Very Wideband Radar Imaging

with the Airborne SAR Sensor PAMIR

Andreas R. Brenner, Joachim H. G. Ender

Department Electronics - Research Institute for High Frequency Physics and Radar Techniques

FGAN Forschungsgesellschaft für Angewandte Naturwissenschaften e.V.

53343 Wachtberg-Werthhoven, Germany

brenner@fgan.de, ender@fgan.de

Abstract—At FGAN a new very wideband radar is under

development. The system will serve as an experimental airborne

platform operating in the X-band and is called PAMIR (Phased

Array Multifunctional Imaging Radar). In this paper high

resolution SAR and ISAR images acquired within the first flight

campaign are presented. A spatial resolution in the sub-

decimetre regime could be achieved.

Keywords—synthetic aperture radar, high resolution radar

imaging, sub-decimetre resolution, SAR, ISAR

I. INTRODUCTION

Air- and spaceborne imaging radar systems in forthcoming remote sensing and surveillance tasks have to meet increasingly severe demands. The next generation SAR systems will comprise high resolution and long range imaging capabilities, highly sensitive ground moving target indication and a multitude of sophisticated operational modes. At FGAN, where the SAR sensor AER-II has been successfully operated for many years [1], a new experimental X-band radar was designed. The sensor is called PAMIR (Phased Array Multifunctional Imaging Radar). It is envisaged to demonstrate SAR imaging at very high resolution and for long range (1 dm @ 30 km; 1 ft @ 100 km). The fine resolution will also be achieved with ISAR imaging of ground moving objects. Furthermore, the number of receive channels will allow GMTI via space-time adaptive processing, and single-pass interferometric SAR with very high 3D resolution.

II. BRIEF DESCRIPTION OF THE SYSTEM DESIGN

The design of the PAMIR system aims at maximum flexibility and modularity. Some key elements to achieve this are an electronically steerable phased array antenna consisting of 16 reconfigurable and autonomously operating subarrays with a total of 256 T/R modules, and a broadband beamforming network based on switchable true time delays. The management of the total bandwidth of 1.8 GHz is realised, e.g., by means of the synthetic bandwidth technique [2], using an arbitrary waveform generator and five separate local oscillators which can be switched in various manners to the waveform generation and signal acquisition units.

The system further offers five parallel receiving channels, a sophisticated timing unit, an integrated INS/DGPS motion

sensor, a high-speed data recorder, various on-board calibration possibilities, and a flexible object-oriented steering software. The most important design parameters of PAMIR are listed in Table 1. A more detailed description of the system design is given in [3].

III. SAR AND ISAR IMAGING RESULTS

In August 2002 a first flight campaign was conducted. In a preliminary extension stage the sensor was operated with one receive channel and a mechanically steerable antenna array (120 W peak power) and field-tested in various modes like stripmap-, sliding-mode- and spotlight-SAR as well as ISAR.

The evaluation of the corresponding images proves the sensor's capabilities with respect to spatial resolution, range, and image dynamics. E.g., for near range acquisition (up to 10 km), a spatial resolution in the sub-decimetre regime could be achieved. To this end a motion compensation based on autofocus techniques was applied. The image generation itself was performed by a time domain based SAR processor [4].

In comparison with SAR imaging in the metre resolution regime now a wealth of details arise. E.g., a tree turns from a small spot into an extended object with identifiable substructures, and in urban areas building facades and vehicles can clearly be recognised: Fig. 4 displays a large-scale high-resolution SAR image of Berlin, in Fig. 1, a cutout is shown in comparison to the optical counterpart, and in Fig. 2, vehicles waiting at a traffic light can be recognised.

TABLE I. BASIC SYSTEM PARAMETERS OF PAMIR (FINAL STAGE OF

EXTENSION)

Carrier

Center frequency

Bandwidth Resolution

Range

Channels Main antenna

Transmit power

Azimuth scan Polarisation

Basic operational modes

Transall C-160

9.45 GHz

1800 MHz 0.1 m x 0.1 m

100 km

5 parallel rec. channels Active phased array, 256 T/R modules

1280 W peak

+- 45 deg VV

Squinted stripmap SAR

Spotlight and sliding mode SAR Scan MTI (STAP)

ISAR

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Figure 1. High resolution SAR image of an urban area and its optical counterpart

PAMIR also provides an ISAR mode for imaging moving vehicles. The objects to be imaged are illuminated with a high PRF and alternating centre frequencies. Fig. 3 shows two stages of the signal processing: In a sequence of roughly motion compensated Range-Doppler images the desired target can be selected by the operator (see small rectangle) and is tracked in the Range-Doppler plane. After some processing steps making use of the track information and including autofocus procedures the final image is shown (top right).

The ISAR image formation with a system like PAMIR has some additional aspects: Along-track multi-phase centres will enable the rejection of clutter returns by STAP filters; the accurate angular estimation of ground moving targets allows to establish tracks with high accuracy.

ACKNOWLEDGMENT

The authors thank all colleagues who have contributed to the realisation of the PAMIR system. Concerning the flight campaign, we also appreciate the support and the assistance of WTD 61, Manching. This work is funded by the German Federal Ministry of Defense (BMVg) and the Federal Office of Defense Technology and Procurement (BWB).

REFERENCES

[1] J. H. G. Ender, P. Berens, A. R. Brenner, L. Rößing and U. Skupin, "Multi channel SAR/MTI system development at FGAN: From AER to PAMIR," Proceedings of IGARSS'02, Canada, pp. 1697-1701, 2002.

[2] P. Berens, "SAR with Ultra-High Range Resolution Using Synthetic Bandwidth," Proceedings of IGARSS'99, Germany, pp. 1752-54, 1999.

[3] J. H. G. Ender and A. R. Brenner, "PAMIR - a wideband phased array SAR/MTI system," IEE Proceedings - Radar, Sonar, Navigation, in press.

[4] A. R. Brenner and J. H. G. Ender, "First experimental results achieved with the new very wideband SAR system PAMIR," Proceedings of EUSAR2002, Germany, pp. 81-86, 2002.

Figure 3. ISAR image formation of moving vehicles

Figure 2. High resolution SAR image of vehicles waiting at a traffic light

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Figure 4. High resolution wide area SAR image acquired with PAMIR in 2002. The image shows the central part of Berlin

with the Reichstag building in the lower left corner, the River Spree on the left, a part of the park Tiergarten at the bottom and the

Brandenburger Tor and the boulevard Unter den Linden in the middle of the scene (slant range 6500 m, scene dimension 500 x 600 m2).

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