The Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) is a multi-frequency nadir-looking instrument designed to characterize the subsurface structure of Mars to a depth of a few kilometers, provide altimetry and roughness data, and make ionospheric measurements. The primary scientific objectives are to map the distribution of liquid water and ice, in the upper portion of the crust of Mars for studies of Mars' geologic, climatic, and possibly organic evolution.

Picardi, G. et. al., MARSIS: Mars Advanced Radar for Subsurface and Ionosphere
                   Sounding, ESA-SP-1240, September 2004

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.

extracted metadata from PDS3 catalog and
                    modified to comply with PDS4 Information Model

Mars Express Mars Advanced Radar for Subsurface and Ionosphere Sounding

Description

Identifier (LID)

Version