essary integration and threshold estab- lishment functions. Another possible technique would make use of phase orthogonal IF com- ponents-one embodying the input signal-plus-noise and the other a wide- band, locally generated, very stable calibration noise. Just prior to detection, the orthogonal components are phase separated into different detectors. The detector handling the signal-plus-noise component is fed to the integrator and threshold comparator circuits as usual, while the detected calibration noise is used to vary the threshold setting as a function of IF gain changes. Although this technique does not suffer from the 50 percent signal loss of the switched (Dicke) receiver, it still does not account for external, or non- common circuit internal, noise temperature, or gain variations. Maintaining a good quadrature relationship over a very wide IF band may also be quite difficult. The best approach to threshold determination is based solely on the received signal and noise itself. It must be remembered that, for the general LPI scenario, the communicator's signal is present for only a very short time interval relative to the total intercept search period. Thus, most of the time, a noise-only condition exists against wich adaptive statistical measurements may be made to maintain a constant PEA as the statistics slowly change.9 Since the specified PFA is small (e.g., 10-6), it obviously cannot be measured "Slowly" is defined such that the PFA, if uncorrected, would change no more than a factor of 2 upward or a factor of 1/2 downward from one decision period to the next. Low Probability of Intercept Communications directly on a short-term basis. However, measurement of the higher order statistical parameters by means of moments or extreme methods, coupled with appropriate predictive formulas, leads to more accurate estimates of PEA. Further, if this is done using reasonably wideband samples, the results can be made virtually insensitive to the presence of signal (whenver such occurs). The result is that the 