Determination of boat speed is now generally done by measurement with a Global Positioning System (GPS) receiver. Most GPS receivers do not actually make any measurement of speed; the receiver just determines a position. Speed is usually calculated--not measured--by computation of the distance between two position fixes and the time stamp of each position fix. Measurement of time in GPS receivers is done with great precision; position measurements generally have an expectation of some error.
With every position fix calculated by a GPS receiver there are factors that introduce errors. In the case of boats operating on water, the position fixes are really only in two-dimensions. There is no change in elevation on the water. At any particular instant for any particular location on the Earth, the geometry of the satellites that will be visible to a GPS receiver is unique and continually changing. Because all positions are calculated on the basis of pseudo-ranges from the receiver to the satellite using at least three pseudo-ranges, the accuracy of the position solution varies with the geometry of those range lines. This influence is calculated in a metric known as the Horizontal Dilution of Position or HDOP, as the horizontal plane is the only consideration. The higher the metric HDOP becomes, the less accuracy there will be in the position solution. An HDOP of 1.0 (or lower) represents a good geometry for the pseudo-range lines to the satellites from the receiver.
If the geometry of the GPS satellites in view is such that the HDOP metric is high, say 3.0 or more, the position solution will have more error. Because SOG depends on the accuracy of the position solution, SOG will likely have more error with high DHOP position solutions.
The accuracy of GPS position solutions can be enhanced with augmentation systems. For boats, the most practical augmentation system is the Federal Aviation Administration's Wide Area Augmentation System (WAAS) that has been developed and deployed for aircraft. Position fixes with GPS receiver using WAAS will typically have much reduced error in position compared to un-augmented position solution. Again, because SOG is based on position solution, increased accuracy in position solution by use of WAAS augmentation should improved accuracy in SOG calculations.
For best accuracy in measuring SOG with GPS, use WAAS augmentation and conduct the measurements when the HDOP of the satellite geometry is at a value close to 1.0.
Articles about GPS, GLONASS, GALILEO, WAAS and other satellite navigation systems
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