How GPS Works

On of the topic under Module 5: 15 is about a Typical Aircraft Systems: GPS - Global Position System.

1. Introduction
As the name implies, the GPS is about the position of the aircraft on the globe. Before we go into the detail of its operation, let us view the various navigation systems used at different era in our history.

a. Compass - Based on the natural magnetic fields produced in the earth environment. Compass is used to show the direction of the magnetic north. Some of the issues that we face with the compass are:.. - cannot be used above a certain latitude as the magnetic line of force are getting to become vertical. - The accuracy is affected by local magnetism of magnetic material. - The is errors due to the turn and banking of the aircraft.

b. Radio Navigation The radio navigation relies of the radio signal radiated by radio signals from the navigation stations. - VOR - VHF OnmiDirectional Range - produce 360 degrees signal and the aircraft system is able to choose and select the radial. As the VOR station's location is known from the navigation map, the aircraft may compute its position, using 2 stations or DME inputs.

c. INS - Inertial navigation The position of the aircraft is calculated based on its known origin and movements of the aircraft. The movement of the aircraft is calculated by accelerometers.

 2. Different Satellite Navigation System
Currently, there are 2 global navigation systems.

- GPS - by US military
- Glonass - Russian system A third system by European is under development.Refer http://en.wikipedia.org/wiki/Galileo_(satellite_navigation)

3. How GPS Works

24 Navstar satellites

There are 24 active Navstar satellites orbiting the earth at about 12,000 nautical miles above the earth. They are continuously sending signals on L1 channel on 1575.42 MHz for civilian uses. The signals contain the synchronization code, identification code, position and time codes. With the installed aircraft systems, the distant of the aircraft to the satellite is calculated. With four satellites signal received and analysed, the aircraft will be able to locate its position. Other parameters are derived from the instantaneous position of the aircraft.

The amount of time taken for the signal to reach the aircraft and the location of the satellites enable the onboard computer to calculate its relative position. The distant from a satellite is a sphere at a certain distant from the satellite. With 2 satellites, the possible location is at any point forming a circle, where the two sphere meet. With 3 satellies, the position is fixed. The fourth satellite will recheck and confirm the position.