Synthetic Impulse and Aperture Radar (SIAR)

A Novel Multi-Frequency MIMO Radar

Baixiao Chen, Jianqi Wu

• Publisher: Wiley
• Year: 2014
• ISBN:
  9781118609590
• Edition: 1
• Topic: Naturwissenschaften
• Subtopic: Wellen & Wellenmechanik

Analyzes and discusses the operating principle, signal processing method, and experimental results of this advanced radar technology

This book systematically discusses the operating principle, signal processing method, target measurement technology, and experimental results of a new kind of radar called synthetic impulse and aperture radar (SIAR). The purpose is to help readers acquire an insight into the concept and principle of the SIAR, to know its operation mode, signal processing method, the difference between the traditional radar and itself, the designing ideals, and the developing method.

It includes 10 chapters. Chapter 1 gives an introduction to the basic principle of SIAR and its characteristic of four antis. Chapter 2 introduces the operating principles and system constitution of SIAR. Chapter 3 presents the main waveforms and the corresponding signal processing methods. Chapter 4 is about the long-time integration technique. Chapter 5 shows the high-accuracy measurement and tracking of 4D parameters of target in SIAR. The range-angle coupling and decoupling are introduced in Chapter 6, where a criteria for transmit frequency optimization of array elements is studied to overcome the coupling among range, azimuth and elevation. In Chapter 7, detection and tracking of targets in strong interference background is investigated. Chapter 8 analyzes quantitatively the influence of array error on the tracking accuracy of SIAR. Expansion of impulse and aperture synthesis to HF band and microwave band are introduced respectively in Chapter 9 and Chapter 10. The operating principle of the novel bi-static surface wave radar system, as well as the experimental system and the experimental results are included in Chapter 9.

Written by a highly experienced author with extensive knowledge of SIAR (Chen), the book can be used as a reference for engineering technical personnel and scientific research personnel working in the research of SIAR, MIMO radar, digital radar or other new type of radar. It can also be a reference for teachers and students in universities who engage in related professional work.

• Details the operating principle, signal processing method, target measurement technology, and experimental results of synthetic impulse and aperture radar (SIAR)
• Expands the technique of impulse and aperture synthesisfrom the VHF band to the HF band and the microwave band
• Written by a leading author with many years' research and practical experience in sparse array SIAR, a typical MIMO radar

Engineers, researchers and postgraduates working in radar engineering will find this an invaluable resource.

Chapter 1

Introduction

1.1 Development of Modern Radar

With the development of microelectronics, very large scale integrated converters (VLSICs), new materials, and advanced productive technologies, modern radar techniques have progressed dramatically. Major development trends in the modern radar are given as follows:

1. Digitization. Digitization of the modern radar is not only represented by significantly improved speed of radar signal processing as a result of the rapidly developed VLSICs but also radio frequency (RF) digitization. Indeed, phase shifters in conventional phased array radars are now gradually being replaced by direct digital synthesizers (DDSs). Due to the rapid development of DDSs, they can directly generate RF excitation signals and send them to the power amplifier in a radar transmitter. That is to say, RF excitation signals in different initial phases directly generated by DDSs in various channels are sent respectively to each transmitter to amplify their power before being sent to each antenna. After being filtered and amplified in digital receivers, RF signals are sampled directly by high-speed analog-to-digital (A/D) converters. The digital receivers can obtain digital baseband signals via digital quadrature conversion without using mixers.

2. Integration. To work effectively in modern warfare, various radar techniques and tools, such as pulse compression, adaptive frequency agility, coherent integration, constant false-alarm-rate (CFAR) circuit, low-probability intercept (LPI), polarimetric information processing, spread spectrum, ultra-low sidelobe antenna, multiple transmitting waveform design, digital beamforming (DBF) or adaptive digital beamforming (ADBF), and sidelobe cancellation (SLC), should be integrated in the modern radar system.

3. Multifunction. With the rapid improvement of radar techniques, the radar system is required to have good detecting, tracking, and identifying capabilities for various targets. In addition, it should have the capability of guiding and targeting for the weapon system. Furthermore, high survivability should be offered in a complex electromagnetic environment.

With the development of stealth techniques, anti-radiation missiles (ARMs), electronic countermeasures (ECMs), and low-altitude penetration [1–6], new challenges and higher demands are expected. As traditional radars are incapable of dealing with these challenges, new countermeasures must be adopted. In order to deal with these “Four Threats,” modern radar is required to employ a series of advanced techniques, such as pulse compression, SLC, and coherent integration. Since stealth aircraft has been successfully applied in recent local wars, anti-stealth techniques have become a “have-to-solve” issue.

Current stealth technologies are mainly focused on structure stealth design, impedance loading, absorbing material coatings, and absorbing penetrating materials to reduce radar cross-section (RCS). These techniques are widely acknowledged as useful measures for centimeter wave radars. However, it has little impact for electromagnetic waves of longer wavelengths (such as meter waves). Since the RCS of a target is related to the radar wavelength with the form RCS = nλ [7], where n depends on the geometrical shape of the target and has a value between 0 and 2, and λ is the wavelength. At the international conference on radar systems in 1985, Moraitis analyzed the influence of radar frequency on the detection of stealth targets. The results show that the RCS of stealth aircraft is higher at the metric band than at the S-band by 15–30 dB. Meanwhile, the impedance loading cannot be carried out since the metric band is the resonance region of the airframe. Absorbing material coatings are influenced by frequency characteristics. Currently, the effective frequency is between 1 and 20 GHz, with the coating thickness lying between 1/10 and 1/4 wavelength. For the metric wave, it is impossible for the coating thickness to be up to an order of 10 cm. Therefore, the absorbing material coating is not a threat to the metric radar. The absorbing penetrating materials also cannot be applied effectively in the metric wave due to the frequency characteristics of the materials. Thus, the metric wave radar has a good capability in detecting stealth targets.

However, traditional metric radars have difficulties in meeting the requirements of modern warfare due to their wide beams, poor positioning accuracy, and especially their inability to track and guide multiple targets. In recent years, radar researchers are trying to improve the performance of resolution, low-altitude detection, anti-jamming, multiple target detection with the metric radars. However, these efforts are only improvements to the traditional radar system, so it is difficult to meet their desired purposes. Only the Synthetic Impulse and Aperture Radar (SIAR, “RIAS” in French) invented by ONERA in the late 1970s was an entirely new four-dimensional (range, azimuth, velocity, and elevation) multifunction (surveillance and tracking) radar system [8–15] To overcome the inherent weakness of low angular resolution, the large sparse array is employed in this metric wave radar. Due to its new transmitting signal system and advanced signal processing techniques, the isotropic illumination for the whole space can be performed with a large antenna array, which has strong directivity. Aperture synthesis and impulse synthesis for signals with large time widths are performed at the same time, so an LPI can be realized.

1.2 Basic Features of SIAR

The basic concepts of SIAR can be summarized as follows:

1. By encoding the signals of each omnidirectional radiation element, the isotropic radiation of the entire space is ensured with a large antenna array with strong directivity; that is, the beam pattern of the transmit signal is not formed in the spatial domain.

2. Signal components of each transmitting element are separated in the receiving system based on their codes. The time delay is calibrated via the elements in the space, and then the signal components are coherently combined again to generate narrow pulses of target echoes, namely the equivalent transmitting beam patterns.

SIAR uses multiple antennas to transmit orthogonal signals with multicarrier frequencies. These signals have unique characteristics in wavelength selection, antenna types, Doppler processing, and beamforming [16]:

1. Operating at the meter wave band. Though the radar with meter wave is suitable for long-range detection, the angular resolution is low and the accuracy of angular measurement is inaccurate due to the limits of the antenna size. Without taking into account the angular resolution, there are many advantages of meter wave radar:

a. Appropriate pulse repetition frequency (PRF) is selected to ensure that range ambiguity (hundreds of kilometers) and velocity ambiguity (several Mach numbers) can be avoided. In other words, the Doppler processing can be realized with no range ambiguity.

b. It is difficult to significantly reduce the RCS of targets (whether their shape or coating) by using stealth techniques.

c. It is easy to make a filter because of the weak ground clutter and narrow frequency spectrum.

d. It is able to obtain high output power for transmitters at a low cost.

e. It is difficult for enemies to use airborne jammers due to the large antenna aperture at the meter wave band and electromagnetic compatibility.

f. It has better countermeasures against ARM.

2. Using large sparse array antennas. Low angular resolution is the main obstacle for meter wave radars. An SIAR experimental system employs 25 transmitting array elements and 25 receiving array elements, which are uniformly distributed on two circles with diameters of 90 and 45 m respectively. If the wavelength is 3 m, the azimuth angular resolution of the array will be about 1.2°, which ensures the desired angular resolution. Meanwhile, taking into consideration the cost and the complexity of implementation, the number of antenna elements should not be too large. Due to the requirement of detecting targets in all directions, SIAR chooses a large sparse circular antenna array with a limited number of elements.

3. Omnidirectional transmission. Each transmit antenna emit signals simultaneously at different frequencies to ensure that the radiation energy is uniformly distributed in space and coherent speckles cannot be formed. Comparatively, the conventional phased array radar operates at the same carrier frequency so spatial coherent speckles representing the transmitting pattern are formed.

4. Doppler processing. The conventional radar needs to steer the beam. The number of echo pulses at one beam direction is small so only a limited number of pulses can be provided to integrate because of the time constraint. Since SIAR does not adopt physical focusing and beam scanning, and provides a continuous surveillance for the entire airspace, the coherent integration time theoretically is only determined by the system coherent performance and target's velocity. The higher the number of pulses provided for integration, the higher the resolution achieved via Doppler processing. For general surveillance radars, if the antenna rotates at 6 rpm, the data rate is 10 seconds. For SIAR, if the pulse repetition interval is 3 ms and the number of integration pulses is 256, the Doppler resolution is 1.3 Hz and the data rate of each target is 0.768 seconds. It is far above the level of a general surveillance radar.

5. Transmit beamforming. Namely, the transmit pattern is generated at the receiving end through impulse synthesis processing, and “impulse compression” is carried out based on the multicarriers.

6. It is able to simultaneously form multiple searching beams covering the whole spatial space and multiple tracking beams to perform the monopulse measurement for each target. SIAR is particularly suitable for detecting and tracking multiple targets since it incorporates surveillance and tracking as a whole.

7. SIAR is a four-dimensional (4D) radar. It can be used to obtain the range, velocity, azimuth, and elevation of targets.

The most significant technical feature of SIAR is that it can realize nondirectional emission and form multiple “stacked” beams simultaneously. Therefore, the long-time coherent integration can be achieved simultaneously at all beam directions so as to improve the ability to detect dim targets (especially stealth targets).

1.3 Four Anti Features of SIAR

1.3.1 Anti-stealth of SIAR

Modern radar faces the challenge of targets with very small RCS, such as cruise missiles, stealth targets, and reentry InterContinental ballistic missile (ICBM) warheads. Improving the radar detection ability has always been a hot topic, and it becomes especially important with the advancement of stealth techniques. To improve detection ability, it is not enough to increase the transmitted power. This new-style radar principle, waveform design, and signal processing are also needed.

The general radar usually uses coherent integration techniques or noncoherent integration techniques to improve detection ability, but the number of pulses available for integration is mainly limited by antenna scanning due to beam scanning. For example, if the radar beamwidth is 2°, the beam scanning speed will be 6 rpm and the radar repetition frequency will be 300 Hz, so the number of integrated pulses is less than 17. For three-dimensional (3D) radars, the number of pulses available for integration is much lower. In order to suppress the clutter, sometimes only part of the pulses can be integrated so the signal-to-noise ratio (SNR) improvement gained through integration is limited.

Since transmit and receive beamformings are realized through signal processing at the receiving end, impulse synthesis in SIAR can keep the beam at certain directions (even one direction). Therefore, multiple beams or stacked beams (including transmitting beams and receiving beams) can be simultaneously achieved at the receiving end. These beams can even cover the entire spatial space without beam scanning and always track targets. This is equivalent to the “burn-through” operational mode in conventional radar, though the conventional radar only operates in one direction in the “burn-through” mode. Since there is no beam scanning in SIAR, the integration time is only determined by the target's velocity and the radar parameters, independent of the beam scanning time on the target. Therefore, SIAR can obtain a larger number of coherent integration pulses. Furthermore, the signals with a large time-bandwidth can be used in an SIAR system to increase the average power of transmitted signals [16, 17], improving the detection range and resolution capability of the radar.

SIAR has two advantages in anti-stealth as follows:

1. T...