Errors of GPS
1. Ionospheric and Tropospheric delay: Due to the radio waves radiated by the satellite when passing through the different layers of the ionosphere and troposphere, the velocity of the waves varies which affects the time difference measurement and the fix will not be accurate.
2. 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 the 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.
3. Satellite clock error: This error is caused due to the error in the satellite’s clock concerning GPS time. This is monitored by .the ground-based segments and any errors in the satellite’s clock form part of the 30 seconds navigational message.
4. Geometric dilution of precision: The geometry of the position of the satellite determines the angle of cut which in turn governs the quality of the position obtained. The wider the angular separation between the satellites, the more accurate is the fix as the intersection will be almost at right angles.
The lower the GDOP value higher the accuracy of the fix and this GDOP value is indicated on the display unit.
5. Multipath error This error is caused by satellite signals arriving at the ship’s antenna both directly from the satellite and having reflected by some other objects.
6. Deviation of the satellite from the predicted path: The satellites are monitored and their paths are predicted by ground-based segment. However, between two consecutive monitoring of the same satellites, there may be minor drifts from their predicted paths resulting in small position inaccuracy.