Acousto-optic processor to generate laser radar range-Doppler images

The cross-ambiguity function (CAF) is commonly used to analyze the delay-Doppler characteristics of signals in radar, sonar, and communication systems. Accordingly, a CAF relates to the correlation processing of signals in the presence of delays and Doppler shifts. In this paper, we use a metaheuristic approach to address the CAF synthesis problem by jointly designing a pair of waveforms. The CAF of waveforms designed in this way, approximates a desired pre-defined CAF. It turns out that the waveforms designed by this approach have the benefit of low peak-to-average power ratios. Numerical examples are presented to show that nature-inspired metaheuristic algorithms can be used as an effective tool to synthesize different types of CAFs.

Advanced radar receivers having an instantaneous dynamic range exceeding 90 dB are currently under development. These receivers must operate linearly to achieve satisfactory performance in a severe clutter environment. Presently available, state-of-the-art analog-to- digital converters (ADCs) having sufficiently high sampling rates do not have adequate dynamic ranges to digitize the outputs of these receivers. A technique has been developed that effectively reduces the dynamic range of the input signal prior to digitization. The dynamic range reduction is accomplished through a process that predicts the next radar return signal from the previous return signals, generates a replica waveform, and subtracts this replica waveform from the radar return signal prior to digitization. This process allows the radar return signal to be digitized without distortion by an ADC having a limited dynamic range. The full dynamic range of the radar return signal is then restored by adding the replica waveform to the ADC output. Tests and evaluations using both synthetic and recorded radar data have demonstrated in excess of a 30 dB reduction in the dynamic range of the signal at the ADC input when strong clutter is present.

The results of experimental measurement of errors in an acousto-optic (AO) frequency counter are presented. This type of error is caused by temperature effect in AO deflectors (Bragg cells), which are the main elements of AO frequency counter. In the case, when input power is large, some characteristics of AO deflectors change. This causes the common mistake of AO frequency counters. AO deflectors are made of tellurium dioxide (TeO/sub 2/) and lead molybdenum tetra oxide (PbMoO/sub 4/) were tested. The construction temperature conductivity of AO deflectors was different. It is proved, that in the case AO frequency counter works with wide dynamic range of input radio signal's power, this type of error may be equal to some percents.

The passive coherent location (PCL) radar detects potential targets by computing the cross ambiguity function (CAF) between the target echo signal and the direct path wave, which is regarded as the reference signal received from the reference channel. However, a number of the multipath interferences present in the reference signal in most cases. The existence of the multipath interferences can increase the false alarm rate and raise the detection threshold giving rise to lower detection rate. Therefore, the direct path wave should be purified. This paper proposes a direct path wave purification algorithm based on the normalized least mean square (NLMS) algorithm. This algorithm can attenuate the multipath interferences of the direct path wave, which results in the lower false alarm rate and lower detection threshold. This algorithm is validated by the experimental results.

This paper describes realizable acousto-optic configurations for the implementation of phased array beamformers. The simple system is capable of generating a distribution of wideband radio frequency (RF) signals with controlled phases. A laboratory demonstration model of the acousto-optic beamformer was built using a pair of Bragg cells and a 10-channel fiber-detector array. Experimental results show that a large phase excursion is obtainable simply by tuning the frequency of the control signal. 1. INTRODUCTION The practical implementation of beamformers for phased array antennas has long been recognized as a challenging technical issue. Simpler and better beamforming techniques are needed to provide the controlled RF amplitude and phase exitations to be fed to the large number of phased array elements. In the conventional microwave implementation the phased array beamforming is performed by the use of individually controlled phase shifters and variable gain amplifiers in the signal path of each phased array element. For large arrays the required RF hardware becomes bulky and complex. The high power consumption of the phase shifter network also poses a major problem.Recently, there has been growing interest in the optical implementation of the phased array beamformer.1 3 The optical approach offers many potential advantages such as high operating frequency and wide bandwidth, immune to radio frequency interference (RFI), small size, low weight, etc. A few methods for the optical beamformer have been pro­ posed. The basic idea is to perform the beamforming at optical wavelengths using spatial light modulators (SLM) and downconverting to the microwave domain by heterodyning with a frequency-shifted reference light beam. This paper discusses several configurations of a phased array beamformer using acousto-optic (AO) techniques.In the proposed approach, AO Bragg cells were used for both the optical beamforming and frequency-shifting purpose. Experimental results were obtained with a ten-channel, one-dimensional beamforming breadboard using two Bragg cells. Both the RF carrier frequency and the phases are independently controlled by tuning the Bragg cell frequencies. The method can be extended to two-dimensional beamforming.

We demonstrated in this work the possibility of application of the effect of resonant acousto- optical (AO) interaction for the creation of wide band and high efficiency Bragg cells on gigahertz frequencies. The main principle of this effect consists in heat there exists the optimum photon energy (very close to the band gap of semiconductor sample) where the optical absorption is still not big enough but the figure of merit already several times higher in comparison with one when the photon energy is fat from the band gap. We have shown theoretically that the optimization of multilayer thin film piezoelectric transducers allows to create AO Bragg cells (deflectors and modulators) working in resonant conditions and having very high diffraction efficiency (more than 10%) and very wide frequency band (up to 2 GHz). This optimization based on the use of the acoustical matching scheme-asynchronous transform, analogous to one in electrotechnics. Experimental devices represent the cells with crystal plates of GaAs and InP of (100) and (110) cut having the multilayer thin film transducer on the butt-end that has been matched with microstrip transmission line. Characteristics of created AO cells reach: 10% per 1 Watt of electrical power; 2 GHz frequency band with 8 MHz frequency resolution. The obtained experimental results show a good agreement with theoretical predictions.

This paper addresses the issue of drones and helicopters classification from radar return using point clouds features. In order to classify helicopters or drones, most published researches estimate the length of blades, the number of rotors, the period of their micro-Doppler signatures or try to extract information from spectrograms (or any other image). Differently this work generates a point cloud from radar return signal. Features are extracted from point cloud and they are used as input of an Artificial Neural Net (ANN) to classify different models of helicopters and drones.

We propose a novel space-time waveform coding (STWC) scheme for joint radar and wireless communications (RadCom). In particular, we consider a frequency-modulated continuous-wave (FMCW) multiple-input multiple-output (MIMO) radar for near-range to medium-range radar applications. In order to establish the required orthogonal transmit signals, up- and down-chirp FMCW waveforms are combined with Alamouti space-time coding across two transmit antennas. We demonstrate that, from a radar perspective, the cross-ambiguity function is improved significantly by the orthogonal Alamouti code, while the FMCW waveforms ensure robustness in the presence of Doppler shifts. Regarding the communications part, the generic orthogonality of the Alamouti code allows us to embed random information symbols within the radar signal, enabling wireless communications in the 100 kb/s regime. To this end, we propose a suitable communications receiver algorithm for multiple antennas with corresponding maximum-ratio combining, which is able to extract the embedded information symbols while acquiring spatial diversity gains.

The use of ground-based radar stations for space surveillance makes it possible to solve problem of detecting new and tracing previously known targets around the clock. The quality of operation ground-based radar station for space surveillance can be significantly reduced when solving problems of radar surveillance in complex phono-target and signal-interference environment. To improve effectiveness of radar surveillance it is advisable to carry out joint use of radar station for space surveillance, in particular as part of multi-position complex with possibility of joint coherent radar signal processing, including signals separated by frequency. It is necessary to develop models for evaluating effectiveness joint use of deferent-band radar stations for space surveillance when solving problems of radar surveillance in complex phono-target and signal-interference environment based on implementation of coherent processing two radar signals. The purpose of study is increase potential accuracy of radar measurements target trajectory parameters in conditions of relayed interference based on implementation of coherent processing two radar signals at ground radar systems for space surveillance. A model of target monitoring has been developed in system of two different-range radar stations for surveying space according to probabilistic indicator and dependences on linkage target trajectories were obtained based on implementation of coherent processing two radar signals in various conditions of signal-interference environment. The result of study can be used in evaluating effectiveness joint use of several radar stations for surveying space as part of multi-position radar systems when observing targets in conditions of noise and relayed interference.

In this paper, by using compressed sensing techniques, a new approach to achieve robust high resolution detection in sparse multipath channels is presented. Currently used sparse reconstruction techniques are not immediately applicable in wireless channel modeling and radar signal processing. Here, we make use of the cross-ambiguity function (CAF) and transformed the reconstruction problem from time to delay-Doppler domain for efficient exploitation of the delay-Doppler diversity of the multi-path components. Simulation results quantify the performance gain and robustness obtained by this new CAF based compressed sensing approach.

Conventional digital signal processing scheme in noise radars has certain limitations related to combination of high resolution and high dynamic range. The bandwidth of radar signal defines range resolution of any radar: the wider the spectrum the better the resolution. In noise radar with conventional processing the sounding and reference signals are to be digitized at intermediate frequency band and to be processed digitally. The power spectrum bandwidth of noise signal which can be digitized with ADC depends on its sampling rate. In currently available ADCs the faster is sampling rate the smaller is its depth (number of bits) available. Depth of the ADC determines relation between the smallest and highest observable signals and thus limits its dynamic range. Actually this is the main bottleneck of high resolution Noise Radars: conventional processing doesn't enable getting high range resolution and high dynamic range. In the paper we discuss ways to go around this drawback by changing signal processing ideology in noise radar. We present results of our consideration and design of two types of high resolution Noise Radar which uses slow ADCs: noise radar with digital generation of sounding signal and analog evaluation of cross-correlation and stepped frequency noise radar. We describe main ideas of these radar schemes and results of experimental tests of the approaches.

A study of the potentials of Global Positioning System (GPS) satellites as passive radar illuminators for ground target detection is presented. GPS signals are promising due to their global coverage and the availability of multiple satellites wherever over the world. The main drawback is the low power at ground level that limits system coverage. For coherent processing, the Cross Ambiguity Function (CAF) is generated using reconstructed reference signals. Signal to noise ratio can be increased by coherent integration in the CAF domain. This approach has been studied for commercial air-planes and ships of different sizes (from ferries to fishing boats). In this paper, the detection of cars in a semi-urban environment is considered. The demonstrator IDEPAR was updated for GPS reception using commercial hemispherical GPS antennas. During the measurement campaign, a cooperative target was employed for validation purposes. Results confirmed the system capabilities for ground target detection in low range conditions. These first results are a probe of concept and motivate the design of directive GPS surveillance antennas and multistatic processing for improving the achievable coverages.

Numerous advancements have been made in planar guided-wave acoustooptics in recent years. These advancements include analytical treatment of complex interaction geometry, design and fabrication of wideband Bragg modulators and deflectors (Bragg cells), and demonstration of a number of novel applications. The continuing progress in the fabrication and performance of other components including optical waveguides, waveguide lenses, diode laser sources and photodetector arrays, and their integration has significantly advanced the prospects for realization of integrated acoustooptic (AO) device modules and circuits. The most notable examples of such modules are the hybrid integrated optic RF spectrum analyzer and correlator in LiNbO3 substrate. In this paper some of the most recent advancements on guided-wave acoustooptics in LiNbO3 channel-planar and spherical waveguides and in GaAs planar waveguides made in the author's laboratory are reported. Emphasis is placed on realization and measurement of titanium-indiffused proton-exchanged (TIPE) microlens-based integrated Bragg modulator modules in LiNbO3 channel-planar composite waveguides. These integrated AO modules have been shown to take up a small substrate dimension (up to 2.0 cm) along the optical path and are also inherently of high modularity and versatility. Some applications of the LiNbO3 channel-planar waveguide- and spherical waveguide-based device modules for wideband multichannel integrated- and fiber-optic communications, signal processing, and computing are also described.

Continuing progress in the fabrication and performance of passive and active components including optical waveguides, waveguide lenses, diode laser arrays, acousto-optic (AO) Bragg cells, electro-optic (EO) Bragg modulator arrays, and photodetector arrays, and their integration has significantly advanced the technology for the realization of integrated optical device modules and circuits. In this paper some of the most recent advances in the realization of integrated AO, EO, and AO-EO device modules in LiNbO3 substrate that are being made in the author's laboratory are reported. Emphasis is placed on the realization and measurement of titanium-indiffusion proton-exchange (TIPE) microlens-based integrated Bragg modulator modules in LiNbO3 channel-planar composite waveguides. These integrated optical modules have been shown to take up a small substrate dimension (up to 2·0 cm) along the optical path and are also, inherently, of high modularity and versatility. Some applications of such device modul...

Pulse-limited radar data taken March 1978 with the 13.9 GHz AAFE (Advanced Applications Flight Experiment) altimeter from 1500-m altitude over ice in the Beaufort Sea are registered to high-quality photography. The variations of the radar return waveform shape and signal level are correlated with the variation of the ice type determined from photography. The AAFE altimeter has demonstrated that the return waveform shape and signal level of an airborne pulse-limited altimeter at 13.9 GHz respond to sea ice type. The signal level responded dramatically to even a very small fracture in the ice as long as it occurred directly at the altimeter nadir point. Shear zones and regions of significant compression ridging consistently produced low signal levels. The return waveforms frequently evidenced the characteristics of both specular and diffuse scattering. >