Speed Derived from GPS Fixes
As discussed in a previous note on Global Positioning System (GPS), it argues that the accuracy of position fixing could be limited. The accuracy of derived information such as course over ground (COG) and speed over ground (SOG) could be even worst.
A velocity of one meter per second can be translated into a speed of about two knots. As GPS produces fixes every second, any difference of one meter between subsequent fixes could result a speed of two knots. A one meter eastward and another meter westward will appear as a course change of 180 degrees. Similar to many other navigation systems, GPS receivers attempt to minimize the effect of these random errors by averaging its readings over a longer period of time. The downside of the technique is the delay in reflecting changes.
At open sea while a vessel is sailing over a steady course, the delay will not cause any practical problem. Indeed, any differences caused by the averaging technique, or damping, is not of any significant.
However, it is a very different story in port or within a harbour. Course and speed of vessels are changing constantly. Thus, the readings from any system deploying damping in reducing the effects of random errors are likely wrong.
In the radar screen on the left, the observer’s ship was making a turn to starboard rounding an island. The radar was set to present true motion picture in which fixed objects such as land were stabilised with GPS inputs. Echo trails after moving targets should be showing path of those targets in true motion. Fixed objects should not have any echo trail.
As shown, there were clearly echo trails built up for fixed objects such as land and bridges. In other words, the corrections used to calculate motion were wrong. Amongst the others, it was caused by damping. It was causing errors in both course and speed of the observer’s ship. The echo trails of the fixed objects in the picture were reflecting an error of its GPS derived speed between two to three knots.
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