Sierra Wireless EV-DO - Manual (Page 85)

Global Positioning System (GPS)
Rev 3.0B Feb.08 80
from the difference between local time and the time the
satellite signals were sent (this distance is called
psuedoorange). The locations of the satellites are decoded
from their radio signals and a database internal to the receiver.
This process yields the location of the receiver. Getting
positioning information from fewer than four satellites, using
imprecise time, using satellites too closely positioned together,
or using satellites too close to the Earth’s curve will yield
inaccurate data.
The GPS data is then transmitted to a central location which
uses a tracking application to compile information about
location, movement rates, and other pertinent data.
Note: Depending on the location of the satellites in relation to the
modem’s location and how many signals are being received, the
PinPoint-E may encounter “GPS drift”. The PinPoint-E may report it is
in a location a few feet from its actual location because it does not
employ differential GPS.
PinPoint-E Supported Protocols
The PinPoint‐E supports three different GPS reporting
protocols.
Remote Access Protocol (RAP)
The Remote Access Protocol (RAP) is a proprietary binary
message format developed by Sierra Wireless AirLink
Solutions. RAP was originally designed to work specifically
with AirLink Tracking System (ATS), but other 3rd party appli‐
cations have been developed to take advantage of the RAP
messaging format.
In the original RAP, a PinPoint line modem uses the UDP (User
Datagram Protocol) to communicate with the host server.
In RAP‐based AVL, each PinPoint line device sends its
command status and responses to the Host server and the Host
sends commands to one or more PinPoint line devices. For
reliability, the Host expects each command to be acknowl‐
edged within a time‐out period. If the acknowledgement
packet (ACK) is not received within the time‐out period, the
Host will retransmit the command.
The RAP messages are in Hex and are referred to by their
message ID. Reports can include GPS data alone, as well as
GPS data with the date and time, radio frequency data, and
state changes of I/O as well as sending reports based on power
states.