MARSIS: Mars Advanced Radar for Subsurface and Ionosphere Sounding
MARSIS (Mars Advanced Radar for Subsurface and Ionosphere Sounding) is a low frequency, nadir-looking pulse limited radar sounder and altimeter with ground penetration capabilities, which uses synthetic aperture techniques and a secondary receiving antenna to isolate subsurface reflections.
The operation altitudes for MARSIS are up to 800 km above the Martian surface for subsurface sounding and up to 1200 km for ionospheric sounding. In its standard operating mode, the instrument is capable of making measurements in 1 MHz wide bands centred at 1.8, 3.0, 4.0 and 5.0 MHz.
MARSIS functions by transmitting a linear frequency modulated chirp using a nadir-looking dipole antenna. The return signal is received on both the dipole antenna and a secondary monopole antenna oriented along the nadir axis. The secondary antenna has a null in the nadir direction and receives primarily the off-nadir surface reflections. This signal can be subtracted from the main received signal during ground processing to reduce surface clutter. Both received signals are down converted to range offset video signals before being passed to an analogue to digital converter. The resultant data are formatted by the MARSIS on-board digital processor and passed to the spacecraft for transmission to Earth.
MARSIS operates in the following modes:
MARSIS will perform Subsurface Sounding when the spacecraft is less than 800 km above the Martian surface. In the highly eccentric orbit selected for Mars Express, this corresponds to a period of about 26 minutes, allowing mapping of about 100 degrees of arc on the Martian surface per orbit. Over the nominal mission lifetime, extensive coverage at all latitudes will be possible. To achieve this global coverage MARSIS supports both dayside and nightside operations, although performance is maximised during the night (solar zenith angle above 80 degrees) when the ionosphere plasma frequency drops significantly and the lower frequency bands, which have greater ground penetration capabilities, can be used.
Active Ionospheric Sounding will be carried out during certain orbital passes when the orbiter is less than 1200 km above the surface, in order to gather scientific data on the Martian ionosphere.
Receive only mode will mainly be used to characterise, from an electromagnetic point of view, the environment in which MARSIS is working.
MARSIS will be operated in calibration mode periodically throughout the operational phase of the mission. The purpose of this mode is to acquire a limited amount of data in an unprocessed format. The unprocessed data is used to determine the characteristics of the adaptive matched filter computation that is used by the MARSIS processor to compress the dispersed echo signals from the planet surface and subsurface boundaries.
MARSIS is composed of three subsystems:
The receivers and digital electronics are housed together within the spacecraft. The transmitter electronics is housed in a separate box, also within the spacecraft.
The main transmit and receive antenna is a deployable dipole with two 20 metre elements, arranged so that its peak gain is in the spacecraft nadir direction. The clutter cancellation antenna is a seven metre long deployable monopole, arranged so that its gain null is in the spacecraft nadir direction. The clutter cancellation antenna is equipped with a low-noise preamplifier. Due to severe limitations on the available mass, the antennas are of a novel design, each consisting of a folding composite tube that supports a pair of wires constituting the conductive element of the antenna. The antennas are deployed by pyrotechnic release mechanisms.
The transmitter is connected to the primary antenna through an impedance matching network. The nominal operating frequency of the transmitter in the subsurface sounder modes is 1.3 to 5.5 MHz, with an instantaneous bandwidth of 1 MHz. For ionospheric sounding, the operating frequency varies between 0.1 and 5.4 MHz. The transmitter takes the chirp generated by the receiver/local oscillator electronics and amplifies it, delivering 5 W of RF power to the antenna.
The receiver electronics consists of the chirp generator/local oscillator and a dual channel receiver that down converts the received echoes. Each receiver channel has a selectable bandpass filter, a mixer, an amplifier chain, low-pass filtering and an analogue to digital converter. The output of the analogue to digital converters is passed to the digital electronics for processing prior to being sent to the ground station via the spacecraft's on-board data handling system.
The digital electronics is responsible for: