Figure 1. The functional units of Ginger.
Résumé
Ginger (pour "Guidance and INto the Ground Exploration Radar") est un nouveau type d'instrument radar à émission continue destiné à de petits véhicules, comme les engins d'exploration de la Lune ou de Mars. Il peut mesurer la vitesse du véhicule et détecter les obstacles. Il peut aussi déterminer si le sol est trop meuble ou s'il est praticable, et déterminer l'épaisseur et les propriétés électromagnétiques des couches sous-jacentes. Un prototype de cet instrument a été construit et testé tant au laboratoire que sur le terrain.
Contractors:
Raumfahrt Systemtechnik AG
(CH) EUROSPACE Techn.
Entwicklungen GmbH (D)
Funding:
Basic Technology Research Programme
Small unmanned vehicles - planetary rovers - will be essential elements in scientific missions to explore the Moon or Mars. In addition to carrying scientific instruments for in situ investigations, a rover will need quite complex sensors to control its own movements over largely unknown and possibly dangerous terrain.
Ginger (Guidance and INto the Ground Exploration Radar) has been developed for planetary rovers. This instrument can provide several different sensing functions with essentially the same hardware. It provides the sensing needed for safe piloting (or guidance) of the rover itself, and can also measure the geophysical properties of the planetary soil to a depth of several meters.
Ginger has four distinct operating modes:
The first three modes are important for piloting the rover, while the fourth yields useful scientific data about the planetary sub-surface (e.g. underground ice on the Moon). Detection of soil softness is an original feature and the combination of all four functions in one instrument is also unique.
Ginger is a continuous-wave (CW) dual-frequency microwave radar operating in the 1 GHz and 60 GHz bands. Ginger comprises several functional blocks (Figure 1). The first is a frequency synthesiser (FS), which generates the required modulation spectrum as well as the in-phase and quadrature reference spectra for the detection process, and which provides the local carrier signal needed to down-convert the intermediate frequency (IF) to baseband. The microwave unit (MU) up-converts the modulation spectrum to 60 GHz and down-converts the received echo signals to IF. An electronics unit (EU) transmits the 1 GHz signals and receives and down converts the echoes to baseband and detects them. It also digitises both echo signals. Finally a control computer (CC) manages the operation of the instrument and handles all digital data.
Figure 1. The functional units of Ginger.
The forward and lateral velocities of the mounting platform are measured by detecting the Doppler shifts in the frequencies of the echo signals transmitted and received by the MU, and the forward and sidewards velocities are extracted by the CC using Fourier transform methods. Both transmit and receive channels use antennas whose beamwidths are narrow in the horizontal plane and wide in the vertical plane. The 60 GHz antenna is small and is capable of measuring a velocity of 1 m/s to an accuracy of 1%, updated at 1 s intervals.
Processing the return radar echoes also allows obstacles in front of the rover to be detected, and since well-defined obstacles give sharp response peaks, digital post-processing is not required. The vertical spatial resolution is determined by the antenna beam width and is typically 0.2 m at a distance of 2 metres for a 5-degree fan beam. Horizontal spatial resolution is enhanced by Doppler or phase processing and using the 60 GHz radar, a horizontal resolution of 0.2 m at 2 m distance can be achieved with the rover moving at 0.5 m/s. The antennas are mounted on the front corners of the rover, with the right-hand antenna covering the left half of the swath and vice versa. This yields a forward-looking cone-of-view which can detect obstacles within an angle 70 degrees wide.
Both the 60 GHz and 1 GHz channels are used in this mode, with a frequency line pair for modulation in both channels. One transmit antenna is used at each frequency, covering the area of concern in front of the rover with approximately equal footprints. By comparing the phases of the 60 GHz and 1 GHz echoes and noting radar reflectivity at these frequencies, the softness and depth of the soil is measured to determine if it will support the weight of the vehicle. This is a very new technique which still must be optimised and calibrated empirically by field tests. Look-up tables can be generated for different soil types, allowing dangerous and benign soils to be distinguished.
In this mode, the vehicle is stationary or moves at less than 0.1 m/s and the instrument works as a CW ground penetrating radar. A signal made up of 32 equally-spaced discrete frequencies in the band 660 to 970 MHz is used to measure unambiguous depth to a maximum of approximately 8 m (depending on the dielectric constants of the soil layers)
with a resolution of better than 0.25 m in a typical lunar or Martian soil. Two antennas are used. One illuminates the ground, while the other receives the echoes from the soil. The basic technique is to transmit the discrete frequencies sequentially and compute the reflected power versus the penetration depth, using the inverse Fourier transformation.
A laboratory prototype of Ginger has been built to demonstrate its feasibility and to calibrate its operation in a realistic environment. To reduce costs, the 60 GHz frequency intended for the planetary instrument was changed to 10 GHz. The equipment is consequently much larger than the expected flight system, and appropriate extrapolation and scaling has to be applied to the measurement results. The prototype measures approx. 50 x 30 x 30 cm and weighs close to 20 kg. This can be drastically reduced by going to 60 GHz and space quality components.
Figure 2 shows the prototype mounted on a remote-controlled toy car which was used as a first testbed. The 1 GHz Yagi antennas and the slotted- waveguide microwave antennas are clearly visible.
Figure 2. The Ginger instrument and its developers during system testing.
Both indoors and outdoors tests have been performed. Speed measurements and obstacle detection have given good results over a variety of reflecting surfaces. Soil property measurements were performed on well-defined sand and gravel profiles at the Bergakademie in Freiberg (D), and compared well with those taken by a commercial high-resolution ground penetration radar. When looking through dry sand, Ginger could detect bedrock 2.5 m down, a sand density transition at a depth of 1 m, and buried obstacles at depths of 0.5 m. Tests of surface softness determination are continuing.
A prototype of Ginger has given very encouraging results, which suggest the suitability of this device piloting a planetary rover and for subsurface geophysical characterisation of martian or lunar terrains. A science team is evaluating the results of the first field tests. Further tests in the ESTEC Lunar Utilisation Testbed and in the GEROMS rover test site at Toulouse (F) are planned to further refine the measurement processes and to obtain the precise specifications for the development of a flight unit. Ginger also appears suitable for terrestrial applications, such as detecting underground water, mines, avalanche victims, or antarctic meteorites.
Preparing for the Future Vol. 7 No. 2