Measurements from two pairs of stations yield intersecting lines of position which can provide a position fix. Below, hyperbolic line A of station pair M-S1 intersects line B of pair M-S2. That intersection of two lines of position (LOPs) is a fix.
Loran C was the main, everyday radio navigator. Michelson had two Loran receivers, initially the SPN-28, originally intended for aircraft use, requiring a 400 hertz AC power supply. Later these were replaced with WPN-3A receivers, with optical analog to digital converter outputs feeding the NAVDAC computer, which calculated position data and sent to a printer for the on duty oceanographer.
There was good coverage throughout the ship's operating areas. Loran C was somewhat susceptible to sky wave interference at longer distances from the transmitting stations. In the receivers at that time, ten microsecond errors could be a problem despite transmitted phase coding and receiver envelope matching. Phase comparison of the signals was made at the 100 khz transmitter frequency, 10 microseconds being the period of one cycle. Overall, Loran C had long range and good accuracy, there being nothing better available until the advent of satellite based systems.
Most radio navaids typically referred their transmitters as master and slave stations, because the slaves were phase locked to the master station's signal, either through over the air synchronization or later using ultra stable cesium beam clocks. Now, in the world of non offensive speech, the generally acceptable, politically correct terms are master and secondary stations.
Two other radio position fixing systems were installed on Michelson at different times:
Decca Navigator in the early 1960s. Widely used in Europe and the UK, Decca stations transmitted continuous wave signals in the range of around 70 to 125 khz, each being a multiple of a basic frequency called "f" (about 14 khz). Decca receivers made phase comparisons at f. Four Decca transmitters, master (6 times f), red (8f), green (9f) and purple (5f), comprised a chain. Signals from two pair of stations were required to fix a position. Once a minute, each master/secondary station pair transmitted lane identification signals. Decca lanes and lines of position were printed on nautical charts of their service areas.
First used in the 1940s, Decca was widely thought to be the ancestor of Loran C because it was the original radio navigator to use signal phase comparison. Regrettably, it suffered from skywave signal contamination making night time use less than reliable.
- A Decca Navigator system descripton can be found here.
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| Position fixing using Decca. Phase differences between master/secondary station pairs were displayed on decometers. A lane identification feature located the receiver between the correct pair of adjacent hyperbolic lines of postion. |
Lorac was a proprietary system owned by Seismograph Service Corporation, then an Oklahoma oil exploration services company. Three stations, a red, green and center station transmitted continuously in the 1.7 to 2.5 mhz spectrum. Red and green frequencies differed from that of the center station by two audio frequencies, typically 240 hz and 600 hz. These difference signals were detected at the reference station and used to amplitude modulate that station's transmitter. Phase comparison at the receiver was made between the center/red and center/green difference frequencies and those transmitted by the reference station.
The Navy's underwater test range in the Bahamas had a Lorac chain, which Michelson used while conducting seemingly endless sea trials in 1964. Readings were taken manually from the Lorac receivers and position fixes computed off line with the Bendix computer. The biggest problem I noticed with Lorac was its lack of lane identification. Two receivers furnished came with strip chart recorders which helped maintain lane count. A Seiscor tech rep came along too.
Once we had the Lorac lane count flown out to us from Patrick Air Force Base. A Navy ET who had previously been aboard Michelson brought us the correct count in a Lorac equipped helicopter.
