Digital Matter Support

Common Concepts - GPS Settings

PDOP, Position & Speed Accuracy: These settings allow you to configure the minimum accuracy accepted for a fix. 

The PDOP is a measure of how imprecise a GPS fix is. This is due to the satellites that are used being too close together to triangulate effectively. Please note that values lower than 2.0 for PDOP will be difficult to achieve in practice. Default Position Accuracy is 50 metres.

Static Hold: If a device is measured to be going less than this speed, it will set it to stationary and it's position fixed.

GPS Model: This selects a statistical module used by the GPS hardware when filtering out noise during GPS fixes. 

Generally choose Automotive

Discard First N Fixes: This option discards a configurable number of points from the GPS if the PDOP is above 2.3. 

This gives the GPS some time to improve its accuracy, and lowers the chance of outlying GPS fixes slipping through the filters.

Fix Fail Flag Timeout(s): Time in seconds of no GPS fix after which the fail flag will be set on the declare Digital Input



Notes on Dilution of Precision (DOP) And Specifically PDOP

DOP is a description of the purely geometrical contribution to the uncertainty in a position fix and has a number of specific applications. Some examples are PDOP, GDOP, HDOP, VDOP, TDOP and RDOP.


DOP is a function expressing the mathematical quality of solutions based on the geometry of the satellites.

Position Dilution of Precision (PDOP) is the most common and we use it as a useful indication of the uncertainty of the 3 dimensional position (3 coordinates) - i.e. how accurate do we expect the position to be? 


PDOP has a best case value of 1, with higher numbers being less accurate. 

A low number of PDOP (2) is good, a high number (typically > 10) is considered to be bad. The best PDOP would occur with one satellite directly overhead and three others evenly spaced about the horizon.


PDOP could theoretically be infinite, if all the satellites were in the same plane.

Most Digital Matter devices have a parameter for the maximum PDOP value to consider for a valid fix. This is generally a high value as default and you can 'tighten' up this parameter if needed. Note that this might mean that position fixes take longer resulting in higher consumption of energy which might impact on battery-powered devices, so it is a trade-off between accuracy and battery usage.


For more info on the details of GPS performance see this article by UBLOX: https://www.u-blox.com/sites/default/files/the_gps_dictionary.pdf


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