Any random errors in the fixes used to calculate COG and SOG will produce errors in speed and course displayed on the GPS.
The rate at which the GPS position is updated is very rapid, but to minimise the effect of random errors, COG and SOG are averaged over about 5 seconds, by default, although the user may alter this time. The longer the time interval, the steadier the reading, but the slower the response to a real alteration of heading or speed.
If the error between two fixes were 15 metres, one to port and the next to starboard, the error in COG over a 5-second period at 6-knots speed could be greater than 45 degrees. Similarly, with similar errors, but in the direction of movement, the SOG displayed could be in error by 6 knots! With selective availability switched off, the normal
situation, random errors are likely to be very small, and COG and SOG are generally stable and accurate. With the default setting for the 'averaging time', watch the COG and SOG at a constant speed and heading to get an idea of how they respond in normal conditions.
If selective availability is switched on by the US military, the accuracy of COG and SOG will deteriorate significantly.
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