A Closer Look at MARSIS, the Radar that Discovered Lakes on MarsSeptember 30, 2020 by Jake Hertz
This week, the European Space Agency discovered underground lakes of liquid water on Mars. What circuit-level hardware made this discovery possible?
Just this week, scientists at the ESA announced the discovery of several ponds of liquid water buried underneath ice on Mars. The discovery was made possible thanks to a special instrument called the Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS).
Scientists believe that ancient Mars had free-flowing water on its surface. (Each number represents years in billions.) Image used courtesy of Science ABC
In this article, we’ll explore how MARSIS operates on a larger system and sub-system level.
MARSIS: the Tool that Found Water on Mars
MARSIS is a low frequency, pulse-limited radar sounder and altimeter with ground penetration capabilities.
Working principle of MARSIS. Image used courtesy of the Max Planck Institute for Solar System Research
The tool works by first transmitting a frequency modulated (FM) signal using a downward-facing dipole antenna. The system contains a secondary clutter-cancellation monopole antenna, also oriented along the downward axis. The primary dipole antenna then works with the secondary antenna to receive the return signal off the nadir surface.
The received signals from both antennas are then down-converted to range offset video signals before being passed to an ADC. The following data is modified by an on-board processor, at which point it is passed to the spacecraft and transmitted to earth.
A Deeper Look at MARSIS' Hardware
From a hardware perspective, MARSIS consists of three main subsystems: the antennas, the RF system (TX and RX), and the digital electronics.
The previously-discussed primary antenna is a dipole with two 20-meter elements, oriented such that its peak gain is in the spacecraft's downward direction. The secondary antenna is a seven-meter monopole, arranged so that its gain null is in the spacecraft's downward direction. The secondary antenna is also equipped with a low-noise pre-amplifier.
MARSIS’ transmitter is impedance matched to the antennas and operates at nominal frequencies of 0.1 MHz to 5.5 MHz with an instantaneous bandwidth of 1 MHz, depending on the mode of operation.
The TX takes the signal generated by the receiver/local oscillator and amplifies it to deliver 5 W of RF power to the antenna.
A conceptual block diagram of the MARSIS system. Image from R. Oroesi et al.
The receiver consists of a dual-channel receiver to perform downconversion of the received signals. Each receiver channel has a selectable bandpass filter, a mixer, an amplifier chain, low-pass filtering, and an ADC. ADC output is fed to the digital electronics for processing prior to being sent to the transmission back to earth.
Finally, the digital electronic subsystem includes a signal generator, timing and control unit, and processing unit. This final subsystem's responsibility is to:
- Synthesize the transmit chirp and local oscillator signals
- Control the transmitter and receivers
- Process the digital data from the receivers
- Receive and execute commands from the spacecraft
- Transmit formatted data
Different Modes of Operation
MARSIS has four working modes: subsurface sounding, active ionospheric sounding, receive only, and calibration.
Subsurface sounding mode occurs when MARSIS detects that it is less than 800 km above the Martian surface. Given MARSIS’ orbit, it has a period of about 26 minutes in subsurface sounding mode, allowing mapping of about 100 degrees of arc per orbit on the Martian surface.
Rendering of MARSIS. Image used courtesy of the Smithsonian
Active ionospheric sounding is activated only during certain orbital passes in order to gather scientific data on the Martian ionosphere.
Receive-only mode is used to obtain EM data about the operating environment of MARSIS.,
And finally, calibration mode occurs periodically throughout the mission. This mode serves to acquire data in an unprocessed format. According to the ESA, this mode “allows scientists 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.”
Sophisticated Tech Leads to Groundbreaking Discoveries
The recent discovery of more water on Mars is a significant stride in space exploration and was only possible thanks to the work of dedicated engineers who helped develop MARSIS. This instrument is a great example of how scientific exploration can drive EE innovation, requiring a complex electrical system to automatically and reliably gather, transmit, and receive data.