GPS

Global

Positioning

System

The GPS is broken in 3 segments:

  1. Space segment cloud of satellite orbiting the earth
  2. Control segment consist of network of control station updating the satellite
  3. User segment

Each GPS satellite orbit the planet twice a day at an altitude of 10900 miles. To locate the GPS must know how far away from the satellite in view? it is being done by calculating the amount of time it takes for a signal transmitted by the satellite to be received.

So all the satellite operate on the same time baseline each contains a highly accurate atomic clock.

They are 24 satellites plus spares and at least 6 are in view at any given time and any point on earth.

GPS receivers only receive the data they don’t transmit anything back to the satellite.

Each satellite transmit microwave signals, these signals are timestamped, when received by the GPS in airplane it will compute the amount of time, therefore the distance between the aircraft and the satellite.

Performing this operation with 3 satellite simultaneously, the GPS can locate itself in 2 dimensions: the position of the aircraft in latitude and longitude, this calculation is called Trilateration .

Airplane travel in 3 dimensions so by adding 4th satellite the GPS unit can also determine the airplane altitude.

RAIM: Receiver Autonomous Integrity Monitoring (monitors GPS accuracy)

It needs at least 5 satellites or 4 satellites and altitude encoding transponder.

All IFR approved GPS must have RAIM capability.

WAAS: Wide Area Augmentation System (increases GPS resolution)

GPS receivers contain Chart Databases