Two GPS on ship show slight different positions due to following reason mentioned below –
it’s not because their antennas are separated by some distance.
– 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 Δt
where ‘s’ is speed of light (3 X 10 meters per second) and ‘Δt’ is the time difference(Δt) between data transmitted from different satellites and its reception at the receiver. ‘Δt‘ varies for each GPS resulting in the different values of ‘d ‘& hence slight difference in the position.
ERRORS OF GPS: –
- 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°.
- 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.
- satellite clock error –
This error is caused due to the error in the satellite’s clock w.r.t. GPS time. This is monitored by the ground based segments and any error in the satellite’s clock forms part of the 30 seconds’ navigational message.
- Geometric dilution of precision (GDOP) –
The geometry of the satellite determines the angle of cut which in turns governs the quality of the position obtained. Wider the angular separation between the satellites, more accurate is the fix as the intersection will be almost at right angles.