Loran
Loran
Interpreting LORAN measurements
Sources Of Loran Measurement Error
LORAN (Long Range Navigation) is a radio-based navigational aid first used during World War II to locate ships and planes with greater accuracy than could be achieved with conventional techniques. LORAN determines location by comparing accurately-synchronized powerful radio pulses originating from different reference transmitter sites. Pulses from nearby transmitters arrive earlier than pulses from distant transmitters since radio signals travel at a constant speed.
At least three different LORAN signals must be received to determine latitude and longitude. In practice, the distance to more than the minimum three LORAN signals increases accuracy.
The first LORAN systems were in use before computers were sophisticated enough to perform the complex calculations needed to process the timing comparisons. Early LORAN installations required highly-skilled operators to interpret the radio pulses. A half century later technical innovations eliminated the need for much of the skill once required to use LORAN for navigation.
LORAN has evolved through three distinct phases, LORAN A, LORAN B, and the present version, LORAN C. The A and B versions were designed for navigational assistance over relatively short distances. LORAN A and LORAN B transmissions operated in a range of frequencies just slightly higher than the standard AM broadcast band in the United States. The present version, LORAN C, is assigned to 100 kHz, is a frequency well below the AM standard broadcast band, and 100 kHz is a frequency where long-distance radio propagation is very dependable. In contrast to LORAN A and LORAN B, LORAN C is reliable over distances of many hundreds of miles.
The Principle Of LORAN
The phenomenal accuracy of LORAN is possible because radio signals travel at a constant known speed. Each coordinated LORAN transmitter sends out a continuous succession of sharp radio pulses. If the LORAN receiver is equidistant from two transmitters, the pulses will be coincident. If the pulses from one station are received earlier than the pulses from the other station, the difference in the time of arrival of the two pulses contains information about the difference in distance to the two transmitters.
Radio signals travel a distance of almost exactly 984 ft (300 m) a microsecond. If a LORAN receiver measures a 100-microsecond time delay between pulses from two identified transmitters, the receiver is somewhere along a line corresponding to all the locations that are 9,843 ft (30,000 m) closer to the station transmitting the pulses received first than to the other transmitter site. That is, the receiver is not necessarily located 9,843 ft from the closest station, but it is 9,843 ft closer to this station than to the other station. If pulses from a different pair of stations is measured, with at least one signal source not involved in the first measurement, the difference in the distance to these new transmitter sites can be determined similarly. If this second comparison reveals that one of these transmitter sites is 16,405 ft (50,000 m) closer than the other, the LORAN receiver will be along a different line where the difference in distance equals 16,405 ft (50,000 m). The coordinates of the point where these two lines cross satisfy both measurement pairs. A third pair of signals must be measured to remove a final ambiguity.
Interpreting LORAN measurements
Early LORAN operation required the use of a previously-prepared map, covered with curved lines that corresponded to various distance differences from sets of received signal sources. The early LORAN devices indicated which map lines to use, the operator found the point on the map where the lines intersected to learn the location.
The latest versions of LORAN C receivers no longer require the use of a special map to determine location. These updated units contain a more sophisticated computer that calculates longitude and latitude directly, displaying in a format that does not need interpretation.
The LORAN C receiver automatically tunes first one then another and another of as many LORAN signals that can be received well enough to provide good data. After a short calculation delay the latitude and longitude is displayed.
As an illustration of the great locating accuracy achieved by LORAN C systems, commercial fishers sometimes use LORAN C when looking for buoys marking submerged crab traps left unattended in the open ocean.
Sources Of Loran Measurement Error
There is a limit to the accuracy with which the relative timing of radio pulses can be measured. For greater precision, pulses need to have a very steep wavefront. That is, they must start very quickly. Pulses with steep wavefronts must have a high harmonic content, and this means that the transmitted signals will have sideband components far to either side of the assigned frequency. The LORAN signals must be confined within a fairly narrow band of frequencies to avoid interference with other services, and this limitation blurs the definition of the start of each pulse. The result is a compromise in the accuracy of measurements of pulse timing.
There is less variation in the radio-signal propagation path taken by LORAN C signals at 100 kHz, but there are path variations that cannot be measured. The effect is to further reduce the quality of the information available to the LORAN computer. These effects are small, but they nevertheless set a limit on the available precision of navigation information that can be obtained from LORAN techniques.
KEY TERMS
AM —Amplitude Modulation.
Coincident —Events timed to have a constant time difference.
Global Positioning System (GPS) —A system of satellites whose signals can be used to locate objects on Earth (including below sea level) very precisely.
Latitude —Number of degrees north or south of Earth’s equator.
Longitude —Number of degrees east or west of Earth’s prime meridian.
Microsecond —One-millionth part of a second.
100 kHz —100,000 Hz, radio-frequency with a 2 mi (3-km) wavelength.
Pulse —Signal that rises to a peak abruptly, with a steep waterfront.
Synchronized —Occurring with the same frequency.
After LORAN C
A relatively new development in electronically-supported navigation systems, the Global Positioning System (GPS), seems destined to replace LORAN C. During the years from 1978 through 1995 the United States launched more than two dozen specialized navigational satellites that each orbit Earth twice every day. These satellites transmit data that permit even portable handheld receivers and decoders to display latitude and longitude with great accuracy. The GPS system provides better information than can be achieved using LORAN so it seems likely that the GPS system will soon render the LORAN system obsolete. LORAN will someday be found only in the history of electronics-based navigational systems but it will have served the world well for better than a half century.
Resources
OTHER
ProQuest Information and Learning. “LORAN: Creating a Viable Backup for GPS (dissertation by Gregory William Johnson).” June 22, 2006. <http://www.umi.com/proquest>=(accessed November 3, 2006).
United States Coast Guard. “LORAN C General Information.” March 6, 2006. <http://www.navcen.uscg.gov/loran/Default.htm> (accessed November 3, 2006).
US Department of Defense. “2005 Federal Radionavigation Plan.” 2005. <http://www.navcen.uscg.gov/pubs/frp2005/2005%20FRP%20WEB.pdf>(accessed November 3, 2006).
Donald Beaty
LORAN
LORAN
LORAN (Long Range Navigation) is a radio-based navigational aid first used during World War II to locate ships and planes with greater accuracy than could be achieved with conventional techniques. LORAN determines location by comparing accurately-synchronized powerful radio pulses originating from different reference transmitter sites. Pulses from nearby transmitters arrive earlier than pulses from distant transmitters since radio signals travel at a constant speed.
At least three different LORAN signals must be received to determine latitude and longitude . In practice, the distance to more than the minimum three LORAN signals increases accuracy.
The first LORAN systems were in use before computers were sophisticated enough to perform the complex calculations needed to process the timing comparisons. Early LORAN installations required highly-skilled operators to interpret the radio pulses. A half century later technical innovations eliminated the need for much of the skill once required to use LORAN for navigation.
LORAN has evolved through three distinct phases, LORAN A, LORAN B, and the present version, LORAN C. The A and B versions were designed for navigational assistance over relatively short distances. LORAN A and LORAN B transmissions operated in a range of frequencies just slightly higher than the standard AM broadcast band in the United States. The present version, LORAN C, is assigned to 100 kHz, is a frequency well below the AM standard broadcast band. 100 kHz, a frequency where long-distance radio propagation is very dependable. In contrast to LORAN A and LORAN B, LORAN C is reliable over distances of many hundreds of miles.
The principle of LORAN
The phenomenal accuracy of LORAN is possible because radio signals travel at a constant known speed. Each coordinated LORAN transmitter sends out a continuous succession of sharp radio pulses. If the LORAN receiver is equidistant from two transmitters, the pulses will be coincident. If the pulses from one station are received earlier than the pulses from the other station, the difference in the time of arrival of the two pulses contains information about the difference in distance to the two transmitters.
Radio signals travel a distance of almost exactly 984 ft (300 m) a microsecond. If a LORAN receiver measures a 100-microsecond time delay between pulses from two identified transmitters, the receiver is somewhere along a line corresponding to all the locations that are 9,843 ft (30,000 m) closer to the station transmitting the pulses received first than to the other transmitter site. That is, the receiver is not necessarily located 9,843 ft from the closest station, but it is 9,843 ft closer to this station than to the other station. If pulses from a different pair of stations is measured, with at least one signal source not involved in the first measurement, the difference in the distance to these new transmitter sites can be determined similarly. If this second comparison reveals that one of these transmitter sites is 16,405 ft (50,000 m) closer than the other, the LORAN receiver will be along a different line where the difference in distance equals 16,405 ft (50,000 m). The coordinates of the point where these two lines cross satisfy both measurement pairs. A third pair of signals must be measured to remove a final ambiguity.
Interpreting LORAN measurements
Early LORAN operation required the use of a previously-prepared map , covered with curved lines that corresponded to various distance differences from sets of received signal sources. The early LORAN devices indicated which map lines to use, the operator found the point on the map where the lines intersected to learn the location.
The latest versions of LORAN C receivers no longer require the use of a special map to determine location. These updated units contain a more sophisticated computer that calculates longitude and latitude directly, displaying in a format that does not need interpretation.
The LORAN C receiver automatically tunes first one then another and another of as many LORAN signals that can be received well enough to provide good data. After a short calculation delay the latitude and longitude is displayed.
As an illustration of the great locating accuracy achieved by LORAN C systems, commercial fishers sometimes use LORAN C when looking for buoys marking submerged crab traps left unattended in the open ocean .
Sources of LORAN measurement error
There is a limit to the accuracy with which the relative timing of radio pulses can be measured. For greater precision, pulses need to have a very steep wavefront. That is, they must start very quickly. Pulses with steep wavefronts must have a high harmonic content, and this means that the transmitted signals will have sideband components far to either side of the assigned frequency. The LORAN signals must be confined within a fairly-narrow band of frequencies to avoid interference with other services, and this limitation blurs the definition of the start of each pulse. The result is a compromise in the accuracy of measurements of pulse timing.
There is less variation in the radio-signal propagation path taken by LORAN C signals at 100 kHz, but there are path variations that cannot be measured. The effect is to further reduce the quality of the information available to the LORAN computer. These effects are small, but they nevertheless set a limit on the available precision of navigation information that can be obtained from LORAN techniques.
After LORAN C
A relatively new development in electronically-supported navigation systems, the Global Position Satellite system, seems destined to replace LORAN C. During the years from 1978 through 1995 the United States launched more than two dozen specialized navigational satellites that each orbit the earth twice every day. These satellites transmit data that permit even portable handheld receivers and decoders to display latitude and longitude with great accuracy. The Global Positioning Satellite system provides better information than can be achieved using LORAN so it seems likely that the GPS system will soon render the LORAN system obsolete. LORAN will someday be found only in the history of electronics-based navigational systems but it will have served the world well for better than a half century.
Resources
books
The 1995 ARRL Handbook. The American Radio Relay League, 1995.
Jacobs, George, and Theodore J. Cohen. The Shortwave Propagation Handbook. Cowan Publishing Corp., 1970.
Now You're Talking. The American Radio Relay League, 1994.
periodicals
Stix, Gary, "Aging Airways." Scientific American (May 1994).
Walker, Paul F., "Precision-guided Weapons." Scientific American (August 1981).
Donald Beaty
KEY TERMS
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .- AM
—Amplitude Modulation.
- Coincident
—Events timed to have a constant time difference.
- Global Positioning System (GPS)
—A system of satellites whose signals can be used to locate objects on Earth (including below sea level) very precisely.
- Latitude
—Number of degrees north or south of the earth's equator.
- Longitude
—Number of degrees east or west of the earth's prime meridian.
- Microsecond
—One-millionth part of a second.
- 100 kHz
—100,000 Hz, radio-frequency with a 2-mi (3-km) wavelength.
- Pulse
—Signal that rises to a peak abruptly, with a steep waterfront.
- Synchronized
—Occurring with the same frequency.