Global Positioning System Reference
In-Depth Information
nas were employed for RDF aboard aircraft. As expected, the nulls were
sharper (extending over a narrower range of angles) than the peaks, so
navigators preferred to use nulls for obtaining an LoP from a loop antenna.
LORENZ SYSTEMS
You may think that you have escaped from TLAs, at least for one section of
this chapter, given the name of this German RDF system developed during
World War II (though Lorenz was the name given to it by the Allies). You
would be wrong: Lorenz is one of many examples of LFR (low-frequency
receiver) devices developed during the 1930s and 1940s, in which radio
direction finding was achieved by transferring part of the task to the trans-
mitter. That is, instead of the receiver and the navigator performing all the
work of estimating transmitter direction, as with my Mercedes receiver in
figure 8.4, a significant part of the task was carried out by modifying the
transmitted signal in some way. This trend would continue into later de-
cades, when all of the direction-estimating work would be carried out by
the transmitter. This eased the navigator's task, as we will see.
The Lorenz system was used by German bombers to guide them toward
British cities during the later stages of the Battle of Britain. It worked as
shown in figure 8.5. Two narrow-beam transmitters—each with a beam-
width of only a few degrees—overlapped a little. Each beam transmitted a
modulated signal, but (here is the clever part) the signals in the two beams
were out of phase. The very slight overlap between them was pointed in the
direction of the target city. An aircraft that flew in one beam or the other
would detect a modulated signal, whereas one that flew in the overlap
region would detect a constant signal. German aircraft would fly along the
beams, and their navigators would know that they were on the right bear-
ing when their Lorenz receivers produced a constant signal. If the received
signal was modulated, they were too far to the left or right and would make
a course correction.
Note how the onus of direction estimation is moving from the re-
ceiver to the transmitter. The complicated equipment was back at base, on
friendly soil; as a consequence, the airborne receivers were small and
simple. The Lorenz system was successful for a while, until British elec-
tronic countermeasures defeated it. 10
10. Actually, the Lorenz transmitters were in France, which was not so friendly to the
Germans; but at that period the French were not overtly hostile, having been conquered a
few months previously. The Battle of Britain saw the first large-scale adoption of electronic
warfare and electronic countermeasures, aimed at guiding German bombers to their target
 
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