Dynamic Platform Tracking and Information System (DPTIS)
Communication system
At first glance Vehilce tracking marketplace may seem to offer a host of communication technologies for vehicle tracking applications. However, just by taking into account the application requirements one or two technologies mat come into focus. The requirements mainly cover aspects such as reporting rate, throughout capacity geographic coverage and cost. Through the systems currently implemented in DTPIS involve UHF transceivers, a brief overview of the different types of communication technologies is warranted in order to understand the comparative advantages and limitations.
Conventional Radio
To date, the most common type of communication system used for tracking has been the conventional radios. The frequency ranges normally used are UHF (400-500 Mhz) and VHF 25-175 Mhz, although others have been used. Conventional radios are portable and can handheld units or dashboard mountable units. They are intended to operate in defined geographical regions. Users of these radios share a common radio channel and compete for airtime. As a result, RF collisions can occur if two users transmit at the same time. To curb collisions, a radio etiquette must be established wherein the users must monitor a channel and announce a line clear before attempting a call. Alternatively the control centre can adopt the polling technique to enable a response-on-request mode of operation. Hence only a half duplex radio will suffice.
Cellular
Due to its wide acceptance and coverage cellular technology has attracted significant interest in the tracking industry. Though the cellular system can guarantee a better link between the control centre and the vehicles, considerable time needs to be expended in establishing a correction hence reducing the position update rates per vehicle. Also the running cost implementing this technology is high. Considering the above points it can be said that for applications with infrequent position reporting rates and requiring voice communications, a cellular system is ideal.
Trunked Radio
In North America, trunked radio is a very popular communication technology, used in 17 different Vehicle tracking systems. In Europe, it is used by five. A trunked radio system is a two way radio communications system with a central controller that acts as an automatic switchboard, handling many users and talk groups. A trunked radio system can have as many as 28 channels, one of which is a dedicated control channel that links all users and assigns open channel automatically. When a user from a single "talk group" connects, the control channel assigns an open channel for the user automatically and lets the rest of the group know which channel is in use. All the radios in that talk group automatically change to that channel, and communication commences. Each user talk group has exclusive use of the same channel to maintain privacy.
As was the case for conventional radio, trunked systems were traditionally designed for voice communications, and can become overtaxed when dealing with data. Digital trunked radio systems that are just rolling out are much better suited for voice and data.
SATCOM Technology
Worldwide, 23 Automatic Vehicle location systems now include satellite-based communications. The relatively high number is due to a significant number of open architecture-type designs that can accommodate satellite communications as well as terrestrial-based communications. Some satellite-based system manufacturers see their systems as complementing existing terrestrial-based system; for example, satellite communications can be used in remote areas not covered by cellular and specialised mobileradio (SMR) techniques (which would require a dual-equipment design). Satellite communications systems typically fall into two categories; those that utilise geostationary satellites and those that plan to incorporate low earth orbit (LEO) satellites..
To date, most satellite communication systems involve geosynchronous satellites. Orbiting at altitude of 36,000 kilometers in synchronization with the earth's surface near the equator. Due to their vast distance above the earth, they require relatively large antennas to send and receive signals, making the equipment somewhat bulky. This technology is effective for tracking applications that require wide-area coverage like ocean going vessels. The equipment and services are relatively costly. INMARSAT is a popular Satcom link for marine communication & tracking.
Control Center
The Control centre is the nerve centre of the whole system polling the vehicles for their positions and receiving and sending data message from and to the vehicles. Apart from this the control centre also transmits the DGPS error correction messages.
The main PC runs software which includes an efficient operating system, a relational data base management system, a geographical information system (GIS) software to display the positions of the vehicles over a local detailed digitised map, a communication control software and a dispatching software.
At the time of deployment of the system each vehicles information is centred into a database at the control centre. These sats can be entered in a specific format to include vehicle description, vehicle identification, in-vehicle equipment details and any pertinent information or special needs the vehicles may have. The database software is also used to keep up-to-date information on the movements of the vehicles along with their defined schedules and destinations.
The graphical software provides a host of features to enable the operator to make quick position related assessments. The software provides zooming feature to enable a close-up view of the vehicle of interest and also an overall view by zooming out to be able to see all the vehicles on the screen. Editing function on the digitised map provides demarcation of certain areas which can be hazardous to the vehicles and if a vehicle ventures into such areas an audio visual alarm is activated which the operator proceeds to convey to the specific errant vehicle. Multiple window views provide both an overall view and a close up view on the same screen.
Messages from the control centre to the vehicles can be both predefined or freeform entry since the entire resources of the powerful computer is available to the operator. Emergency messages from the vehicles generate audio visual alarms at the control centre the attract the operators attention and demand an acknowledgement to ensure that the message has been received and read.
The basic data format employed for message exchanges between the control centre and the vehicles includes predefined codes in the headers of each message to stimulate the desired response from the specific member of the system.
Polling technique is employed in DPTIS. In this technique, the control centre firs transmits the DGPS corrections which will be received by all the mobile GPS receivers via the communication links. Then the control centre will poll the vehicles for their positions using their identity codes in a sequential manner to cover all the vehicles in the database. If for reasons of communication link masking, a particular vehicle does not respond the first time, the control centre will execute retries for a finite number of attempts which can be preset by the operator depending on the need.
Challenges
Among the different DPTIS components-communications, positioning, maps and computers-the communication link is the chief impediment to successful widespread implementation of DPTIS. As stated at the outset, the challenges of selecting the appropriate communication solution for a particular application still remains in view of optimising reporting rate, throughput capacity, geographic coverage and reducing cost.
Summary
Vehicle tracking systems have evolved rapidly and are being currently used in many western countries. In Japan and Singapore it has already been commercialised. The tracking option has provides a tremendous advantage in all areas involving Air, Marine and Land transport.
GPS represents one of the most significant technological developments in air navigation, for establishing more efficient routes to save on fuel costs, and for managing air corridors.
At sea, GPS along with satellite based communication links help vessels to navigate safely and efficiently-even in adverse situations-while keeping in touch with the home office.
On land the goal of a GPS based tracking system is to significantly improve the efficiency of commercial and private transportation by keeping public safety organisations, maintenance crews, and drivers informed in real time about the status of the roadways. The system has also enabled cargo companies to ensure on-time schedules. Dispatchers and switchers can use the position given by GPS to adjust the routes of the vehicles to allow optimal usage of the cargo service with maximum returns.
As mentioned earlier DPTIS is a flexible and expandable system that can be easily customised to support any configuration to suit a variety of applications.