EPE = estimated position error

[Horsa]

GlobalTop offers a special firmware option which allows users to enable the geo-fencing function and set the center and the radius of a circular geo-fencing zone. Once set, the module will output a special proprietary serial protocol that lets the user know whether or not the receiver is inside or outside this zone. In addition, the distance between the current location and the center of the geofencing zone, as well as other information such as estimated position error and minimum recommended geofencing radius are calculated to give developers a higher level of flexibility when implementing this function.

MediaTek did not disclose much details about how it derives the EPE, but it is probably derived from a combination of factors such as C/N (carrier to noise ratio), HDOP (horizontal dilution of precision), and some other internal values.

Basically in our tests we found that, in order to prevent false alarms due to GPS position "drift" when the reception is less than ideal, it is recommended to set the size of the geo-fencing zone  larger than

   

recommended minimum geofencing radius = EPE value x 10.

This value is for reference only and will not restrict users in any way on setting a geo-fencing radius smaller than this.