Friday, November 13, 2009

Low-Frequency Navigation Stations

Your vocabulary word for the day is "Multilateration." Rather than mangle the defintion I am taking it directly from Wikipedia.
"The process of locating an object by accurately computing the time difference of arrival (TDOA) of a signal emitted from that object to three or more receivers. It also refers to the case of locating a receiver by measuring the TDOA of a signal transmitted from three or more synchronised transmitters."
It's like triangulation, except distances are determined by the time it takes a radio signal to travel a known distance. LORAN-C stands for LOng RAnge Navigation. LORAN-C is just one version of Loran. It was preceded (of course) By LORAN-A and LORAN-B. LORAN-D was used exclusively by the United States Airforce. LORAN-A transmitted on 1750 kHz, 1850 kHz, 1900 kHz and 1950 kHz. the System wasn't very accurate and LORAN-B was a phase comparison variation that tried to improve on the model. There was infact a LORAN-F system that Motorla experiemnted with. This was never launched. the migration to a digital "eLORAN" system has already begun. More here.

The present LORAN system is harshly effected by weather, and changes in solar radiation including sunrise and sunset. Most LORAN-C transmitters use mast radiators insulated from ground with heights between 190 and 220 meters. Some of these stations broadcast in excess of 1000 kW. They each broadcast a precisely timed 1Hz signal and all of them are perfectly synchronized to UTC within 100 nanoseconds.

A chain of LORAN-C radio navigation stations transmitting pulse-coded signals on the same frequency. Provides navigation location and timing services for air, land and marine users. The Loran-C system serves the 48 continental states, their coastal areas, and parts of Alaska. Each station has a Group Repetition Interval and has an ID number starting with GRI to indicate which chain they are a part of. Obviously in order to determine your location through multilateration you need at least 3 of these stations. That's why you'll see the same ID three or more times below. Inside the United States, the following stations broadcast:

GRI 7960 - Tok, AK
GRI 7960 - Narrow Cape, AK
GRI 7960 - Port Clarence, AK

GRI 7980 - Carolina Beach, NC

GRI 7980 - Raymondville, TX

GRI 7980 - Malone, MS

GRI 8290 - Gillette, WY

GRI 8290 - Baudette, MN

GRI 8290 - Havre, MT

GRI 8970 - Dana, IN

GRI 8970 - Malone, MS

GRI 9610 - Gillette, WY

GRI 9610 - Las Cruces, NM

GRI 9610 - Raymondville

GRI 9610 - Searchlight, NV

GRI 9940 - George, WA

GRI 9940 - Middletown, CA

GRI 9940 - Searchlight, NV

GRI 9960 - Nantucket Island, MA

GRI 9960 - Carolina Beach, NC

GRI 9960 - Caribou, ME

GRI 9990 - Attu Island, AK

Strangely in 2008 George Bush Jr. threatened to shut down the highly effective LORAN system. Instead after much hand-wringing the administration of the LORAN system was transferred from the Coast Guard to the Dept. of Homeland Security.