User's Guide
Model: VSM3
21
Positioning Universal, Inc. - Confidential & Proprietary
10 Pinning, GPS Quality Filter, and AGPS
The Quality Filter is needed to prevent poor quality gps data from triggering erroneous
events. Only good quality locations which meet the quality filter are used in move
locations. The GPS module reports locations on a 5 HZ interval.
10.1 Pinning
Typically, the location reported by a GPS device is an estimate of location based on the
calculations inside the GPS module. This estimate can vary over time, even for a
stationary vehicle because of the effects of the moving satellites and atmospheric and
local conditions. This can result in the module reporting a stationary object as moving
slowly. In addition, occasional interferences or satellite signal reflections can result in
errant location reporting.
GPS quality is used to filter out suspect information based on the number of satellites
whose signals are reaching the GPS module at any given time, in addition to estimates the
module makes of how accurate the fix might be. To reduce the risk of falsely reporting
that a stationary device is moving, the GPS module “pins” the device location to a fixed
point when the fix appears to be stable and not moving a significant distance.
10.2 Filter Parameters
A location will be considered good if it meets the following criteria:
● has more than or equal to the minimum number of satellites
● has at least the minimum Horizontal Accuracy (HAC) estimate. Note that the
minimum can vary based on Stopped or Moving state. A tighter HAC threshold is
typically used in Stopped state to reduce the incidence of “false” indication of
movement
10.3 Adding Events In Poor Coverage
If a location is below the quality threshold then it will not be used for event reporting.
Instead the last good location will be used for new events. This means that when a
vehicle enters an underground garage, it will appear to stop at the entrance to the garage
since the device probably hasn’t received any good locations after it entered.
10.4 AGPS
The device attempts to download GPS assist data from the DMAN server using current
cell tower information when the following conditions are met:
● The device has transitioned to Moving state
● The device doesn’t have a valid GPS fix