Loran could be used for Alaska GPS corrections

 - May 8, 2008, 6:37 AM

At press time, technical experts from the FAA, the U.S. Coast Guard and researchers from Ohio and Stanford Universities were due to begin a two-week flight-test program in Alaska to assess the use of loran transmitters to send out GPS WAAS messages across the state.

WAAS was designed to transmit GPS integrity (such as failure, warnings and accuracy corrections) to surface and airborne GPS users from 25,000-mi-high geostationary satellites (GEOs) above the equator. Because their orbital speed is equal to the earth’s rotational speed, GEOs appear in the northern hemisphere to be fixed in the southern sky, where they perform a number of key tasks, including ground and airborne telephone relays, TV retransmission and various long-distance data-handling requirements.

However, at increasingly higher latitudes the equatorial orbits of the GEOs cause them to appear lower and lower on the southern horizon. For example, an observer at 35 deg North in the lower-48 states would see a GEO around 55 deg above the southern horizon, whereas at 80 deg North the same GEO would be just 10 deg above the horizon.

At such low elevations, the GEO can occasionally be completely blocked by intervening high ground, and even when it is completely visible its signals can become easily degraded by local interference. As a result, WAAS is unable to provide GPS users in high latitudes with the necessary integrity and accuracy corrections to enable GPS to be used as a primary navaid.

In the flight-test program, the loran transmitter at Tok, Alaska, was to transmit the WAAS integrity and accuracy correction data on the 100-kHz loran frequency, which was to be received by loran sets aboard an FAA Technical Center Convair
CV-580 and an Ohio University King Air. The loran/WAAS signals would then be converted to conventional GPS formats and fed to the onboard GPS receivers and also recorded for later analysis.

Loran was chosen for this task because of its powerful long-range signals, which extend from the surface to above jet altitudes and also penetrate mountain passes and valleys where other higher-frequency signals are blocked. The system is also impervious to intentional jamming. Should the tests prove successful–and ground and air tests performed in New England earlier this year suggest this is highly likely–Alaska’s aviation community will undoubtedly press the FAA to accelerate its development of combined loran/GPS receivers, so they can also partake in  the performance and safety benefits that WAAS will provide to the continental U.S. Preliminary results of the Alaska tests are expected later this year, with a full report planned for the U.S. Institute of Navigation annual technical meeting in San Diego in January.

Both the FAA Convair and the Ohio University King Air will carry earlier “legacy” model loran receivers in addition to new “all in view” prototype lorans, which can–at virtually any location–simultaneously receive and process the signals from a large number of transmitters in the worldwide loran network. This technique ensures high loran signal availability and performance, while enhancing accuracy.

As an indication of the reception potential of these units, a Locus Inc. all-in-view loran receiver installed in the FAA’s Convair was recently reported as performing poorly since it appeared to be receiving only five or six stations–roughly the same as a legacy receiver. Further investigation showed that the loran antenna had become disconnected, and the receiver was just using the antenna cable as its antenna.
Reconnecting the antenna while the aircraft was parked on the ramp caused the receiver to start tracking 19 worldwide loran stations immediately.

The antennas used with the all-in-view loran receivers are also new H-field designs, which eliminate the precipitation static problem that plagued earlier loran installations. The new antennas have been installed inside low-profile Bendix/King ADF antenna housings, which can also accommodate a GPS antenna for operation with combined loran/GPS receivers now being investigated under FAA sponsorship. One proposed combined unit would use the current multimode ILS/GPS/MLS landing guidance receiver, where a loran chipset would occupy the unused MLS slot.

The all-in-view loran receivers are also demonstrating much higher nationwide accuracy–generally well within the 0.3 RNP required for Lnav approaches–than their legacy predecessors, due to their vastly increased number of usable signals, much more advanced data-processing capabilities and the improving stability of the U.S. loran transmitter network, which has greatly benefitted from congressional appropriations.

Routinely, Congress has added funding for loran upgrades to the DOT’s annual budget requests, even during years when the department requested none.