User's Guide

Teletrac, Inc. - Prism TM Information and Installation Guide
16 1622-0300 B1 3/18/04
there, the NCC contacts the customer’s vehicles via the Internet. The vehicles that use GPS to
determine their location send that information directly to the NCC and it is in turn sent back to the
eClient workstation.
The GPS receiver built into the Prism TM works to determine the location of itself. As long as the
receiver is able to see enough satellites it can tell the Prism TM where it is. If a vehicle drives
into an underground garage, inside a warehouse or even under an overpass, the receiver may
not be able to see enough, if any, satellites to determine it’s location. Since the signals coming
from the satellites to the receiver are very low they can easily be blocked, even dense cloud
cover can reduce the actual signal.
The GPS receiver will determine its
location every few seconds and store the
information. When the Prism TM
Controller is contacted through the GPRS
modem, the Controller contacts the GPS
receiver and a request is made for its
location at a certain time. Once the
Controller receives the location information
from the GPS receiver, it relays the locate
to the Teletrac NCC via the GPRS modem.
Even if the GPRS modem cannot be
contacted by the NCC, the GPS receiver is
still collecting the information on where it is
located. When the GPRS modem is able
to communicate with the NCC, the Prism
TM will download the location information
that the GPS receiver has been providing.
Now, lets say your driver is taking a lunch break under the awning of a drive-up restaurant. In
this location the GPS receiver probably cannot see enough satellites to determine it’s location. In
this event, when the controller requests a locate from the GPS receiver, the last known location
will be used. Since the receiver takes it’s own readings every few seconds the last known
location is probably just outside the restaurant awning. When it’s time to send in a locate to the
NCC, the Prism TM can still “pick up” the GPRS modem and contact the NCC. But, the only
location that will be returned is the last known location reported to the Prism TM Controller, which
was probably just outside the awning. This location will be reported as a poor quality locate and
display as the last known location.
Even though the GPS receiver is blocked, a dispatcher can still send messages to a driver. Since
the messages travel over the GPRS system they will be sent to the Prism TM and simply a poor
locate (last known location) will be returned to the dispatcher.
Lastly, the Prism TM can be set up to store events such as ignition on/off, messages and location
information when the GPRS modem is out of it’s coverage area. The events, messages and
locations can be stored in a memory buffer and later transmitted once the modem is able to
communicate. See the following chart to help explain what happens when each system is able to
operate or is blocked.
GPRS Communication to NCC
Satellite
GPRS ModemControl BoardGPS Receiver
Location Unit