Thursday, July 06, 2006

Radio for Navigation (pre tomtom)

Throughout history men have gotten lost on long trips, flubbed their ability to use maps and refused to ask for directions. About 100 years ago it got harder to get lost.

I see this as developing in three major steps
1. radio wave detection
2. first attempt at a navigable beacon
3. first large scale implementation attempt.

In 1894 British physicist Sir Oliver Lodge demonstrated the possibility of signaling using radio waves. He did this with a device called a coherer [pic below], a vacuum tube filled with iron filings. He did not invent this device. In fact the Coherer had been invented 10 years earlier by Temistocle Calzecchi-Onesti. The coherer had gone through revisions in the hands of Edouard Branly and Alexander Popov. [Popov actually invented a radio communication system based on the coherer, is yet another candidate for inventor of radio]

The Coherer was a radio-wave detector and the heart of the early radiotelegraph receiver. It functioned as a rough gauge of resistance. I So now we could measure the radio wave, and thus distance from the transmitter by signal stregnth ... a fixed point. [later revisions to sensitivity allowed this to decode the first dots and dashes]
Around 1900, only 6 years later, Tesla opens the Wardenclyffe Tower facility and advertises it's services. He intended for the massive Wardenclyffe tower facility to be a radio beacon for navigation, and even time synchronization. But by 1905 Tesla had to shut it down. It was sold for $20 grand in debts.
In WWII radio navigation matures into practical use. The need to blow more people up more efficiently begets the need to pinpoint aircraft position. RDF is born. The Radio Direction Finder works by pointing a directional antenna in "various directions" and then listening for the direction in which the signal from a known station comes through most strongly. (this skips the triangulation needed to make an estimate with a coherer.) In WWII pictures of aircraft the RDF antennas are easy to spot on German World War II aircraft, as wire loops under the rear section of the fuselage. In Allied aircraft it was enclosed in a small teardrop-shaped fairing.
RDF using aircraft tuned into brodcasts of morse code letters. This technique is still used today by commercial aircraft. Most modern detectors can also tune in any commercial radio stations, which is particularly useful due to their high power and location near major cities.