Ultra-High Resolution and Long Range X-Band Airborne SAR System

Rémi Baqué, Olivier Ruault du Plessis, Nicolas Castet, Patrick Fromage, Joseph Martinot-Lagarde, Jean-François Nouvel, Hélène Oriot, Hubert Cantalloube, Martine Chanteclerc, Philippe Martineau

Electromagnetism and Radar Department - ONERA FRANCE Remi.Baque@onera.fr

Abstract — Since a few years ONERA has been investing in high performance radar demonstrators. It has been contracted by the French MoD to develop a X-Band high peak power and large bandwidth sensor for airborne experiments dedicated to UHR (Ultra High Resolution) imagery. This paper will describe the RAMSES-NG X-UHR sensor design and results with airborne High Resolution Imagery.

Keywords — SAR, Airborne Imagery, Ultra High Resolution.

I. INTRODUCTION

With the objective of updating research sensors for defense remote sensing applications, the DGA (French MoD) contracted ONERA to develop a new Ultra High Resolution and Long Rang X-Band SAR/GMTI Sensor. This development is a part of the DGA RAMSES-NG project.

RAMSES-NG sensors are tested onboard the SETHI platform [1]. SETHI is a research platform based on a Dassault Aviation Falcon 20 used as an airborne remote sensing laboratory dedicated to civilian applications (as Forest measurements and vegetation characterization, chemical products detection and maritime survey) but also to defense applications (as high resolution for target imagery, moving target detection and change detection). It combines two pods under wings which are able to carry heavy and cumbersome payloads of different kinds ranging from VHF-UHF to X band and/or optical sensors with a wide range of acquisition geometries. The pod-based concept allows the easy integration and testing of new systems under the single certification of the pods by authorities (Fig. 1).

Fig. 1. SETHI (top left) with view of radar antennas inside the Pod (top right) and cabin layout (bottom)

II. SENSOR GLOBAL SPECIFICATIONS

The main objectives of the RAMSES-NG new X-Band sensor are to investigate Ultra High Resolution and very long range geometry applications.

Existing ONERA X-Band sensor has a 1.5 GHz bandwidth capacity used for 10 cm resolution imagery in a full polarimetric mode. This new sensor overcomes this limit to be able to study less than 5 cm resolution imagery. With this objective the bandwidth is specified to 4 GHz.

ONERA SAR sensors (VHF/UHF-, L- and X-Band) are used with typical 10 km range geometry. To be able to study waveform and geometry adapted to stand-off configuration, the new X-Band sensor range specification is more than 50 km.

III. DIGITAL CORE

In 2017 ONERA invested in a new digital core based on National Instrument PXI-Express chassis (Fig. 2).

Fig. 2. SETHI PXI-Express digital core

This 19 inches chassis supports: • 2 Vectorial Signal Transmitters, able to generate

any arbitrary waveform at 2.5 GS/s each with frequency range 9 kHz to 6 GHz, 1 GHz of instantaneous bandwidth and a 16 bits dynamic. For UHR applications the transmitted code is a chirp, generated with 1.5 GHz centre frequency and 720 MHz bandwidth. The up-conversion and frequency diversity necessary to obtain 4 GHz bandwidth is detailed in Microwave Design chapter.

• 4 receive channels with 1 GS/s I/Q sampling frequency each and 12 bits of dynamic.

• 1 high data rate controller (PCIe x8) that centralises sampled data and fed an external 19 inches recorder at 3.4 GB/s. The 2U 19 inches record system supports 16 SSD disks 512 GB each, for a 7 TB total capacity.

• 2 optic fiber 10 Gb/s Ethernet controllers that send sampled data to an onboard SAR processor.

• 1 GPS receiver used for raw data time stamping with a very high precision (1 µs). This function is very important for data post-processing and aircraft motion compensation, more specifically with ultra-high resolution applications.

• 1 reference clock based on OCXO oscillator used as reference for all sensor oscillators (sampling frequency, local oscillators …).

• 1 digital I/O generation board that drives all microwave components (switches, triggers…)

IV. MICROWAVE DESIGN

The microwave part is composed by four elements (Fig. 3): • A 5U 19 inches rack in cabin (Fig. 4) for transmit

signal up-conversion, received signals down-conversion, filtering and pre-amplification,

• a 2U 19 inches rack in cabin (Fig. 5) for local oscillators generation, filtering and selection,

• a 8 kW peak, 400 W mean, 4 GHz bandwidth TWT (Traveling-Wave Tube) (Fig. 6) localized in Pod under the wing close to the transmit antenna,

• a front-end receive box with LNA (Low Noise Amplifiers) and internal calibration chain (Fig 7), localized in Pod under the wing close to the receive antenna.

Fig. 3. X-Band UHR Microwave Synoptic

Fig. 4. X-Band UHR in cabin Tx/Rx rack

Fig. 5. X-Band UHR in cabin LO rack

Fig. 6. X-Band UHR TWT

Fig. 7. X-Band UHR in-Pod receive box

V. ANTENNAS

The system is equipped with two parabolic antennas, the first for transmission, which transmits a peak power of 8 kW, thus ensuring long range applications, second is dedicated to reception. These antennas meet the integration constraints imposed by the SETHI pod (Fig. 8)

Fig. 8. Receive (left) and transmit (right) parabolic antennas

Beamwidth has been specified to 5° on azimuth axis and 7° on elevation axis offering more than 50 km range objective.

Narrow beamwidth, and so small on-ground beam pattern, can be an issue when used for on-board applications. Indeed real aircraft trajectories can be disrupted and not straight due to atmospheric effects. In order to ensure beam illumination on

clearly identified targets on the ground, these antennas have been integrated on 2-axes motorized turrets (elevation and azimuth).

The study of turrets performances in term of angular speed and acceleration was conducted using the statistical behaviour of SETHI platform during acquisition tracks (Fig. 9).

Fig. 9. Exemple of cumulative histogrammes of roll angular speed (left) and

acceleration (right)

Analyses of a large amount of data from different campaigns, flight and geometries (strip map, circular SAR …) and future mode estimations concluded to angular speed specifications for the motorized antennas: 45°/s on azimuth axis and 7°/s on elevation.

Precision specification for antenna pointing over the two axes is 10% of 3dB-beamwidth : 0.5°.

Fig. 10. 2-axes motorized Receive antenna

After a large number of laboratory and flight tests, this dynamic 2-axes motorization is now able to provide Strip Map and Spot Light configurations with or without real-time platform motion compensation and also cooperative ground target tracking.

Fig. 11 presents circular real flight trajectory with representation of the targeted point during the entire acquisition circular track with (right) and without (left) motion compensation by motorized antennas.

Fig. 11. Targeted point with (right) and without (left) real circular trajectory

Fig.12 shows a specific use of these capabilities with a “double spotlight” during same acquisition. We can see on the produced SAR image the two areas acquired in Spot Light mode for high resolution imagery and between each, a transition area on ground scanned by the antennas during their movement from back squint (first zone) to front squint (second zone) producing an unintentional SAR image.

Fig. 12. Double Spot Light imagery (up) and ground truth optical image

(down)

Last example of real time motorized antenna is presented Fig. 13. During this flight we received from the boat target its AIS information (Automatic Identification System transmitted by VHF radio) and extracted from this signal the boat position, heading and speed. These target trajectory data are then processed in real time in order to track the target with the motorized antennas and also to provide to pilots a predicted aircraft trajectory in a circular configuration taking in account the movement of the target during acquisition. In this example (Fig.13) circular imagery of the boat with a straight line movement is obtained with an aircraft “solenoid” trajectory.

Fig. 13. Real time onboard visualisation of boat trajectory (yelow), aircraft

trajectory (red) and theoretical trajectory corridor for moving boat circular imagery (blue)

Last issue for antenna development concerns the peak power transmission. The high frequency and high power signal is transmitted from the TWT to the antenna via waveguides. As TWT is fixed, and transmit antenna is motorized, the waveguide path is provided by means of rotary joints: one for elevation axis and one for azimuth one (Fig. 14).

Fig. 14. Elevation and azimuth waveguide rotary joints

The long range and/or wide swath geometry is reached with high altitude flight tracks. SETHI is certified up to 30,000 feet. One issue with this altitude is pressure and air moisture control to avoid any electric arc. Pressure and air moisture are controlled inside the waveguide by using a radome placed on the antenna waveguide source, and with a dry air pressurizer for all the waveguide chain (Fig. 15).

Fig. 15. Waveguide chain pressurizer system

VI. POD IMPLEMENTATION

All the presented radar elements have to be implemented on-board SETHI. Digital core, LO and Tx/Rx racks are inside the cabin whereas Receive and Transmit antennas, TWT and receiving microwave box are inside the pod under the wing. Fig. 16 presents the in pod implementation of these elements which corresponds to 110 kg of payload and 1500 VA electric consumption.

Fig. 16. In-Pod implementation

The nose of the pod is equipped with an air inlet which ensures the cooling of the TWT with aircraft motion speed (Fig. 17).

Fig. 17. SETHI Pods with air inlet for RAMSES-NG X-UHR TWT cooling

VII. AIRBORNE EXPERIMENT AND RESULTS

RAMSES-NG X-UHR long range sensor has been tested, validated and used during many flight test campaigns. Fig. 18 presents example of geometry used for acquisition tracks with 30° depression angle (60° incidence angle). Other geometries have been used during these flights, for example 45° incidence angle.

Fig. 18. X-UHR 60° incidence angle geometry

Projected swath is around 1900 m (antenna pattern limitation) and acquisition tracks are 10 km long. Results presented here below are full SAR image zooms expected for the first long range SAR image (Fig. 19) with a 18 km swath at 60 km range [2].

Fig. 19. Long range (60 km) wide swath (18 km) 20 cm resolution X Band

imagery (zoom on 2 tanks and agricultural farm compared to ground truth optical image)

Fig. 20 is a zoom on UHR imagery (7.5 cm range and 10 cm cross-range resolution) of electric pylons, a water tower and train rails after autofocus.

Fig. 20. Electric pylons and water tower SAR imagery at 45° incidence angle

(up) and ground truth optical image (down)

The entire lines are visible between the two poles due to large integration angle (9°). Semi-circular responses are echoes from top and foot of the cylindrical water tower whose radar shadow can be clearly seen.

Interest of resolution is demonstrated Fig. 21 with an urban area imagery. Zoom over one house roof is presented at different resolutions.

Rrange = 15 cm Rcross = 20 cm

Rrange = 15 cm Rcross = 10 cm

Rrange = 7.5 cm Rcross = 10 cm

Fig. 21. SAR imagery of an urban area with zoom on house roof for differents range (Rrange) and cross-range (Rcross) resolutions

With full resolution imagery we can clearly distinguish the shape of the tiles and count 56 rows of it.

The following and last examples show circular SAR

benefit (combined with high resolution) for target recognition. On Fig. 22 we can see all the target (Falcon 20) details as engines, main landing gear, nose radar, fuselage and wing details, etc. On Fig. 23 we can clearly see the man shape holding a mass, and all hand tools on ground. An other zoom shows a chair and a radio on ground, unfocused lines are a walking man signature.

These images are color compositions of 36 SAR images acquired in circular configuration at 9 km range. Each color corresponds to an azimuth angle [3].

Range and cross-range resolutions are respectively 7.5 cm and 10 cm on Fig. 23 (Fig. 22 resolutions are confidential).

Rrange = 7.5 cm Rcross = 10 cm

Fig. 22. Circular SAR imagery of a Falcon 20 and ground truth optical images

Fig. 23. Ultra High Resolution circular imagery (< 10 cm) with zooms on

Human, object and tool detection

ACKNOWLEDGMENT

Authors wish to acknowledge DGA (French MoD) for its financial support to RAMSES NG X-UHR sensor development and airborne campaigns.

REFERENCES [1] Ruault du Plessis, Dreuillet, "The ONERA Multi-Frequency SAR

Imaging Systems" Proc. Radar 2013 International Conference on Radar, Adelaide Australia (Septembre 2013)

[2] Cantalloube, "Very High Resolution Airborne Synthetic Aperture Radar Imaging at Long Range" Proc. 14th International Radar Symposium (IRS), Dresden Germany (June 2013). Book Series: International Radar Symposium Proceedings (Pages: 224-229), Published: 2013.

[3] Oriot et al., “RAMSES-NG X-Band UHR capability” Proc. 12th European Conference on Synthetic Aperture Radar, Aachen Germany (June 2018)