Direction finding
These wavelengths are nevertheless used for marine radio navigation as they can travel very long distances "over the horizon", which is valuable for ships when the line-of-sight may be only a few tens of kilometres.
Modern systems often used phased array antennas to allow rapid beamforming for highly accurate results, and are part of a larger electronic warfare suite.
Early radio direction finders used mechanically rotated antennas that compared signal strengths, and several electronic versions of the same concept followed.
[7] A major improvement in the RDF technique was introduced by Robert Watson-Watt as part of his experiments to locate lightning strikes as a method to indicate the direction of thunderstorms for sailors and airmen.
When the office was moved, his new location at a radio research station provided him with both an Adcock antenna and a suitable oscilloscope, and he presented his new system in 1926.
Improved vacuum tubes and the introduction of the transistor allowed much higher frequencies to be used economically, which led to widespread use of VHF and UHF signals.
The various procedures for radio direction finding to determine position at sea are no longer part of the maritime safety system GMDSS, which has been in force since 1999.
Early systems used mechanically rotated antennas that compared signal strengths from different directions, and several electronic versions of the same concept followed.
Modern pseudo-Doppler direction finder systems consist of a number of small antennas fixed to a circular card, with all of the processing performed by software.
However, the terminology was not inaccurate; the Chain Home systems used separate omnidirectional broadcasters and large RDF receivers to determine the location of the targets.
So simply turning the antenna to produce a minimum in the desired signal will establish two possible directions (front and back) from which the radio waves could be arriving.
With a sufficient number of shorter "director" elements, a Yagi's maximum direction can be made to approach the sharpness of a small loop's null.
Starting in the 1950s, these beacons were generally replaced by the VOR system, in which the bearing to the navigational aid is measured from the signal itself; therefore no specialized antenna with moving parts is required.
Due to relatively low purchase, maintenance and calibration cost, NDBs are still used to mark locations of smaller aerodromes and important helicopter landing sites.
The service is based on a number of radio DF units located at civil and military airports and certain HM Coastguard stations.
As the commercial medium wave broadcast band lies within the frequency capability of most RDF units, these stations and their transmitters can also be used for navigational fixes.
In World War II considerable effort was expended on identifying secret transmitters in the United Kingdom (UK) by direction finding.
Even with the expanded network, some areas were not adequately covered and for this reason up to 1700 voluntary interceptors (radio amateurs) were recruited to detect illicit transmissions by ground wave.
The HF Adcock stations consisted of four 10 m vertical antennas surrounding a small wooden operators hut containing a receiver and a radio-goniometer which was adjusted to obtain the bearing.
In 1941, RSS began experimenting with spaced loop direction finders, developed by the Marconi company and the UK National Physical Laboratories.
The Royal Navy introduced a variation on the shore based HF DF stations in 1944 to track U-boats in the North Atlantic.
[12] A comprehensive reference on World War II wireless direction finding was written by Roland Keen, who was head of the engineering department of RSS at Hanslope Park.
The transmissions from mobile telephone handsets are also located by a form of direction finding using the comparative signal strength at the surrounding local "cell" receivers.
In Naval systems, the DF capability became part of the Electronic Support Measures suite (ESM),[17]: 6 [18]: 126 [19]: 70 where the directional information obtained augments other signal identification processes.
Over time, it became necessary to improve the performance of microwave DF systems in order to counter the evasive tactics being employed by some operators, such as low-probability-of-intercept radars and covert Data links.
Earlier in the century, vacuum tubes (thermionic valves) were used extensively in transmitters and receivers, but their high frequency performance was limited by transit time effects.
[42] Amplitude comparison has been popular as a method for DF because systems are relatively simple to implement, have good sensitivity and, very importantly, a high probability of signal detection.
Although the gains of the antennas and their amplifying chains have to be closely matched, careful design and construction and effective calibration procedures can compensate for shortfalls in the hardware.
As all systems generate thermal noise[49][50] then, when the level of the incoming signal is low, the signal-to-noise ratios in the receiver channels will be poor, and the accuracy of the bearing prediction will suffer.
By considering the signal levels on a logarithmic scale, as provided by the DLVAs, a large dynamic range is achieved [56]: 33 and, in addition, the direction finding calculations are simplified when the main lobes of antenna patterns have a Gaussian characteristic, as shown earlier.
