Measuring Distances

Radio Record, Volume III, Issue 29, 31 January 1930, Page 8

Measuring Distances

Radio Methods ‘ALTHOUGH many attempts have been made to devise a simple method of ascertaining the exact distance (as distinct from the bearing) of a ship or aeroplane from a given spot by wireless, the problem is by no means as easy as might be imagined. Of course, directional methods can be used to find the ship’s bearing relative to a known wireless beacon’ or , transmitter. Then, by repeating the ‘game operation on a second. beacon, station, it is possible to ascertain precise distance or location by . method of triangulation. Then there are systems in which "a transmitting station sends ont two signals simultaneously, one being a sound signal and the other a wireless signal. Sound waves travel through the air at a velocity of approximately 830 metres a second, whilst for all practical purposes we can regard the wireless signals as arriving instantaneously. If, therefore, the operator on the ship or aeroplane notes the interval which elapses between the receipt of the wireless signal and the subsequent arrival of the sound signal, and then multiplies this time in seconds by 830 (the .velocity of sound), he will get a very fair approximation of his actual distance in metres from the beacon station. This method is, of course, only applicable: over comparatively short distances, though it has distinct possibilities when navigating near the coast in foggy weather, or for preventing collisions between ships at sea under similar conditions.

MORE ingenious suggestion. has recently been made to solve the "distance" problem by utilising the well-known fact that true wireless radiation does not set in for a distance of approximately a quarter of a wavelength from the transmitting aerial. It is known that for ordinary or true radiation the signal or field strength diminishes with distance according to a straight-line law. In the case of pseudo-radiation (i.e., within a distance of a quarter of the wavelength from the transmitter) this law does not hold good. Accordingly a distinction can be made between two such signals received simultaneously from the same transmitter, and in this way a measure of the actual distance of the transmitter can be obtained. For instance, suppose the transmitting beacon sends out the letter "a" on a wavelength of 20,000 metres, interlaced with the letter "n"’ on 1000 metres. Adjustments are so made that at a radius of 3 kilometres -both signals are received at equal strength. Then, as a ship approaches the transmitter, the operator will first hear the shorter wavelength (the letter "n’’) at greater strength. As the ship draws nearer the letter: "n" weakens, until at the critical distance of 3 kilometres both signals merge into a continuous dash. At still closer distances the long-wave signal "a". . predominates. This allows the ship’s operator at any, time to ascertain within reasonable limits his relative distance from the warning station,