LORAN-C radionavigazione superstite

Un giornale locale della Lousiana ha pubblicato un bell'articolo sulla stazione LORAN-C di Grangeville. Il titolo del quotidiano è un po' fuorviante perché a differenza del predecessore Omega, Il LORAN-C dà una copertura dei 48 stati continentali e dell'Alaska, non è un sistema globale. Il sito di The Advocate con l'articolo originale pubblica anche una bella foto dell'antenna di circa 200 metri. Il LOng RAnge, Navigation System - C è un sistema di radioposizionamento di tipo iperbolico. In pratica il ricevitore del mezzo (nave, ma anche aereo) che deve determinare la posizione si sintonizza su un triangolo di tre stazioni, una master e due secondarie e calcola la leggera differenza temporale tra i segnali ricevuti. Questo consente di tracciare una doppia iperbole con una coppia master-secondary A e B come fuochi. Il ricevitore si troverà nel punto di intersezione delle due iperboli. La preciione, un quarto di miglio marino, non è quella del GPS, ma il sistema è affidabile ed economico. Creato una trentina d'anni fa, il LORAN-C è ancora attivo, ma il suo destino è probabilmente segnato. Trovate tutte le informazioni su questo affascinante sistema di radioposizionamento terrestre sul sito della Guardia Costiera americana. La frequenza operativa LORAN è di 100 kHz.

LORAN-C sent from St. Helena to world

Site keeps ships, planes on course

By DAVID J. MITCHELL
Advocate Florida parishes bureau
Published: Dec 31, 2007

GRANGEVILLE — Hidden far away in the piney woods of St. Helena Parish, a 640-foot transmission tower pulsing 20,000 volts helps ships and aircraft find their way as far south as Mexico’s Yucatan Peninsula.
Only a sign off La. 63 north of La. 16 lets passers-by know that the U.S. Coast Guard operates the facility 150 miles north of the Gulf of Mexico.
The facility — on about 300 acres since 1978 — is part of two chains of LORAN C (LOng-RAnge Navigation) transmission towers set up in rural, inland locales across the Southeast, Midwest and Southwest.
Coast Guard Petty Officer 1st Class Thornton Dixon was assigned to the facility about seven months ago from the harbor town of Portsmouth, Va. He smiled when asked about his land-based duty.
Dixon, 34, a native of Cincinnati, said area residents are not sure what happens where he works and have come to him with several theories.
“You know what the No. 1 question is? ‘Does the Amite River run through there?’ ” he said.
It doesn’t. The river runs about two miles to the west.
Dixon said the LORAN C stations were set up away from the coast to protect the radio towers from high winds and in rural areas to limit damage in case a tower falls.
The towers — there are 24 stations across the United States — use low-frequency radio waves that help aviators and sailors — civilian and military — navigate, fix their positions and keep time.
LORAN C is a precursor of the now-ubiquitous Global Positioning Satellite system. LORAN C is more reliable — but less accurate — than GPS, Dixon said.
With an increase in the use of GPS, the LORAN C has been proposed for the congressional budget axe since the mid-1990s but has not received the final blow.
In the meantime, a four-man crew keeps the Grangeville station transmitting signals 24 hours a day, seven days a week, 365 days a year.
Behind the pines, a white cinder-block building and a collection of trailers sit in the center of a broad grassy field framed by a gravel ring road, trees and ground-contact points for tower guy wires.
The building entrance greets visitors with a three-flag yardarm and a “Welcome Aboard” sign.
Inside, electronic equipment, including three cesium atomic clocks, create, shape, monitor, amplify and transmit signal pulses, Dixon said.
Behind a door that warns the visitor to cover his ears, a static, metallic, skipping sound — the sound of the signal — fills the room.
“The PA (public address) system at my church will pick it up once and a while. I will occasionally smile because I’m the only one who knows what it is,” Dixon said.
In that noisy room, two banks of 28 pull-out cabinets filled with electronics, amplify the signal before it is sent to the transmission tower via lines that run through the wall.
The room cannot get warmer than 70 degrees, Dixon said, and special fire-suppression equipment shaped like spheres line the back of the electronics banks, just in case.
“There’s a lot of voltage in there,” Dixon said.
The Coast Guard says the system’s future is under evaluation for the long term.
In 2001, the U.S. Department of Transportation’s Volpe National Transportation Systems Center recommended LORAN C be upgraded as a GPS backup because the center found GPS could be jammed or otherwise knocked out.
The Grangeville facility has gotten some of those upgrades, Dixon said, and Congress recently authorized funding the system for 2008.
But in January and in July, the Coast Guard sought public comment on plans that could include eliminating all LORAN C facilities.
Dixon said LORAN C seems to be safe through 2010, but after that he is not sure. “I don’t want to see it go away,” he said.