What is GPS, its errors and details?

1. A master control station (MCS),
2. An alternate master control station,
3. Four dedicated ground antennas, and
4. Six dedicated monitor stations.

The satellites are controlled by keeping them on their correct orbital paths and updating them regularly with information so that data transmitted by them are accurate.

The Control Segment is the sole responsibility of the Department of Defense (USA).

USER SEGMENT

• The user segment consists of the antenna and the receiver which is fitted on board a ship.
• There are multiplier supplier of these equipment, who pay license fees to USA for use of positional information broadcast by the GPS satellites.

How position fixing is done by  GPS?

GPS uses the ranging principle to fix the ship.

The GPS concept is based on time and the known position of specialized satellites. The satellites carry very stable atomic clocks that are synchronized to each other and to ground clocks. Any drift from true time maintained on the ground is corrected daily. Likewise, the satellite locations are known with great precision. GPS receivers have clocks as well; however, they are not synchronized with true time, and are less stable. GPS satellites continuously transmit their current time and position. A GPS receiver monitors multiple satellites and solves equations to determine the precise position of the receiver and its deviation from true time. The time difference between data transmitted from the satellite and its reception at the receiver is measured (which is in nano-seconds) and integrated with speed of light (3 X 10 meters per second). i.e. d= s x ∆t

At a minimum, four satellites must be in view of the receiver for it to compute four unknown quantities (three position coordinates and clock deviation from satellite time).