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From your friends at  2007 - Reproduction without permission is prohibited.

Fourteen years after the FAA ordered its first five Precision Runway Monitor (PRM) systems from Raytheon Company, almost ten years after the first PRM installation was commissioned at Minneapolis-St. Paul International, PRM has finally arrived at Atlanta's Hartsfield-Jackson International (ATL) and is now officially operational after almost a year of testing.

What PRM does is as simple as the technology empowering it is complex:  It converts, at least as far as inbound pilots and passengers on final approach to a PRM-equipped airport are concerned , foul weather into flying weather by enabling independent, simultaneous approaches on closely spaced parallel runways during instrument meteorological conditions (IMC). 

In other words, PRM frees adjacent runways - like 9R/27L and 10/28 at Hartsfield-Jackson - from the FAA prohibition against simultaneous IFR (instrument flight rules) approaches on runways spaced less than 4300-feet from centerline to centerline. 

Technically, PRM is what it's developer, Raytheon, describes as a "non-rotating, non-mechanical circular phased array antenna" that is "electronically scanned to an extremely high-azimuth (1 milliradian) monopulse secondary radar system for increased airport capacity and safety." 

While accurate, the word "secondary" in that description can be misleading to the uninitiated because PRM installations are completely self-contained air traffic control systems consisting of dedicated antennas with computer-controlled electronic-sensor scanning beams, high-resolution displays with integrated alerts for such problems as an aircraft going off course or about to penetrate a no-transgression zone, and standalone tracking and monitoring electronics. 

In operation, PRM systems are very, very quick; their one-second scanning update rates five times higher than conventional radar and almost two-and-a-half times faster than FAA update-rate requirements for parallel runways spaced between 4300 and 3000 feet.  PRM systems also come in two varieties, ILS and SOIA (Simultaneous Offset Instrument Approach). 

ILS-based PRM systems, deployed where runways, such as those at Hartsfield-Jackson, are less than 4300 but more than 3000 feet from centerline to centerline, uses PRM monitoring, scanning and reporting technology in conjunction with an airports' existing ILS system and localizer.
SOIA PRM systems, which allow simultaneous IMC landings on runways with centerlines between 700 and 3000 feet, also interface with the airport's conventional ILS system but add an offset Localizer Direction Aid with Glidescope for the second runway in the pair.
Though the primary benefits of PRM in either of its forms are operational - increased runway capacity of up to 30 percent in reduced visibility conditions, decreases in late arrivals, jet fuel savings inherent in eliminating delayed landings and excessive holding patterns, and reductions in crew flying time per flight segment - PRM systems can, depending on the topography and approach patterns at specific airports, offer considerable community benefits by limiting weather-delay-caused late-night arrivals and departures and enabling continuous descent approaches in place of louder stepped-down approaches.

The recently commissioned PRM installation at Atlanta's Hartsfield-Jackson, in conjunction with the new $1.28 billion runway opened in 2006, allows Atlanta to join Chicago O'Hare, Denver, and Dallas-Fort Worth in a very elite club: U.S. airports authorized to accept simultaneous landings on three parallel runways.

With around 70 percent of Hartsfield-Jackson's 85 million annual passengers connecting to other flights, the estimated 34 percent increase in the field's adverse-weather capacity to 224-231 flights per hour (only 20-30 flights below good-weather capacity) has, as might be expected of any operational change at the world's busiest airport, implications for the entire North American air travel system. 

As FAA Administrator Marion Blakey has stated, delays at Hartsfield-Jackson "can create a national and even worldwide ripple effect moving through the entire aviation system." Likewise, eliminating delays at Hartsfield can produce an entirely more pleasant kind of ripple, a projected $5 million a week savings for airlines serving Atlanta. visitors following the live TRACON Radar action will notice that spacing between incoming aircraft will be visibly tighter when the PRM system is in use than under normal ILS operations.

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