AVL Overview

Thorcom implements Automatic Vehicle Location (AVL) across a range of its radio modems and in-vehicle location units based on an intelligent ‘rules based’ system.

In Thorcom's AVL systems, position data is determined using a high performance embedded GPS receiver from Trimble Navigation. The position data is processed locally in the modem (or in-vehicle unit) against a set of ‘rules’ that determine when a position update is transmitted.

So, the responsibility for generation and transmission of AVL updates is moved to the mobile equipment - the mobile is "intelligent" and does not have to rely on "polling" from a central site, which is inefficient and wasteful of network bandwidth by comparison.



AVL Rule Sets and Profiles

The rules that govern the transmission of position reports include the following:

	Time since last report generated
	Distance travelled since last report
	Response to a 'poll' command from the communications centre
	Manually generated report (from the user/equipment)
	Whether the vehicle has started/stopped moving
	Reaching a reporting point ('waypoint')
	Change of status of vehicle crew (entered through the MDT)
	Operation of 'panic' button or switch input
	Change of status of other input signals
	Loss or restoration of GPS coverage
	Change of AVL operating profile

The mobile unit processes GPS data and data gathered from other sensors and inputs and decides when it should make an AVL transmission. The mobile unit can have one or more sets of these rules, referred to as "profiles", and the central control station can command the mobile to change profiles.

The rule and profile based AVL approach is very flexible and powerful allows a wide range of configurations to be implemented to meet the customer requirements.



Information transmitted in AVL updates

The information transmitted in a standard AVL update includes the following items

	Vehicle callsign or identity
	Date and time of position report (accurate to within one second)
	Position (lat itude and longitude)
	Speed
	Heading (direction of travel)
	Vehicle status (16-bit value: range 0-65535)
	Reason code (indicates reason for the transmission)
	Checksum (to ensure the data is delivered correctly)

The data is compressed into around 20 bytes of binary data, which makes it small enough to be transmitted efficiently over a number of radio/network systems (see below).

Additional information (for example accuracy of position, or other customer data) can also be transmitted along with the location update if required - please consult Thorcom.