As part of its evaluation of loran as a potential backup to GPS, the FAA has contracted Rock-well Collins to build a combined GPS/loran variant of its standard multimode navigation and landing receiver. The unit’s primary function will be to provide GPS navigation, with automatic switchover to loran should GPS signals be lost or degraded, and automatic reversion to GPS when normal service resumes. Flight tests are scheduled for late summer.

The concept reflects the concerns about GPS interference and jamming expressed in a report published last September by the DOT’s Volpe Technical Center. While less accurate than GPS, loran is being considered as a backup candidate because of its continental U.S.-wide coverage from ground level to above jet altitudes, its independence from other systems and its resistance, for all practical purposes, to jamming.

The FAA’s loran evaluation was described in a paper jointly written by the agency, Boeing, Rockwell Collins and loran manufacturer Locus Inc., and presented at a recent NASA conference on integrated navigation, communications and surveillance (CNS). The paper, “Loran-C as a secondary navaid to complement GPS,” reviewed the developments in loran technology over the past several years, including anti-static antennas and “all-in-view” digital receivers that typically use signals from more than 30 separate loran stations to derive position, rather than the three or four used by earlier units.

Can the ‘New’ Loran Cut It?
FAA project engineer Mitchell Narins said the evaluation will assess whether the “new” loran can provide the accuracy, availability, integrity, continuity and coverage to support lateral navigation throughout all phases of flight, down to nonprecision approaches. Also, the FAA has assessed loran’s ability to provide an alternate and robust backup means of transmitting GPS WAAS corrections. In FAA test flights to Alaska’s North Slope, where conventional WAAS reception is degraded, WAAS corrections over loran were successfully received.

The evaluation will also investigate loran’s capability as the nation’s backup “Stratum 1” timing source should GPS become unavailable. Stratum 1, the highest level of timing accuracy, is used by government agencies and the telecommunications, utility, finance and other industries.

Time dissemination is one of GPS’s most widely used features– far in excess of navigation guidance–and is a critical part of the U.S.’s civil and military infrastructure. (It is also one of the main reasons why the Europeans have decided to develop their GPS-like Galileo satnav system– to avoid reliance on a timing source controlled by a foreign military power.)

Before the adoption of GPS timing, loran–which also meets Stratum 1, but at slightly lower accuracy–was a widely used government timing standard. During the FAA’s Alaska loran/WAAS tests, certain loran transmitters were temporarily shut down, much to the concern of Anchorage ARTCC officials, since the center still uses loran timing for certain functions.

The FAA is due to complete its loran evaluation by December 31.