User guide

ALEOS 4.2.3 User Guide
206 20080616
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 AirLink device may encounter “GPS drift”. The
AirLink device may report it is in a location a few feet from its actual location because it
does not employ differential GPS.
AirLink Device Supported Protocols
The AirLink device 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
applications 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
acknowledged 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.
Examples of tracking applications using RAP:
• Air-Trak
• TrackStar
• CompassCom
• Zoll Data
• HTE
• Spillman