Chirp Radar Research Articles

Range/Doppler ambiguity elimination in high-frequency chirp radars

Range/Doppler ambiguity is an important problem in chirp radar operations with all the existing schemes for ambiguity elimination relying on multiple waveform switching. Now, a new method for ambiguity elimination is proposed. It is based on a single waveform and only involves a series of software computations associated with the instantaneous amplitudes of the target under consideration at adjacent range bins. Besides the ambiguity factor, the instantaneous range of the target is also estimated. The method can greatly improve the target detection and tracking capability of high-frequency chirp radars. Both simulation and real data processing results are demonstrated to validate of the method.

Dense radio frequency interference suppression in HF radars

Radio frequency interference (RFI) is often a major problem in high-frequency (HF) radar operation, and until now, few existing methods worked very well to suppress dense RFI. In this letter, a theoretical analysis of the range-domain correlation of RFI in HF chirp radar is presented, and the relation between the correlation functions of RFI in range and fast time domain is given; based on this, a new method for RFI suppression is developed. The echo signals at the reserved range bins are used to construct an interference subspace, and then, the signals at the range bins of interest are projected onto its orthogonal subspace to suppress the RFI. When applied to radar signal processing, the method suppressed dense nighttime RFI effectively. It greatly improves the radar's detection performance in dense RFI.

Digital false-target image synthesiser for countering ISAR

The paper presents the design of a pipelined, all-digital image synthesiser capable of generating false-target images from a series of intercepted inverse synthetic aperture radar (ISAR) chirp pulses to provide a novel RF imaging decoy capability. The image synthesiser modulates the phase samples from a phase-sampling digital RF memory (DRFM) that stores intercepted ISAR pulses. The synthesiser contains a parallel array of (identical) complex digital modulators with one modulator for each false-target range bin. The binary phase samples from each intercepted ISAR pulse are applied one at a time to the modulator array. To synthesise the image, each modulator requires a set of phase and gain coefficients that are derived from the range–Doppler description of the false-target to be synthesised. An ISAR image compression algorithm is presented to reconstruct the image using the synthesiser output pulses. A ship with 32 range bins is presented as an example of a false-target input, and simulations are used to quantify the image synthesis capability of the architecture. The image quality as a function of the number of ISAR pulses integrated is also numerically evaluated. The programmable design permits real-time alteration of modulation coefficients, allowing rapid and adaptive shifting among different types of targets offering a low-cost decoy capability using readily available DRFM technology.

Time domain analysis and its practical application to the measurement of phase noise and jitter

The precise determination of phase is necessary for a very large set of measurements. Traditionally, phase measurements have been made using analog phase detectors which suffer from limited accuracy and dynamic range. This paper describes how phase digitizing, which uses time domain techniques, removes these limitations. Phase digitizing is accomplished using a time interval analyzer, which measures the signal zero-crossing times. These zero-crossing times are processed to compute phase deviation, with the reference frequency specified as a numerical value or derived from the times themselves. Phase digitizing can be applied even in the presence of modulation, as the underlying clock can be reconstructed in software to fit the data. Measurements derived from this phase data such as phase noise, jitter analysis, Allan variance (AVAR), maximum time interval error (MTIE), and time deviation (TDEV) are applied to such applications as the characterization of oscillators, computer clocks, chirp radar, token ring networks, and tributaries in communication systems.

Design, Development, Field Observations, and Preliminary Results of the Coherent Antarctic Radar Depth Sounder (CARDS) of the University of Kansas, U.S.A.

AbstractA modern coherent Antarctic radar depth sounder for probing the ice sheets of Antarctica and Greenland has been designed and developed by the University of Kansas. It was successfully tested during the austral summers of 1987 and 1988 at Downstream Β and Upstream B, Antarctica. Ground-based measurements were made with the radar in a mobile hut hauled by a Sno-cat in 1987 and in a Spryte vehicle in 1988.The coherent Antarctic radar depth sounder (CARDS) is an unfocussed synthetic-aperture chirp radar where the along-track resolution is improved by extensive coherent integration. Surface acoustic wave (SAW) devices are used to implement pulse expansion and compression. A common stable oscillator for the transmitter and the receiver establishes coherency. The system signal-to-noise ratio is enhanced by pulse compression and coherent integration. Antennas for the ground-based measurements are configured with an array of eight dipole elements, four active and four passive, the latter acting as reflectors. The aircraft antennas also consist of four active elements hung underneath the two wings. The wings serve as reflectors. A PC facilitates system control and data recording on a high-density recorder. Α-scope plots of selected records allow frequent field checks on system performance. More descriptive display facilities have been incorporated in the latest version of the system.The radar transmits 60 ns, 20 W peak power at 150 MHz. The number of coherent integrations is selectable up to 64 k. The system is capable of 5 m range resolution and 5 km range in ice. A programmable sensitivity time control (STC) increases the receiver dynamic range. System parameters such as pulse-repetition frequency, number of integrations, and display modes can be chosen during field operations by user-friendly software.This paper describes the design and field operations of the system. Some results of the 1987 operations at Downstream Β are presented.

Design, Development, Field Observations, and Preliminary Results of the Coherent Antarctic Radar Depth Sounder (CARDS) of the University of Kansas, U.S.A.

AbstractA modern coherent Antarctic radar depth sounder for probing the ice sheets of Antarctica and Greenland has been designed and developed by the University of Kansas. It was successfully tested during the austral summers of 1987 and 1988 at Downstream Β and Upstream B, Antarctica. Ground-based measurements were made with the radar in a mobile hut hauled by a Sno-cat in 1987 and in a Spryte vehicle in 1988.The coherent Antarctic radar depth sounder (CARDS) is an unfocussed synthetic-aperture chirp radar where the along-track resolution is improved by extensive coherent integration. Surface acoustic wave (SAW) devices are used to implement pulse expansion and compression. A common stable oscillator for the transmitter and the receiver establishes coherency. The system signal-to-noise ratio is enhanced by pulse compression and coherent integration. Antennas for the ground-based measurements are configured with an array of eight dipole elements, four active and four passive, the latter acting as reflectors. The aircraft antennas also consist of four active elements hung underneath the two wings. The wings serve as reflectors. A PC facilitates system control and data recording on a high-density recorder. Α-scope plots of selected records allow frequent field checks on system performance. More descriptive display facilities have been incorporated in the latest version of the system.The radar transmits 60 ns, 20 W peak power at 150 MHz. The number of coherent integrations is selectable up to 64 k. The system is capable of 5 m range resolution and 5 km range in ice. A programmable sensitivity time control (STC) increases the receiver dynamic range. System parameters such as pulse-repetition frequency, number of integrations, and display modes can be chosen during field operations by user-friendly software.This paper describes the design and field operations of the system. Some results of the 1987 operations at Downstream Β are presented.

Use Of Speckle For Determining The Response Characteristics Of Doppler Imaging Radars

The synthetic aperture radar (SAR) aboard Seasat in 1978 demonstrated a unique sensitivity to oceanic and geologic features imaged over a 100 km swath with 25 m resolution. The ability of the remote sensor to resolve the fine details of large environmental systems resulted in the orbiting of a similar system, the Shuttle Imaging Radar (SIR-B), aboard the Space Shuttle Challenger during October 1984. Coherent speckle noise observed for these Doppler imaging-radar systems is caused by random correlations of the illuminating radar chirp with the surface reflectance downrange. Radar speckle is similar to optical speckle in that respect, but it is also influenced by along-track sampling statistics. The Rayleigh statistics of coherent scattering and the Poisson statistics of radar pulse detection are employed to model the speckle observed in spatially random data samples. Speckle degradation of a radar scene may obstruct interpretations of scene detail, but it can also be useful in determining the spatial response of the remote sensor and scene correlator. Randomly speckled scenes that are otherwise featureless provide a white-noise input to the Doppler imaging process. Several such scenes have been processed with fast Fourier transform methods to estimate the point spread function and its Fourier-domain equivalent, the wavenumber response function. These measure-ments of spatial resolution are used to compare the Seasat SAR and the Challenger SIR-B remote sensors. In addition, two ground-scene correlators are compared in terms of the point spread estimate of spatial resolution for common input data from the SIR-B sensor.

Saw Filters for TV Receivers

The surface acoustic wave (SAW) technology has existed in many laboratories for a number of years. It has already found a place in many military and professional systems. Perhaps its most successful application to date has been in providing pulse compression filters for chirp radar. Recently, however, SAW bandpass filter design has reached the state where a single device can perform all the filtering necessary in the I.F. stage of colour T.V. receivers. The added advantages of the SAW solution over traditional LC methods are immediately obvious. No manual alignment is necessary. The SAW filter is compact, rugged and reproducible.

Quasiholographic techniques in the microwave region

Following the intransification of holographic activity in the 1960s, various researchers reported experiments that were direct microwave counterparts of the optical holography which preceded them. This work, which has recently become extensive, may be termed true microwave holography. As the title of our paper implies, we deal not with this rather restrictive field, but with a much broader one which embodies holographic-like techniques. There exists in the field of communication science a variety of techniques that resemble holography in various degrees, both in concept and in their mathematical formulation. Yet these processes have developed quite independently of holography and in no way depend upon principles originating in holography. We select four such techniques for our discussion: the synthetic aperture, the chirp radar, the rotating-target imaging system, and the beamforming technique. Each in its own way resembles holography, and viewing them as holographic processes offers, in some cases, new insights that lead to new implementations.

Theory of the dispersive modulator

The recent observations by Loy of amplitude modulation and compression of infrared laser pulses caused by passage through the dispersive modulator (a cell containing dilute NH3 vapor with a Stark−modulated resonant frequency) are explained. It is shown that there is a formal equivalence between (i) the passage of a near−resonant light beam through a vapor with a time−dependent resonant frequency, and (ii) the passage of a frequency−modulated light beam through a vapor with a constant resonant frequency. For low−intensity light this equivalence reduces the problem to the optical analog of chirp radar.

Effects of Range-Doppler Coupling on Chirp Radar Tracking Accuracy

Attention is called to the phenomenon of "range-Doppler coupling," characteristic of linear FM (chirp) waveforms, and the strong effect this coupling may have on tracking accuracy. Numerical results are presented for simple three-state filters and for a realistic reentry vehicle tracking problem.

Introduction to Chirp Concepts with a Cheap Chirp Radar

The elements of pulse compression of a chirp in a dispersive delay are reviewed and chirp and pulse modes are contrasted. A simple microwave Doppler-shift apparatus is modified info a chirp system to permit distance measurement in a laboratory experiment that is rich in fundamental concepts of physical parameters which are related by the Fourier transform, and which involve the convergence of ideas from several disciplines.

Ultrasonic imaging system by using optical pulse compression.

Conventional ultrasonic imaging techniques, including holographic, have high resolution in angle, but relatively poor resolution in range, because resolution in range is obtained only by use of selective focus or parallax, which is useful only with small focal ratio. The paper presents the principle and some experimental results of new ultrasonic imaging techniques that use a linear FM ultrasonic pulse and a pulse of light. This method gives high resolution both in angle and in range; it may be considered to be an application of pulse-compression techniques, used in chirp radar, to ultrasonic imaging, but it has image-forming as well as range-discriminating capability.

光学的パルス圧縮を用いる超音波映像法

Conventional ultrasonic imaging techniques, including holographic, have high resolution in angle, but relatively poor resolution in range, because resolution in range is obtained only by use of selective focus or parallax, which is useful only with small focal ratio. The paper presents the principle and some experimental results of new ultrasonic imaging techniques that use a linear FM ultrasonic pulse and a pulse of light. This method gives high resolution both in angle and in range; it may be considered to be an application of pulse-compression techniques, used in chirp radar, to ultrasonic imaging, but it has image-forming as well as range-discriminating capability.

An introduction to the principles and applications of holography

Holography has strong historical ties with electrical engineering and potential application to many electrical engineering problems. The basic problem addressed by holography is introduced in both physical and mathematical terms. The analogy between the hologram of a point-source object and the linear FM signals of chirp radar is stressed, and the first-order imaging properties of holograms recorded in arbitrary geometries are derived. Various types of holograms are described, including thin, thick, transmission, reflection, amplitude, and phase holograms. The important properties of each type of hologram are introduced. A survey of various applications of holography is presented, with introductions to the use of holography in interferometry, microscopy, imaging through distorting media, optical data processing, and optical data storage. The use of simple holograms as optical elements is also described.

A Computer-Aided Design of a Microwave Delay Equalizer

The folded-tape meander-line configuration that has been used in the design of compression filters for chirp RADAR applications can also be used in the design of microwave delay equalizers for communication systems. Specifically, it can be utilized in the design of delay equalizers for use in the repeaters of a long distance waveguide transmission system. In this paper, the synthesis of such L-band delay equalizers using a general purpose successive approximation computer program, SUPROX is described. In this procedure, both the locations of the resonant frequencies and the number of meander-line turns at each of these resonant frequencies are optimized to give the final design. Linear as well as certain higher order delay characteristics have been synthesized with a high degree of accuracy. Experimental results show excellent agreement with the theoretical designs.

Compression of optical pulses

A scheme, similar to one used in the chirp radar, is proposed to compress in time, without loss of energy, the light pulse generated by a mode-locked laser. On the basis of preliminary experimental results obtained with a mode-locked He-Ne laser and a LiNbO 3 phase modulator, it is predicted that compression of 50-ps pulses derived from a Nd:YAG laser to 0.4 ps or less is feasible.

Matched-Filter Responses of the Linear FM Waveform

Although the properties of the linear FM signal have been studied previously in considerable detail, such studies have involved rather narrow aspects of the theory. This paper extends the work in several respects. By presenting three-dimensional projections of the conventional ambiguity function of the linear FM signal in more detail than was available before, we can study the sidelobe behavior off as well as on the axes, without weighting, with unilateral weighting in the receiver, and with bilateral weighting. These plots reveal interesting properties related to the signal symmetry in time and frequency. The matched-filter response is then extended to include Doppler distortions of the modulation function. The results show that Woodward's ambiguity function is valid only for signals with relatively modest sophistication, even though in most practical situations one is interested only in those undistorted parts of the matched-filter response in the vicinity of the delay axis. Plots of the response are presented for various degrees of distortion, for signals with and without weighting. Lastly, we consider the effects of a mismatch in range acceleration, again for the various cases of interest. The results convey a thorough insight into the properties of chirp radar under a broad range of operational conditions.

Sonic-Pulse Compression in Bats and People: A Comment

Bats have been reported to be extremely resistant to noise jamming. Since they navigate on a sonar consisting of FM chirps, G. K. Strother suggested that their auditory system contains “collapsing filter” similar in principle to that used in chirp radars. This paper examines that possibility in more detail and reports on some informal experiments with people; the conclusion is reached that people do not have collapsing filters, and that bats very probably don't either. It is further concluded, on the basis of an analysis of basilar-membrane responses, that at the hair-cell level the time order in the stimulus is preserved and that the basilar membrane as a whole preserves the envelope structure of a complex stimulus. Concerning the question of why, then, the bat chirps, the suggestion is made that an FM signal acts as a feature-extracting probe signal, informing the bat not only of the location but also of the size, and possibly texture, of objects in its environment.

A simple optical filter for chirp radar

A simple optical filter for chirp radar