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Introduction
Thorcom implements automatic vehicle location (AVL) across a range of of its radio modems and in-vehicle controller units based on its original TrakSys concept.
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-vehcile unit) against a set of 'rules' which determine when a position update is transmitted.
In the TrakSys concept resonisbility 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 which is inefficient and wasteful of channel resources.
AVL rule sets and profiles The rules which govern the transmission of position reports include the following:
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Time since last report generated |
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Distance travelled since last report |
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Response to a 'poll' command from the communications centre |
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Response to a 'poll' command from the communications centre |
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Manually generated report (from the user/equipment) |
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Whether the vehicle has started/stopped moving |
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Reaching a reporting point ('waypoint') |
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Change of status of vehicle crew (entered through the MDT) |
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Operation of 'panic' button or switch input |
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Change of status of other input signals |
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Loss or restoration of GPS coverage |
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Restoration of coverage of radio network |
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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 approch 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:
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Vehicle callsign or identity |
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Date and time of position report (accurate to within one second) |
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Position (latitude and longitude) |
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Speed Heading (direction of travel) |
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Vehicle status (16 bit value: range 0-65535) |
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Reason code (indicates reason for the transmission) |
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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.
Delivery mechanism The method of delivering the position update varies with the type of radio system in use:
- MDS700 and conventional PMR systems
- The ETS-300.230 Short Data Message (SDM) is used to deliver the data to the central site
- MPT1327 trunked radio systems
- The MPT1327 Short Data Message (SDM-2) is used to deliver the data on the control channel
- GSM Cellular networks
- The Short Message Service (SMS) of GSM Phase-II is used to deliver the data
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