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Long Range Navigation (LORAN)

in Navigation Systems

LORAN uses a network of land-based transmitters to provide an accurate long-range navigation system. The FAA and the United States Coast Guard (USCG) arranged the stations into chains. The signal from station is a carefully structured sequence of brief RF pulses centered at 100 kHz. At that frequency, signals travel considerable distances as ground waves, from which accurate navigation information is available. The airborne receiver monitors all of the stations within the selected chain, then measures the arrival time difference (TD) between the signals. All of the points having the same TD from a station pair create a line of position (LOP). The aircraft position is determined at the intersection of two or more LOPs. Then the computer converts the known location to latitude and longitude coordinates. [Figure 7-26]


Figure 7-26. A control panel from a military aircraft after LORAN was first put into use. The receiver is remotely mounted and weighs over 25 pounds. Its size is about six times that of the LORAN fully integrated receiver.

Figure 7-26. A control panel from a military aircraft after LORAN was first put into use. The receiver is remotely mounted and weighs over 25 pounds. Its size is about six times that of the LORAN fully integrated receiver.

While continually computing latitude/longitude fixes, the computer is able to determine and display:

  1. Track over the ground since last computation;
  2. Groundspeed by dividing distance covered since last computation by the time since last computation (and averaging several of these);
  3. Distance to destination;
  4. Destination time of arrival; and
  5. Cross-track error.

The Aeronautical Information Manual (AIM) provides a detailed explanation of how LORAN works. LORAN is a very accurate navigation system if adequate signals are received. There are two types of accuracy that must be addressed in any discussion of LORAN accuracy.

Repeatable accuracy is the accuracy measured when a user notes the LORAN position, moves away from that location, then uses the LORAN to return to that initial LORAN position. Distance from that initial position is the error. Propagation and terrain errors will be essentially the same as when the first position was taken, so those errors are factored out by using the initial position. Typical repeatable accuracy for LORAN can be as good as 0.01 NM, or 60 feet, if the second position is determined during the day and within a short period of time (a few days).

Absolute accuracy refers to the ability to determine present position in space independently, and is most often used by pilots. When the LORAN receiver is turned on and position is determined, absolute accuracy applies. Typical LORAN absolute accuracy will vary from about 0.1 NM to as much as 2.5 NM depending on distance from the station, geometry of the TD LOP crossing angles, terrain and environmental conditions, signal-to-noise ratio (signal strength), and some design choices made by the receiver manufacturer.

Although LORAN use diminished with the introduction of Global Navigation Satellite Systems such as the United States’ GPS, its use has since increased. Three items aided in this resurgence:

  • In 1996, a commission called the Gore Commission evaluated GPS’ long-term use as a sole navigation aid. Although GPS was hailed originally as the eventual sole NAVAID, which would replace the need for most currently existing NAVAIDs by the year 2020, the Commission questioned single-link failure potential and its effect on the NAS. For this reason, the forecasted decommissioning of the VOR has been amended and their expectant lifecycle extended into the future. Additionally, the use of LORAN continues to be evaluated for facilitating carrying GPS corrective timing signals.
  • The GPS is controlled by the DOD presenting certain unforecasted uncertainties for commercial use on an uninterrupted basis.

As a result of these and other key factors, it was determined that LORAN would remain. In recognition of GPS vulnerabilities as a GNSS, there are plans to maintain other systems that could provide en route and terminal accuracy such as LORAN. Therefore as LORAN is further modernized it’s a possibility that it may be used to augment GPS and provide backup to GPS during unlikely but potential outages. Or if combined with GPS and other systems such as newer miniaturized lowcost inertial navigation systems (INS), superior accuracy and seamless backup will always be available.

LORAN Components The LORAN receiver incorporates a radio receiver, signal processor, navigation computer, control/display, and antenna. When turned on, the receivers go through an initialization or warm-up period, then inform the user they are ready to be programmed. LORAN receivers vary widely in their appearance, method of user programming, and navigation information display. Therefore, it is necessary to become familiar with the unit, including programming and output interpretation. The LORAN operating manual should be in the aircraft at all times and available to the pilot. IFR-approved LORAN units require that the manual be aboard and that the pilot be familiar with the unit’s functions, before flight.

Function of LORAN

After initialization, select for the present location WP (the airport), and select GO TO in order to determine if the LORAN is functioning properly. Proper operation is indicated by a low distance reading (0 to 0.5 NM). The simplest mode of navigation is referred to as GO TO: you select a WP from one of the databases and choose the GO TO mode. Before use in flight, verify that the latitude and longitude of the chosen WP is correct by reference to another approved information source. An updatable LORAN database that supports the appropriate operations (e.g., en route, terminal, and instrument approaches) is required when operating under IFR.

In addition to displaying bearing, distance, time to the WP, and track and speed over the ground, the LORAN receiver may have other features such as flight planning (WP sequential storage), emergency location of several nearest airports, vertical navigation capabilities, and more.


LORAN Errors
System Errors

LORAN is subject to interference from many external sources, which can cause distortion of or interference with LORAN signals. LORAN receiver manufacturers install “notch fi lters” to reduce or eliminate interference. Proximity to 60 Hz alternating current power lines, static discharge, P-static, electrical noise from generators, alternators, strobes, and other onboard electronics may decrease the signal-to- noise ratio to the point where the LORAN receiver’s performance is degraded.

Proper installation of the antenna, good electrical bonding, and an effective static discharge system are the minimum requirements for LORAN receiver operation. Most receivers have internal tests that verify the timing alignment of the receiver clock with the LORAN pulse, and measure and display signal-to-noise ratio. A signal will be activated to alert the pilot if any of the parameters for reliable navigation are exceeded on LORAN sets certified for IFR operations.

LORAN is most accurate when the signal travels over sea water during the day and least accurate when the signal comes over land and large bodies of fresh water or ice at night; furthermore, the accuracy degrades as distance from the station increases. However, LORAN accuracy is generally better than VOR accuracy.

Operational Errors

Some of the typical pilot-induced errors of LORAN operation are:

  1. Use of a nonapproved LORAN receiver for IFR operations. The pilot should check the aircraft’s POH/ AFM LORAN supplement to be certain the unit’s functions are well understood (this supplement must be present in the aircraft for approved IFR operations). There should be a copy of FAA Form 337, Major Repair and Alteration, present in the aircraft’s records, showing approval of use of this model LORAN for IFR operations in this aircraft.
  2. Failure to double-check the latitude/longitude values for a WP to be used. Whether the WP was accessed from the airport, NDB, VOR, or intersection database, the values of latitude and longitude should still be checked against the values in the A/FD or other approved source. If the WP data is entered in the user database, its accuracy must be checked before use.
  3. Attempting to use LORAN information with degraded signals.

 

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