Jacqueline Hochheiser, Corporate Communications
When LORAN, long range navigation, arrived on the scene in the 1940s, it was the first technology of its time capable of pinpointing a receiver’s location. LORAN survived nearly 60 years before it was finally declared obsolete by the United States government. However, this low frequency hyperbolic radio navigation system may be set to make a come-back in the near future.
LORAN was developed during the early years of World War II to satisfy the needs of the United States Air Force for an airborne, long range navigational system. Alfred Lee Loomis, an American scientist and physicist, was responsible for inventing LORAN and led the mission along with members from the USAF and the United States Navy who had also taken interest in the new system. The USAF commissioned the project with set standards; accuracy within a 1,000 ft (304 m) margin of error and a range of 200 miles (320 km).
The project underwent development at the MIT Radiation Laboratory, where the first version of LORAN was born. Ultimately, the system was granted to both the US navy and the USAF to made navigation easier during the war. Although LORAN was employed on both seafaring vessels and aircraft, it is best known for its maritime applications.
LORAN, a hyperbolic radio navigation system, consisted of a mobile receiver, mounted on a ship or aircraft, that detected low frequency (1850 – 1950 kHz) radio waves transmitted from land-based beacons. LORAN was revolutionary for its time due to its ability to pinpoint its location based on the time it took for a fixed transmitter’s radio waves to reach the mobile receiver. This principal is known as Multilateration or MLat. The transmitters were referred to as beacons, which were organized into specific groups. Each group consisted of a master transmitter and from two to five secondary transmitters.
Each transmitter within the group had its own pulse sequence, a series of radio pulses timed at regular intervals. The master pulsed nine times for identification and each secondary pulsed eight times. The pulses of the master transmitter are followed at specific time intervals by each of the secondaries. The time interval between the reoccurrence of the master pulse is called a Group Repetition Interval (GRI). For the sake of determining whereabouts, each transmitter group had a unique GRI.
LORAN calculates the time difference of arrival (TDOA) between a master signal and each of its secondary signals as a pair. The TDOA between each master-secondary pair corresponds to a specific hyperbolic line of position (LOP). The intersection of two or more master-secondary LOPs establishes the position of the receiver. It was the end-user’s job to take the detected signals the receiver picked up and to calculate that point of intersection. The first version of LORAN was weak providing position accuracy within tens of miles and lacked the range later models would achieve. This was mainly because technology did not allow the transmitters within a group to be accurately synchronized. Nor could the transmitters be located far from each other as this would risk losing accuracy in the LOPs, therefore hindering the range.
In 1958, the system was transferred to the United States Coast Guard where it was renamed LORAN-A, although the capabilities remained the same. It wasn’t until 1974 that a new version was released, known as LORAN-C that operated over the 90 – 110 kHz band. With the availability of solid-state electronics, the components used in LORAN’s systems became much more affordable. Not only did the coast guard upgrade to the new system, but it was openly available to civilians for use on ships and pleasure boats. LORAN-C proved to be a popular model and its usage peaked in the 1970s and early 80s right before the release of GPS. LORAN-C provided worldwide coverage with transmitters in North America, Europe and Japan, while also boasting compatibility with the Russian system CHAYKA.
LORAN-C was able to determine its position within 100s of feet, rather than 10s of miles, due to more advanced technology that allowed more accurate synchronization of transmitter pulses. The improved synchronization allowed the distance between each transmitter to be increased, permitting more range. The new system also employed phase shift capabilities that allowed for more accurate readings of the received signals.
However, LORAN-C’s glory days came to an end when GPS was introduced as a more sophisticated positioning system offering more range and increased accuracy. GPS automatically computed a mobile receiver’s position rather than leaving the calculation up to human intervention. LORAN was declared obsolete in 2009 and decommissioned in 2010. Despite the obvious advantages of the high frequency GPS system, national security concerns have led engineers to revive LORAN as a backup for GPS.
High frequency operation, while providing higher performance, is largely more susceptible to jamming and hacking than lower frequency systems like LORAN. In a low-frequency system, the signal integrity is much stronger, therefore requiring a more powerful transmitter to cause interference. GPS satellites, on the other hand, are easier to jam and are more prone to disruption from space weather, EMP (electromagnetic pulse) events, and can be blocked in areas with large buildings.
As a result, the US Coast Guard and Federal Aviation Administration took on the Modernization and Recapitalization program in the 1990s to create an enhanced version of LORAN, otherwise known as eLORAN. This new system, which is still in development, will be more self-sufficient than its predecessor, LORAN-C, as well as more accurate with longer range. Where end-users would have to take the intersection of LOPs and calculate the position themselves, eLORAN will have the capability to calculate the receiver’s position on its own, making it a competitive system with GPS. eLORAN’s self-sufficiency will also allow for missions to function unmanned, for example, using a drone for surveillance.
While LORAN remained an integral part of maritime and arial navigation for many decades, it seems that the once antiquated technology is making a smart comeback. With better technology that makes it more competitive with GPS, eLORAN provides a secure backup option for radio navigation both overseas and in the skies.