Costas arrays with small number of cross-coincidences

A simple method is described for obtaining two or more Costas arrays with a maximum of one mutual bit or cross-coincidence, assuming that the vertical components of their relative shifts are appropriately bounded. The result can be useful for designing multiple Costas waveforms with small cross-ambiguities, if the potential Doppler shifts are restricted to a small portion of the waveform bandwidth     

Cross-correlation properties of algebraically constructed Costasarrays

The problem of determining the cross-correlation properties of signals based on algebraically constructed Costas arrays is addressed by examining the discrete cross-correlation of the algebraically constructed Costas arrays for a given construction and dimension. Finding two arrays that minimally correlate implies that the signals based on these arrays also minimally correlate. The properties of finite fields are reviewed, and the major algebraic constructions for Costas arrays are presented, i.e. the Welch construction and the Golomb construction. The discrete cross-correlation properties of the Costas arrays are derived for arrays of the same dimension derived from the same construction. The use of Costas arrays in the signal design problem is discussed, and examples are given to show the cross-correlation of the signals based on the algebraically constructed arrays     

On cross-ambiguity properties of Welch-Costas arrays

We discuss cross-ambiguity properties of a specific family of Costas arrays called Welch-Costas (W-C) arrays. These properties are of interest in multiuser radar and sonar system, especially since Costas arrays are known to possess ideal auto-ambiguity functions. The theory of W-C arrays is reviewed. It is then proved that only pairs of W-C arrays can have at most two hits in their cross-ambiguity function (best possible case). The maximum number of hits in the cross-ambiguity functions of a family of W-C arrays is shown to be a function of the number of W-C arrays in the family. The upper bound on the number of hits in the cross ambiguity functions for a family of W-C arrays is also derived. Specific examples of how reducing the number of W-C arrays improves the cross-ambiguity properties are given for various types of prime numbers     

Ambiguity properties of quadratic congruential coding

The ambiguity characteristics of multiple access frequency hop codes based on standard quadratic congruences are investigated in the light of results obtained for codes based on Costas arrays and extended quadratic congruences. While the autoambiguity properties are found to be very similar to those of Costa codes, i.e. nearly ideal, the cross-ambiguity properties of quadratic congruential codes are much better. These results are valid across the whole class of code sets considered, but they are obtained at some expense in the pulse compression characteristics of the codes. A uniform upper bound is placed on the entire cross-ambiguity function surface, and bounds are placed on the amplitude of spurious peaks in the autoambiguity function. These bounds depend on the time-bandwidth product and code length exclusively and lead naturally to a discussion of the design tradeoffs for these two parameters. Examples of typical autoambiguity and cross-ambiguity functions are given to illustrate the performance of quadratic congruential coding with respect to Costas coding     

Family of multicarrier bi-phase radar signals represented by ternary arrays

A K times L ternary array, comprised of the elements {0, 1, - 1}, with some unique features, represents a multicarrier radar signal with favorable autocorrelation and ambiguity functions. Constructing such an array using Galois fields is described. As in a Costas binary array, only one frequency is transmitted at any time slot, but in our array the same frequency is repeated in several time slots, yielding a signal with considerably larger pulse compression than a Costas signal that uses the same number of frequencies     

Digital and Analog Subcomplementary Sequences for Pulse Compression

Golay's complementary pairing has been a method to increase the utility of binary sequences, because of the temporal sidelobe suppression in the autocorrelation vector summation. Complementary sets of Tseng and Liu and of Hollis exhibit the same effect when several autocorrelations are combined. These complementary pairs and sets of sequences can be extended into long complementary chains by a simple transformation. This transformation is extended here to all pulse compression waveforms. By this method, even though analog complementary sequences cannot be formed, a new class of waveforms, called subcomplementary waveforms, can be formed. Following these rules, repetition of waveforms such as linear frequency modulation (LFM) or linear stepped frequency modulation (LSFM) in a prescribed manner is possible without creating autocorrelation grating lobes or repetitive sidelobes. This method is equally applicable to all analog or digital pulse compression waveforms.     

Radar Waveforms for Suppression of Extended Clutter

A common problem in waveform design is to adapt the transmitted signal to the target environment in order that the interference from extended fields of scatterers is reduced. This problem is investigated here for the special case of detection of a single target in the ``vicinity' of an extended clutter space. The paper considers the possibility of confining the matched-filter response in delay and Doppler, or ambiguity function, to a narrow strip with arbitrary orientation in the delay-Doppler plane. It is shown that strict confinement of the response is achievable only with waveforms that are unlimited in both time and frequency domain. With practical waveforms, which are necessarily of finite extent, one merely can trade close-target separability against detectability in the background clutter. Thus, one form of the resolution problem is exchanged against the other. The paper examines these effects quantitatively.     

Limiter-Discriminator Detection Performance of Manchester and NRZ Coded FSK

The theory of limiter-discriminator detection of a frequency shift keyed (FSK) carrier is reviewed and this theory is used to predict the bit error probability performance of Manchester coded and nonreturn-to-zero (NRZ) coded data. A major result of the study is that the predetection time bandwidth product BT and the devia tion ratio h needed to give optimum performance for Manchester coding are seen to be larger than optimum NRZ FSK. Specifically, BT ~ 2 and h ~ 1 will result in Manchester performance about 2 dB worse than optimum NRZ.     

The hit array: an analysis formalism for multiple access frequencyhop coding

A formalism is presented for the analysis of general frequency hop waveforms, such as those suitable for use in coherent active radar and sonar echolocation systems as well as multiple-access spread-spectrum communications. This formalism is based on the concept of coincidence, or `hit', between two frequency hopping patterns. The collection of all possible hits, together with their locations, is recorded in time-frequency space, which produces the high array associated with the two patterns considered. If the code length is sufficiently small with respect to the time-bandwidth product chosen, the hit array can be viewed as a digital representation of the corresponding ambiguity function. Salient properties of the hit array formalism are derived, including simple relationships between hit arrays resulting from basic symmetry-preserving transformations. These properties make it possible to predict the performance of a given set of frequency hop waveforms directly from the associated set of frequency hopping patterns     

Pseudorandom Frequency Modulation in Range-Doppler Radar

In many radar systems, efficient use of transmitter power requires the transmission of a constant-amplitude signal for a substantial fraction of time; for a monotonic transmission, however, the range resolution is restricted by the length of the transmitted pulse. Linear frequency modulation removes this constraint for targets with negligible, or known, radial velocities; it is not suitable, however, for simultaneous observations of range and radial velocity (Doppler shift). This paper describes a class of waveforms suitable for simultaneous measurement of range and Doppler shift. These waveforms are characterized by a uniform distribution in frequency and by pseudorandom frequency changes. Uniform frequency distribution is attained by a uniform spacing of frequencies with each frequency present for an identical length of time. Frequency changes are effected by sequencing the frequencies with a pseudorandom number generator. Ambiguity functions are computed for pseudorandom frequencymodulated waveforms designed for ionospheric backscatter studies. By suitable choice of parameters, the ambiguity function becomes a narrow central peak surrounded by a plateau whose height varies randomly between zero and approximately twice its average. Waveform generation by means of a digital frequency synthesizer and data reconstruction considerations are described.     

Waveform fusion in sonar signal processing

In active sonar systems, proper selection of the transmitted waveform is critical for target detection and parameter estimation, especially with the existence of clutter (reverberation). Two commonly used waveforms (constant frequency (CF) and linear frequency modulated (LFM)) are studied. Their characteristics are complementary both with respect to their accuracies and their sensitivity to the blind zero-Doppler ridge. Several fusion schemes of the two kinds of waveforms are explored and fusion results are studied both analytically and from simulation. It is concluded that fusion of the information of different waveforms can be not only more robust, but in some cases outright preferable, in term of detection probability and estimation accuracy.     

THE WIDE-BAND PLASMA WAVE INVESTIGATION

As part of the Cluster Wave Experiment Consortium (WEC), the Wide-Band (WBD) Plasma Wave investigation is designed to provide high-resolution measurements of both electric and magnetic fields in selected frequency bands from 25 Hz to 577 kHz. Continuous waveforms are digitised and transmitted in either a 220 kbit s^-1 real-time mode or a 73 kbit s^-1 recorded mode. The real-time data are received directly by a NASA Deep-Space Network (DSN) receiving station, and the recorded data are stored in the spacecraft solid-state recorder for later playback. In both cases the waveforms are Fourier transformed on the ground to provide high-resolution frequency-time spectrograms. The WBD measurements complement those of the other WEC instruments and also provide a unique new capability for performing very-long-baseline interferometry (VLBI) measurements.     

基于自适应滤波的PCM／FSK软件解调方法

提出了一种新的PCM／FSK数字信号解调方法。通过采用最小均方误差自适应滤波算法，跟踪信号的单一频率，分析输出信号的包络，实现PCM／FSK信号的解调。详细介绍了此种解调方法的基本原理及实现过程，分析了影响解调性能的因素。计算机仿真结果表明，此方法比差分解调性能优越，抗干扰能力强，便于软件编程及DSP器件实现，具有一定的应用价值。     

False-Lock Detection in Costas Demodulators

False lock occurs in Costas demodulators with sweep frequency acquisition because sideband energy is detected in the data channel. It is shown that utilizing the data and error channels to make the lock decision will prevent false lock from occurring.     

Sequences and arrays with perfect periodic correlation

Properties and methods for synthesizing sequences with perfect periodic autocorrelation functions and good energy efficiency are discussed. The construction is extended to two-dimensional perfect arrays. The construction methods used are based mainly on a search in the frequency domain and on a multiplication theorem for periodic sequences and arrays     

Rapid Aquisition Signal Design in a Multiple-Access Environment

Acquisition in a communication receiver is the operation of determining the arrival time of a transmitted periodic timing marker. This is generally accomplished by sending a known acquisition waveform to aid in the arrival time measurement. Certain classes of waveforms are called rapid acquisition waveforms and are advantageous for reducing the time to acquire when sequential methods are used. A multiple-access acquisition environment occurs when a multiplicity of transmitter-receivers are attempting to each individually perform an acquisition operation, each pair interfering with all others. In this paper, a study is made of the interference effects when rapid acquisition techniques are used in a multiple-access environment. The prime object is to determine the manner in which the actual acquisition waveform structure effects the overall system performance. When interference variance is used as a criterion, it is shown that rapid acquisition waveforms made up of component waveforms having disjoint, flat, wideband spectra produce minimal interference. The result suggests the use of coded tone acquisition waveforms. The results have primary application to ranging, surveillance, or navigation operations performed in a many-user situation.     

Computer Simulation of VLF Power Amplifiers under Reactive Loading

Recent efforts have been made to achieve substantially increased baud rates with the U. S. Navy's very low frequency (VLF) communication system. Because of the constraints imposed by existing transmitter and antenna structures it was necessary to reexamine the operation of every major system component. Each particular VLF transmitter was subjected to an extensive theoretical analysis followed by a detailed on-site measurement program. Following a review of three major systems, Cutler, Lualualei, and North West Cape, a sufficient body of data was available to substantiate the validity of the computer-simulated transmitter analysis presented here. With this method it is possibe to predict frequency-shift keying (FSK) and minimum shift keying (MSK) performance limitations for particular VLF transmitting systems based primarily on the 0 of the antenna at the operating frequency.     

The Galileo Plasma wave investigation

The purpose of the Galileo plasma wave investigation is to study plasma waves and radio emissions in the magnetosphere of Jupiter. The plasma wave instrument uses an electric dipole antenna to detect electric fields, and two search coil magnetic antennas to detect magnetic fields. The frequency range covered is 5 Hz to 5.6 MHz for electric fields and 5 Hz to 160 kHz for magnetic fields. Low time-resolution survey spectrums are provided by three on-board spectrum analyzers. In the normal mode of operation the frequency resolution is about 10%, and the time resolution for a complete set of electric and magnetic field measurements is 37.33 s. High time-resolution spectrums are provided by a wideband receiver. The wideband receiver provides waveform measurements over bandwidths of 1, 10, and 80 kHz. These measurements can be either transmitted to the ground in real time, or stored on the spacecraft tape recorder. On the ground the waveforms are Fourier transformed and displayed as frequency-time spectrogams. Compared to previous measurements at Jupiter this instrument has several new capabilities. These new capabilities include (1) both electric and magnetic field measurements to distinguish electrostatic and electromagnetic waves, (2) direction finding measurements to determine source locations, and (3) increased bandwidth for the wideband measurements.Deceased     

FOPEN SAR imaging using UWB step-frequency and random noisewaveforms

The detection and identification of targets obscured by foliage have been topics of great interest. Several synthetic aperture radar (SAR) experiments have demonstrated promising images of terrain and man-made objects obscured by dense foliage, by using either linear frequency modulation (LFM) or step-frequency waveforms. We present here the methodology and results of a comparative study on foliage penetration (FOPEN) SAR imaging using ultrawideband (UWB) step-frequency and random noise waveforms. A statistical-physical foliage transmission model is developed for simulation applications. The foliage obscuring pattern is analyzed by means of the technique of paired echoes. The results of the comparative study demonstrates the ability of a UHF band UWB random noise radar to be used as a FOPEN SAR. Advantages of the random noise radar system include covert detection and immunity to radio frequency interference (RFI)     

Staggered Costas Signals

A radar signal, based on coherent processing of a train of staggered Costas bursts, is suggested and investigated. The selection of sequences of each burst is based on a minimum number of collocation of their individual ambiguity function sidelobe peaks. The resulting ambiguity function combines qualities of both "thumbtack" and "bed of nails" signals. Comparison with linear-FM, V-FM, and complementary phase coded (CPC) signals is given, as well as comparison with hybrid signals consisting of both phase and frequency coding.