hen the system has officially acquired and the search comes to an end. If the hit is a false alarm, then verification cannot be consummated and the search must continue. In either case, we shall assume that the verifica- tion is characterized by an extended dwell time (e.g., KT d sec; K 1) assumed to be fixed and an entering into the code tracking loop mode. Understanding that a true hit corresponds to a single code phase position and that this can occur only once per search through the code, we can regard the time interval KTd sec as the "penalty" of obtaining a false alarm, since a false alarm can occur on any code phase position. If the I & D output falls below the preset threshold, then the local PN code generator steps to its next position and the search proceeds. Thus, at each test position (aside from the single true code phase position), one of the two events can take place, each characterized by a probability of occurrence; namely, a false alarm can hap- pen, i.e., an indication that acquisition has occurred when the PN codes are actually misaligned, with probability PFA, causing a penalty of KTd sec, or no false alarm occurs with probability (1 - PFA), resulting in only a single dwell time of Td sec-hence, the Markov chain model. Furthermore, at the true SQUARE-LAW DETECTOR SAMPLE AT pzied RECEIVED SIGNAL T 1/4 d / ( ) dt THRESHOLD COMPARISON NOISE SQUARE-LAW DETECTOR LOCAL PN CODE PHASE UPDATE GENERATOR Figure 1.7. An equivalent low-pass representation of the single dwell time PN acquisition system. The Single Dwell Serial PN Acquisition System code phase position, either a correct detection can happen, i.e., an indica- tion that acquisition has occurred when the PN codes are indeed aligned, with probability PD, or no detection occurs, with probability (1 - PD). In the absence of any a priori information regarding the true code phase position, the uncertainty in misalignment between the received PN code and the local replica of it could be as much as a full code period. Thus, for long PN codes, the cor