| DATA_SET_DESCRIPTION |
Data Set Overview
=================
MARSIS Level 1b data products consist of the data produced by the
instrument reconstructed from the scientific telemetry, sorted by
instrument state and data type, and provided with spacecraft position,
velocity and attitude information. Any other spacecraft telemetry
relevant for calibration and processing (e.g. temperature of the
receiver) is also be included.
Parameters
==========
MARSIS data are organized into groups of echoes called frames. A frame
contains one or more echoes, with or without on-board processing. Each
echo, depending on the kind of processing it underwent, is recorded
either as a time series of signal samples, or as the complex spectrum of
the signal itself produced by means of a FFT. Scientific data in a frame
are complemented by a set of ancillary data, produced by the instrument
and recording parameter values used in pulse transmission, echo
reception and on-board processing.
Processing
==========
Level 1b processing starts from the telemetry data, as produced by the
C&DH system on the spacecraft and passed to the telemetry subsystem:
data are still in the form of transfer frame packets organised by
contacts or ground tests data.
Processing starts by cleaning, merging and time-ordering the packets.
This means that duplicate data have been deleted, missing packets are
padded out, and the data are organised by days (or orbits). Data then
need to be sorted by instrument data types and instrument modes.
MARSIS Level 1b processing orders data in a useful way for the intended
users (i.e. radar scientists) and applications (i.e. quick look to
monitor hardware performance and higher-level processing), altering and
manipulating them as little as possible to avoid the risk of introducing
errors and, at the same time, including all necessary information from
all relevant sources. Level 1b data are in scientifically useful form,
i.e. individual spectra. These data are still uncalibrated.
MARSIS Level 1b data products consist of the data produced by the
instrument reconstructed from the scientific telemetry, sorted by
instrument state and data type, and provided with spacecraft position,
velocity and attitude information. Any other spacecraft telemetry
relevant for calibration and Processing (e.g. temperature of the
receiver) is also be included.
Level 1b processing requires the acquisition of MARSIS scientific
telemetry and any relevant spacecraft auxiliary data from the Mars
Express Data Disposition System (DDS) in ESOC, and of SPICE kernels
describing the spacecraft state and attitude from the Auxiliary Data
Conversion System (ADCS) in ESTEC.
Both instrument telemetry and ancillary data are stored at the PI
processing facility as they accumulate over the course of the mission,
to provide the capability to reprocess data in case of errors or to
accommodate new information referring to existing data sets.
Level 1b data distribution to the Co-Is and to the Mars Express mission
archive is performed by ASDC. It is required by ESA that data products
are delivered in batches of six-months worth of data within six months
from the last data take (i.e. one year after the beginning of that
particular data collection period), but it is necessary that level 1b
processing be completed in a much shorter period, to allow enough time
for level 2 data processing and data analysis within the MARSIS team
before the expiration of the data proprietary period (which is the same
six-month time span).
Data
====
The MARS EXPRESS MARS MARSIS EXPERIMENT DATA RECORD V1.0 Data Set
contains scientific telemetry generated by the instrument, edited to
remove duplications, zero-padded for missing packets, and correlated
with geometric information needed to locate observations in space and
time. No other kind of processing is applied to the data.
The list of EDR data Products is :
AIS_EDR : Active Ionosphere Sounding data frames with geometry
information.
CAL_EDR : Data frames acquired in Calibration mode with geometry
information.
RXO_EDR : Data frames acquired in Receive Only mode with geometry
Information.
SSx_yyy_CMP_EDR : On-board processed Subsurface Sounding data
(either in Acquisition or Tracking state) with
geometry information
SSx_yyy_IND_EDR : Individual Echoes from Subsurface Sounding data
(either in Acquisition or Tracking state).
SSx_yyy_UNC_EDR : Uncompressed Subsurface Sounding data
(either in Acquisition or Tracking state).
SSx_yyy_RAW_EDR : Raw Subsurface Sounding data
(either in Acquisition or Tracking state).
Where x stands for a number between 1 and 5, while yyy stands
either for ACQ or TRK.
AIS_EDR, CAL_EDR, RXO_EDR and SSx_yyy_CMP_EDR Data Products is made by
two files, each of which contains a PDS binary TABLE object preceded by
a PDS attached label describing its structure. The first file, called
Frame file (FRM) contains the instrument data proper, exactly in the
same format (bit by bit) as they were produced by the instrument.
Each frame corresponds to a record in the file, which is also a row in
the PDS binary TABLE object into which frames are organised. A Data
Product contains all frames acquired using the same instrument mode, in
the same instrument state and after the same type of on-board processing
during a single orbit. The second file constituting an EDR is called a
Geometry file (GEO), and contains one record, corresponding to one line
of the PDS binary TABLE object into which data are organised, for every
frame in the corresponding FRM file.
Subsurface data from Individual Echoes are the unprocessed version of
data that are also down-linked in processed form. A frame of Individual
Echoes consists of a variable number of raw echoes, because, to produce
a constant along-track ground resolution, synthetic aperture (Doppler)
processing performed on board requires a number of echoes that increases
with altitude of the spacecraft. The Data Product consists in just one
file containing a PDS header and the data themselves, because any
additional geometric information would just duplicate similar
information already provided in SSx_yyy_CMP_EDR GEO data files. File
names contain the first ten digits of the spacecraft clock count
corresponding to the time at which data were acquired.
Each record in a file contains a single unprocessed echo, preceded by
auxiliary data and by a counter starting from 0 and increasing by one at
each new echo. Echoes are ordered according to the time at which they
were collected. Ancillary information is produced once per frame, and is
thus the same for all echoes in a file: this duplication has been deemed
necessary to simplify the data structure as much as possible.
The number of echoes required in Doppler processing is a function of
frequency, as well as of spacecraft altitude. In a dual-frequency mode,
the exact number of echoes collected at each frequency is contained in
the Ancillary data and echoes within a frame are ordered by frequency
before being ordered in time.
Subsurface UNC data are the uncompressed version of data that are also
down-linked in fully processed form. Because they are structured exactly
as CMP data, the only difference being the compression of individual
echo samples from 4-byte IEEE real numbers to 1-byte integers, it has
been chosen to produce one Data Product per orbit, just as for CMP data.
The Data Product consists in just one file containing a PDS header and
the data themselves, because any additional geometric information would
just duplicate similar information already provided in SSx_yyy_CMP_EDR
GEO data files. File names contains the four-digit number of the orbit
at which data were acquired. The structure of UNC Data Products is
identical to the one used in CMP FRM files. Because UNC data are down-
linked through a dump of the flash memory, rather than through the
scientific telemetry of the instrument, they do not come with a full set
of ancillary data
Subsurface Raw Data are the unprocessed version of data that are also
down-linked in processed form. A frame of Raw Data consists of a
variable number of raw echoes, because, to produce a constant along-
track ground resolution, synthetic aperture (Doppler) processing
performed on board requires a number of echoes that increases with
altitude of the spacecraft.
Because of this, it has been chosen to produce one Data Product per each
frame of Raw Data, rather than one per orbit, similarly to Individual
Echoes. The Data Product consists in just one file containing a PDS
header and the data themselves, because any additional geometric
information would just duplicate similar information already provided in
SSx_yyy_CMP_EDR GEO data files. File names contain the first ten digits
of the spacecraft clock count corresponding to the time at which data
were acquired.
The structure of RAW Data Products is identical to the one used in IND
files. Because RAW data are down-linked through a dump of the flash
memory, rather than through the scientific telemetry of the instrument,
they do not come with a full set of ancillary data.
Ancillary Data
==============
Each frame of MARSIS data (with the exception of frames stored in flash
memory) carries a 228 byte header of ancillary data, containing
necessary information for subsequent analysis of the data and further
processing. The exact content of the ancillary data depends on
instrument mode. There are four major structure types of the ancillary
data : acquisition, tracking / individual echoes for subsurface modes,
calibration / receive only and active ionosphere
Coordinate System
=================
Locations on the surface of Mars are expressed in planetocentric
coordinates. Longitude is comprised in the range 0 - 360 degrees.
Software
========
Marsis data products can be read by the PDS software NASAView, which
reads a PDS label and displays the associated image or table. NASAView
has been tested on both EDR and RDR data products.
Media/Format
============
The standard distribution format for the data is transfer through
Internet from the Planetary Science Archive of ESA, which can be
accessed at the following URL:
http://www.rssd.esa.int/psa
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