| 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.
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