Abstract 

In this paper recent technical advances for future radar are outlined. Technological challenges such as ultra-broadband radar and multifunctional RF applications along with its operational benefits are discussed. New trends such as the path towards future fully digital frontends as well as distributed aperture radar systems including MIMO and Passive Radar are addressed. In addition the need for efficient and optimized resource management to cope with all the radar sensor flexibilities is addressed as well as advantages in emerging technologies on knowledge based and cognitive radar are pointed out.

1. Introduction

HENSOLDT and its predecessor companies have a long history in radar development of Airborne, Spaceborne, Naval and Ground-based applications and is a frontrunner in Europe for setting technological trends for future radar developments and operations.

The continuously growing complexity of military and commercial scenarios calls for a progressive development of novel radar technologies along with its sophisticated algorithms and mode operations. The variety in mission roles and tasks of future operational scenarios require highest radar sensor performance, multi-mode and multi-band capabilities as well as flexible sensor and waveform control.

Figure 1 shows such a complex scenario where a multitude of different functions are needed to cope with all necessary tasks. These functions include Surveillance in Air-to-Air, Air-to-Ground and Air-toSea in Volume & Surface Search, Tracking, Reconnaissance with SAR Spot/Strip and Maritime ISAR, Electronic Warfare applications (including Electronic Support, Electronic Intelligence and Electronic Attack) as well as real-time Data Links for Network Centric Operations (NCO). All these functions impose enormous requirements on sensor hardware and software, demanding a steady progression and perusing of technological trends.

In the following section several of these technological trends with their operational benefits are discussed.

2. Technical Trends and Achievements

2.1 Broadband Radar and Multifunctional RF-Systems

One of the major trends in radar is the continuous increase of operational radar frequency ranges towards applications for broadband Multifunctional RF-Systems. One of the advantages of broadband and wide frequency range applications in radar is that effective jamming and interference with radar signals becomes more difficult, when the available operating frequency range increases, since jammed frequency bands can easier be avoided and more RF power of the jamming signal is needed to cover the larger bandwidth with equal RF power density, thus making jamming of broadband radar systems significantly more difficult. Furthermore, the increasingly complex operational scenarios demand for more detailed ultra-high resolution (UHR) Synthetic Aperture Radar (SAR) images of fixed targets for classification support in all-weather, day and night applications to be acquired from large stand-off ranges. Such UHR SAR images require very high bandwidths in the range of several GHz, thus supporting the need for broadband Multifunctional RFSystems.

The idea behind Multifunctional RF-Systems (MFRFS) is that common hardware is used to provide not only radar but also Electronic Warfare (EW) functionalities, such as Electronic Support (ES) and Electronic Attack (EA), as well as Communications using Data Link functionalities. These functions operate via shared AESA antennas. which allow to achieve novel and unprecedented operational capabilities and make use of higher RF output power, higher antenna gain and higher sensitivities compared to traditional EW antenna systems for ES and EA in use today. These advantages enable earlier detection and recognition, sooner jamming of opposed targets, higher sensitivities beyond the ordinary ES performance, and therefore allow Low Probability of Intercept (LPI) operations and, in general, higher operational flexibilities thanks to an additional spatial degree of freedom provided by such AESA antennas.¹

However, fully operational MFRFS sensors are not available yet due to a still insufficient bandwidth of AESA antennas based on today’s conventional semiconductor technology. To satisfy the bandwidth needs of an operational EW system, a bandwidth from at least C- to Ku-Band is needed depending on the respective application. Most promising to cope with such broadband requirements in combination with sufficiently high RF power and adequate Power Added Efficiency (PAE) yield is the GaN semiconductor technology.

HENSOLDT has built-up TR-Modules (TRM) in GaN technology, applying broadband Frontend chips together with Core chips developed in BiCMOS SiGe technology. They satisfy the requirements for multi-frequency band applications and high-density integration of complex RF core functionalities with TRM system-on-chip solutions.² In Figure 2 a standard Hensoldt TRM is depicted.

Moreover, an ultra-broadband fully polarimetric GaN AESA Frontend demonstrator has been built-up by HENSOLDT to practically examine Multifunctional RF-Systems requirements in experimental setups with aperture and RF manifolds and operating frequencies covering C- to Ku-Band.³

2.2 Digital Front Ends

A strong trend in radar applications is the progression of signal digitization along with its processing capabilities towards the frontend for both the waveform generation in transmit as well as the echo discretization on receive. Radar antenna frontends are split-up in ever more sub-apertures, leading to an increase of RF-channels to supply the radar processing chain. They enable advanced processing techniques, such as Adaptive Nulling of antenna directivities toward jammers, Space Time Adaptive Processing (STAP) to effectively suppress clutter and interferences, while maintaining target detection sensitivities. Furthermore multiple beams-on-receive and generation of multiple transmit beams can be formed to enable simultaneous mode operations, enhanced target tracking and increased target detection verification performance.

The broadband techniques pointed out above and the early digitization in the signal chain enable higher flexibilities in waveform generation, signal processing as well as in command and control, so that advanced radar sensors can take over a large variety of different multi-functional modes with multi-mission capabilities such as required in the scenario of Figure 1.

Figure 4 shows one of the latest advanced broadband air-to-ground surveillance radar sensors developed by HENSOLDT with a fully TRM-populated Active Electronically Scanned Array (AESA) antenna on a 360 degree gimbal system. It offers both a 360° access to the operation area, independent of the platforms flight direction, and simultaneously an inertia-free beam steering capability in azimuth and elevation direction to provide highest possible flexibility. This sensor already enables processing techniques as antenna diagram Adaptive Nulling for hardening against jamming and interferences and STAP processing for effectively suppressing ground and sea clutter. Moreover thanks to the very high sensor bandwidth, exceptionally high resolution Synthetic Aperture Radar (SAR) images can be computed in nearly real-time. These advanced features are only achievable by broadband multi-channel, high data rate digitization with prompt data pre-processing.⁴

HENSOLDT is deploying these advanced techniques not only for airborne applications, but also in its Naval & Ground radar TRS-4D and TRML-4D product families to enable digital beamforming as well as multiple-beam-on-receive techniques. In Figure 5 HENSOLDT’s TRS-4D Non Rotator (NR) Radar, comprising four AESA antenna faces mounted on the German Frigate F125, is shown with track beam and Multiple-Beam-on-Receive functionality.

These techniques can be regarded as predecessors towards fully digitized RF frontends, where transceiver modules are placed behind each radiation element (RE), digitizing echo signals from each RE and generating waveforms via digital-analog conversion for each RE. Such digital frontends are the enabler for fully software defined radars in the future.⁵

In this regard HENSOLDT developed a single System on Chip (SoC) in SiGe BiCMOS technology (as presented in Figure 6) for the reception and narrow band digitization after I/Qdemodulation of RF signals from each RE of the AESA aperture, covering a frequency range from L- to Ku-Band. To improve RF signal quality digitally- assisted analogue techniques are used to compensate for unwanted artefacts, caused by the on-chip analogue components and linearize the Rx-chain and A/D receive path including I/Q mismatching.⁶ Exemplary results of these compensation techniques with respect to Spurious Free Detection Range (SFDR) are shown on the right in Figure 6.

Such techniques can also be applied for digital pre-distortion on-transmit to maximise the Tx-path SFDR on the one hand and additionally reduce out-of-band emissions on the other.

2.3 Distributed Aperture Radar Systems

Modularity and scalability of modern radar sensors are progressive trends and one of the important factors to design cost effective sensors such as HENSOLDT’s TRS, TRML, Spexer and PrecISR radar families. These trends are even extended beyond single radar operation, through networked operational capabilities and distributed radar aperture systems, in order to increase diversity of detection information, to lower time on targets, to improve target revisit time and to provide better and more complete situational awareness.

Several types of distributed aperture radar systems can be distinguished:

1. Aperture sets angularly arranged such that very large electronic scan angles of more than 90° up to a full scan angle of 360° can be achieved without any mechanically moving parts;
2. Multi-static radar set-ups located far apart from one another to transmit RF signals and receive target echoes from different aspect angles. Also Passive (Coherent Location) Radar belongs to this type of radar set-ups;
3. Multiple-In, Multiple-Out (MIMO) distributed aperture radar set-ups;
4. Combinations of types listed at items 1 to 3.

The first type of distributed aperture radar is applied on HENSOLDT’s TRS-4D NR radar version, which makes use of a four-face electronic scanning antenna aperture set, where the antenna faces are mechanically aligned to one another in a rectangular manner.

2.3.1 Passive Coherent Location Radar Systems

For the second type of distributed aperture radar systems Network Capable Operations (NCO) can effectively be used in both active and passive surveillance systems. It is the driving principle of passive coherent locating radars, which is still an emerging radar technology. They utilize public broadcast stations as target illuminator in FM, DAB and DVBT (so called “transmitters-ofopportunity”) and are inherently very difficult to detect and thus to jam, thanks to their inherent passive nature. HENSOLDT has been working on such systems since more than a decade and has an advanced mobile passive radar demonstrator system in its portfolio as shown in Figure 7.

Usually numerous radio and TV broadcast stations are available to build-up multi-static transmitterreceiver set-ups with one or more passive receiver stations. The upper left of Figure 7 shows the transmitter-receiver bi-static range equation with its target bearing in azimuth and elevation. Such a single transmitter-receiver is ambiguous. To remove such target ambiguities, more than one ellipsoid is needed to retrieve the three-dimensional target geo-location as shown in Figure 7.

There are several advantages using Passive Coherent Location Radar (PCL) detection techniques. Apart from the advantage of being difficult to detect with no emissions, the multiple illuminatorsreceiver set-up and the low broadcasting frequencies in FM, DAB and DVBT, improve detection performance for low flying targets. By networked operation of multiple passive radar receivers a further enhanced detection performance can be achieved through higher transmitter diversity and angle-of-incidence diversity.

In Figure 8 on the right a typical multiple transmitter-receiver PCL Radar set-up with eight FM transmitter stations (marked in red) and one receiver located near the village of Schwaighofen in the southern part of Germany is presented. On the left, a typical detection scene of tracked air targets is shown, as provided by HENSOLDT’s mobile Passive Radar demonstrator system.

HENSOLDT’s Multiband (FM/DAB/DVB-T) mobile multi-illuminator system provides 360° coverage of targets in 3-D through transmitter diversity for optimum detection and tracking performance.

Targets from FM, DAB and DVB-T broadcasting can be merged in a multi-hypothesis tracker system with real-time processing and targets display.⁷

PCL radars are highly cost-effective compared to active radar systems with roughly the same performance, thanks to the saving on expensive transmitters, which make out a large part of the costs, especially in active AESA radars. Operational strengths are amongst others the relatively easy relocation of individual sensors and their ability to combine and fuse dislocated multiple passive radar sensors for improved geographical performance. Promising areas of application are wide-area air surveillance, ATC for regional airports as well as complementary ATC for major airports (gapfilling) as well as local surveillance tasks & critical infrastructure protection.

2.3.2 Multiple-In, Multiple-Out Radar Systems

As the third type of distributed aperture radar systems there is an emerging trend of MIMO antenna aperture set-ups, using orthogonal waveforms on usually very sparsely populated antenna arrays. This technique may be a cost-efficient approach for short and medium range applications, where high angular resolutions are required. HENSOLDT has developed a MIMO radar demonstrator as shown in Figure 9 to investigate the performance of such systems. On the right the built-up of a virtual MIMO array is depicted from the physical arrangement of Tx and Rx radiating elements making use of orthogonal waveforms.⁸

There are several advantages but also restrictions in using MIMO radar for compact multifunction sensors. Apart from the better angular resolution compared to conventionally built radar of equivalent size, the MIMO approach has the general advantage that an instantaneous Field of View (FoV) can be covered without the need of scanning, disregarding beam focusing in Tx and making use of frequency diverse orthogonal waveforms, so that orthogonal echoes of respective targets can be processed in parallel. On the other hand disregarding beam focusing leads to higher background clutter levels, so that applications are rather constrained to noise limited scenarios of short and medium ranges.⁸

2.4 Cognitive Approaches

Early signal digitization enables already today’s radars to implement knowledge based processing schemes and self-learning approaches of the environment and thus to improve detection probabilities and reduce false alarms. HENSOLDT is making use of a priori information in its various radar systems, such as ground topology, vegetation, buildings, coastlines, roads, railways, wind mills etc. as well as self-learning maps such as ground und Doppler clutter maps in order to reduce false alarm rates and improve detection probability performance. The use of such a priori and self-learning information is a steadily growing trend towards cognitive radar functionalities.

2.5 Multifunctional Resource and Sensor Management

The variety of new possibilities provided by the future radar trends needs to be optimized by a flexible and modern radar resource management which assigns resources by quality needs of certain radar tasks rather than by fixed rules. One of the latest highly advanced real-time capable Quality of Service (QoS) based resource management schemes which also allows finding globally optimized solutions (under certain conditions) is the Quality of Service Resource Allocation Method called QRAM.⁹

The QRAM method allows the optimization of resources allocation such as time, RF power, frequency bandwidth, computational power, antenna sub-apertures, RF-channels etc. on given radar tasks, according to certain quality criteria which may even depend in the actual mission situation.

Hence with an optimized allocation of frequency and bandwidth as well as partitioning of time slots among the different radar tasks, faster reaction times and shorter blind times of the sensor can be achieved. Based on the achieved higher availability through optimized inter-coordinated radar functions, the overall radar system performance will be enhanced significantly.

3. Conclusions

The trends in next generation radars have been outlined, from making use of ultra-broadband multifunctional RF-system capabilities, digital frontend AESA antennas with distributed aperture systems in a multiple radar architectures, connectivity via network capable operations up to orthogonal waveforms to parallelize processing in combination with the progression in real-time computing, allows for highly advanced and performant radar applications in the future. They may only be limited by the price for flexibility and processing power, customers are willing to spent.

As presented in this paper, HENSOLDT is at the forefront of developing these new technologies and is committed in participating to further shape the future on these novel radar technologies and trends.

Acknowledgement

Many results shown in this paper were only possible thanks to funding by the German Bundeswehr Technical Center for Information Technology and Electronics (WTD81) and the German Federal Ministry of Defence (BAAINBw). This financial support is gratefully acknowledged.

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