PDS_VERSION_ID = PDS3
LABEL_REVISION_NOTE = "2004-11-16, Roberto Orosei, first draft
2006-09-17, R.Noschese, revision"
PRODUCT_ID = "INST.CAT"
RECORD_TYPE = STREAM
RELEASE_ID = 0001
REVISION_ID = 0000
OBJECT = INSTRUMENT
INSTRUMENT_HOST_ID = MEX
INSTRUMENT_ID = MARSIS
OBJECT = INSTRUMENT_INFORMATION
INSTRUMENT_NAME = "MARS ADVANCED RADAR FOR SUBSURFACE AND
IONOSPHERE SOUNDING"
INSTRUMENT_TYPE = "RADAR"
INSTRUMENT_DESC = "
Instrument Overview
===================
MARSIS is a multi-frequency nadir-looking pulse-limited radar sounder
and altimeter, which uses synthetic aperture techniques and a
secondary receiving antenna to enhance subsurface reflections. MARSIS
can be effectively operated at any altitude lower than 800 km in
subsurface sounding mode, and below 1200 km in ionosphere sounding
mode. The instrument consists of two antenna assemblies and an
electronics assembly. Maximum penetration depths are achieved at the
lowest frequencies, and penetration is in the order of a few
kilometres, depending on the nature of the material being sounded. On
the dayside of Mars, the solar wind-induced ionosphere does not allow
subsurface sounding at frequencies below approximately 3.5 MHz, as
the signal would be reflected back at the radar without reaching the
surface. To achieve greater subsurface probing depths, operations on
the night side of Mars are thus strongly preferred.
Scientific Objectives
=====================
The primary objective of MARSIS is to map the distribution of water,
both liquid and solid, in the upper portions of the crust of Mars.
Detection of such reservoirs of water addresses key issues in the
hydrologic, geologic, climatic and possible biologic evolution of
Mars, including the current and past global inventory of water,
mechanisms of transport and storage of water, the role of liquid
water and ice in shaping the landscape of Mars, the stability of
liquid water and ice at the surface as an indication of climatic
conditions, and the implications of the hydrologic history for the
evolution of possible Martian ecosystems.
Three secondary objectives are defined for the MARSIS experiment. The
first objective consists in probing the subsurface of Mars, to
characterise and map geologic units and structures in the third
dimension. An additional secondary objective consists in acquiring
information about the surface of Mars: the specific goals of this
part of the experiment are to characterise the roughness of the
surface at scales of tens of meters to kilometres, to measure the
radar reflection coefficient of the surface and to generate a
topographic map of the surface at approximately 15-30 kilometres
lateral resolution. A final secondary objective is to use MARSIS as
an ionosphere sounder to characterize the interactions of the solar
wind with the ionosphere and upper atmosphere of Mars.
Calibration
===========
In order to get the predicted performances in the dual antenna
clutter cancellation procedure, and consequently to reach the
expected penetration depth, the null of the monopole antenna has to
be determined.
An estimation of the direction of the null in the monopole channel
can be obtained by acquiring calibration data over a rough (related
to the wavelength) area (range - azimuth transform to detect the null
direction). This requires MARSIS to operate at full power with
the pitch set at zero degree and over a rough terrain to get a strong
surface clutter and with proper illumination condition in order to
use all the frequencies.
After data analysis, the pitch (along track) null region direction
is identified with a coarse accuracy; around this point we
require S/C manoeuvre to get the 1 degree accuracy required. The
following procedure has been applied over a smooth area:
- Every orbit had a different roll (cross track) pointing:
from -2 to 2 degrees with steps of 1 degree
- In each orbit the pitch pointing has been varied continuously (with
steps of 1 degree) during the pericenter passage from -4 to 1
degrees.
Operational Considerations
==========================
MARSIS has been designed to perform subsurface sounding at each orbit
when the altitude is below 800 Km. A highly eccentric orbit such as
the baseline orbit places the spacecraft within 800 km from the
surface for a period of about 26 minutes. This would allow mapping of
about 100 degrees of arc on the surface of Mars each orbit, allowing
extensive coverage at all latitudes within the nominal mission
duration. To achieve this global coverage MARSIS has been designed to
support both day side and night side operations, although
performances are maximized during night time (solar zenith angle ❯ 80
degrees), when the ionosphere plasma frequency drops off
significantly and the lower frequency bands, which have greater
subsurface penetration capability, can be used.
Active Ionosphere Sounding is also carried out by MARSIS at
certain passes when the spacecraft is below 1200 Km of altitude, both
during day and night time.
The instrument is commanded by means of two tables, the Operations
Sequence Table and the Parameters Table, which are up-linked from
ground as part of the instrument programming and commanding, and
loaded in the instrument memory at switch-on.
Detectors
=========
MARSIS antenna assembly consists of two antennas, a dipole and a
monopole. The primary dipole antenna, parallel to the surface and to
the direction of spacecraft motion, is used for transmission of
pulses and for reception of pulse echoes reflected by the Martian
surface, subsurface and ionosphere. The secondary monopole antenna,
oriented along the nadir, has a null in the nadir direction, and it
is thus sensitive to off-nadir surface returns. Such surface returns
could mask subsurface echoes arriving at the same time, and are thus
an undesired contribution to the received echoes (clutter): the
monopole antenna is used in subsurface sounding to remove clutter
from the signal received by the dipole.
Electronics
===========
Due to limits in permitted data rate for data transmission between
the instrument and the solid state mass memory of the spacecraft, and
constraints on the data volume that can be down-linked to Earth, most
data processing is performed within the instrument itself. Major
tasks performed by MARSIS digital processing unit are Doppler
processing, range processing, and multi-looking. Different operative
modes requires all, some or none of these capabilities.
Conceptually, Doppler processing of pulse echoes consists in
artificially adding a delay, corresponding to a phase shift of the
complex signal, to the samples of each pulse, and then in summing the
samples so as to allow the constructive sum of the signal component
whose delay (phase shift) from one pulse to the next corresponds to a
desired direction (usually nadir or close to nadir). This is called
also synthetic aperture processing, and is used to improve both
horizontal resolution in the along-track direction and signal-to-
noise ratio: horizontal resolution becomes that of an equivalent
antenna whose length is equal to the segment of the spacecraft
trajectory over which pulse echoes are summed coherently, whereas
signal-to-noise ratio improves by a factor equal to the number of
coherently summed pulses.
Range processing consists in computing the correlation between the
transmitted pulse and received echoes. If the transmitted amplitude
is constant during the pulse, the correlation with an echo identical
to the transmitted signal takes the form of a (sin x)/x pulse. This
process, called also range compression, is performed on ground for
most subsurface sounding modes, on the digitally sampled data, to
properly compensate ionospheric effects: accurate coherent pulse
compression requires in fact detailed knowledge of the modulation of
the returning signals, whose phase structure is distorted in their
(two-way) propagation through the ionosphere.
Multi-look processing is the non-coherent sum of echoes (that is,
phase information in the complex signal is ignored), after both
Doppler and range processing, performed to increase the signal-to-
noise ratio and reduce speckle, this last being the effect of random
fluctuations in the return signal observed from an area-extensive
target represented by one pixel. Because this process requires that
multiple observations of the same area are available for the summing,
it spans across several frames in which the same spot on the surface
is observed at slightly different angles of incidence in different
adjacent synthetic apertures.
Filters
=======
In MARSIS subsurface sounding, the same group of echoes undergoing
synthetic aperture processing can be used to focus multiple points on
the surface, by changing the phase shift from echo to echo so as to
produce constructive interference in different directions. The
resulting processed echoes are also called Doppler filters.
Operational Modes
=================
For subsurface sounding, a chirp signal is generated and
transmitted at each operating frequency for a period of about 250
microseconds. The instrument then switches to a receive mode and
records the echoes from the surface and subsurface. The total
transmit-receive cycle lasts a few milliseconds, depending on
altitude. The received signals are passed to a digital-to-analogue
converter and compressed in range and azimuth. The azimuth
integration accumulates a few seconds of data and results in an
along-track footprint size of 10 km. The cross-track footprint size
is on the order of 20 km. Digital on-board processing greatly reduces
the output data rate to 75 kilobits per second or less. For each
along-track footprint, echo profiles show the received power as
a function of time delay, with a depth resolution of 50-100 m,
depending on the wave propagation speed in the crust.
Active ionosphere sounding consists of transmitting a pulse from
MARSIS at a frequency f, and then measuring the intensity of the
reflected radar echo as a function of time delay. For a radar signal
incident on a horizontally stratified ionosphere, a strong specular
reflection occurs from the level where the wave frequency is equal to
the electron plasma frequency. By measuring the time delay for the
reflected signal (controlled by the group delay), the plasma
frequency, and therefore the electron density can be derived as a
function of height. The frequency of the transmitted pulse is
systematically stepped to yield time delay as a function of
frequency.
In addition to subsurface and ionosphere sounding, MARSIS is capable
of two more data collection modes that are not science-related, but
are rather used for the testing of the instrument. Hardware
calibration mode and receive-only mode are identical in their
sequencing of data acquisition, differing only for the fact that in
receive-only mode no pulses are actually transmitted. In both modes,
80 echoes are collected from both antennas at one of the frequency
bands used in subsurface sounding, stored in a buffer as they come
out of the analogue-to-digital converter, and, because the resulting
data rate would be too high for the spacecraft data bus, transferred
to the spacecraft mass memory over a time span eighty times longer
than the one used for data acquisition.
Subsystems
==========
From the functional point of view, the instrument can be split into
three subsystems:
- Antenna (ANT)
- Radio Frequency Subsystem (RFS)
- Digital Electronics Subsystem (DES)
From the mechanical point of view, DES and the receiver section (RX)
of the RFS subsystem are allocated in the same box inside the
spacecraft. Inside the spacecraft is also allocated the mechanical
box for the transmission electronics (TX). The dipole antenna and
monopole antenna are located outside the spacecraft.
Measured 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.
"
END_OBJECT = INSTRUMENT_INFORMATION
OBJECT = INSTRUMENT_REFERENCE_INFO
REFERENCE_KEY_ID = "PICARDIETAL2004"
END_OBJECT = INSTRUMENT_REFERENCE_INFO
END_OBJECT = INSTRUMENT
END
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