The long-range radar equipment used in controlled airspace to manage traffic is the air route surveillance radar (ARSR) system. There are approximately 100 ARSR facilities to relay traffic information to radar controllers throughout the country. Some of these facilities can detect only transponder-equipped aircraft and are referred to as beacon-only sites. Each air route surveillance radar site can monitor aircraft flying within a 200-mile radius of the antenna, although some stations can monitor aircraft as far away as 600 miles through the use of remote sites.

The direction and coordination of IFR traffic in the U.S. is assigned to air route traffic control centers (ARTCCs). These centers are the authority for issuing IFR clearances and managing IFR traffic; however, they also provide services to VFR pilots. Workload permitting, controllers will provide traffic advisories and course guidance, or vectors, if requested.


Precision runway monitor (PRM) is a high-update-rate radar surveillance system that is being introduced at selected capacity-constrained U.S. airports. Certified to provide simultaneous independent approaches to closely spaced parallel runways, PRM has been operational at Minneapolis since 1997. ILS/PRM approaches are conducted at Philadelphia International Airport. Simultaneous Offset Instrument Approach (SOIA)/PRM operations are conducted at San Francisco International and Cleveland Hopkins International Airports. Since the number of PRM sites is increasing, the likelihood is increasing that you may soon be operating at an airport conducting closely spaced parallel approaches using PRM. Furthermore, St. Louis Lambert International Airport began SOIA/PRM operations in 2005, and Atlanta Hartsfield International Airport will begin PRM operations in 2006. PRM enables ATC to improve the airport arrival rate on IFR days to one that more closely approximates VFR days, which means fewer flight cancellations, less holding, and decreased diversions.

PRM not only maintains the current level of safety, but also increases it by offering air traffic controllers a much more accurate picture of the aircraft’s location on final approach. Whereas current airport surveillance radar used in a busy terminal area provides an update to the controller every 4.8 seconds, PRM updates every second, giving the controller significantly more time to react to potential aircraft separation problems. The controller also sees target trails that provide very accurate trend information. With PRM, it is immediately Figure 1-13. ARTS-III Radar Display. apparent when an aircraft starts to drift off the runway centerline and toward the non-transgression zone. PRM also predicts the aircraft track and provides aural and visual alarms when an aircraft is within 10 seconds of penetrating the non-transgression zone. The additional controller staffing that comes along with PRM is another major safety improvement. During PRM sessions, there is a separate controller monitoring each final approach course and a coordinator managing the overall situation.

PRM is an especially attractive technical solution for the airlines and business aircraft because it does not require any additional aircraft equipment, only special training and qualifications. However, all aircraft in the approach streams must be qualified to participate in PRM or the benefits are quickly lost and controller workload increases significantly. The delay-reduction benefits of PRM can only be fully realized if everyone participates. Operators that choose not to participate in PRM operations when arriving at an airport where PRM operations are underway can expect to be held until they can be accommodated without disrupting the PRM arrival streams.