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Why two GPS on ship show slight different positions?

Have you noticed that two GPS kept on bridge has a slight difference in their calculated position & its not because their antennas are separated by some distance. To understand this, first understand

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

Please note that Each GPS receiver has its own clock which are less stable which means each receiver has its own clock error and when it solves the equations to determine the precise position of the receiver i.e. d= s x ∆t where ‘s’ is speed of light (3 X 10 meters per second) and ‘∆t’ is the time difference(∆t) between data transmitted from different satellites and its reception at the receiver. ∆t‘ varies for each GPS resulting in the different values of ‘d‘ & hence slight difference in the position.

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ANAND GAUTAM

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