f the input state. Once it is determined that a correct estimate was made, the lock detector thresh- old crossing inhibits further reloading of the local shift register. This regis- ter is then closed within the PN tracking loop which is responsible for maintaining code phase from that time on. At this point, several features of the RASE system are worthy of note. Clearly, since the success of this acquisition scheme depends on the ability to make credible estimates of the received PN code chips in the presence of noise, for a PN modulated carrier as an input signal, the estimation process would consist of a simple demodulation and hard-limiting detec- tion of this signal in the same way that one would demodulate and detect any PSK data stream.6 As such this scheme falls into the class requiring coherent detection and is thus of limited use in most spread-spectrum applications. Nevertheless, it warrants discussion since it does have the advantage of being a rapid acquisition technique for moderate input signal- to-noise ratios, and in fact, when compared with the simplest serial search (stepping correlator) system to be discussed next, it offers significant improvement in acquisition time even for signal-to-noise ratios down to -15 dB [4]. Despite its rapid acquisition capability, the RASE technique has the drawback of being highly vulnerable to noise and interference sig- nals. The reason for this vulnerability stems from the fact that the estima- tion process is performed on a chip-by-chip basis and as such makes no use of the interference rejection capabilities of PN signals. A modification of the RASE system, which was given the acronym RARASE (Recursion-Aided Rapid Acquisition Sequential Estimation), was reported more than a decade later by Ward and Yiu [5]. Here additional esti- mated code chips are summed to form a sync-worthiness indicator (SWI) 6In the original version of the RASE as suggested by Ward, the incoming signal was an unmod- ulated PN waveform and the code chip estimator consisted of a low-pas