User guide

ALEOS 4.2.3 User Guide
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computes the distance to each satellite 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 device’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, the AirLink device uses the UDP (User Datagram Protocol) to
communicate with the host server.
In RAP-based AVL, each device sends its command status and responses to the
Host server and the Host sends commands to one or more 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 include:
• Air-Trak
• TrackStar
• CompassCom
• Zoll Data
• HTE
• Spillman