Influence of GNSS Receiver Update Rate

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jimh
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Influence of GNSS Receiver Update Rate

Postby jimh » Mon Dec 26, 2016 11:08 am

A GNSS receiver updates its deduced position at a particular rate. For some time, an update rate of 1-Hz was considered a standard. More modern GNSS receivers can now easily provide position update rates of 5-Hz or 10-Hz. The significance of an increased update rate depends mostly on the speed of motion of the vehicle or boat carrying the GNSS receiver. As the speed of travel of the receiver increases, the interval between position updates causes the distance travelled between position updates to lengthen. During this time interval between position solutions, there will be uncertainty about the location of the GNSS receiver. This distance between position solutions is easily calculated on the basis of time and speed.

For a GNSS receiver that is moving slowly, say at a rate of 1-MPH, an interval of 1-second between position updates (1-Hz) calculates to a distance of

(1-mile/1-hour) × (5280-feet/1-mile ) × (1-hour/3600-seconds) × (1-second/1-position-solution) = 1.47-feet of travel/position solution

If the speed of the GNSS receiver increases to 10-MPH, an interval of 1-second between position updates (1-Hz) calculates to a distance of 14.7-feet of travel/position solution.

If the speed of the GNSS receiver increases to 60-MPH, an interval of 1-second between position updates (1-Hz) calculates to a distance of 88-feet pf travel/position solution.

If the GNSS receiver can produce position solutions at a faster rate, say 10-Hz or once every 0.1-seconds, then the distance traveled between position solutions will be reduced to one-tenth. I summarize these relationships in a table below:

Distance Traveled between Position Solutions
as a function of receiver speed of travel

GNSS          GNSS      Distance traveled 
Update Rate Speed between position Solutions

1-Hz 1-MPH 1.5-feet
1-Hz 10-MPH 15.0-feet
1-Hz 60-MPH 88.0-feet
10-Hz 1-MPH 0.15-feet
10-Hz 10-MPH 1.5-feet
10-Hz 60-MPH 8.8-feet


An interesting trend to observe: as the GNSS receiver update rate increases, the variation with GNSS speed of travel becomes less. In the case of the 1-Hz update rate, the difference between 1-MPH and 60-MPH speed is quite significant, about 85-feet. When a 10-Hz update rate is used, this difference is reduced to just 8.6-feet. In navigation of a boat, it is unlikely that an uncertainty of the position while the boat continues to travel a modest distance of 8-feet will cause any particular danger in navigation.

Another factor to be considered is the accuracy of the position solution, the accuracy of the Earth geodesy model, and the accuracy of the electronic chart in use. For the GPS GNSS, a user of the L1 C/A signal, even with SBAS augmentation from WAAS, should expect the position solution to be only accurate to about 10-feet. The translation of that position onto the Earth's geodesy adds further uncertainty. And the mapping of the actual Earth to a representation on an electronic chart adds even more uncertainty.

The GPS position solution is initially calculated by reference to the satellite position in an inertial reference plane. This position must be transferred to an Earth-reference Earth-centered position according to a particular datum. The datum used must be entered by the user into the GNSS receiver in order for this translation of position reference planes to be properly calculated. While the error in the reference datum may be small, if the user has not entered the correct datum associated with the mapping to be used, the position solution can be off by a distance greater than any uncertainty due to intervals between position solutions.

The accuracy of mapping of Earth locations is also subject to error, and it is often seen, particularly in the case of detached land masses like islands for which historical mappings depended on position finding by relatively coarse methods, that electronic chart data is not precisely accurate to an error less than the uncertainty due to intervals between position solutions.

The accumulation of all these other errors inherent in GNSS navigation tend to make the relatively small uncertainties about location that exist between successive position solutions due to GNSS receiver update rate to be of not much concern in recreational boat navigation, even at speeds of 60-MPH.