Loran and GPS Vunerability
Loran Gets a Witness
Mar 1, 2007
GPS World
The Radio Technical Commission for Maritime Services (RTCM) has joined more than 900 other organizations and individuals in responding to the request for comments on Loran. RTCM comments verbatim:
RTCM's Board of Directors has adopted a position in support of maintaining the Loran-C system, and its modernization to e-Loran standards. Loran provides an important backup to the Navstar Global Positioning System (GPS) and other Global Navigation Satellite Systems (GNSS), not only for position and navigation purposes, but for timing applications as well. In deciding on this position, RTCM has taken the following into consideration:
GPS/GNSS augmentation: The accuracy of GPS/GNSS systems is improved by the availability of additional satellites. In a combined GPS/Loran receiver, Loran signals can function like an additional satellites, thereby improving positional accuracy. Combined GPS and Loran receivers are currently available, and the accuracy provided by the envisioned e-Loran system will enhance their usefulness.
Vulnerability of signals: The signals from GNSS satellites are very weak at the earth's surface. Although it is difficult, jamming of civil GNSS signals is possible. Loran's stronger signal makes jamming more difficult. The following is from the 2005 Federal Radionavigation Plan:
1.6.2.1 Vulnerability of GPS in the National Transportation Infrastructure
The Final Report of the President's Commission on Critical Infrastructure Protection concluded that GPS services and applications are susceptible to various types of radio frequency interference, and that the effects of these vulnerabilities on civilian transportation applications should be studied in detail.
Shielding is another potential issue. Loran's stronger signal would enable land navigation and tracking systems to continue to operate with a combined GNSS/Loran receiver in some places such as tunnels and areas of dense vegetation, where GNSS reception is shielded.
Vulnerability of satellites: As has been recently demonstrated, satellites may be vulnerable to destruction by a future adversary. Loran stations are land-based, and easier to defend. Should the GNSS system be compromised as the result of a conflict, Loran receiver input to shipboard systems such as the Automatic Identification System (AIS) and Long Range Identification and Tracking (LRIT) System, would enable those systems to continue to function. Both systems are considered essential to maritime security, a concern that would certainly take on added importance in a time of conflict. Recent experiments have shown e-Loran capable of meeting Harbor and Harbor Entrance positioning requirements.
Loran is needed as a reliable backup to GPS for timing purposes. Modern communications systems (e.g. cellular telephone) depend on timing derived from GPS and/or Loran, and would not be able to function without them.
Legacy users: Many recreational boaters and smaller commercial vessels have Loran-C receivers, which provide them with sufficient navigational accuracy for their purposes. Termination of Loran-C service would render their equipment useless and their investment in it would be lost. Many commercial boat operators use both GPS and Loran-C, and consider Loran-C a vital supplement to GPS. They are quite familiar with periods of GPS unavailability, and charter boat operators have to use Loran-C under those conditions to find wrecks, rocks, or reefs for fishing or SCUBA diving.
International compatibility: Loran providers in Europe, Asia, and the Middle East have committed to continuing Loran service. The United States needs to commit to Loran in the interest of a seamless international position, navigation, and timing service.
For the Radio Technical Commission for Maritime Services,
— R. L. Markle, President
Sprint Nextel Chimes In
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