PDS_VERSION_ID = PDS3 RECORD_TYPE = STREAM LABEL_REVISION_NOTE = "2004-11-16, Roberto Orosei, first draft" 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 will be 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 will address 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 will require 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 will be identified with a coarse accuracy; around this point we require S/C manouevre to get the 1 degree accuracy required. The following procedure should be applied over a smooth area: - Every orbit should have a different roll (cross track) pointing: from -2 to 2 degrees with steps of 1 degree - In each orbit the pitch pointing shall be 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 will be 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 will be performed within the instrument itself. Major tasks performed by MARSIS digital processing unit are Doppler processing, range processing, and multi-looking. Different operative modes will require 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 will be 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 will 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