Tobias Rommel

Digital Beam-Forming reconfigurable Radar System demonstrator

Synthetic Aperture Radar (SAR) based on the Digital Beam-Forming (DBF) concept belongs to the family of Multi-Modal Radar Systems (MMRS) offering higher operational flexibility and improved performance compared to conventional radar systems using analog beam steering. DBF SAR, in particular, overcomes the fundamental limitation of classical SAR systems and can deliver high resolution and simultaneously wide swath images. The purpose of this paper is to present a laboratory prototype of such a MMRS architecture - a reconfigurable radar demonstrator based on the DBF architecture. A hardware configuration of the radar as well as advanced concepts to be experimentally verified using it are considered and discussed.

CEBRAS: Cross elevation beam range ambiguity suppression for high-resolution wide-swath and MIMO-SAR imaging

In this paper we propose a new hybrid technique to suppress range ambiguities in spaceborne SAR systems with multiple elevation beams. First, conventional scan-on-receive (SCORE) is performed in real-time onboard the satellite by employing a set of dispersive beams that maximize the collected signal energy for each transmitted pulse. The range ambiguities are then removed in a second step by a joint processing of the signals collected by the multiple elevation beams. The suggested two-stage approach has the advantage that a more robust range ambiguity suppression, which may involve advanced nulling techniques to account for local topography as well as satellite attitude and instrument phase errors, can be performed on ground without tremendously increasing the onboard processing demands or the data downlink volume.

On the Pulse Extension Loss in Digital Beamforming SAR

One of the benefits of synthetic aperture radar (SAR) systems utilizing digital beamforming is the ability to increase the receive power. The relevant SAR technique is known as SCan-On-REceive (SCORE), which steers the receive antenna beam such that it follows the radar pulse echo traversing the ground. This allows the use of a narrow receive beam in elevation, and therefore, the height of the receive antenna can be increased, resulting in a higher gain, which explains the higher receive power. Although advantageous, this technique has some pitfalls, which impose an upper limit on the antenna size and constrain the selection of SAR operation parameters. These limitations (which are often neglected in the system conception) are caused by the pulse extent on the ground and the way it is modulated by the receive antenna pattern. This letter addresses and quantifies the effects caused by the transmit pulse length (here denoted as pulse extension loss) through a rigorous analysis, with the purpose of introducing an important SAR performance figure. Closed expressions are derived for the simplified case of a uniform linear antenna array.

Radar 2020: The future of radar systems

The first radar has been patented 110 years ago. Meanwhile the applications became numerous and the system concepts have been adopted to the available technologies. Typical applications are speed control, air traffic control, synthetic aperture radar, airborne and spaceborne missions, military applications and remote sensing. Research for medical radar applications is well progressing for breast cancer detection and tumor localization. Automobile radar for save and autonomous driving are meanwhile produced in millions per year. In the next years the state-of-the-art radar system concepts will experience almost a revolution. Despite the significant advancements, the radar system technology did not develop like communications or other technologies during the last 20 years. Some of these new technologies will within a few years penetrate radar and revolutionize radar system concepts. This will then allow for new radar features and radar signal processing approaches.

Low-Cost X/Ku/Ka-Band Dual-Polarized Array With Shared Aperture

This paper presents a novel tri-band (X/Ku/Ka-band) planar antenna array with dual polarizations and shared aperture. Compared with traditional dual-polarized arrays, the proposed array has advantages of low cost, low profile, and high integration. Three types of antennas resonating at different frequencies, including the perforated patch, stacked patch, and slim crosspatch, are innovatively interleaved in the same aperture. The crosspatch fed by proximity coupling is presented as Ku-band element for its advantages of compact size, high isolation, and pure polarization. The techniques such as series feed and reverse feed are utilized to implement the six feed networks in a compact size with reduced cross polarizations. Measured results agree well with the simulations, showing three operation bands at X-, Ku-, and Ka-bands with the bandwidths of 3.6%, 6.7%, and 5.3%, respectively. The antenna also exhibits excellent radiation performance with the cross-polarization discrimination over 25 dB at the three bands. To the best of the author’s knowledge, this is the first shared-aperture X/Ku/Ka-band dual-polarized antenna array reported, which is useful for potential synthetic aperture radar applications.

Multiple-input multiple-output circular SAR

Interest on circular and multicircular SAR acquisitions has been increased in the past few years due to the inherent potentials they provide, e.g., full 3-D reconstruction and subwavelength resolution. However, one main limitation is the size of the spotlighted region, which is basically defined by the range ambiguities, the pulse repetition frequency (PRF), and the minimum half-power beamwidth (HPBW) in the range and azimuth directions. This paper introduces multiple-input multiple-output (MIMO) techniques in order to tackle this limitation, thereby being able to extend the applications of CSAR to much wider areas. The basic principle of the proposed solution is based on three methods, namely scan-on-receive (SCORE), multi-channel reconstruction in azimuth (MCRA) and the use of orthogonal waveforms for quad-polarized systems. The impact and dependency of the PRF, HPBW and the range ambiguities is also investigated.

An orthogonal waveform for fully polarimetric MIMO-SAR

Synthetic aperture radar (SAR) with multiple transmit and receive channels (MIMO-SAR) has a higher flexibility and an improved efficiency compared to a conventional SAR system with a single channel. The multiple receive channels can be used, among other things, to increase the swath width at constant azimuth resolution or to suppress spatial interferences. However, multiple transmit channels, which transmit simultaneously in the same frequency band provide currently a challenge. Therefore, in this paper a modified chirp waveform is introduced which extends in combination with digital beam-forming (DBF) on receive the orthogonality condition to another degree of freedom, thus allowing in theory perfect orthogonality. Furthermore, the hardware design of the MIMO-Radar Demonstrator, a multichannel measurement system for radar and SAR applications is depicted. The shown measurement result demonstrates the possibility of obtaining all four parameters of the scattering matrix at the same time by transmitting orthogonal waveforms in different polarizations.

Development of a MIMO Radar System demonstrator - Calibration and demonstration of first results

Multi-Modal Radar Systems (MMRS) using Digital Beam-Forming (DBF) concept offer higher operational flexibility and improved performance compared to the conventional radar systems using analog beam steering. In particular, Synthetic Aperture Radar (SAR) based on DBF concept overcomes the fundamental limitation of classical SAR systems delivering high resolution and simultaneously wide swath images. This paper presents a laboratory prototype of the next generation MMRS system - a reconfigurable Multiple Input Multiple Output (MIMO) radar demonstrator based on the DBF architecture. The hardware configuration of the demonstrator is described in detail, and the system calibration and signal processing procedures are considered. The first measurement results confirming its functional capabilities are presented and discussed.

Low-profile aperture-shared X/Ka-band dual-polarized antenna for DBF-SAR applications

In this paper, a novel low profile aperture-shared X/Ka-band dual-polarized antenna array is proposed for spaceborne digital beamforming (DBF) synthetic aperture radar (SAR) applications. To begin with, the DBF concept and its operation mechanism for SAR system are discussed. In this work, every 2 × 2 Ka-band elements are combined as a subarray and connected to a channel while each one X-band element is connected to a channel. Then, a novel slot-antenna based on substrate-integrated-cavity (SIC) is proposed. The resonant frequency can be controlled by adjusting the length of the U-shaped slot. To share a same aperture, the X-band antenna is interlaced with the Ka-band subarray on the same plane. To verify the concept, an antenna operating at 9.6/35.75 GHz is designed, prototyped and tested. The results demonstrate good performance in terms of impedance matching, channel isolation, and cross polarization level. Such highly integrated low profile antenna can significantly reduce the potential cost of the DBF-SAR system.

A 60-Channels ADC board for space borne DBF-SAR applications

A 60-Channels ADC (Analog to Digital Converter) board for space borne Digital Beam Forming (DBF) Synthetic Aperture Radar (SAR) applications is described. The purpose of the board is to digitize analog signals detected by a dual band SAR receiving array operating at X and Ka band. It contains 48 high speed ADCs, which sample synchronously the incoming data of the antenna front end at Ka band. Other 12 ADCs are used to sample the coming data from the X band antennas. The board is composed by an analog section, a digital section and a clock distribution network used to synchronize the ADCs. Output digital signals from the board are routed to digital boards were are processed in the Digital Beamforming Network (DBFN).