9: GPS Configuration
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The GPS tab that displays in ACEmanager is applicable across all
Sierra Wireless AirLink devices.
GPS
This group includes commands specific to GPS features and the
AirLink device.
The AirLink device is equipped with a Global Positioning System receiver (GPS) to ascertain its position and track the movements of a vehicle or other devices which move. The AirLink device relays the information of its location as well as other data for use with tracking applications.
Tracking Applications used with Sierra Wireless PinPoint line devices include:
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Air-Trak
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Track Your Truck
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Track Star
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DeLorme Street Atlas USA
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Microsoft Streets and Trips
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CompassCom
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Zoll Data
GPS Overview
The Global Positioning System (GPS) is a satellite navigation system used for determining a location and providing a highly accurate time reference almost anywhere on Earth. The US military refers to GPS as Navigation Signal Timing and Ranging Global Positioning System
(NAVSTAR GPS).
GPS consists of a “constellation” of at least 24 satellites in 6 orbital planes. Each satellite circles the Earth twice every day at an altitude of 20,200 kilometers (12,600 miles). Each satellite is equipped with an atomic clock and constantly broadcasts the time, according to its own clock, along with administrative information including the orbital elements of its motion, as determined by ground-based observatories.
A GPS receiver, such as the AirLink device, requires signals from four or more satellites in order to determine its own latitude, longitude, and elevation. Using time synced to the satellite system, the receiver
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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: RAP, NMEA, and TAIP.
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 device 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:
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Air-Trak
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TrackStar
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CompassCom
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Zoll Data
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HTE
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Spillman
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GPS Configuration
National Marine Electronics Association (NMEA)
National Marine Electronics Association (NMEA) is a protocol by which marine instruments and most GPS receivers can communicate with each other. NMEA defines the format of many different GPS message (sentence) types, which are intended for use by navigational equipment.
Example of a tracking application using NMEA:
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Microsoft Streets and Trips
Tip:
For more information on the AirLink device supported NMEA message formats, please refer to the Appendix.
Trimble ASCII Interface Protocol (TAIP)
Trimble ASCII Interface Protocol (TAIP) is a digital communication interface based on printable ASCII characters over a serial data link. TAIP was designed specifically for vehicle tracking applications but has become common in a number of other applications, such as data terminals and portable computers, because of its ease of use.
Example of a tracking application using TAIP:
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DeLorme Street Atlas USA
Tip:
For more information on TAIP message formats, refer to the Appendix and to the
Sierra Wireless MP 3G device TAIP Reference.
Datum
The GPS datum is the method of ascertaining the position of the GPS device using a specific reference point location. The datum used can influence the accuracy of the GPS positioning.
In addition to different reporting protocols, the AirLink device supports the most widely used GPS datum:
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WGS84
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NAD83
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NAD27
Before You Configure GPS
To decide what configuration you need for your AirLink device, there are some fundamental considerations you should determine:
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Protocol: What is the GPS protocol used by your tracking application, and what type of reports will you need?
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Datum: What is the datum supported by your tracking application?
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Dynamic IP Address: Will you need DNS support to handle a dynamic IP address account?
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