
change. Since the rise and fall of the peak deceleration pulse
due to the atmosphere varies approximately exponentially with
time, extrapolation and interpolation through the transients was
performed using the logarithm of the accelerations for gain
changes occurring during the deceleration pulse.
Only two data values were missing from the accelerometer
measurements; one z-axis measurement (at SCLK 1246726322.53) and
one x-axis measurement (at SCLK 1246726361.41). These two points
were replaced by using linear interpolation across the adjacent
points.
Limitations
===========
Refinements for the effects of the small variations in the angle
of attack have not yet been incorporated. This correction should
only affect the smallest amplitude (less than or similar to a few
Kelvin) structures. Improvements in the calibration of the
spacecraft clock indicate about a 1.5 second shift in the UTC
corresponding to the first data point of the file. Hence, at
time T0 (SCLK 1246725418.00000) the UTC time was
1997-07-04T16:36:50.172Z. This shift has not been incorporated
in the profiles presented here and should have only a small
effect (few degrees Kelvin). Further work to incorporate these
refinements is in progress.
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