eLoran named best backup for satnav

 - August 29, 2007, 10:57 AM

A study commissioned by the NGATS Institute on behalf of the FAA’s JPDO and prepared by the advanced engineering and sciences division of ITT determined that eLoran (for enhanced) has “the highest overall preference rating…particularly in the U.S.” as a backup for satnav receivers in the event of failure or interference. The system is a modernized version of loran-C, where the user’s receiver–employing a GPS-like “all in view” technique–continuously tracks signals from every loran transmitter within a radius of about 2,000 miles and then selects the best of those to determine position.

The 180-page study, which had not been released at press time, analyzed DME/DME/INS; GNSS/INS; eLoran; VOR; “hardened” GNSS; terrain mapping; and multilateration.

GNSS was used generically in the report to include GPS and other satnav systems, such as Europe’s Galileo; “hardened” means that a GNSS receiver would incorporate circuitry to resist interference and jamming attacks. Terrain mapping is a military technique in which onboard altimetry and other sensors compare the terrain below with a TAWS-like digital terrain database. Multilateration is a relatively new development, proposed to replace secondary radar eventually, where widely spaced and unmanned “listening posts” continuously monitor aircraft transponder replies to accurately determine an aircraft’s position, which is then transmitted to the nearest ARTCC.

The prime assessment criteria measured each system’s ability to support RNP values of 2.0, 1.0 and 0.3, for en route, terminal and nonprecision approach respectively, its technical readiness by 2015 to 2025 and its complete independence from satnav. The study also reviewed each candidate’s ability to provide high-accuracy time signals to key elements of the nation’s terrestrial infrastructure that depend on GPS.

In its conclusion, the report stated, “eLoran scored significantly highest for the general aviation segment, and eLoran integration into GNSS/eLoran FMS systems for general aviation and certain air carrier segments appears to be a viable and capable solution.”