Hyperbolic navigation
Encyclopedia
Hyperbolic navigation refers to a class of radio navigation
systems based on the difference in timing between the reception of two signals, without reference to a common clock. Calculating the distance from the stations based on these timings produces a series of hyperbolic lines. Taking two such measurements and looking for the intersections reveals the receiver's location to be in one of two locations. Any form of additional navigation information can be used to eliminate this ambiguity and determine a fix.
The first such system to be used was the WWII-era Gee
system introduced by the RAF for use by Bomber Command
. This was followed by the more accurate Decca Navigator System
in 1944 by the Royal Navy
, along with LORAN-A by the US Navy for long-range navigation at sea. Post war examples including the well-known US Coast Guard LORAN-C, the international Omega system, and the Soviet Alpha
. All of these systems saw use until their wholesale replacement by satellite navigation systems like the Global Positioning System
(GPS).
Consider a portable receiver located on the midpoint of the line drawn between the two stations. In this case, the signals will, necessarily, take 0.5 ms to reach the receiver. From this information, they could determine that they are precisely 150 km from both stations, and thereby exactly determine their location. If the receiver moves to another location along the line the timing of the signals would change. For instance, if they time the signals at 0.25 and 0.75 ms, they are 75 km from the closer station and 225 from the further.
If the receiver moves to the side of this "baseline", the delay from both stations will grow. At some point they will measure a delay of 1 and 1.5 ms, which implies the receiver is 300 km from one station and 450 from the other. If one draws circles of 300 and 450 km radius around the two stations on a chart, the circles will intersect at two points. With any additional source of navigation information, one of these two intersections can be eliminated as a possibility, and thus reveal their exact location, or "fix".
However, it was possible to accurately measure the difference between two signals. Much of the development of suitable equipment had been carried out between 1935 and 1938 as part of the efforts to deploy radar
systems. The UK, in particular, had invested considerable effort in the development of their Chain Home
system. The radar display
systems for Chain Home were based on oscilloscope
s (or oscillographs as they were known at time) triggered to start their sweep when the broadcast signal was sent. Return signals were amplified and sent into the 'scope display, producing a "blip". By measuring the distance along the face of the oscilloscope of any blips, the time between broadcast and reception could be measured, thus revealing the range to the target.
With very slight modification, the same display could be used to time the difference between two arbitrary signals. For navigational use, any number of identifying characteristics could be used to differentiate the master from the slave signals. In this case, the portable receiver triggered its trace when it received the master signal. As the signals from slaves arrived they would cause a blip on the display in the same fashion as a target on the radar, and the exact delay between the master and slave easily determined.
In the second example the receivers determined the timing to be 0.25 and 0.75 ms, so this would produce a measured delay of 0.5 ms. There are many locations that can produce this difference - 0.25 and 0.75 ms, but also 0.3 and 0.8 ms, 0.5 and 1 ms, etc. If all of these possible locations are plotted, they form a hyperbolic curve centred on the baseline. Navigational charts can be drawn with the curves for selected delays, say every 0.1 ms. The operator can then determine which of these lines they lie on by measuring the delay and looking at the chart.
A single measurement reveals a range of possible locations, not a single fix. The solution to this problem is to simply add another slave station at some other location. In this case two delays will be measured, one the difference between the master and slave "A", and the other between the master and slave "B". By looking up both delay curves on the chart, two intersections will be found, and one of these can be selected as the likely location of the receiver. This is a similar determination as in the case with direct timing/distance measurements, but the hyperbolic system consists of nothing more than a conventional radio receiver hooked to an oscilloscope.
Because a slave could not instantaneously transmit its signal pulse on receipt of the master signal, a fixed delay was built into the signal. No matter what delay is selected, there will be some locations where the signal from two slaves would be received at the same time, and thus make them difficult to see on the display. Some method of identifying one slave from another was needed. Common methods included transmitting from the slaves only at certain times, using different frequencies, adjusting the envelope of the burst of signal, or broadcasting several bursts in a particular pattern. A set of stations, master and slave, was known as a "chain", and similar methods are used to identify individual chains as well.
in 1941. Gee was used both for bombing over Germany as well as navigation in the area of the UK, especially for landing at night. Several Gee chains were built in the UK, and after the war this expanded for four chains in the UK, two in France, and one in northern Germany. For a period following the formation of the International Civil Aviation Organization
in 1946, Gee was considered as the basis for a worldwide standard for navigation, but the VOR
system was selected instead, and the last Gee chain was eventually shut down in 1970.
Gee signals from a given chain were all sent on a single frequency. The master station sent two signals, the "A" signal that marked the beginning of a timing period, and the "D" signal which was essentially two "A"s to mark the end. In every period, one of the two slaves would respond, alternating their "B" and "C" signals. The resulting pattern was "ABD…ACD…ABD…" A wide-band receiver was used to tune in chain and the output set to the operator's oscilloscope
. As the stations were closely spaced in frequency, this sometimes resulted in the signals from several stations appearing on the display. To distinguish the chains in these cases, a second "A" signal, the "A1" or "ghost A", was sometimes keyed in, and the pattern of flashing on the display could be used to identify the chain.
The operator initially tuned in their receiver to see a stream of pulses on the display, sometimes including those of other chains which were nearby in frequency. He would then tune a local oscillator that started the trigger of the oscilloscope's trace so that it matched the clock at the master station (which could, and did, change over time). Next he would use a variable delay to move the start of the signal so one of the "A" pulses was at the very left side of the 'scope (the action is identical to the "horizontal hold" dial on an analog television). Finally the speed of the trace across the display would be tuned so the D pulse was just visible on the right. The distance of the B or C pulse from the A pulse could now be measured with an attached scale. The resulting delays could then be looked up on a navigational chart.
The display was relatively small, which limited resolution, and thus the determination of the delay. A measurement accuracy of 1 microsecond was quoted, which resulted in an accuracy of the determination of the correct hyperbolic to about 150 meters, and when two such measurements were combined the resulting fix accuracy was around 210 m. At longer ranges, 350 miles for example, the error ellipse was about 6 miles by 1 mile. The maximum range was about 450 miles, although several long-range fixes were made under unusual circumstances.
navigation in particular. R. J. Dippy, inventor of Gee, moved to the US in mid-1942 to help with this project, which eventually emerged as LORAN, for LOng RAnge Navigation. LORAN became LORAN-A when the design of its replacement started, this was initially the LORAN-B concept, but eventually replaced by the very long-range LORAN-C starting in 1957.
LORAN-A was essentially a version of Gee with a new selection of frequencies
suitable for long-range transmission over water, eventually selecting 1.950 MHz. 7.5 MHz was selected for daytime use as an additional channel, but never used operationally. In comparison to Gee's 450 mile range through air, LORAN-A had a range of about 1500 miles over water, and 600 miles over land. Operation was generally similar to Gee, but only one of the slave signals was displayed at a time. A fix required the operator to measure one delay, then the other, and then look up the resulting delays on the charts. The accuracy was quoted as 1% of range.
LORAN-A used two methods to identify a chain. One was the operational frequency, with four "channels", as in Gee. The second was the rate at which the pulses were repeated, with "high", "low" and "slow" rates. This allowed for up to 12 chains in any given area. Additionally, the originally steady repetition of the pulses was later modified to create another eight unique patterns, allowing a total of 96 station pairs. Any given chain could use one or more pairs of stations, demanding a large number of unique signals for widespread coverage.
as an accurate adjunct to naval versions of Gee, Decca was first used on 5 June 1944 to guide minesweeper
s in preparation for the D-Day
invasions. The system was developed post-war and competed with GEE and other systems for civilian use. A variety of reasons, notably its ease-of-use, kept it in widespread use into the 1990s, with a total 42 chains around the world. A number of stations were updated in the 1990s, but the widespread use of GPS led to Decca being turned off at midnight on 31 March 2000.
Decca was based on comparing the phases of continuous signals instead of the timing of their pulses. This was more accurate, as the phase of a pair of signals could be measured to within a few degrees, four degrees in the case of Decca. This greatly improved inherent accuracy allowed Decca to use much longer wavelengths than Gee or LORAN while still offering the same level of accuracy. The use of longer wavelengths gave better propagation than either Gee or LORAN, although ranges were generally limited to around 500 miles for the basic system. However, Decca also had the inherent disadvantage that the signal repeated over space, and could only identify the location within what they referred to as "lanes". These were relatively small, so additional information was needed to identify which lane the receiver was located in.
Decca solved this problem though the use of an odometer
-like display known as "decometers". Prior to leaving on a trip, the navigator would set the decometer's lane counter to their known position. As the craft moved the dial's hand would rotate, and increment or decrement the counter when it passed zero. The combination of this number and the current dial reading allowed the navigator to directly read the current delay and look it up on a chart, a far easier process than Gee or LORAN. It was so much easier to use that Decca later added an automatic charting feature that formed a moving map display. Later additions to the signal chain allowed the zone and lane to be calculated directly, eliminating the need for manual setting of the lane counters and making the system even easier to use.
As each master and slave signal was sent on a different frequency, any number of delays could be measured at the same time; in practice a single master and three slaves were used to produce three outputs. As each signal was sent on a different frequency, all three, known as "green", "red" and "purple", were simultaneously decoded and displayed on three decometers. The slaves were physically distributed at 120 degree angles from each other, allowing the operator to pick the pair of signals on the display that were sent from stations as close to right angles to the receiver as possible, further improving accuracy. Maximum accuracy was normally quoted as 200 yards, although that was subject to operational errors.
In addition to greater accuracy and ease of use, Decca was also more suitable for use over land. Delays due to refraction can have a significant effect on pulse timing, but much less so for phase changes. Decca thus found itself in great demand for helicopter use, where runway approach aids like ILS
and VOR
were not suitable for the small airfields and essentially random locations the aircraft were used. One serious disadvantage to Decca was that it was susceptible to noise, especially from lightning
. This was not a serious concern for ships, who could afford to wait out storms, but made it unsuitable for long-range air navigation where time was of the essence. Several versions of Decca were introduced for this role, notably DECTRA and DELRAC, but these did not see widespread use.
of the signal, which increases at lower frequencies - in other words, using a lower frequency would necessarily lower the accuracy of the system. Hoping for the best, early experiments with "LF Loran" instead proved that accuracy was far worse than predicted, and efforts along these lines were dropped.
Several halting low-frequency efforts followed, including the Decca-like Cyclan and Navarho concepts. A key development was the introduction of the low-cost phase-locked loop
in the 1950s, which allowed a receiver to "lock on" to a signal and maintain the phase and frequency very accurately. This allowed a carrier signal to be re-constructed in a local oscillator by observing received pulses, a process that was accurate enough for phase comparisons against the local oscillator. This allowed a single system to combine the features of pulse-based and phase-based systems.
Re-using the Cyclan transmitters, the US Navy started experiments with such a system int the mid-1950s, and turned the system on permanently in 1957. Numerous chains followed, eventually providing around-the-world coverage near US allies and assets. Although less accurate that Decca, it offered the combination of reasonable accuracy and long ranges, keeping it in operation until GPS finally led to its shutdown on 8 February 2010.
In basic operation, LORAN-C is more similar to Decca than Gee or LORAN-A, as its main way determining location was the comparison of phase differences between signals. However, at low frequencies and long ranges it would be difficult to know whether you are looking at the current phase of the signal, or the phase of the signal one cycle ago, or perhaps one reflected off the ionosphere
. Some form of secondary information is needed to reduce this ambiguity. LORAN-C achieved this by sending unique details in the pulses so each station could be uniquely identified.
The signal was started off when the Master broadcast a sequence of nine pulses, with the precise timing between each pulses being used to identify the station. Each of the Secondary stations then sent out their own signals, consisting of eight pulses in patterns that revealed which station they were. The receivers could use the signal timings to select chains, identify secondaries, and reject signals bounced off the ionosphere.
LORAN-C chains were organized into the Master station, M, and up to five Secondary stations, V, W, X, Y, Z. All were broadcast at 100 kHz, a much lower frequency than earlier systems. The result was a signal that offered a daytime ground wave range of 2,250 miles, nighttime ground wave of 1,650 miles and skywaves out to 3,000 miles. Timing accuracy was estimated at 0.15 microseconds, offering accuracies on the order of 50 to 100 meters.
Where the phase-locked loop made LORAN-C a possibility, for Omega it was the introduction of inertial navigation system
s (INS) that offered a solution - the INS was accurate enough to resolve any ambiguity about which lane the receiver was in. Experiments continued throughout the 1950s and 60s, in parallel with Decca's development of their almost identical DELRAC system. It was not until the 1960s, when ice-breaking ballistic submarines became a main deterrent force, that there was a pressing need for such a system. The US Navy authorized full deployment in 1968, reaching a complete set of 8 stations in 1983. Omega would also prove to be one of the shortest-lived systems, shutting down on 20 September 1997.
Omega stations broadcast a continuous-wave signal in a specific time-slot. In order to maintain precise timing of the slots for stations distributed around the world, stations were equipped with synchronized atomic clock
s. These clocks also ensured that their signals were sent out with the right frequency and phase; unlike previous systems, Omega did not need to have a master/slave arrangement as the clocks were accurate enough to trigger the signals without an external reference. To start the sequence, the station in Norway
would initially broadcast on 10.2 kHz for 0.9 seconds, then turned off for 0.2 seconds, then broadcast on 13.6 kHz for 1.0 seconds, and so on. Each station broadcast a series of four such signals lasting about a second each, and then stood silent while other stations took their turn. At any given instant, three stations would be broadcasting at the same time on different frequencies. Receivers would select the set of stations that were most suitable for their given location, and then wait for the signals for those stations to appear during the 10 second chain. Calculation of the fix then proceeded in precisely the same fashion as Decca, although the much lower operating frequency led to much less accuracy. Omega's charts quote accuracies of 2 to 4 nautical miles.
's counterpart to LORAN-C, and operates on similar principles and the same frequency. It differs primarily in details of the pulse envelopes. There are five CHAYKA chains distributed around the former Soviet Union, each with a master and between two and four slaves.
starting in 1962. The initial system used only three transmitters running roughly in a line in Krasnodar, Revda and Novosibirsk, the later being the master station. In 1991 two additional stations came online at Khabarovsk and Seyda. The stations use frequencies between 11 and 14 kHz.
Radio navigation
Radio navigation or radionavigation is the application of radio frequencies to determine a position on the Earth. Like radiolocation, it is a type of radiodetermination.The basic principles are measurements from/to electric beacons, especially...
systems based on the difference in timing between the reception of two signals, without reference to a common clock. Calculating the distance from the stations based on these timings produces a series of hyperbolic lines. Taking two such measurements and looking for the intersections reveals the receiver's location to be in one of two locations. Any form of additional navigation information can be used to eliminate this ambiguity and determine a fix.
The first such system to be used was the WWII-era Gee
GEE (navigation)
Gee was the code name given to a radio navigation system used by the Royal Air Force during World War II.Different sources record the name as GEE or Gee. The naming supposedly comes from "Grid", so the lower case form is more correct, and is the form used in Drippy's publications. See Drippy 1946....
system introduced by the RAF for use by Bomber Command
Bomber Command
Bomber Command is an organizational military unit, generally subordinate to the air force of a country. Many countries have a "Bomber Command", although the most famous ones were in Britain and the United States. A Bomber Command is generally used for Strategic bombing , and is composed of bombers...
. This was followed by the more accurate Decca Navigator System
Decca Navigator System
The Decca Navigator System was a low frequency hyperbolic navigation system that was first deployed during World War II when the Allied forces needed a system which could be used to achieve accurate landings...
in 1944 by the Royal Navy
Royal Navy
The Royal Navy is the naval warfare service branch of the British Armed Forces. Founded in the 16th century, it is the oldest service branch and is known as the Senior Service...
, along with LORAN-A by the US Navy for long-range navigation at sea. Post war examples including the well-known US Coast Guard LORAN-C, the international Omega system, and the Soviet Alpha
Alpha (radio navigation)
Alpha is a Russian system for long range radio navigation. RSDN in Russian stands for , i.e., radio-technical long-distance navigation system....
. All of these systems saw use until their wholesale replacement by satellite navigation systems like the Global Positioning System
Global Positioning System
The Global Positioning System is a space-based global navigation satellite system that provides location and time information in all weather, anywhere on or near the Earth, where there is an unobstructed line of sight to four or more GPS satellites...
(GPS).
Timing-based navigation
Consider two ground-based radio stations located at a set distance from each other, say 300 km so that they are exactly 1 ms apart at light speed. Both stations are equipped with identical transmitters set to broadcast a short pulse at a specific frequency. One of these stations, called the "slave" is also equipped with a secondary receiver. When this receiver hears the signal from the other station, referred to as the "master", it triggers its own broadcast. The master station can then broadcast any series of pulses, with the slave hearing these and generating the same series after 1 ms delay.Consider a portable receiver located on the midpoint of the line drawn between the two stations. In this case, the signals will, necessarily, take 0.5 ms to reach the receiver. From this information, they could determine that they are precisely 150 km from both stations, and thereby exactly determine their location. If the receiver moves to another location along the line the timing of the signals would change. For instance, if they time the signals at 0.25 and 0.75 ms, they are 75 km from the closer station and 225 from the further.
If the receiver moves to the side of this "baseline", the delay from both stations will grow. At some point they will measure a delay of 1 and 1.5 ms, which implies the receiver is 300 km from one station and 450 from the other. If one draws circles of 300 and 450 km radius around the two stations on a chart, the circles will intersect at two points. With any additional source of navigation information, one of these two intersections can be eliminated as a possibility, and thus reveal their exact location, or "fix".
Absolute vs. differential timing
There is a serious practical problem with this approach - in order to measure the time it took for the signals to reach the receiver, the receiver must know the precise time that the signal was originally sent. With modern electronics this is a trivial exercise, and forms the basis of all modern navigation systems, including GPS. In the 1930s, however, such precise time measurements simply weren't possible; a clock of the required accuracy was difficult enough to build in fixed form, let alone portable.However, it was possible to accurately measure the difference between two signals. Much of the development of suitable equipment had been carried out between 1935 and 1938 as part of the efforts to deploy radar
Radar
Radar is an object-detection system which uses radio waves to determine the range, altitude, direction, or speed of objects. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations, and terrain. The radar dish or antenna transmits pulses of radio...
systems. The UK, in particular, had invested considerable effort in the development of their Chain Home
Chain Home
Chain Home was the codename for the ring of coastal Early Warning radar stations built by the British before and during the Second World War. The system otherwise known as AMES Type 1 consisted of radar fixed on top of a radio tower mast, called a 'station' to provide long-range detection of...
system. The radar display
Radar display
Modern radar systems typically use some sort of raster scan display to produce a map-like image. In the past, notably during the early days of radar development, such displays were difficult to produce for a number of reasons. Several different display types were developed during this...
systems for Chain Home were based on oscilloscope
Oscilloscope
An oscilloscope is a type of electronic test instrument that allows observation of constantly varying signal voltages, usually as a two-dimensional graph of one or more electrical potential differences using the vertical or 'Y' axis, plotted as a function of time,...
s (or oscillographs as they were known at time) triggered to start their sweep when the broadcast signal was sent. Return signals were amplified and sent into the 'scope display, producing a "blip". By measuring the distance along the face of the oscilloscope of any blips, the time between broadcast and reception could be measured, thus revealing the range to the target.
With very slight modification, the same display could be used to time the difference between two arbitrary signals. For navigational use, any number of identifying characteristics could be used to differentiate the master from the slave signals. In this case, the portable receiver triggered its trace when it received the master signal. As the signals from slaves arrived they would cause a blip on the display in the same fashion as a target on the radar, and the exact delay between the master and slave easily determined.
Hyperbolic navigation
Consider the same examples as our original absolute-timed cases. If the receiver is located on the midpoint of the baseline the two signals will be received at exactly the same time, so the delay between them will be zero. However, the delay will be zero not only if they are located 150 km from both stations and thus in the middle of the baseline, but also if they are located 200 km from both stations, and 300 km, and so forth. So in this case the receiver cannot determine their exact location, only that their location lies somewhere along a line perpendicular to the baseline.In the second example the receivers determined the timing to be 0.25 and 0.75 ms, so this would produce a measured delay of 0.5 ms. There are many locations that can produce this difference - 0.25 and 0.75 ms, but also 0.3 and 0.8 ms, 0.5 and 1 ms, etc. If all of these possible locations are plotted, they form a hyperbolic curve centred on the baseline. Navigational charts can be drawn with the curves for selected delays, say every 0.1 ms. The operator can then determine which of these lines they lie on by measuring the delay and looking at the chart.
A single measurement reveals a range of possible locations, not a single fix. The solution to this problem is to simply add another slave station at some other location. In this case two delays will be measured, one the difference between the master and slave "A", and the other between the master and slave "B". By looking up both delay curves on the chart, two intersections will be found, and one of these can be selected as the likely location of the receiver. This is a similar determination as in the case with direct timing/distance measurements, but the hyperbolic system consists of nothing more than a conventional radio receiver hooked to an oscilloscope.
Because a slave could not instantaneously transmit its signal pulse on receipt of the master signal, a fixed delay was built into the signal. No matter what delay is selected, there will be some locations where the signal from two slaves would be received at the same time, and thus make them difficult to see on the display. Some method of identifying one slave from another was needed. Common methods included transmitting from the slaves only at certain times, using different frequencies, adjusting the envelope of the burst of signal, or broadcasting several bursts in a particular pattern. A set of stations, master and slave, was known as a "chain", and similar methods are used to identify individual chains as well.
Operational systems
Meint Harms was the first to have attempted the construction of a hyperbolic navigation systems, starting with musings on the topic in 1931 as part of his master's examination at Seefahrtschule Lübeck (Navigation College). After taking the position of Professor for Mathematics, Physics and Navigation at the Kaisertor in Lübeck, Harms tried to demonstrate hyperbolic navigation making use of simple transmitters and receivers. On 18 February 1932 he received Reichspatent-Nr. 546000 for his invention.Gee
The first operational hyperbolic navigation was UK's Gee, first used experimentally by Bomber CommandBomber Command
Bomber Command is an organizational military unit, generally subordinate to the air force of a country. Many countries have a "Bomber Command", although the most famous ones were in Britain and the United States. A Bomber Command is generally used for Strategic bombing , and is composed of bombers...
in 1941. Gee was used both for bombing over Germany as well as navigation in the area of the UK, especially for landing at night. Several Gee chains were built in the UK, and after the war this expanded for four chains in the UK, two in France, and one in northern Germany. For a period following the formation of the International Civil Aviation Organization
International Civil Aviation Organization
The International Civil Aviation Organization , pronounced , , is a specialized agency of the United Nations. It codifies the principles and techniques of international air navigation and fosters the planning and development of international air transport to ensure safe and orderly growth...
in 1946, Gee was considered as the basis for a worldwide standard for navigation, but the VOR
Vör
In Norse mythology, Vör is a goddess associated with wisdom. Vör is attested in the Prose Edda, written in the 13th century by Snorri Sturluson; and twice in kennings employed in skaldic poetry...
system was selected instead, and the last Gee chain was eventually shut down in 1970.
Gee signals from a given chain were all sent on a single frequency. The master station sent two signals, the "A" signal that marked the beginning of a timing period, and the "D" signal which was essentially two "A"s to mark the end. In every period, one of the two slaves would respond, alternating their "B" and "C" signals. The resulting pattern was "ABD…ACD…ABD…" A wide-band receiver was used to tune in chain and the output set to the operator's oscilloscope
Oscilloscope
An oscilloscope is a type of electronic test instrument that allows observation of constantly varying signal voltages, usually as a two-dimensional graph of one or more electrical potential differences using the vertical or 'Y' axis, plotted as a function of time,...
. As the stations were closely spaced in frequency, this sometimes resulted in the signals from several stations appearing on the display. To distinguish the chains in these cases, a second "A" signal, the "A1" or "ghost A", was sometimes keyed in, and the pattern of flashing on the display could be used to identify the chain.
The operator initially tuned in their receiver to see a stream of pulses on the display, sometimes including those of other chains which were nearby in frequency. He would then tune a local oscillator that started the trigger of the oscilloscope's trace so that it matched the clock at the master station (which could, and did, change over time). Next he would use a variable delay to move the start of the signal so one of the "A" pulses was at the very left side of the 'scope (the action is identical to the "horizontal hold" dial on an analog television). Finally the speed of the trace across the display would be tuned so the D pulse was just visible on the right. The distance of the B or C pulse from the A pulse could now be measured with an attached scale. The resulting delays could then be looked up on a navigational chart.
The display was relatively small, which limited resolution, and thus the determination of the delay. A measurement accuracy of 1 microsecond was quoted, which resulted in an accuracy of the determination of the correct hyperbolic to about 150 meters, and when two such measurements were combined the resulting fix accuracy was around 210 m. At longer ranges, 350 miles for example, the error ellipse was about 6 miles by 1 mile. The maximum range was about 450 miles, although several long-range fixes were made under unusual circumstances.
LORAN
The US had also considered hyperbolic navigation as early as 1940, but only halting progress had been made by the time they were introduced to Gee. Gee was immediately selected for the 8th Air Force and their attention turned to other uses, eventually considering convoyConvoy
A convoy is a group of vehicles, typically motor vehicles or ships, traveling together for mutual support and protection. Often, a convoy is organized with armed defensive support, though it may also be used in a non-military sense, for example when driving through remote areas.-Age of Sail:Naval...
navigation in particular. R. J. Dippy, inventor of Gee, moved to the US in mid-1942 to help with this project, which eventually emerged as LORAN, for LOng RAnge Navigation. LORAN became LORAN-A when the design of its replacement started, this was initially the LORAN-B concept, but eventually replaced by the very long-range LORAN-C starting in 1957.
LORAN-A was essentially a version of Gee with a new selection of frequencies
Frequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...
suitable for long-range transmission over water, eventually selecting 1.950 MHz. 7.5 MHz was selected for daytime use as an additional channel, but never used operationally. In comparison to Gee's 450 mile range through air, LORAN-A had a range of about 1500 miles over water, and 600 miles over land. Operation was generally similar to Gee, but only one of the slave signals was displayed at a time. A fix required the operator to measure one delay, then the other, and then look up the resulting delays on the charts. The accuracy was quoted as 1% of range.
LORAN-A used two methods to identify a chain. One was the operational frequency, with four "channels", as in Gee. The second was the rate at which the pulses were repeated, with "high", "low" and "slow" rates. This allowed for up to 12 chains in any given area. Additionally, the originally steady repetition of the pulses was later modified to create another eight unique patterns, allowing a total of 96 station pairs. Any given chain could use one or more pairs of stations, demanding a large number of unique signals for widespread coverage.
Decca Navigator
The Decca Navigation System was originally developed in the US, but eventually deployed by the Decca Radio company in the UK and commonly referred to as a British system. Initially developed for the Royal NavyRoyal Navy
The Royal Navy is the naval warfare service branch of the British Armed Forces. Founded in the 16th century, it is the oldest service branch and is known as the Senior Service...
as an accurate adjunct to naval versions of Gee, Decca was first used on 5 June 1944 to guide minesweeper
Minesweeper (ship)
A minesweeper is a small naval warship designed to counter the threat posed by naval mines. Minesweepers generally detect then neutralize mines in advance of other naval operations.-History:...
s in preparation for the D-Day
D-Day
D-Day is a term often used in military parlance to denote the day on which a combat attack or operation is to be initiated. "D-Day" often represents a variable, designating the day upon which some significant event will occur or has occurred; see Military designation of days and hours for similar...
invasions. The system was developed post-war and competed with GEE and other systems for civilian use. A variety of reasons, notably its ease-of-use, kept it in widespread use into the 1990s, with a total 42 chains around the world. A number of stations were updated in the 1990s, but the widespread use of GPS led to Decca being turned off at midnight on 31 March 2000.
Decca was based on comparing the phases of continuous signals instead of the timing of their pulses. This was more accurate, as the phase of a pair of signals could be measured to within a few degrees, four degrees in the case of Decca. This greatly improved inherent accuracy allowed Decca to use much longer wavelengths than Gee or LORAN while still offering the same level of accuracy. The use of longer wavelengths gave better propagation than either Gee or LORAN, although ranges were generally limited to around 500 miles for the basic system. However, Decca also had the inherent disadvantage that the signal repeated over space, and could only identify the location within what they referred to as "lanes". These were relatively small, so additional information was needed to identify which lane the receiver was located in.
Decca solved this problem though the use of an odometer
Odometer
An odometer or odograph is an instrument that indicates distance traveled by a vehicle, such as a bicycle or automobile. The device may be electronic, mechanical, or a combination of the two. The word derives from the Greek words hodós and métron...
-like display known as "decometers". Prior to leaving on a trip, the navigator would set the decometer's lane counter to their known position. As the craft moved the dial's hand would rotate, and increment or decrement the counter when it passed zero. The combination of this number and the current dial reading allowed the navigator to directly read the current delay and look it up on a chart, a far easier process than Gee or LORAN. It was so much easier to use that Decca later added an automatic charting feature that formed a moving map display. Later additions to the signal chain allowed the zone and lane to be calculated directly, eliminating the need for manual setting of the lane counters and making the system even easier to use.
As each master and slave signal was sent on a different frequency, any number of delays could be measured at the same time; in practice a single master and three slaves were used to produce three outputs. As each signal was sent on a different frequency, all three, known as "green", "red" and "purple", were simultaneously decoded and displayed on three decometers. The slaves were physically distributed at 120 degree angles from each other, allowing the operator to pick the pair of signals on the display that were sent from stations as close to right angles to the receiver as possible, further improving accuracy. Maximum accuracy was normally quoted as 200 yards, although that was subject to operational errors.
In addition to greater accuracy and ease of use, Decca was also more suitable for use over land. Delays due to refraction can have a significant effect on pulse timing, but much less so for phase changes. Decca thus found itself in great demand for helicopter use, where runway approach aids like ILS
ILS
Ils may refer to:*Ils , an electronic music producer and DJ*Them , a French horror filmILS may refer to:In Organizations:...
and VOR
Vör
In Norse mythology, Vör is a goddess associated with wisdom. Vör is attested in the Prose Edda, written in the 13th century by Snorri Sturluson; and twice in kennings employed in skaldic poetry...
were not suitable for the small airfields and essentially random locations the aircraft were used. One serious disadvantage to Decca was that it was susceptible to noise, especially from lightning
Lightning
Lightning is an atmospheric electrostatic discharge accompanied by thunder, which typically occurs during thunderstorms, and sometimes during volcanic eruptions or dust storms...
. This was not a serious concern for ships, who could afford to wait out storms, but made it unsuitable for long-range air navigation where time was of the essence. Several versions of Decca were introduced for this role, notably DECTRA and DELRAC, but these did not see widespread use.
LORAN-C
LORAN-A was designed to be quickly built on the basis of Gee, and selected its operating frequency based on the combination of the need for long over-water range as well as accuracy of the resulting fix. Using much lower frequencies, in the kHz instead of MHz, would greatly extend the range of the system. However, the accuracy of the fix is a function of the wavelengthWavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
of the signal, which increases at lower frequencies - in other words, using a lower frequency would necessarily lower the accuracy of the system. Hoping for the best, early experiments with "LF Loran" instead proved that accuracy was far worse than predicted, and efforts along these lines were dropped.
Several halting low-frequency efforts followed, including the Decca-like Cyclan and Navarho concepts. A key development was the introduction of the low-cost phase-locked loop
Phase-locked loop
A phase-locked loop or phase lock loop is a control system that generates an output signal whose phase is related to the phase of an input "reference" signal. It is an electronic circuit consisting of a variable frequency oscillator and a phase detector...
in the 1950s, which allowed a receiver to "lock on" to a signal and maintain the phase and frequency very accurately. This allowed a carrier signal to be re-constructed in a local oscillator by observing received pulses, a process that was accurate enough for phase comparisons against the local oscillator. This allowed a single system to combine the features of pulse-based and phase-based systems.
Re-using the Cyclan transmitters, the US Navy started experiments with such a system int the mid-1950s, and turned the system on permanently in 1957. Numerous chains followed, eventually providing around-the-world coverage near US allies and assets. Although less accurate that Decca, it offered the combination of reasonable accuracy and long ranges, keeping it in operation until GPS finally led to its shutdown on 8 February 2010.
In basic operation, LORAN-C is more similar to Decca than Gee or LORAN-A, as its main way determining location was the comparison of phase differences between signals. However, at low frequencies and long ranges it would be difficult to know whether you are looking at the current phase of the signal, or the phase of the signal one cycle ago, or perhaps one reflected off the ionosphere
Ionosphere
The ionosphere is a part of the upper atmosphere, comprising portions of the mesosphere, thermosphere and exosphere, distinguished because it is ionized by solar radiation. It plays an important part in atmospheric electricity and forms the inner edge of the magnetosphere...
. Some form of secondary information is needed to reduce this ambiguity. LORAN-C achieved this by sending unique details in the pulses so each station could be uniquely identified.
The signal was started off when the Master broadcast a sequence of nine pulses, with the precise timing between each pulses being used to identify the station. Each of the Secondary stations then sent out their own signals, consisting of eight pulses in patterns that revealed which station they were. The receivers could use the signal timings to select chains, identify secondaries, and reject signals bounced off the ionosphere.
LORAN-C chains were organized into the Master station, M, and up to five Secondary stations, V, W, X, Y, Z. All were broadcast at 100 kHz, a much lower frequency than earlier systems. The result was a signal that offered a daytime ground wave range of 2,250 miles, nighttime ground wave of 1,650 miles and skywaves out to 3,000 miles. Timing accuracy was estimated at 0.15 microseconds, offering accuracies on the order of 50 to 100 meters.
Omega
One of the last hyperbolic navigation systems to enter operational use was one of the earliest to be developed. Omega traces its history to work by John Alvin Pierce in the 1940s, working on the same basic idea as the Decca phase-comparison system. He imagined a system specifically for medium-accuracy global navigation, and thus selected the extremely low frequency of 10 kHz as the basis for the signal. However, the problem with phase ambiguity, as in the case of Decca, meant that the system was not practical at the time.Where the phase-locked loop made LORAN-C a possibility, for Omega it was the introduction of inertial navigation system
Inertial navigation system
An inertial navigation system is a navigation aid that uses a computer, motion sensors and rotation sensors to continuously calculate via dead reckoning the position, orientation, and velocity of a moving object without the need for external references...
s (INS) that offered a solution - the INS was accurate enough to resolve any ambiguity about which lane the receiver was in. Experiments continued throughout the 1950s and 60s, in parallel with Decca's development of their almost identical DELRAC system. It was not until the 1960s, when ice-breaking ballistic submarines became a main deterrent force, that there was a pressing need for such a system. The US Navy authorized full deployment in 1968, reaching a complete set of 8 stations in 1983. Omega would also prove to be one of the shortest-lived systems, shutting down on 20 September 1997.
Omega stations broadcast a continuous-wave signal in a specific time-slot. In order to maintain precise timing of the slots for stations distributed around the world, stations were equipped with synchronized atomic clock
Atomic clock
An atomic clock is a clock that uses an electronic transition frequency in the microwave, optical, or ultraviolet region of the electromagnetic spectrum of atoms as a frequency standard for its timekeeping element...
s. These clocks also ensured that their signals were sent out with the right frequency and phase; unlike previous systems, Omega did not need to have a master/slave arrangement as the clocks were accurate enough to trigger the signals without an external reference. To start the sequence, the station in Norway
Norway
Norway , officially the Kingdom of Norway, is a Nordic unitary constitutional monarchy whose territory comprises the western portion of the Scandinavian Peninsula, Jan Mayen, and the Arctic archipelago of Svalbard and Bouvet Island. Norway has a total area of and a population of about 4.9 million...
would initially broadcast on 10.2 kHz for 0.9 seconds, then turned off for 0.2 seconds, then broadcast on 13.6 kHz for 1.0 seconds, and so on. Each station broadcast a series of four such signals lasting about a second each, and then stood silent while other stations took their turn. At any given instant, three stations would be broadcasting at the same time on different frequencies. Receivers would select the set of stations that were most suitable for their given location, and then wait for the signals for those stations to appear during the 10 second chain. Calculation of the fix then proceeded in precisely the same fashion as Decca, although the much lower operating frequency led to much less accuracy. Omega's charts quote accuracies of 2 to 4 nautical miles.
CHAYKA
CHAYKA is the Soviet UnionSoviet Union
The Soviet Union , officially the Union of Soviet Socialist Republics , was a constitutionally socialist state that existed in Eurasia between 1922 and 1991....
's counterpart to LORAN-C, and operates on similar principles and the same frequency. It differs primarily in details of the pulse envelopes. There are five CHAYKA chains distributed around the former Soviet Union, each with a master and between two and four slaves.
Alpha
Alpha, more correctly known by its Soviet name, RSDN-20, is essentially a version of Omega deployed in the former Soviet UnionSoviet Union
The Soviet Union , officially the Union of Soviet Socialist Republics , was a constitutionally socialist state that existed in Eurasia between 1922 and 1991....
starting in 1962. The initial system used only three transmitters running roughly in a line in Krasnodar, Revda and Novosibirsk, the later being the master station. In 1991 two additional stations came online at Khabarovsk and Seyda. The stations use frequencies between 11 and 14 kHz.