Ultra-high Range Resolution Research Articles

High-Resolution and Wide-Swath SAR Imaging With Sub-Band Frequency Diverse Array

High-resolution and wide-swath imaging serves as an important task for synthetic aperture radar (SAR). However, the tradeoff between high azimuth resolution and wide unambiguous swath coverage has not been well optimized in traditional SAR system. Meanwhile, the ultrawideband signal is required to obtain ultrahigh range resolution imaging. It is not easy to increase the bandwidth directly due to the difficulty of system design. To this end, this article makes notable contribution on a sub-band frequency diverse array framework to realize range ambiguous echoes separation and wideband signal synthesis from narrowband signals. By introducing a small frequency increment across the elements within each subarray, the transmit steering vector within each subarray is range-angle-dependent, making it feasible to resolve range ambiguity in the spatial frequency. In addition, the transmitted waveforms of different subarrays occupy different frequency bands in range frequency domain, which play a pivotal role in achieving high-resolution imaging. With phase compensation and sub-band frequency spectrum splicing technique, the wideband signal can be obtained. Simulation results have verified the effectiveness of the proposed approach.

Waveform design for target classification in MIMO radar systems

MIMO Radar sensors are relatively immune to environmental conditions and can work in complete darkness and also bright daylight. Also, because of ultra-high range resolution (ability to discriminate closely spaced objects) and sensitivity (ability to detect weak signals), these sensors are ideal for different applications, including healthcare and security, provided that appropriate transmit and receive strategies are employed in the sensor. Because of their high frequency, with appropriate signal processing, these sensors potentially can classify and sometimes identify different objects. In this paper, we propose a waveform design framework to enhance target classification performance for the MIMO radar systems. To this end, we enhance object discrimination by maximizing the differences between received signal from every target. We show that this difference depends to the radar transmit waveform. Thus, we design the probing signal, by solving a non-convex optimization problem while considering unimodular/discrete phase, and similarity constraints in the design stage. To solve the optimization problem, we propose Coordinate Descent (CD) framework, and iteratively design the code entries. The results show that the performance of the proposed framework enhances the discrimination performance for automotive MIMO radar systems.

Ultra-high resolution microwave photonic radar with post-bandwidth synthesis

We demonstrate microwave photonic radar with post-bandwidth synthesis, which can realize target detection with ultra-high range resolution using relatively small-bandwidth radio frequency (RF) frontends. In the proposed radar, two temporal-overlapped linear frequency-modulated (LFM) signals with the same chirp rate and different center frequencies are transmitted. By post-processing the de-chirped echoes in the receiver, a signal equivalent to that de-chirped from an LFM signal with the combined bandwidth is achieved. In a proof-of-concept experiment, two LFM signals with bandwidths of 8.4 GHz are exploited to achieve radar detection with an equivalent bandwidth of 16 GHz, and a range resolution of 1 cm is obtained.

Ultrahigh Range Resolution ISAR Processing by Using KT–TCS Algorithm

Nowadays, the slant-range resolution of inverse synthetic aperture radar (ISAR) can be improved to 1.5 cm by the application of microwave photonics technology. With the great improvement in range resolution, the ISAR images may contain more information on targets. However, ultrahigh resolution also induces severer space-variant range cell migration (RCM). In this paper, we propose an algorithm based on Keystone transform (KT) and time-domain chirp scaling (TCS) to deal with the space-variant RCM in ISAR imaging with ultrahigh range resolution. The algorithm consists of two steps. In the first step, the azimuth-variant linear components in the space-variant RCM are compensated for by KT. In the second step, the TCS is performed to remove the range-variant quadratic RCM. The results acquired from the processing of both the simulation and the real data validate the effectiveness of the proposed algorithm.

Photonics-based MIMO radar with high-resolution and fast detection capability

A photonics-based multiple-input-multiple-output (MIMO) radar is proposed and demonstrated based on wavelength-division-multiplexed broadband microwave photonic signal generation and processing. The proposed radar has a large operation bandwidth, which helps to achieve an ultra-high range resolution. Compared with a monostatic radar, improved radar performance and extended radar applications originated from the MIMO architecture can be achieved. In addition, low-speed electronics with real-time signal processing capability is feasible. A photonics-based 2 × 2 MIMO radar is established with a 4-GHz bandwidth in each transmitter and a sampling rate of 100 MSa/s in the receiver. Performance of the photonics-based multi-channel signal generation and processing is evaluated, and an experiment for direction of arrival (DOA) estimation and target positioning is demonstrated, through which the feasibility of the proposed radar system can be verified.

Ultra-high range resolution demonstration of a photonics-based microwave radar using a high-repetition-rate mode-locked fiber laser

We experimentally demonstrate the ultra-high range resolution of a photonics-based microwave radar using a high repetition rate actively mode-locked laser (AMLL). The transmitted signal and sampling clock in the radar originate from the same AMLL to achieve a large instantaneous bandwidth. A Ka band linearly frequency modulated signal with a bandwidth up to 8 GHz is successfully generated and processed with the electro-optical upconversion and direct photonic sampling. The minor lobe suppression (MLS) algorithm is adopted to enhance the dynamic range at a cost of the range resolution. Two-target discrimination with the MLS algorithm proves the range resolution reaches 2.8 cm. The AMLL-based microwave-photonics radar shows promising applications in high-resolution imaging radars having the features of high-frequency band and large bandwidth.

Photonics-based broadband radar for high-resolution and real-time inverse synthetic aperture imaging.

A photonics-based radar with generation and de-chirp processing of broadband linear frequency modulated continuous-wave (LFMCW) signal in optical domain is proposed for high-resolution and real-time inverse synthetic aperture radar (ISAR) imaging. In the proposed system, a broadband LFMCW signal is generated by a photonic frequency quadrupler based on a single integrated electro-optical modulator, and the echoes reflected from the targets are de-chirped to a low frequency signal by a microwave photonic frequency mixer. The proposed radar can operate at a high frequency with a large bandwidth, and thus achieve an ultra-high range resolution for ISAR imaging. Thanks to the wideband photonic de-chirp technique, the radar receiver could apply low-speed analog-to-digital conversion and mature digital signal processing, which makes real-time ISAR imaging possible. A K-band photonics-based radar with an instantaneous bandwidth of 8 GHz (18-26 GHz) is established and its performance for ISAR imaging is experimentally investigated. Results show that a recorded two-dimensional imaging resolution of ~2 cm × ~2 cm is achieved with a sampling rate of 100 MSa/s in the receiver. Besides, fast ISAR imaging with 100 frames per second is verified. The proposed radar is an effective solution to overcome the limitations on operation bandwidth and processing speed of current radar imaging technologies, which may enable applications where high-resolution and real-time radar imaging is required.

Imaging approach for airborne stepped-frequency synthetic aperture radar in the squinted mode

University of Chinese Academy of Sciences, Haidian, Beijing, ChinaAbstract. For synthetic aperture radar (SAR) imaging, a stepped-frequency chirp signal iswidely used to obtain ultrahigh range resolution. There are mainly two challenges in thestepped-frequency SAR imaging. One is the subbands synthesis, and the other is the estimationand compensation of amplitude andphase errors of the subbandscaused by the radar system andthe propagation. An imaging approach for the airborne squinted sliding-spotlight SAR withstepped-frequency chirps is proposed to achieve ultrahigh two-dimensional (2-D) resolution.Due to the squint angle, a Doppler centroid shift is introduced and then a modified space-variantcompensationmethodfortherangedifferencebetweenthesubbandsisrealizedviamultiplyingaphaserampinthe2-Dfrequencydomain.Moreover,arange-dependentphasegradientautofocusalgorithm based on raw data is proposed to estimate and compensate the residual phase errorafter the internal calibration. Afterward, the frequency-domain synthetic bandwidth method isused to complete the bandwidth synthesis. Finally, the extended Omega-K algorithm is appliedto focus the synthesized data. The simulation results and the real data processing results provethe validity of the proposed approach.

Accurate estimation of stitching error for high‐resolution multi‐sub‐bands SAR system with image sharpness analysis

Multi-sub-bands stitching technique is an effective method to achieve ultra-high range resolution in synthetic aperture radar (SAR) system. However, sub-bands stitching error results in artefacts in multi-sub-bands SAR images, e.g. defocused targets, higher sidelobes, which seriously degrade SAR image quality and cannot be mitigated by weighting function. A novel stitching error estimation method was presented. The stitching error is modelled by regarding phase error as function of range frequency, which will be estimated and compensated to reach the maximum image sharpness. First, amplitude balance is applied to single look complex SAR image in range frequency domain. Subsequently, based on image sharpness, an iterative optimisation algorithm is designed to estimate and compensate the stitching phase error. Finally, experimental results using both real RADARSAT-2 and airborne SAR dataset validate proposed method perform well in arbitrary scenes, and it outperforms the existing methods.

A Synthetic Bandwidth Method for High-Resolution SAR Based on PGA in the Range Dimension.

The synthetic bandwidth technique is an effective method to achieve ultra-high range resolution in an SAR system. There are mainly two challenges in its implementation. The first one is the estimation and compensation of system errors, such as the timing deviation and the amplitude-phase error. Due to precision limitation of the radar instrument, construction of the sub-band signals becomes much more complicated with these errors. The second challenge lies in the combination method, that is how to fit the sub-band signals together into a much wider bandwidth. In this paper, a novel synthetic bandwidth approach is presented. It considers two main errors of the multi-sub-band SAR system and compensates them by a two-order PGA (phase gradient auto-focus)-based method, named TRPGA. Furthermore, an improved cut-paste method is proposed to combine the signals in the frequency domain. It exploits the redundancy of errors and requires only a limited amount of data in the azimuth direction for error estimation. Moreover, the up-sampling operation can be avoided in the combination process. Imaging results based on both simulated and real data are presented to validate the proposed approach.

Physical constraints in sonar design

The dynamic range and processing gains achievable within a simple passive sonar will be reviewed. It will be demonstrated that the constraints of modern electronic hardware components do not necessarily limit the performance of a passive sonar system when compared with the physical constraints of the acoustic channel. By contrast it will be demonstrated that the pulse design of an active sonar places significant limitations on the capability of the system. For example, range resolution, Doppler resolution, range sidelobes, ambiguities and reverberation performance all depend on the design of the sonar transmission. Techniques available to the designer for suppressing range sidelobes and pulse designs with both range and Doppler sensitivity will be reviewed. The constraints imposed by the transducer will be considered, particularly in relationship to ultrahigh range resolution capabilities. Finally, a comparison between an early 1960s sonar and a modern equivalent will be made in terms of performance, power consumption and size.

Ultrawide-band radar observations of multipath propagation over the sea surface

An ultrawide-band radar system centered at 10 GHz has been developed for sea-scatter research and was recently deployed on a research pier in the North Atlantic. The radar is based on a time-domain reflectometer module for a sampling oscilloscope. Using transient excitation of a traveling wave-tube amplifier, the system generates 200-ps wide pulses with a 10-GHz center frequency and a peak power of approximately 1 KW. The resulting range resolution is approximately 3 cm. The system was used to investigate low-grazing angle, multipath effects in the ocean environment using a trihedral corner reflector, mounted 45 cm above the water surface. The ultrahigh-range resolution of the system allows spatial separation of the direct and indirect echoes from the trihedral. In addition, a comparison of the indirect vertically- (VV) and horizontally- (HH) polarized echoes illustrates the effects of Brewster angle damping. The implications of these effects for sea-clutter statistics are briefly discussed.