Global Positioning System Reference
In-Depth Information
late 1930s to sense the range, height, heading, and velocity of aircraft and ships by
bouncing radio signals off remote targets and sensing the reflections. The time
taken for a pulse to travel to a target and back again provides a distance
measurement (known as lateration). The angle of the antenna (known as
angulation) when it illuminates the target provides a heading and height. Speed is
found by finding the Doppler shift in the received radio signal frequency that
could be measured by mixing it (or heterodyning it) with the radio frequency
carrier of the transmitted signal. Secondary radar was also used to determine the
identity of the target and is active rather than passive. The first systems were
military “identification friend or foe” (IFF) systems. A radio transponder fitted to
the “friend” squarks [sic] a valid digital code when polled by the system (unlike a
foe that would remain inactive). This signal would also be picked up by the highly
directional primary radar antenna. The “blip” on the screen from the primary
system could thus be labeled with its identity. Identity is of fundamental
importance to positioning. Today we may see the same transponder approach
being used by air traffic controllers and in shops and warehouses with goods fitted
with small RFID tags.
Further advances in the Second World War (WW2) led to aircraft guidance
systems that used a combination of radio timing measurements from different
transmitters to provide an automatic means to find the location of exactly where to
drop bombs at night. These electronic advances set the stage for the space age
when electronic navigation matured and systems such as GPS became possible.
Autonomous guidance systems or autopilots were developed for both aircraft and
rockets that used gyroscopes to provide a stable orientation platform much more
reliable than a compass. Accelerometers and ground topography following radar
added further sophistication, accuracy, and automation.
The underwater equivalent of radar is sonar, which uses much the same
approach but uses sound pressure waves in water rather than electromagnetic
radiation. Indoor positioning is currently being researched and developed using
ultrasonics in air. The main differences between sound and radio are that sound
wave propagation is much slower and the range is less.
Passive radio techniques for positioning were also first used in WW2 and
were generally known as direction finding (DF). When a radio signal was
intercepted, an array of receiving antennae was used to create a highly directional
system that had high antenna gain in one direction. This could be steered to find
the bearing of the signal. If repeated in other places, then the intersecting beam
directions could be plotted on a map and the approximate location of the
transmitter found. A more automatic approach to this method involved the
invention of the goniometer, where a static fixed circular array of receiving
antennae could be made to “rotate” electronically by continuously switching the
antennae to the receiver. Many modern radar and radarlike systems use static
antenna arrays with complex computer controlled electronic beam forming
techniques to create and steer beams rapidly without moving physically.
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