Changing atmospheric conditions change the speed of the GPS signals as they pass through the Earth’s atmosphere and this affects the time difference measurement and the fix will not be accurate. Each satellite transmits its message on two frequencies and hence a dual frequency receiver receives both the frequencies and correction is calculated and compensated within the receiver thus increasing the accuracy of the fix.
- Effect is minimised when the satellite is directly overhead.
- Becomes greater for satellites nearer the horizon. The receiver is designed to reject satellites with elevation less than 9.5°.
User Clock Error
If the user clock is not perfectly synchronised with the satellite clock, the range measurement will not be accurate. The range measurement along with the clock error is called pseudo range. This error can be eliminated within the receiver by obtaining 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 satellites clock forms part of the 30 seconds navigational message.
The satellites are monitored and their paths are predicted by the ground based segment. However, between two consecutive monitoring of the same satellite, there may be minor drifts from their predicted paths resulting in small position inaccuracy.
The GDOP of a satellite determines the angle of cut which in turn governs the quality of the position obtained. Wider the angular separation between the satellites, better the accuracy of the fix. Or, conversely said, the lower the GDOP value, the greater the accuracy of the fix. The GDOP value is indicated on the display unit.
This error is caused by the satellite signals arriving at the ship‟s antenna both directly from the satellite and those that get reflected by some objects. Thus two signals are received simultaneously which will cause the distortion of signal from which range measurement is obtained. Siting the antenna at a suitable place can minimize this error.