This document is disseminated under the sponsorship of the Department of Transportation in the interest of information exchange. The United States Government assumes no liability for its contents or use thereof. 6. Abstract This report documents the scan-scan correlator (tracker) algorithm developed for Phase II of the ASR-9 Processor Augmentation Card (9-PAC) project. The improved correlation and tracking algorithms in 9-PAC Phase II decrease the incidence of false-alarm tracks and increase the detection of real aircraft. The tracker processing for 9-PAC Phase II definedin this document builds upon the prototype 9-PAC Phase II tracker describedin ATC-245. Tracker algorithms from Mode S (ATC-65) are also used in Phase II. This document describes the three main processing tasks of the tracker: initialization, input/output, and the actual correlation/tracking. The tracker itself is further broken down into four main functions: report-to-track association, report-to-track correlation, track update, and track initiation. Each of these functions is described in detail and is further broken down into sub-functions. In addition to the algorithm descriptions, the 9-PAC Ph ase II tracker system requirements are reviewed, and main data structures used in the 9-PAC Phase II tracker are defined. This report documents the Scan-Scan correlator Phase II algorithms developed for the ASR-9 Processor Augmentation Card (9-PAC) project. The 9-PAC is a processor card that serves as a processing enhancement to the existing ASR-9's post-processor system. It provides increased speed and memory capabilities to the processor, which allows for the introduction of more complex scan-scan correlator algorithms. These more complex algorithms improve the ASR-9's system performance through decreased false alarms, and increased detection of aircraft. The 9-PAC Scan-Scan Phase II correlator, also known as the Tracker, consists of three basic processing tasks: initialization, input/output, and the actual Tracker. The Tracker can be broken down further into four main processing functions: report-to-track association, report-to-track correlation, track update, and track initiation. These four 9-PAC Tracker functions are the same as in the original ASR-9 processor, but with different algorithms. Each of these functions is addressed individually in this report, and is further broken down into sub-functions for more detailed discussion.