Analysis of a Signal Processor for an Antenna Array

A signal processor that provides ratio-squared predetection combining, has been investigated for application in an adaptive antenna array. The analysis and experimental data presented here pertain to the signal processing gain when the antenna array is illuminated by a coherent signal source and a partially coherent noise source. For a noise source which is coherent, the processing gain depends on relative strength of the signal and noise, relative directions of arrival, and the usual "array factor." The array exhibits capturing effects much as in an FM receiver. The effective antenna pattern is a superposition of two beams of different magnitudes, one directed to the signal source and the other to the noise source. When the noise is partially coherent, the behavior of the signal processor is quite complex. Analytical prediction and experimental simulation measurement on a four-channel system indicate that the partially coherent noise may be regarded as the source of an incoherent noise component plus a coherent noise component with the magnitude of the latter determined by the coherence coefficient for the noise source.     

An Optical Technique for Simultaneous Beamforming and Cross-Correlation

This paper presents a method of greatly simplifying the processing of received signals from antenna arrays through the use of a coherent optical system for signal processing. It is shown that a coherent optical system is ideally suited for carrying out beamforming operations. Several other advantages of coherent optics for this application are also discussed. A major result is a technique for forming several unambiguous beams simultaneously by correlating the signals from two linear arrays. The coherent optics technique permits this operation to be carried out with extreme simplicity.     

Frequency-Agile Radar Signal Processing

Modern radars may incorporate pulse-to-pulse carrier frequency modulation to increase probability of detection, to reduce Vulnerability to jamming, and to reduce probability of interception. However, if coherent processing is used for clutter rejection, the frequency of N consecutive pulses must be held constant for N-pulse clutter cancellation or Doppler filtering. If M pulses are transmitted during the time the antenna illuminates a target, there are M/N coherently integrated echoes available for noncoherent integration in the computer or the operator's display to further improve the signal-to-noise ratio (SNR). In this paper, analytical and simulation methods are employed to determine the balance between coherent and noncoherent integration that yields the greatest SNR improvement. Attention is focused upon a model using peak selection of fast Fourier transform (FFT) Doppler channels and is compared to a reference model involving only a single Doppler channel. Curves of detectable SNR as a function of M and N are presented for both models.     

ETS-II Experiments Part II: Earth Station Facilities

A part of the Earth station at the Kashima Branch of the Radio Research Laboratories has a highly sensitive receiving system newly designed for receiving 3 coherent and weak beacon signals transmitted from the Engineering Test Satellite Type-II (ETS-II) of Japan and obtaining propagation data on quasimillimeter and millimeter wavelengths. The ground system includes a main receiving station which has a 10-m diameter antenna for multifrequencies, highly sensitive receivers, a rain radar, which has many unique functions, a radiometer, meteorological instruments, and data processing computers. The facilities of the main receiving station for ETS-II propagation experiments, except for the rain radar, are described.     

Hard Limiting in Synthetic Aperture Signal Processing

In synthetic aperture radar a large linear phased array is formed from the rapid movement of a single element through each position in the array. Storage and coherent combining of the successive radar echoes are central to the array-forming process. Optical processing is the most common technique because of the efficiency with which Fourier transformation may be accomplished with simple optics. Real-time operation, however, requires all-electronic processing, which is difficult to accomplish because of the huge quantity of data to be manipulated. Dynamic range compression by hard limiting may ease the problem by reducing the number of bits per frame. The effects of hard limiting are analyzed in this paper. It is shown that large targets simultaneously illuminated by the radar antenna will produce image targets or ghosts displaced in angle. Statistically homogeneous clutter will "linearize" the hard-limited receiver and suppress the ghosts without loss in contrast, as does thermal noise if it is larger than the target echoes. Pulse compression reduces the probability of images from prominent targets. Judicious choice of the pulse-compression waveform is a powerful tool for destroying coherent buildup of images from all large targets not in the same range resolution cell. Linear FM, the most common choice, unfortunately does not exhibit this desirable property.     

Robust adaptive beamforming for HF surface wave over-the-horizon radar

Adaptive beamforming is used to enhance the detection of target echoes received by high frequency (HF) surface wave (HFSW) over-the-horizon (OTH) radars in the presence of spatially structured interference. External interference from natural and man-made sources typically masks the entire range-Doppler search space and is characterized by a spatial covariance matrix that is time-varying or nonstationary over the coherent processing interval (CPI). Adaptive beamformers that update the spatial filtering weight vector within the CPI are likely to suppress such interference most effectively, but the intra-CPI antenna pattern fluctuations result in temporal decorrelation of the clutter which severely degrades subclutter visibility after Doppler processing. A robust adaptive beamformer that effectively suppresses spatially nonstationary interference without degrading subclutter visibility is proposed here. The proposed algorithm is computationally efficient and suitable for practical implementation. Its operational performance is evaluated using experimental data recorded by the Iluka HFSW OTH radar, located near Darwin in far north Australia.     

Optical Processing Techniques for Simultaneous Pulse Compression and Beam Sharpening

A technique is described in which the separate techniques of beam sharpening (by synthetic antenna methods) and pulse compression are converted into a single two-dimensional operation, which is carried out with a coherent optical processor.     

Results from a Four-Element Adaptive Antenna Experiment

Experimental results from a four-element, linear, half-wavelength spacing, adaptive-array antenna under the control of the least mean square (LMS) algorithm are presented. The array is found to be capable of nulling a 70-MHz signal to -35 dB below a desired signal over a 5-MHz bandwidth. The antenna processing gain is constant over a desired signal-to-jammer signal power ratio range from -20 dB to 5 dB. A sharp reduction in processing gain is observed for angular separations between jammer and desired signal of less than 10°. Antenna patterns taken with weights set in 300 iterations of the LMS algorithm show that the one strong, one weak jammer combination has a longer weight convergence time and reduced processing gain compared with a two strong jammers combination. Contours of constant desired signal-to-jammer signal power ratio, after adaptive antenna processing, reveal a complex shape for communication between air and ground due to the finite angular resolution of the adaptive antenna.     

Walker Model for Radar Sensing of Rigid Target Fields

The objective of this primarily tutorial item is to describe a general model for the observable data and the appropriate data processing involved in sensing rigid target fields with coherent radars. Any number of radars may be involved, and the scene and each radar may be in any kind of motion-with no restrictions on motion through resolution cells during the coherent processing time of the radars. The motions are assumed to be known. To some extent motion parameters can be estimated from the radar data, e.g., by adaptive parameter adjustments in the data processing; however, this subject is beyond the scope of this discussion. In large measure, the analysis in this item highlights the central conceptual result obtained by J.L. Walker as described in [1] -a major work in radar theory.     

WARLOC: A High-Power Coherent 94 GHz Radar

This paper describes the development of a high-power, coherent radar system at W-band and discusses potential applications of radars with this new capability. Previous radars in this frequency band were limited by available power-amplifier technology to about 500 W of average power; WARLOC radar represents an increase in power, by 20 times, over previous coherent radars at 94 GHz. This performance improvement is possible due to the development of a gyroklystron amplifier specifically for this and future radars in this frequency band. The gyroklystron amplifier tubes deliver 100 kW peak power and 10 kW of average power at a center frequency of approximately 94 GHz. Other novel features of this radar include the use of highly overmoded waveguides and rotary joints for the transmission of power from the final power amplifier (FPA) to the antenna, and a high-power quasi-optical duplexer. The system uses a relatively large 1.8 m diameter (580-wavelength) Cassegrain antenna, which required the development of an antenna with an rms surface accuracy of 0.0025 in, to obtain long-range detection and identification of small objects. Test data show an antenna gain of 62.5 dB, confirming that the needed surface accuracy was achieved. Two mobile shelters house the radar system, permitting relocation to various test sites. WARLOC is presently operational at the Naval Research Laboratory's Chesapeake Bay Detachment facility, Maryland. It is being employed in radar imaging of airborne and surface objects, and in the scientific study of propagation effects and atmospheric physics phenomena.     

Pulse Compression in Optical Radar

An analysis of the application of pulse-compression techniques in optical radar systems is presented. The particular case of pseudorandom on-off amplitude coding is chosen for ease of analysis. The roles of the siqnal and photodetector properties and the processing method are examined. The qualitative relationship between the concepts of coherent and incoherent integration in microwave radar and linear and nonlinear processing in optical radar is demonstrated, thus validating the application of pulse-compression techniques with optical signals. Finally, an experimental simulation of an optical rangefinder has been constructed to illustrate various findings of the analysis.     

Bandpass Signal Sampling and Coherent Detection

Coherent detectors in radar and communications receivers are generally implemented in the form of two parallel baseband channels which form in-phase (I) and quadrature (Q) components of a received RF/IF signal. Phase errors of several degrees due to imperfect matching of these separate channels limit the performance achievable from signal processors such as moving target indicators (MTI), coherent integrators, Doppler filters, antenna array processors, and coherent sidelobe cancellers. Thus methods in which a single analog to digital (A/D) converter samples and digitizes the IF signal directly, eliminating the need for IF to baseband conversion, have been of recent interest and are the subject of this paper. To obtain accurate coherent detection from IF samples taken near the Nyquist rate requires interpolation based upon a number of stored samples. An algorithm derived from sampling theory is defined and used to demonstrate accurate reconstruction of the original IF signal from digitized samples. In-phase and quadrature components of the signal are shown to be available from processed samples with demonstrated phase errors less than 0.2°.     

Doppler Return from a Random Rough Surface

A theoretical analysis of the Doppler return from a random rough surface ace shows that the Doppler spectrum is composed of three distinct components. The methods of analysis for determining the average power return for the static case and an application of the ergodic hypothesis for a stationary surface show that the assumption of a single Doppler component is based on a smooth surface. In addition to this coherent component, the incoherent components of the random rough surface produce two additional frequency components. The amplitudes of these latter components depend on the variance of the surfaceand the antenna beamwidth averaging. Radar measurements were made, utilizing a scatterometer, in order to measure the surface characteristics. s. Simultaneous measurements from a Doppler radar system were analysed to identify the additional frequency components.     

Spectrum Distortion Due to Frequency Instabilities in a Two-Way Coherent Doppler Radar

The power spectral density of the intermediate frequency signal in a coherent Doppler navigation radar is derived. The effects of antenna parameters, periodic frequency instabilities, signal two-way transit time, and transmitter frequency modulation noise are considered Several examples based on the measured frequency modulation noise of a solid-state source transmitter are presented. The results indicate the degree of loss in signal-to-noise ratio, and spectrum broadening due to an increase in signal transit time and/or frequency modulation noise.     

Direction finding with a four-element Adcock-Butler matrix antennaarray

The conventional analog Adcock-Butler matrix (ABM) antenna array direction finder suffers from systemic errors, component matching problems, and bandwidth limitations. Three digital bearing estimators are developed as candidates to replace the analog signal processing portion of the ABM. Using the same antenna array, they perform all signal processing in the frequency domain, thereby benefitting from the computational efficiency of the fast Fourier transform (FFT) algorithm. The first estimator requires two analog-to-digital converters (A-D) and three antenna elements. It multiplies the difference between the discrete Fourier transforms (DFTs) of the output signals from two antenna elements with that from a third antenna element. At each frequency component, the phase of this product is a function of the bearing. A weighted least squares (LS) fit through all the phase components then gives a bearing estimate. The second estimator is similar to the first but uses three A-D and all four antenna elements. The output signal from the additional antenna element provides an independent estimate of the weights for the LS fit, giving an improvement in accuracy. The third estimator applies the physical constraint existing between the time-difference-of-arrival (TDOA) of a signal intercepted by two perpendicular sets of antenna elements. This yields a better estimator than simple averaging of the bearing from each set of antenna elements. The simulation studies used sinusoids and broadband signals to corroborate the theoretical treatment and demonstrate the accuracy achievable with these estimators. All three direction finders have superior performance in comparison with the analog ABM     

A Generalized Clutter Computation Procedure for Airborne Pulse Doppler Radars

A general procedure for analyzing ground clutter effects in airborne pulse Doppler radars is described. The quantity computed is the expected clutter power at the output of any specified range gate/ Doppler filter processing cell. The procedure has been computerized and is quite general with respect to antenna gain pattern, clutter cross section variation, PRF, pulse and range gate shapes, and the various receiver processing functions. It is applicable only to distributed ground clutter and linear processing, and excludes the dynamic effects of continuous antenna scanning. To exemplify the use of the procedure, two studies conducted for a postulated high PRF radar are described, and the results are presented.     

An Experimental Radar Facility

A coherent CW superheterodyne radar system operating at frequencies of 9, 17, 35, and 70 GHz is described. The radars are installed on a free-flight range to study backscattering from wakes of hypersonic-velocity projectiles. Each radar is equipped with a focused-lens antenna oriented at an angle of approximately 45° to the flight axis. Amplitude and phase of the received signal are recorded separately. Some typical results are given to demonstrate the capabilities of the equipment.     

利用旋转双天线载波相位双差的欺骗干扰检测技术

有效的欺骗干扰检测是防止卫星导航接收机被欺骗干扰攻击的前提。提出了一种基于旋转双天线载波相位双差的卫星导航接收机欺骗干扰检测技术,在对接收机双天线匀速旋转时输出载波相位测量值进行载波相位双差处理后,利用广义似然比检验实现了对单一发射天线输出欺骗干扰信号的检测。进一步分析了旋转半径和数据长度对检测性能的影响,并与旋转单天线和天线阵载波相位双差欺骗干扰检测方法的性能进行了对比。最后,通过蒙特卡罗方法进行了仿真验证,结果表明了该检测方法和检测性能分析的正确性。     

Adaptive processing at the subarray level

This paper describes a class of partially adaptive arrays with adaptive processing applied to the outputs of steered subarrays. The problem is to detect a signal or estimate its direction of arrival in the presence of jammers. The advantage of applying adaptive processing to subarrays is that it requires much less CPU time than the corresponding fully adaptive processing. The subarray processing equations for the two kinds of problem are described. In this paper, we compare partially adaptive processing performance with fully adaptive processing performance in the case of the following antenna and signal sources:

• •- array with regularly spaced sensors ;
• •- between 20 and 100 elements ;
• •- between 3 and 20 subarrays ;
• •- a single jammer ;
• •- desired signal from the antenna zenith.

We suggest a method for determining the optimal subarray configurations in this case. An example is given to show that the performance of an antenna with five subarrays is comparable to that of a fully adaptive thirty-element array for eliminating a single jammer with a target at the zenith.     

Array antennas for DGPS

Multipath interference limits the speed and accuracy of determining position by “differential” GPS techniques. A geodetic surveyor, for example, requires multipath interference rejection of about 36+20 log₁₀ sin ϵ dB, where ϵ is the elevation angle of the satellite being observed. Signal processing in a GPS receiver cannot satisfy this requirement. A receiving antenna is required that can sufficiently reject signals arriving from below the horizon. Available antennas have inadequate rejection, and brute-force methods of improving them, by enlarging their ground-planes, are impractical. A compact, ground-planeless, dual-band, GPS antenna with improved multipath rejection has been designed and field-tested. This antenna resembles a vertical post rather than a horizontal platter; within its 0.1-m diameter, 0.4-m tall radome is a vertical array of turnstile elements. In field tests, a three-element array antenna rejected multipath better than a 0.5-m diameter ground-plane antenna by an average of 5 dB. A five-element array antenna appears superior to a 0.9-m diameter ground-plane antenna