| DATA_SET_DESCRIPTION |
Data Set Overview
=================
The Clementine Bistatic Radar Intermediate and Reduced Data
Archive (BSR-IRDA) is a collection of partially and fully
reduced products from bistatic radar scattering experiments
conducted using the Clementine spacecraft while it orbited
the Moon.
The spacecraft radio system was used as a signal source,
and the spacecraft high-gain antenna beamed those signals
toward surface targets on the Moon. The transmit polarization
along the boresight of the spacecraft antenna was right-
circular; the nominal wavelength was 13 cm (S-band). Echo
signals were received coherently in both right- and left-
circular polarizations (RCP and LCP, respectively) using
70-m antennas at stations of the NASA Deep Space Network
(DSN) on Earth.
The scattering experiments were conducted in two
configurations. For 'spotlight' experiments a single
target was identified on the Moon; the spacecraft attitude
was controlled during the experiment so that the HGA always
pointed toward that target location.
Spotlight experiments, conducted on polar targets were
designed to detect and measure enhanced backscatter from
possible ice deposits in areas permanently shaded from
solar illumination. Such enhancements have been seen in
Earth-based radar studies of the Galilean satellites
[CAMPBELLETAL1978], Mars [MUHLEMANETAL1991], and Mercury
[HARMONETAL1994]. Although Earth-based radar studies of
the Moon [STACY1993] have had considerably better
sensitivity and surface resolution than this experiment,
the Clementine geometry uniquely allowed measurements as a
function of the bistatic angle -- the separation angle
between transmitter and receiver when viewed from the target
[SIMPSON1993]. The angular variation of any enhancement may
be related to the distance the radar signal travels through
the ice and, hence, to the thickness and/or clarity of the
ice at 13 cm wavelength.
For 'quasi-specular' observations, the HGA was aimed toward
the point on the lunar surface where mirror-like (specular)
reflections were expected. Signals transmitted by the
spacecraft would carom from the lunar surface and be received
at the DSN stations. Because the (specular) image of
Earth moved as Clementine orbited the Moon, the HGA
illuminated a swath across the surface as the spacecraft
advanced in its orbit. A single quasi-specular track was
obtained -- in the southern hemisphere during the last set
of bistatic radar observations.
The first analysis of spotlight data was published by
[NOZETTEETAL1996]. The authors identified an enhancement
in right-circularly polarized echo power near the time
when the bistatic angle was zero in permanently shaded
regions near the South Pole. A second analysis
[SIMPSON&TYLER1999] focused on the data with the reported
enhancement but was unable to reproduce the earlier results.
This data set includes intermediate products from the second
analysis. These are large files of double precision complex
time samples in both RCP and LCP. The data cover the time
period 1999-04-09T18:36:45 to 18:52:45 - the time at which the
bistatic angle was 0 nearest the South Pole was 18:46:36.5.
The intermediate files are processing output after correction
for ripples in the receiver filters (equalization), after
correction for gain changes in both channels, after digitally
centering the carrier in the passband, and after changing the
frequency reference so that the South Pole frequency bin
would be in the center. These 8 files are in the DATA
directory, which is spread over all volumes in the data set.
Also in the data set are more highly processed data, including
a subset of power spectra from the final processing stage
described above. Measured echo powers in these spectra were
sorted to construct empirical scattering functions at 72
locations near the South Pole. The spectra, the sorted power
values, and a table containing two versions of the scattering
function are in the SORT directory on the final volume.
In order to carry out the sorting, it was necessary to know
the mapping of Doppler frequencies onto the Moon's southern
hemisphere. Grids of frequency and other parameters were
computed at 100 second time steps. These are stored in PDS
IMAGE format in the IMG directory, which appears on all
volumes.
Users of the Clementine data interested in fully understanding
the signal processing might prefer to begin with the raw data,
which have been archived separately on 13 CD-R volumes as PDS
data set CLEM1-L-RSS-1-BSR-V1.0. Users interested in other
orbits necessarily must start with the raw data, since only
part of a single orbit is included in this archive.
Users of the partially processed data could pick up the
processing at any stage and follow it through to completion.
The calibration files which were used during the Stanford
processing to equalize the filter response, adjust the gain,
and modify the frequency of the data are stored in the CALIB
directory. Two versions of the list of target points for
constructing the scattering function(s) are included in the
GEOMETRY directory.
Users interested only in the two scattering functions
derived in the Stanford analysis will find them in the
file RATIOFN.TAB in the SORT directory. Note, however, that
these are functions giving the RATIO of RCP to LCP power
rather than the individual powers themselves, that they were
derived in only two of several possible ways, and that the
uncertainties (based on noise in the original measurements of
echo power) are large.
Parameters
==========
Files in the DATA directory contain double precision complex
time samples of receiver output in RCP and LCP between
1994-04-09T18:36:45 and 18:52:45, after corrections and
adjustments for various effects.
Files RCP.IMG and LCP.IMG in the SORT directory are calibrated
arrays (in PDS IMAGE format) giving echo power in RCP and LCP,
respectively.
Files SRTPWRR.TAB and SRTPWRL.TAB contain power measurements
from the arrays above, sorted according to proximity to one of
72 surface target points and bistatic angle at that point.
File SRTNPWR.TAB gives the number of measured values that
have been associated with each target point at each bistatic
angle.
File RATIOFN.TAB gives the RCP to LCP ratio as a function of
bistatic angle for the 13 target points poleward of 88 S
latitude. The ratio is computed in two different ways.
Ephemerides were used to compute the observing geometry for a
700x700 grid. Each grid point was related to a surface point
orthographically projected onto a plane tangent to the lunar
surface at the South Pole. Various observing parameters were
computed at 100 second time steps and stored in PDS IMAGE
format. These include:
BET bistatic angle (degrees)
DAR area (square meters)
DBR offset from boresight at the (DSN) receiving
antenna (degrees)
DPR incremental received power (watts) from the
surface element
FQZ Doppler frequency relative to the South Pole (Hz)
GAM tilt of the surface element (degrees)
GRX receiving antenna gain at the surface element (dB)
GTX spacecraft transmitting antenna gain at the surface
element (dB)
RRX distance from the receiving antenna (meters)
RTX distance from the spacecraft transmitting
antenna (meters)
SBR offset from boresight at the spacecraft
antenna (degrees)
SG0 assumed specific radar cross section for
calculating DPR
THI incidence angle at the surface element (degrees)
THS scattering angle at the surface element (degrees)
VAL validity mask (1 = grid point within lunar disk;
0 = outside)
DFQ time derivative of FQZ (Hz/sec)
Processing
==========
Processing is described in more detail in the file PROCSSNG.TXT
in the DOCUMENT directory. The DOCUMENT directory also
contains other files which may be useful in understanding the
processing carried out at Stanford.
The raw 8-bit samples from the analog-to-digital converters
were stored on tape during data acquisition. Those samples
were separated into two computer files, corresponding to RCP
and LCP (also referred to as Channel 1 and Channel 3,
respectively, in some contexts). This program (called PREPMO)
also padded the samples to 16 bits.
The two files of 16-bit samples were then used as input
(separately) to program PREPFND which carried out digital
filtering. The output of this stage was a stream of double
precision complex time samples at half the original rate for
each polarization (channel). The filtering operation in
PREPFND was 'equalization', in which signal amplitude in
the center 80 percent of the passband was normalized by the
filter response with only noise as input.
The output from PREPFND was fed to program GAIN which adjusted
the sample amplitude in each channel for changes in attenuator
settings during data acquisition, gain drift in the receiving
system, and gross differences in levels between the two
channels.
The output from GAIN was passed to program POWERFIT, which
located the peak signal in the passband and estimated both its
amplitude and frequency. The frequency estimates were used to
create a file of coefficients for a piecewise continuous
linear approximation to the carrier frequency.
Using the file of coefficients, the program STEER adjusted
the phase of the complex output samples from GAIN so that the
carrier appeared to be at the center of the passband
(12500 Hz).
Using ephemerides for the spacecraft and planetary bodies,
the expected Doppler offset between the carrier signal and a
hypothetical echo from the lunar South Pole was computed.
This offset was converted to a second set coefficients for a
piecewise linear approximation to the difference frequency.
The second set of coefficients was used in a second
application of the STEER program to move the estimated
frequency of the South Pole to the center of the passband
(12500 Hz). From this point, the South Pole frequency
became the reference.
Program FNDLOOK was used to compute 16384-point power spectra
from the second-order STEER results. There was no averaging,
so spectra were spaced by 0.65536 seconds and frequency
resolution was approximately 1.5259 Hz. A subset of the
frequency range (bins 7356-8379) was saved for further analysis.
This array of 1464 1024-point spectra had the South Pole echo
in bin 838. A smoothed version of the bin 838 response (and
21 of its neighbors) appeared in [SIMPSON&TYLER1999] as
Figure 10, from which the authors concluded there appeared to
be nothing unusual about the response of the South Pole.
The ephemerides were used to locate points in the
vicinity of the South Pole where bistatic angle went to 0
during the course of this observation. 72 such points,
spaced by 1 second, were identified, forming a track which
passed very close to the Pole (Figure 2 in [SIMPSON&TYLER1999]).
The values stored in the 700x700 grids could be used to map
Doppler contours onto the lunar surface and to compute expected
echo spectra (for an assumed scattering function). As tools
for interpreting the measured values, they could be used to
determine which of the 72 target points (if any) contributed
to a frequency bin at a certain offset from the South Pole
frequency. And they could be used to determine the bistatic
angle at such a target point when the measurement was made.
Such an inversion was accomplished, through a number of steps,
leading to the sorted echo power measurements in files
SRTPWRR.TAB and SRTPWRL.TAB in the SORT directory. The
arrays of sorted measurements for 72 targets and 101 bistatic
angles (with up to 42 values in each bin) were then reduced
to scattering functions for each of the 72 targets and for
aggregations of those targets. RATIOFN.TAB in the SORT
directory contains two such possibilities.
Data
====
The data set contains two each of the following files (one for
each polarization). Each file is accompanied by a detached PDS
label which completely describes both the format and content
of the file. Each data file has 384002048 bytes and is located
in the DATA directory.
A. PREPFND output file after filter equalization. File
names: PF1.TAB (RCP) and PF3.TAB (LCP)
B. GAIN output file after gain corrections. File names:
GN1.TAB (RCP) and GN3.TAB (LCP)
C. First-order STEER output file after centering carrier
at 12500 Hz. File names: S11.TAB (RCP) and S13.TAB (LCP)
D. Second-order STEER output file after centering South
Pole echo. File names: S21.TAB (RCP) and S23.TAB (LCP)
The data set contains two each of the following files (one for
each polarization). Each file is accompanied by a detached PDS
label which completely describes both the format and content
of the file. These files are in the SORT directory.
E. FNDLOOK output file converted to PDS IMAGE format. Each
is a 1464x1024 array of echo power density with the
South Pole at bin 838. File names: RCP.IMG and LCP.IMG
F. Output array from the echo sorting process. Each file is
a table but represents data in a three-dimensional
array (up to 42 valid power measurements in a 72 target
by 101 angle array). File names: SRTPWRR.TAB (RCP)
and SRTPWRL.TAB (LCP).
Accompanying the sorted data are two other arrays. Each is
accompanied by a detached PDS label which completely describes
the format and content of the data file. These files are in
the SORT directory.
G. An array giving the number of valid measurements in
the two data arrays SRTPWRR.TAB and SRTPWRL.TAB.
File name: SRTNPWR.TAB
H. A table containing two empirical scattering functions.
File name: RATIOFN.TAB.
The data set contains 59 sets of 15-16 grids. The sets are
spaced by 100 seconds in time, starting at 65696 seconds past
0 h on 1994-04-09 and covering the time during which DSS 14
tracked the Clementine spacecraft. Each file occupies 980000
bytes and is accompanied by a PDS detached label. Grids are
ordered chronologically on CD-R volumes (but with no more
than 256 grids per volume) and are stored in the IMG directory.
Because the DFQ grid is formed by differencing the EQZ grids at
the immediately adjacent time steps, there is no DFQ grid for
the first or last time step. At each of the other steps there
are 16 grids.
Ancillary Data
==============
Files in the CALIB directory include the following. Each is
accompanied by a PDS detached label which describes the format
and content of the data file.
A. Filter equalization files with names of the form
FLssbpnn.SPC where
ss is the DSN antenna number (14 or 43)
f is the frequency band (S)
p is the polarization (R or L)
nn is the sequence number (01 or 02)
Only data from DSS 14 (ss=14) are included in this archive;
only filters with sequence number nn=01 were used in the
Stanford processing.
B. Files containing coefficients for gain correction with
names of the form G099C14c.TAB where
c denotes the channel number (1=RCP, 3=LCP)
C. A file containing coefficients for centering the carrier
frequency with name STEERCF1.TAB
D. A file with coefficients describing the difference in
frequency between the carrier and South Pole bin with
name DF2SCM.TAB
E. A table of noise level measurements for calibrating system
gain and determining system noise temperature with
name TABLE2.TAB (see Table 2 in [SIMPSON&TYLER1999])
Files in the GEOMETRY directory include the following. Each is
accompanied by a PDS detached label which describes the format
and content of the data file.
F. A table summarizing geometry and observing conditions with
name BSRGEOM.TAB
G. Two tables listing coordinates for 72 target points around
which sorting was carried out and scattering functions
derived. BETAZER0.TAB lists the coordinates in radians;
BETAZER1.TAB lists the coordinates in degrees.
Coordinate System
=================
The radio measurements can be understood without reference to
a particular coordinate system. For mapping measurements to
the lunar surface, a selenocentric system with positive east
longitude was used.
Software
========
This data set contains no software.
Media/Format
============
The archival data set is written on CD-WO media using a
Yamaha CRW4260tx CD writer and GEAR software running on a
Sun Ultra-5 workstation.
|
| CONFIDENCE_LEVEL_NOTE |
Overview
========
The calibrations, gain corrections, and frequency adjustments
were done with considerable care. They have been described
by [SIMPSON&TYLER1999] and in several documents in the
DOCUMENT directory. The files in the DATA and CALIB directories
and the RCP.IMG and LCP.IMG files in the SORT directory fall
into this category.
The geometrical calculations were also done carefully and are
documented in [SIMPSON&TYLER1999] and in the DOCUMENT directory
of this volume. Files in the GEOMETRY and IMG directories fall
into this category.
The sorted files (SRTPWRR.TAB, SRTPWRL.TAB, and SRTNPWR.TAB)
and the scattering function (RATIOFN.TAB) have been done once.
The procedures are documented in the DOCUMENT directory and in
in an abstract for Lunar and Planetary Science XXXI (see
LPSC_ABS.LBL in the DOCUMENT directory). But resource
limitations have prevented the careful scrutiny and double
checking that were possible in the earlier analysis. Users
should view these files more skeptically.
Review
======
This archival data set was distributed to a peer review
panel prior to its acceptance by the Planetary Data System
(PDS). The review panel was appointed by the Manager
of the PDS Geosciences (GEO) Discipline Node (DN); the
review was conducted in accordance with PDS GEO
procedures.
Data Coverage and Quality
=========================
The times covered by this data set are listed in the table
below. The depth of coverage is given in the 'Analysis'
column at the right, with entries explained below the table.
Revolution
Start Time (ERT) Stop Time (ERT) Number DSS Analysis
------------------- ------------------- ---------- --- --------
1994-04-09T17:48:45 1994-04-09T18:04:44 233 14 2
1994-04-09T18:04:45 1994-04-09T18:20:44 233 14 2
1994-04-09T18:20:45 1994-04-09T18:36:44 233 14 2
1994-04-09T18:36:45 1994-04-09T18:52:44 233 14 1
1994-04-09T18:52:45 1994-04-09T19:08:44 233 14 2
1994-04-09T19:08:45 1994-04-09T19:24:44 233 14 2
1994-04-09T19:24:45 1994-04-09T19:40:44 233 14 2
1994-04-09T19:40:45 1994-04-09T19:56:44 233 14 2
1994-04-09T19:56:45 1994-04-09T20:05:09 233 14 2
1994-04-09T23:00:55 1994-04-09T23:16:54 234 43 3
1994-04-09T23:16:55 1994-04-09T23:32:54 234 43 3
1994-04-09T23:32:55 1994-04-09T23:48:54 234 43 3
1994-04-09T23:48:55 1994-04-10T00:04:54 234 43 3
1994-04-10T00:04:55 1994-04-10T00:20:54 234 43 3
1994-04-10T00:20:55 1994-04-10T00:36:54 234 43 3
1994-04-10T00:36:55 1994-04-10T00:52:54 234 43 3
1994-04-10T00:52:55 1994-04-10T01:08:54 234 43 3
1994-04-10T01:08:55 1994-04-10T01:24:54 235 43 3
1994-04-10T01:24:55 1994-04-10T01:33:55 235 43 3
1994-04-10T04:02:22 1994-04-10T04:18:21 235 43 3
1994-04-10T04:18:22 1994-04-10T04:34:21 235 43 3
1994-04-10T04:34:22 1994-04-10T04:50:21 235 43 3
1994-04-10T04:50:22 1994-04-10T05:06:21 235 43 3
1994-04-10T05:06:22 1994-04-10T05:22:21 235 43 3
1994-04-10T05:22:22 1994-04-10T05:38:21 235 43 3
1994-04-10T05:38:22 1994-04-10T05:54:21 235 43 3
1994-04-10T05:54:22 1994-04-10T06:10:21 235 43 3
1994-04-10T06:10:22 1994-04-10T06:26:21 236 43 3
1994-04-10T06:26:22 1994-04-10T06:42:21 236 43 3
1994-04-10T06:42:22 1994-04-10T06:46:54 236 43 3
Analysis levels represented in the archive:
1 Exhaustive calibration and correction of data; data
values sorted into empirical scattering functions
2 Filter functions, gain corrections, and grids provided;
no data in the archive, no frequency coefficients
available.
3 Notes on gain variations observed are in the archive;
no calibration/correction files and no data included.
Only very limited work with DSS 63 data from 1994-04-10 was
carried out at Stanford. And virtually nothing was attempted
with data from days 1994-04-23 and 1994-04-24.
Limitations
===========
See section Overview and Data Coverage and Quality above.
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