And it worked pretty well. Lorac, meaning "longrange accuracy", was aproduct of theSeismographService Corporationof Tulsa, Oklahoma,which owned the patents. Equipment was built by a subsidiary calledSeiscor forboth civil and military markets. They also suppliedtechnical support in the field.
Originally intended for oil exploration surveys, semi-permanent Loractransmitterchains covered the US gulf coast from Texas to the Florida panhandleduring the late 1950s to 1960s.
The US Navy was a big user of Lorac for inshore and coastal surveys aswell as in sea searches to locate lost ships and aircraft. Ruggedizedfield deployable tactical transmitters and receivers wereused by regular navy survey ships of that time. A photo of AN/SRN-7,the navy's Lorac receiver, is below.
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| AN/SRN-7US Navy Lorac receiving equipment. From left: antenna coupler,indicator and receiver. |
Lorac transmitters operated in the 1.7 to 2.5 mhzmedium wavespectrum,then also the home of Loran A and the 160 meter amateur (ham) radioband. This is just above AM standard broadcast. Like Loran C, Decca andother radio navaids, Lorac was a "hyperbolic" phase comparisonsystem.
The useful range of a Lorac system was a bit over 100 miles inthe daytime, but perhaps half that at night due to skywaveinterference. Accuracy was said to be within 2 1/2 feet at best.
There appears to have been two somewhat different flavors ofLorac; the drawing below describes the variety used in the Bahamas in1964. This was calledLorac B.
Three land based stations transmitting unmodulatedcontinuous wave signalswere called green, red and center. For example frequencies ofthegreen and red stations might differ from that of the centerstation by600hzand 240hz respectively. These three signals were received at thereference station and the difference signals (audible tones of 600hzand240hz) detected.
Amplitude (AM) modulated by the two tones, the referencestation transmitted on a frequency unrelated to the other three but inthe same band. Green and red stations did not need to be phase lockedto the center station as minor variations would be reflected in thereference station's two modulating tones as well.
Aboard ship, the Lorac receiver measured the difference inphase between the green/center 600hz difference (beat frequency) andthe 600hz modulation on the reference station's signal. Similarly, thered/center 240hz beat frequency was compared with the referencestation's 240hz modulation. Phase meters indicated the green and redphase differences.
Mechanical counters kept track of the lane counts, a lanebeing about 80meters wide along the baselines between station pairs. Width of Loraclanes diverge from the green/center and red/center baselines sincepoints of equal phase describe hyperbolas. Thus, Lorac was potentiallyvery accurate as the smallest movement of the mobile (shipboard)receiver meant a large phase difference.
Being that no means of lane identification was provided, Loracreceivers had to be calibrated at a known geographic position at thebeginning of a survey.
Another version of Lorac did without a reference station. Thegreen and red stations alternately transmitted referenceinformation in a time shared mode. Instead ofradiating a singlefrequency, the center station switched between two different ones atabout 10times per second. Both center station frequencies differedfrom the green and red frequencies byaudio tones, just as in the above diagram. This switched green/red modewas calledLorac A.
Both the green and red stations were furnished with Loracreceivers. Green received the center and red signals and used theaudio tone difference (beat) frequency to modulate its transmissionduring the"green mode". Similarly, in the "red mode" the red station modulatedits signal using the center/green beat frequency.
Another diagram of how the switched mode Lorac A workedcan be found here.
As well as being more economical, Lorac A switched mode wasprobably used to allow a greater choice of operating frequencies in thenarrow band available. It also may have prevented interference from theadjacent Loran A chains which operated in the same frequency band.
All of this was a bit more complex, but very clever, very accurate andworked essentially the same as the four station Lorac B.
UntilGPS came along Lorac was probably the most accurate radio navaid forsurvey control. While it lacked the range of Decca and Loran C it waswidely used by the US Navy as well as by the oil industry.An excellent system description of Lorac can be found inReviewof Marine Navigation Systems and Techniques, archived on theweb. This is dated 1965, a Department of the Navy, Bureau of Shipspublication.Here is a link to thechapter on the Lorac radio navigator.
TheInternational Hydrographic Review, the journal of the IHO(Monaco), has published several articles about Lorac.The basics of Lorac A, theoriginal version of Lorac, can be found inHyperbolicPositioning Systems for Hydrographic Surveys, published in1953. From 1966,Calibration of Loraccovers the more accurate Lorac B system (four station Lorac).This article also describes a then new lane identification feature.
