Data Set Overview
=================

MARSIS Level 2 data products consist of Level 1b data that have been
uncompressed, corrected for Automated Gain Control, aligned to a
reference altitude and, except for data acquired using the SS2 mode,
range-processed after correcting for the distortion of the transmitted
signal caused by the ionosphere. Echoes collected in the acquisition
(ACQ) phase, having a much smaller bandwidth, do not have a vertical
resolution sufficient for scientific analysis, and thus are not used
in generating Reduced Data Records. Geometric information needed to
locate observations in space and time is also provided in the Data
Set.


Parameters
==========

MARSIS data are the result of the processing of groups of echoes
called frames. A frame is produced by the coherent summation of many
tens of echoes. Each frame is recorded as a time series of complex
signal samples. Scientific data in a frame are complemented by a set
of engineering data, produced by the instrument and recording
parameter values used in pulse transmission, echo reception and
on-board processing, and by geometric information needed to locate
observations in space and time.


Processing
==========

Level 2 processing of subsurface sounding data consists of several
steps, needed to convert the instrument telemetry into a collection of
radar echoes that can be used for geological analysis and
interpretation of the Martian subsurface. These are:

 Data de-compression: 8-bit spectrum samples are converted back to
4-byte real numbers.

 AGC compensation: the gain scaling factor, applied during Automated
Gain Compensation, is applied to the data.

 Receiving time compensation: echoes are shifted in time to refer them
to a common altitude above the Martian ellipsoid.

 Range compression: the received signal is convolved with the
transmitted signal to increase range resolution and signal-to-noise
ratio.

 Ionosphere distortion correction: an algorithm to correct ionosphere
dispersion of the signal, called 'contrast method', is applied to the
data.

Depending on the instrument mode, some of the above steps may not
apply (for example, range processing is performed on board in mode
SS2). The final result is a file containing all complex echoes
collected by MARSIS in the course of a single orbit.



Data
====

Data Products need to accommodate the different data structures
produced by different subsurface sounding modes of the instrument.
There is a single type of Data Product specialized into five different
subtypes, one for each subsurface sounding mode:

R_SSx           Subsurface Sounding data that have been uncompressed,
calibrated
and range-processed, together with geometry information

Where 'x' stands for a number between 1 and 5.

SSx_RDR Data Products consist of two files, one of which contains a
PDS binary TABLE object while the other is a PDS label describing its
structure. The first file contains the instrument data proper,
organised into frames. 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 tracking (TRK) state during a single orbit.


Ancillary Data
==============

No Ancillary data are provided.


Coordinate System
=================

Locations on the surface of Mars are expressed in planetocentric
coordinates. Longitude increases to the East and 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.


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.cosmos.esa.int/web/psa/psa-introduction


Confidence Level Overview
=========================

This data set contains all data for the Mars Express MARSIS for the
interval described above. Every effort has been made to ensure that
all data returned by the spacecraft is included and that processing is
accurate.


Review
======

The MARSIS EDR data have been reviewed internally by the Mars Express
MARSIS team prior to release to the PDS. The data set has been also
peer reviewed by the PSA.


Data Coverage and Quality
=========================

Data in this dataset have been acquired only in the last six months of
the nominal mission, because of the delay in the deployment of the
MARSIS antenna. Because of this, coverage is limited mainly to
southern latitudes.

Data quality in a data product label is indicated through the
DATA_QUALITY_ID element, and measures the median of the total echo
power across the orbit. The permitted values of DATA_QUALITY_ID are
the following:

 0: data quality unknown
 1: median data quality index in the bottom 25% of computed values
 2: median data quality index in the bottom 50% of computed values
 3: median data quality index in the top 50% of computed values
 4: median data quality index in the top 25% of computed values

For instrument mode transmitting at two different frequencies,
DATA_QUALITY_ID reports a value for every frequency.


Limitations
===========

There are no known limitations at this time.

Data quality in a data product
                    label is indicated through the DATA_QUALITY_ID
                    element, and measures the total power in radar
                    echoes.