Systems for bistatic monitoring of ocean processes differ radically from monostatic systems searching for targets. The paper discusses basic principles,showing how these effect signal design, signal processing goals and signal processing techniques. In addition, computer advances have a strong influence in changing signal processing choices, sometimes rendering yesterday's optimums today's curiosities, and perhaps converting today's impossibilities into tomorrow's challenges.

A comprehensive study of the performance of the optimal adaptive beamformer in the M-sequence receiving system is presented. Two receiving system arrangements were considered: (1) adaptive beamformer precedes the matched filtering (APM), and (2) matched filtering precedes the adaptive beamformer (MPA). The optimal adaptive beamformer was constructed through the Bayesian approach. Theoretical and Monte Carlo simulation analysis showed that at high array signal-to-noise ratios, since the adaptive beamformer could converge to the steady state, the order of processing did not make a difference in the overall system performance. However, the convergence of the adaptive beamformer at low array signal-to-noise ratios was affected by the change in signal environment due to the change of the order of processing. Simulation showed that the performance of the MPA system had a lower threshold in the array signal-to-noise ratio.>

Quadrature-quadrature phase-shift keying (Q/sup 2/PSK) is a spectrally efficient modulation scheme which utilizes available signal space dimensions in a more efficient way than two-dimensional schemes such as QPSK and MSK (minimum-shift keying). It uses two date shaping pulses and two carriers, which are pairwise quadrature in phase, to create a four-dimensional signal space and increases the transmission rate by a factor of two over QPSK and MSK. However, the bit error rate performance depends on the choice of pulse pair. With simple sinusoidal and cosinusoidal data pulses, the E/sub b//N/sub 0/ requirement for P/sub b/(E)=10/sup -5/ is approximately 1.6 dB higher than that of MSK. Without additional constraints, Q/sup 2/PSK does not maintain a constant envelope, however, a simple block coding can provide a constant envelope. This coded signal substantially outperforms MSK and TFM (time-frequency multiplexing) in bandwidth efficiency. Like MSK, Q/sup 2/PSK also has self-clocking and self-synchronizing ability. An optimum class of pulse shapes for use in Q/sup 2/PSK format is presented. One suboptimum realization achieves the Nyquist rate of 2 b/s/Hz using binary detection.>

The doubly-compound hypothesis detection problem with finite-dimensional parameter vectors is treated in a general context. It is shown that estimation and detection occur simultaneously, with the detector using the a posteriori densities generated by two separate estimators, one for each hypothesis. No assumptions are made on the estimation criterion and very loose assumptions on the detection cfiterion. If sufficient statistics and hence natural conjugate densities exist for the unknown parameters, the procedure is quite tractable. In this case, the optimal detector partitions in such a way that the primary processing can be done without knowledge of the a priori parameter distributions.

Observation-decision procedures which associate a cost with observing are generally sequential in nature in order to reduce the observation length. Presented in this paper is the optimum nonsequential procedure in the sense of minimum expected loss for a terminal decision. The procedure is presented in generality and evaluated for normal observation statistics. Two numerical examples are presented for normal observation statistics.

A general theory of signal detectability, constructed after the model provided by decision theory, is applied to the performance of the human observer faced with the problem of choosing among multiple signal alternatives on the basis of a fixed, finite observation interval. An extension of the theory, previously reported, of deciding among two alternatives, is developed in detail, permitting the treatment of the two simple cases involving multiple alternatives that are studied experimentally. In both cases, a priori probabilities are assigned to the occurrence of the relevant signal alternatives, and values are assigned to the possible decision outcomes. This assignment permits definition of the expected value of a decision, and specifies as the optimum decision criteria those that maximize expected value. The experiments are primarily concerned with determining the ability of the human observer to successively establish optimum decision criteria in accordance with changes in the a priori probabilities and risk functions. The experimental results are portrayed in the form of comparisons of the payoff obtained by the observer and the theoretically maximum payoff attainable, and comparisons of the response-frequency tables generated by the theory and by the observer. The results indicate that a highly simplified theory is adequate for prediction of the obtained payoff and response-frequency tables to within a few percent. They also indicate the fairly large extent to which intelligence may influence a sensory process usually assumed to involve fixed parameters.

The problem of signal detectability treated in this paper is the following: Suppose an observer is given a voltage varying with time during a prescribed observation interval and is asked to decide whether its source is noise or is signal plus noise. What method should the observer use to make this decision, and what receiver is a realization of that method? After giving a discussion of theoretical aspects of this problem, the paper presents specific derivations of the optimum receiver for a number of cases of practical interest. The receiver whose output is the value of the likelihood ratio of the input voltage over the observation interval is the answer to the second question no matter which of the various optimum methods current in the literature is employed including the Neyman - Pearson observer, Siegert's ideal observer, and Woodward and Davies' "observer." An optimum observer required to give a yes or no answer simply chooses an operating level and concludes that the receiver input arose from signal plus noise only when this level is exceeded by the output of his likelihood ratio receiver. Associated with each such operating level are conditional probabilities that the answer is a false alarm and the conditional probability of detection. Graphs of these quantities called receiver operating characteristic, or ROC, curves are convenient for evaluating a receiver. If the detection problem is changed by varying, for example, the signal power, then a family of ROC curves is generated. Such things as betting curves can easily be obtained from such a family. The operating level to be used in a particular situation must be chosen by the observer. His choice will depend on such factors as the permissable false alarm rate, a priori probabilities, and relative importance of errors. With these theoretical aspects serving as an introduction, attention is devoted to the derivation of explicit formulas for likelihood ratio, and for probability of detection and probability of false alarm, for a number of particular cases. Stationary, band-limited, white Gaussian noise is assumed. The seven special cases which are presented were chosen from the simplest problems in signal detection which closely represent practical situations. Two of the cases form a basis for the best available approximation to the important problem of finding probability of detection when the starting time of the signal, signal frequency, or both, are unknown. Furthermore, in these two cases uncertainty in the signal can be varied, and a quantitative relationship between uncertainty and ability to detect signals is presented for these two rather general cases. The variety of examples presented should serve to suggest methods for attacking other simple signal detection problems and to give insight into problems too complicated to allow a direct solution.