ULTRA LIGHT-WEIGHT ANTENNA SYSTEM FOR FULL POLARIMETRIC GPR APPLICATIONS

Dirk Plettemeier1, Stefan Balling1, Wolf-Stefan Benedix1, Valerie Ciarletti2,Svein-Erik Hamran3, Charlotte Corbel2, Stefan Linke4

Email: dirk.plettemeier@tu-dresden.de1Technische Universität Dresden, Germany

2Centre d'Etudes des Environnements Terrestre et Planétaires, Vélizy, France 3Forsvaret forskningsinstitutt, Kjeller, Norway

4INVENT GmbH, Braunschweig, Germany

Abstract: The motivation to develop an ultra light-weight antenna system was driven by a space borne radar ap-plication. The Experiment “Water Ice and Subsurface Deposit Observations on Mars” (WISDOM) is a Ground Penetrating Radar (GPR) selected to be part of the Pasteur payload on board the rover of the ExoMars mission. Among the Pasteur Panoramic Instruments on the ExoMars rover, only WISDOM can provide a view of the subsurface structure. WISDOM is the first GPR on a planetary rover. It has been designed to character-ize the shallow subsurface structure of Mars. WISDOM will for the first time give access to the geological struc-ture, electromagnetic nature, and, possibly, hydrological state of the shallow subsurface by retrieving the layering and properties of the buried reflectors. It will address important scientific questions regarding the planet’s present state and past evolution. The measured data will also be used to determine the most promising locations to obtain underground samples with the drilling system mounted on board the rover. The instrument’s objective is to get high-resolution measurements down to 2 m depth in the Martian crust. The radar is a gated step frequency system covering a frequency range from 500 MHz to 3 GHz. The radar is fully polarimetric and makes use of an ultra wideband antenna system based on Vivaldi antenna elements. The paper describes an-tenna requirements to fulfil for this very specific GPR application and it gives an overview about the light-weight design and its realization. Simulated and meas-ured antenna performance is compared in this paper. Test measurements were performed in permafrost regions on earth.

Index terms: antenna, ground penetrating radar (GPR), Vivaldi tapered slot antenna (TSA), fully polarized, ExoMars, WISDOM

I. INTRODUCTION

The WISDOM instrument is one of the instru-ments that have been selected to be part of the Pasteur payload of the ESA ExoMars mission. The main sci-entific objectives for the Pasteur payload on board the rover are to search for traces of past and present life on Mars and to characterise the shallow subsurface. To do this, the rover is equipped with a drill that can obtain samples of the subsurface down to a depth of approximately 2 m. The exploration of the subsurface of the planet is essential, since the chances that life has survived on Mars increase with increasing burial

depth. The Pasteur payload consists of three sets of instruments: the Panoramic Instruments (a wide angle camera, an infrared spectrometer and the radar WISDOM) that will perform large-scale scientific investigations at the rover location; the contact in-struments, mounted on the Rover robotic arm, that will be used for cm-scale investigations of outcrops, rocks, soils, and the Analytical Laboratory Instru-ments that will analyse samples obtained by the sub-surface drill.

WISDOM is the only instrument able to obtain in-formation about the subsurface before drilling. Its objective is the exploration of the first ~ 3 meters of the soil with a very high range resolution in accor-dance with the objectives and expected capabilities of the drill exploration. It will allow the characterisation of the subsurface environment through the detection of electromagnetic permittivity contrasts as a function of depth along the rover path.

WISDOM will provide help for the identification of sedimentary layers where it is most likely that or-ganic molecules may be well preserved and shall thus support the search for subsurface signs of past life at the rover sites. In addition, the information collected with WISDOM nearby the observed outcrops will be used to drill at locations where the first bedrock layer is within the drill reach. In combination with the drilling and the results obtained from the analytical instruments, WISDOM will give for the first time access to the geological structure, electromagnetic nature, and, possibly, of hydrological state of the shallow subsurface by retrieving and mapping the layering and properties of the buried reflectors. It will address scientific questions regarding the planet pre-sent state and past evolution, such as the depth of the desiccation layer, the presence of water ice close to the surface, and the depth and distribution of ice bodies.

The first section of this paper points out the main requirements for the design of the antenna systems mainly driven by the scientific objectives of WISDOM. The second section is dedicated to the dif-ferent design steps in the realization of the antenna prototype, with particular emphasis on the description of the antenna performance needed to meet the scien-tific objectives of WISDOM. The third section ad-dresses interferences, for instance caused by radiation

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Fig 2 – left: Ultra light-weight Vivaldi structure made of metallised Kapton (total weight: < 15 grams).

right: First realization of a cross polarized Vivaldi structure.

Fig. 1 – Bow tie antennas and current density distributions left: Four bow tie antennas in a full polarimetric

arrangement, e.g. mountable on a dielectric substrate below the base plate of the rover.

right: Bow tie antennas below the solar panels of the rover.

coupling effects with the rover structure and describes the current status of the accommodation study. Results of the antenna performance achieved by simulations and measurements will be shown in sec-tion 4. The last section shows the realization of the antenna structure in light-weight design and points out the design steps that have to be improved to achieve the final space qualified design for the flight models.

II. ANTENNA SYSTEM REQUIREMENTS

The main design requirements of the WISDOM antenna system are driven by the required scientific return of the experiment and by the specific GPR con-figuration and accommodation on the Mars Rover. The resolution of a few centimetres and a penetration depth of more than two meters require a frequency range of 500 MHz to 3 GHz. To be able to study de-polarization effects, a full polarimetric antenna system is required. The full polarimetric two-channel GPR system needs an antenna design that takes into ac-count two perpendicular linearly polarized transmit-ting antennas and two co- and cross-polar oriented antennas for reception. Usually, GPR antennas are placed on ground or accommodated in a close-by ground configuration with respect to wavelength. Considering the requirements for the design of the ExoMars rover, the GPR antenna system has to have a ground clearance of about 30 cm, which is equal to three wavelengths for the highest operating frequency. Considering that the GPR antennas on the rover are not able to use the advantages of a close-by ground arrangement and that, due to mass, volume and planetary protection requirements, the application of absorbing material and reflectors are not appropriate, the antenna pattern of each single antenna element should be directed towards ground. The radiation pattern should be wide in the rover path direction, so that a visibility of point reflectors for long distance is possible. The across path pattern should be narrow. These and other constraints like EMC requirements, as well as pattern deformation due to radiation cou-pling effects with the rover structure, led to an an-tenna design that is based on Vivaldi structures for each single element. To realize the full polarimetric antenna system, two perpendicularly oriented Vivaldi elements are combined in each of the two dual polar-ized antennas. The antennas will be covered by thin dielectric foil to protect the sensitive parts from Mar-tian dust particles. The overall size of the dual polar-ized transmitting and receiving antenna will be less than 20 cm x 20 cm x 20 cm. The available mass for the whole antenna system is less than 400 grams.

The main design requirements can be summarized as follows:

Bandwidth: 500 MHz – 3 GHz Polarization (TX/RX): linear, xx xy yx yy Matching (S11): � -10dB Radiation pattern: homogenous main lobe Beam width: > 20°, H-plane > E-plane Gain function: smooth, increasing with

frequencyAntenna cross talk: < -15 dB (for co- and

cross-pol.)Pointing error: < 5° (for all frequencies) Radiation coupling (rover): as low as possible Mass budget per antenna: 200g Envelope volume per antenna: < 20 cm x 20 cm x 20 cm

III. STAGES OF DEVELOPMENT

Computer simulations were performed for many different kinds of wide-band antennas mounted on different locations of a rover structure that was not clearly defined at the beginning. The bow tie arrangements shown in Fig. 1 illustrate just one other antenna type we investigated for this specific applica-tion.

The antenna arrangements in Fig. 1 show charac-teristic problems for this specific application. From the current distribution illustrated in Fig. 1 it can be seen that many types of antennas cause strong radia-

tion coupling effects within the antenna system itself and with the surrounding rover structure. This leads to strong frequency dependent interferences in return loss, causes strong cross talk and yields significant antenna pattern deformations.

Taking into account the WISDOM design re-quirements, the simulation based analysis showed clearly the strong advantages of Vivaldi structures. Fig. 2 shows the first steps in realising the ultra light-weight design using Vivaldi antenna elements.

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Fig. 3 – Cross-polarized Vivaldi horn in rigid design. Antenna height: 28 cm, size of the radiating aperture: 20 x 20 cm².

left: Inner structure of one dual polarized WISDOM antenna without cover.

right: Exterior view of one of the dual polarized WISDOM antenna.

Fig. 4 – Alternative WISDOM antenna design taking into account a dual-fed structure.

Antenna height: 16 cm, size of the radiating aperture: 20 x 20 cm²

left: Inner structure of one dual polarized WISDOM antenna without cover.

right: Exterior view of one of the dual polarized WISDOM antenna.

Fig. 5 – Prototype of the WISDOM antenna in dual-fed design. (Dimensions see figure 4) left: Micro strip feeding of the tapered slotline structure.right: Prototype on the near field scanner in the anechoic chamber.

Beside rigid structures, foldable designs were studied. The most important stages in development and optimization are listed below.

A. Cross-polarized Vivaldi Horn, Rigid Design The first design that was based on Vivaldi

elements is shown in Fig. 3. The arrangement consists of two Vivaldi structures oriented perpendicular to each other. The perpendicular structures are crossed

along the slot lines. The antennas are covered by thin dielectric foil to protect the sensitive parts from Mar-tian dust particles. The design shown in Fig. 3 is easy to realize in terms of feeding. Both slot lines can be fed by coaxial lines. The mechanical design in terms of stability and thermal requirements are more chal-lenging, due to the small slot line structure. Several laboratory models were built and tested. The electro-magnetic antenna parameters met all requirements. B. Cross-polarized Vivaldi Horn in Rigid Dual-Fed

Design Extensive miniaturization studies based on a de-

tailed electromagnetic field analysis led to the dual-fed design. This design consists of an in-phase fed dual slot line structure for each polarization (see Fig. 4). The signal has to be divided and combined by a broadband 3 dB power divider. Using this structure, a shortening of the antenna height by roughly factor two is possible. Due to the insertion loss of coupler and cable, the dynamic of the GPR system will be reduced by about 2-3 dB. However, a more homoge-

neous current distribution on the radiating aperture leads to a slight increase in directivity.

The inner structure is mounted on a flange on top of the slot line structure (coloured light blue in Fig. 4), which is made of carbon reinforced plastic. The power dividers are placed inside the flange structure. Alternatively, they can be placed outside the flange, depending on the final design of the flange and rover structure, respectively. Feeding can be realized by thin coaxial cables or micro strip lines, as both solu-tions are working equally well. C. First Prototype in Dual-Fed Design

After successful computer simulations of the an-tenna system, a first prototype of the dual-fed design was build. Fig. 5 (left) shows the feeding of the slot line realized in micro strip technology. In this proto-type the power dividers are located below the radiat-ing elements.

The prototype was equipped with external power dividers, realized in ferrite transformer technology. Measurements in the lab have shown that this proto-type reaches the electrical performance of the required final design and validates the results of the computer simulations. These antennas were successfully used for several field tests in permafrost regions. D. Light-Weight Design

To prove the feasibility of the concept, two fully functional Antennas were built (see Fig. 6). These antennas consist of all essential components, which differ slightly regarding the used materials and dimensions according to the final design (see Fig. 22). Main design driving requirements for this light-weight sandwich design were:

1) First global mechanical resonance frequency: >120 Hz

2) Mass for one Antenna: <200 g

To fulfill these requirements the antennas have to be made of lightweight, stiff materials, i.e. fiber rein-forced components. Different mechanical designs were studied. The final design can be characterized by:

� Antenna cross made from glass fiber rein-forced epoxy with copper layers

� Antenna cross additionally reinforced with honeycomb sandwich to raise stiffness

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1

45

3

2

Fig. 6 – WISDOM antenna prototype in the light-weight design. (dimensiona see figure 4)

left: 1: Sandwitch base plate with holes for screws and cables2: Antenna structure reinforced by sandwich plate 3: Kapton casing 4: Reinforcement brackets 5: Sandwich reinforcing element

right: FE model of the WISDOM light weight design.

Fig. 7 – Accommodation of the full polarimetric WISDOM antenna system in the rear of the ExoMars rover. The colors represent the field distribution on ground and the surface current density on the rover structure at 500 MHz.

Antenna height: 16 cm (vertical dimension), size of the radiating aperture: 20 cm x 20 cm (horizontal

plane).

Fig. 8 – 3D radiation pattern and E- (red, dashed), H-plane (green, circles) cuts at 500 MHz

� Sandwich ground plate with holes for cables, screws and venting

� Reinforcement brackets for structural bonding of all components

� Lightweight casing made of Kapton

The development of the light-weight design is based on material qualification results and on the FEM structure analysis. Results of the FEM analysis show the first global mechanical resonance frequency at 159.18 Hz and a calculated mass of 190 grams.

IV.ACCOMODATION STUDY

As a result of accommodation studies performed by the rover designers, the WISDOM antenna system will probably be placed in the rear of the rover. Fig. 7

shows the latest result of the accommodation studies. To have overlapping footprints on the surface and

in the subsurface, the antennas for transmission and reception are next to each other in 1 cm distance. The antenna elements inside the antenna cover are oriented 45 degrees to the rover track, so that the sur-face projected polarization and the shape of the pat-tern are 45 degrees turned related to the rover track.

This minimizes the overall coupling with the rover structure, and also helps to distinguish between echoes arriving from left and right side of the track. A tilt of some degrees backward shall help to distinguish between positive and negative slopes of reflecting subsurface structures.

V. SIMULATIONS AND MEASUREMENTS

First design and optimization steps of all the dif-ferent versions of the WISDOM antenna system are based on computer simulations. Numerical methods like Finite Elements Method (FEM) and Method of Moments (MoM) were applied. Validations of the simulation results were performed by antenna meas-urements in the anechoic chamber, in free space and in field test in Mars analog environment.

3D radiation patterns and corresponding E-plane and H-plane cuts are shown in Fig. 8 to 11 for the characteristic frequencies of 500 MHz, 1 GHz, 2 GHz and 3 GHz. It can be seen that the shape of main lobe is smooth, that the beam width in the E-plane is smaller than the beam width in the H-plane and that there are no side lobes in the field of view (FOV) �30° off-nadir. E-plane and H-plane are oriented 45 degrees out of the direction of movement of the rover, which allows us to distinguish between left and right side echoes along the rover track. The forward to backward ratio is around 10 dB for nearly the whole frequency range. The requirements for the antenna radiation pattern could be achieved.

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Fig. 10 – 3D radiation pattern and E- (red, dashed), H-plane (green, circles) cuts at 2 GHz

Fig. 11 – 3D radiation pattern and E- (red, dashed), H-plane (green, circles) cuts at 3 GHz

Fig. 12 – Simulated return loss (on all 4 feeding points without power divider)

Fig. 13 – Realized gain versus frequency for both polarizations without insertion loss of the power divider.

Fig. 9 – 3D radiation pattern and E- (red, dashed), H-plane (green, circles) cuts at 1 GHz

The simulated return loss and antenna gain func-tion versus frequency are shown in Fig. 12. The return loss is less than -10 dB over the whole frequency range. The gain functions are smooth and mainly in-

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Fig. 16 – Measured return loss of the current version of the WISDOM antenna system (insertion loss of the power divider included).

Fig. 17 – Realized gain versus frequency of the current version of the WISDOM antenna system (insertion loss of the power divider included).

Fig. 14 – Measurements of antenna cross talk without and with relevant parts of the rover structure

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3.0 GHz 2.0 GHz

Fig. 15 – 3D far field Radiation pattern (lower hemisphere towards ground) at 0.5 GHz, 1 GHz, 2 GHz and 3 GHz, measured on a spherical near field scanner in the anechoic chamber.

creasing with frequency. The realized gain curves in Fig. 13 do not take into account the insertion loss of the power diver. The insertion loss of the power divider is less than 2 dB at 3 GHz. Summarizing the simulation results of return loss, realized gain and pattern, it can be concluded that all requirements regarding the electromagnetic antenna performance could be achieved with a very compact antenna structure that was designed for the very specific GPR application on a Mars rover.

Measurements of the WISDOM antenna system were performed on a spherical near field scanner in the anechoic chamber. Fig. 14 shows the antenna arrangement mounted on a simplified rover mock-up.

A projection of the results of the 3D antenna pat-tern measurements are illustrated in Fig. 15 for the hemisphere directed towards the target. The shape of the antenna pattern and the frequency behaviour is consistent with the simulation results shown in Fig. 8 to 11. The E-plane is smaller than the H-plane over the whole frequency range and no side lobes occur

within a FOV � 30° off-nadir. The results of the measured return loss S11 and absolute gain versus frequency are shown in Fig. 16 and 17, respectively. A comparison with the simulated results in Fig. 12 and 13 show a strong coincidence between sim-ulations and measurements, especially if the insertion loss of the power divider, which was not included in the simulations, is taken into account.

The measured maximum gain function shows an increase versus frequency up to about 2.3 GHz. For higher frequencies first side lobes at off nadir angles larger than 60 degrees are reducing the maximum gain. The increase in maximum gain works against the frequency dependence of free space and scattering

losses and partially compensates the gain functions of the amplifiers. S11 is below -10 dB over the whole frequency range for both polarizations, the gain func-tions are smooth and mainly increasing with frequency.

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Fig. 19 – Measurements of antenna cross talk with relevant parts of the rover structure

Fig. 22 – WISDOM antenna (current, improved light-weight design) on the shaker in plane (IP)

WISDOM Pre-PDR, CETP Vélizy, 6-8 october 2008 18

WISDOM

Radar Set Up

Antennas

Radar Electronics

Sampling Wheel

PC-Box

Fig. 21 – Prototype of the WISDOM antenna system together with a GPR mounted on a sled and pulled by a snow scooter on field tests in permafrost regions; Antenna system and GPR

Fig. 20 – Prototype of the WISDOM antenna system and GPR mounted on a light-weight trolley.

Typical results of cross talk measurements between the perpendicular polarized elements of the transmitting and the receiving antenna are illustrated in Fig. 18 and 19. The antenna cross talk was meas-ured for the antenna system itself and additionally together with relevant parts of the rover structure.

A simplified metallic rover mock-up was built to determine the radiation coupling effects with the rover structure. Even if the rover structure causes a slight increase in cross talk, we can summarize that between all ports a cross talk less than -25 dB can be achieved for nearly the whole WISDOM related frequency range (500 MHz – 3 GHz).

Fig. 20 and 21 show the GPR antenna system during field tests in permafrost regions (e.g. on Svalbard). The antenna system together with a GPR mock-up was mounted on a trolley and sled respec-tively. Several GPR profiles were taken in an area in front of two glaciers. The glacier ice was covered with fluvial sediments with a thickness of up to 2 meters. The whole GPR system was successfully tested in a nearly Mars analogue environment and all system requirements could be achieved.

VI.VIBRATION AND THERMAL VACUUM TESTS

In addition to the electromagnetic antenna per-formance tests, mechanical vibration tests and thermal vacuum tests have been performed on the WISDOM antenna system.

Fig. 22 shows a WISDOM antenna mounted on the shaker. Vibration tests (sinus, random, shock re-sponse) have been successfully performed in all 3 axes. Tables 1 and 2 show an example of the load values for sinus and random vibration tests, respec-tively. In Fig. 23 the set-up used for thermal vacuum tests is shown. The antenna was tested in a tempera-ture range between -125°C and +125°C in ambient vacuum (about 1 mbar during all test cycles).

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Fig. 23 – Vacuum cylinder, antenna fixation and thermal chamber

Table 1 – Mechanical loads for vibration tests (IP: in plane; OOP: out of plane)

f [Hz] a [g] IP

a [g] OOP

5 1 1

20 44 74

100 44 74

Table 2 – Mechanical loads for random vibration tests (IP: in plane; OOP: out of plane)

F [Hz] Level [g2/Hz]

IP

Level [g2/Hz] OOP

20 + 6dB + 6 dB

100 0,41 0,96

400 0,41 0,96

2000 - 6dB -6 dB

To monitor the antenna performance under de-fined conditions, a specific vacuum cylinder was built (see Fig. 23). The antenna is mounted with the an-tenna flange on the closure head of the vacuum cylin-der. The antenna is equipped with two temperature sensors, one close to the radiating aperture and the other one close to the flange, near the power divider. Antenna performance, temperatures and pressure are recorded during the test cycles.

VII. CONCLUSIONS

A very compact (envelope < 20 cm x 20 cm x 18 cm), ultra light (< 200g) and powerful space qualified broad band antenna system that provides full po-larimetric performance has been developed and suc-cessfully tested for the shallow subsurface sounding radar WISDOM on board the ExoMars rover. This antenna system is suitable for a wide variety of broad-

band radar applications, especially if in GPR applica-tions a certain ground clearance is needed.

ACKNOWLEDGEMENTS

The authors would like to thank the “Institut für Leichtbau und Kunststofftechnik” at TU Dresden for its support.

This paper presents results of research partly funded by DLR.

References [1] J. Vago, “ExoMars Science Management Plan”, EXM-

MS-PL-ESA-00002, 16th May 2006. [2] Hamran, S-E, T. Berger, L. Hanssen, M. J. Øyan, V.

Ciarletti, C. Corbel and D. Plettemeier, 2007, ”A prototype for the WISDOM GPR on the ExoMars mission” IWAGPR 2007, Naples, Italy.

[3] S.-E. Hamran, V. Ciarletti. C. Corbel D. Plettemeier, M. J. Øyan, T. Berger, L. Hanssen, 2007, ”The WISDOM GPR on the ExoMars Mission”, European Mars Science and Exploration Conference: Mars Express & ExoMars, 2007, ESTEC, Noordwijk, The Netherlands.

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