Concept
WISDOM enables the characterization of Martian soil using a ground-penetrating radar concept. Therefore, electromagnetic waves radiated by the WISDOM antenna penetrate a few metres into the soil and consequently, reflections occur at different obstacles in the soil. The reflected signal can be analysed using the WISDOM electronic unit. A resolution of a few centimetres can be achieved using the WISDOM instrument.
Taking into account the data achieved by the WISDOM instrument, eligible drilling sites are defined and samples will be taken using the drill on board the rover. Moreover, the samples together with the WISDOM data will help answer the question whether there is life on Mars or whether life is possible. Therefore, amongst others the search for organic molecules is of special interest.
All in all, WISDOM helps answer scientific questions regarding the geological environment at the landing site of the rover, and whether there is or ever was water or ice on Mars and its influence on the current landscape.
Basic setup
The WISDOM instrument consists of two subsystems: WISDOM Electronic Unit (WEU) and WISDOM antenna (WAA). WEU provides the frontend for the ground penetrating radar, whereas WAA transmits and receives the signal generated by WEU. Both units connected via four coaxial cables that transmitt the RF signals.
How it works
First, the WEU generates a sinusoidal signal with the frequency f1 (e.g. 500 MHz). This signal is amplified and during the time t (e.g. 1 ms) it is conducted to one single blade of the WAA. From there the signal propagates as electromagnetic wave mainly towards the surface of the soil. At this interface between air and soil a portion of the wave is reflected backwards to the receiving blade of the WAA and received by the WEU. But another part of the wave propagates further into the soil. Whenever the wave encounters an interface of two different materials then again a part of the wave is scattered back to the WAA but with some time delay. At the receiver all the reflected waves superimpose and one single value is stored which represents the strength of the received signal multiplied by the cosine of the phase. After time \Delta t, the WEU generates the next frequency f2. And everything repeats until the last frequency fn (e.g. 3 GHz), which is usually the 1001st step. Then one shot is finished and there are 1001 values, that represent the “echo” from the subsurface in the frequency domain. These values can be processed further. The main part of that processing is done with the so called Fourier Transform that converts all values from the frequency domain into values of the time domain, which can be interpreted more intuitively than those values of the frequency domain. The result is called A-scan. After one shot the rover drives a small distance ahead (e.g. 10 cm) and another shot is done. Combining several A-scans together gives a B-scan, which is the representation of all “echoes” over a certain distance.
WISDOM Antenna Assembly (WAA)
The WAA is made of two antennas (cross structure): one transmitting antenna (Tx) and one receiving antenna (Rx) arranged together to one baseplate. Each cross again is made of two Vivaldi 2-element arrays to enable the transmission and the reception of two orthogonal polarizations. A glas-fibre reinforced plastic (GFRP) is used as the carrier for the few tens of micro meter thick metallic elements. To obtain a light-weight framework that survives the strong vibrations and shocks during the launch to and landing on Mars, the GFRP is arranged as a sandwich: Two thin sheets bound a honeycomb structure. That reduces the weight of the WAA but ensures the stiffness of the antenna at the same time without decreasing the antenna performance.
The WAA is positioned at rear of the rover in order to keep the influence of the rover low. WAA and WEU are connected via four coaxial cables, two for Tx antenna and two for Rx.
One major figure-of-merit regarding the antenna design is its weight and the WAA is optimized to achieve a weight of less than 540 g (inclusive screws and washers).
Another important aspect of the antenna design is to ensure its reliability. To ensure the antenna survives the rocket start and the harsh environment of Mars, several tests such as thermal-vacuum tests as well as shaking tests have been performed. Of course the functionality was tested as well during several field tests.
WISDOM Electronic Unit (WEU)
The WEU is integrated within the rover. The overall size of the housed system is 145 mm x 163 mm x 55 mm. Three subsystems build this unit: a DC/DC module, a Digital Processing Unit (DPU) module, and the RF (radio frequency) module generating the signals to be radiated by the WAA.
The RF module generates and receives RF signals in the frequency range between 500 MHz and 3 GHz. In order to evaluate these signals and to achieve the required data, the following components are included: amplifier chains, polarization selection modules, mixers as well as an analogue-to-digital converter. Moreover, time gating is implemented in order to reduce the influence of unwanted reflections on the received signal.
For the DPU, a stable clock is of importance. Moreover, the overall system is to be controlled by using an FPGA with the integrated microcontroller 8051. The control system fulfils the following tasks: radar control, electronic data processing, CAN bus communication, memory control and synchronization.
The DC/DC module converts the supply voltage provided by the rover to the necessary voltage levels of the WEU.
Regarding the application, the weight of the overall WEU is of importance and meets the requirements with a weight of less than 820 g excluding the coaxial cables. Moreover, a fully operational system with the RF-path switched on consumes only a power of 12.5 W.
