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Local Area Augmentation Systems
A Local area Augmentation System is used to augment Global Positioning Systems
when fine, precise detail is required.
LAAS work via ground stations, which are fixed, permanent structures and who
know their precise global position - no earthquakes, ground subsidence, or other
building movement issues since the position was last worked out.
These are in constant communication with the GPS satellites in orbit.
A GPS receiver of any type calculates its position by measuring the distance
between itself and three or more GPS satellites. It works via a receiver unit at the position to be checked, which has an internal clock, and a microwave receiver to receive signals from orbiting satellite or stratellite systems. The user does not have to transmit anything to the satellite and the satellite does not know the user is there.
The precise time delay for the clock tick of the microwave signal sent gives the distance to each satellite, with an error based on the accuracy of the receiver's clock.
The signal also reports the satellite's location in space. The satellites
continually monitor one another's signals, to reassure one another of their precise position.
By determining the position of, and distance to, at least three satellites,
the receiver can compute its position using triangulation between them. A
problem comes in if the receiver's clock is less than accurate, causing errors
in the distance travelled. Most commercial systems are relatively accurate
for motionless distances (within 10m when the signal may have travelled 200,000m),
although this accuracy is nowhere near enough for fine work - checking positions
centimetres apart is impossible.
If the receiver is in motion, the error generated is multiplied by the speed
of the vehicle. This becomes critical for fast moving vehicles such as aircraft,
where errors in positioning could result in an aircraft ploughing into the
runway in bad weather, or mid-air collisions.
LAAS was first developed for air travel to combat this problem. The ground
structures are typically control towers, which are sweeping the skies with radar
to find all aircraft within range anyway.
It was thus a simple matter to have the LAAS ground station, with two clocks,
one the same as the ones found in aircraft, the other a very accurate clock.
They download the GPS data from all reachable satellites overhead, compare that
data to the tower's own position, its own position and calculate the offset
or error ratio between the two clocks.
This corrected signal, along with the error correction is then sent to every
aircraft within range. This allows each aircraft to receive correction data
to use againt it's own clocks, know for certain the error ranges, and reduce
the error sufficiently to allow close in manoeuvres around an airport.