Errors of Marine GPS
This article aims to shed light on some of the common errors of marine GPS systems. Marine GPS systems have revolutionized navigation at sea, providing mariners with an unprecedented level of information and accuracy. However, like all technological systems, they are not without their errors. The 6 errors of marine GPS are listed below
Ionospheric and Tropospheric Delays
One of the primary sources of error in GPS systems arises from the delay of satellite signals as they pass through the ionosphere and troposphere. These atmospheric layers have different densities, which can cause the velocity of the radio waves to vary. This variation affects the time difference measurement, leading to inaccuracies in the GPS fix.
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Dual frequency receivers, which receive both frequencies transmitted by each satellite, can calculate and compensate for these errors. This results in a more accurate fix than what single frequency receivers can provide. To minimize refraction errors, receivers are designed to ignore satellites with an elevation of less than 9.5°.
User’s Clock Error
The synchronization of the user’s clock with the satellite’s clock is crucial for accurate range measurement. If the user’s clock is not perfectly synchronized, the range measurement, known as pseudo range, will be inaccurate. This error can be eliminated within the receiver by obtaining the pseudo range from three satellites.
Satellite Clock Error
Errors can also occur due to discrepancies in the satellite’s clock with respect to GPS time. These errors are monitored by ground-based segments and are included in the 30-second navigational message transmitted by the satellite.
Geometric Dilution of Precision (GDOP)
The quality of the GPS fix is also influenced by the position and geometry of the satellites. A wider angular separation between the satellites results in a more accurate fix. The lower the GDOP value, the higher the accuracy of the fix. Receivers are programmed to select the best available satellites considering their elevation and geometry.
Multipath Error
Multipath error occurs when satellite signals reach the ship’s antenna both directly and via reflection from other objects. This can distort the signal used for range measurement. This error can be minimized by placing the antenna in a suitable location.
Satellite Path Deviation
Satellites are monitored and their paths are predicted by ground-based segments. However, minor drifts from their predicted paths between two consecutive monitoring can result in small position inaccuracies.
In conclusion, while marine GPS systems have greatly improved navigation at sea, understanding the potential errors can help mariners make the best use of this technology. Always cross-check your position with other navigation methods when possible, and remember that no system is infallible.
