Lunar Prospector Magnetometer Derived Bundle
Lunar Prospector Regional Field Bins at the Spacecraft Altitude Data
Description
PDS3 DATA_SET_ID = LP-L-MAG-5-LUNAR-FIELD-BINS-V1.0
ORIGINAL DATA_SET_NAME = LP MOON MAG LEVEL 5 LUNAR MAGNETIC FIELD
BINS V1.0
START_TIME = 1998-01-16
STOP_TIME = 1999-07-29
PDS3 DATA_SET_RELEASE_DATE = 2003-10-01
PRODUCER_FULL_NAME = LON L. HOOD
DATA_DESCRIPTION =
Overview:
=========
Lunar Prospector Magnetometer (MAG) (NASA Level 3 CODMAC Level 5) Derived
Data. Regional Field Bins at the S/C Altitude. Interpolation
and two-dimensional filtering are applied to the calibrated data to
produce gridded field component data files on a surface defined by
the slowly varying S/C altitude. The spatial resolution of the
grid is chosen to be much less than the mean S/C altitude.
====================================================================
Parameters:
===========
The measured parameters consist of the three vector components
of the lunar crustal magnetic field in East, North, and Radial
(ENR) coordinates, the field magnitude, and the spacecraft
altitude. The units of the field measurements are nanoTeslas (nT).
1 nT = 0.00001 Gauss. The units of the spacecraft altitude are
kilometers (km). ENR coordinates are centered at the instantaneous
spacecraft location with X directed eastward, Y directed northward,
and Z directed radial to the Moon.
====================================================================
Processing:
===========
Step 1: A two-dimensional filtering algorithm is applied to
final selected LP magnetometer calibrated orbit segments to produce
a gridded file suitable for mapping. The program first sorts the
data into latitude / longitude bins and averages values within
a single bin (e.g., 0.5 by 0.5 degrees for data acquired at
altitudes near 100 km). A moving boxcar method is
then applied to two-dimensionally filter the mean gridded
measurements. A 5 bin by 5 bin filter size is
normally chosen. This step is repeated twice, separately for
each field component and the S/C altitude. Finally, the
resulting field magnitude is calculated from the smoothed
component files on the same two-dimensional grid.
The resulting two-dimensionally filtered gridded files are
the derived data. They contain latitude, longitude, the
field component in nT (or the altitude in km), and the number
of data points in each bin.
Step 2: If a significant longitudinal gap between
usable orbit segments is present, linear interpolation between the
two nearest orbit segments is applied to create an interpolated
orbit segment. This interpolated segment is then read into the
two-dimensional filtering code to fill in the missing grid values.
This method is applied only when the gap between usable orbits is
smaller than the 5-bin size of the two-dimensional filter.
Step 3: For visualization purposes, a suitable
graphics utility is applied to construct regional shaded contour
plots of the vector components of the crustal field, the field
magnitude, and the mean S/C altitude. These are contained in a
postscript file that is included with the filtered gridded files.
The file naming convention for the postscript files is the same
as for data files (see below), but with a .PS extension.
====================================================================
File Names and Format:
======================
The two-dimensionally filtered gridded files are named as follows:
For 1998 data:
MAP98_mmp_xxx.TAB
where mm indicates the month (01 for January, etc.),
p (optional) indicates the map number for this
month,
xxx is either RAD, EAS, NOR, TOT or ALT, indicating
what type of data are in the file (radial B component,
east B component, north B component, total B, or
altitude).
For 1999 data:
MAP99_dddp-dddp_xxx.TAB or MAP99_dddp_xxx.TAB
where ddd indicates a day (and thus ddd - ddd is a range of days),
p and xxx are as for 1998 data.
Each file (in ascii format) contains the latitude, east longitude,
the field component (or total field, or altitude) at the grid
point, and the number of data points in the bin centered at the
grid point. The altitude is estimated by subtracting the mean lunar
radius (1738 km) from the radial distance of the spacecraft to the
lunar center of figure.
The columns in each data file are as follows:
Column 1: Latitude at this grid point (South is negative;
North is positive).
Column 2: East longitude at this grid point.
Column 3: Magnetic field value in nanoTeslas or altitude in
kilometers. The last part of the file name before the dot
indicates what is contained in this column (EAS = East field
component; NOR = North field component; RAD = Radial field
component; TOT = total field; ALT = Altitude).
Column 4: Number of data points in this latitude/longitude bin.
The bin size can be deduced from the difference between
successive latitude and longitude values (e.g., 0.5 degrees
by 0.5 degrees for altitudes near 100 km; 0.25 degrees by
0.25 degrees for altitudes near 25 km).
CONFIDENCE_LEVEL_NOTE =
======================================================================
Review:
=======
These data have completed peer review and are certified.
======================================================================
Limitations:
============
The quality of the measurements is a
strong function of the spacecraft altitude, which varies from a
minimum of about 80 km during the first year of the mission to a
minimum of about 15 km during the final six months of the
mission. The best crustal field measurements were therefore
obtained during the final ''extended mission'' phase when the
orbit periapsis was lowest. The surface resolution of the
measurements is normally of the same order as the spacecraft
altitude. However, at the lowest altitudes (about 15 km), the
resolution is limited also by the orbit track separation, which
is approximately 30 km at the lunar equator. The accuracy to
which the crustal field is measured at a single point along the
spacecraft trajectory is limited by residual external field
variations and is approximately 0.1 nT.
======================================================================
Data Coverage:
==============
The coverage of the measurements over the lunar surface is limited
to those areas on the near and far side when the Moon and the LP
spacecraft were favorably located to allow direct measurements of
the crustal magnetic field.
|