Why two GPS on ship show slight different positions ?

ERRORS OF GPS: – 

1. Ionospheric and tropospheric delay –

The radio waves radiated by the satellite when passing through the different layers of the ionosphere and troposphere, the velocity of the waves vary which affects the time difference measurement and the fix will not be accurate.

In case of signals from a satellite with low elevation, the refraction of radio waves will be more which will increase this error. Hence the receiver is designed not to select the satellites whose elevation is less than 9.5°.

1. Users clock error –

If the user clock is not perfectly synchronized with the satellite’s clock, the range measurement will not be accurate and the range measurement along with the clock error is known as PSEUDO RANGE.

This error can be eliminated within the receiver on board by obtaining the pseudo range from three satellites and is done automatically within the receiver.