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1997-04-16
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7KB
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146 lines
Keeping track in a fast moving world
(Big Brother is guiding you!)
Film fans will no doubt remember how James Bond was always
able to track the movements of the villain from a red flashing
screen mounted on the dashboard of his Aston Martin.
But despite Q's technical brilliance, this form of automatic
vehicle location (avl) was before its time. It is only in the
past couple of years that it has been possible to accurately
monitor the position of a moving car as it travels around the
country, passing through all sorts of terrain from open fields to
built-up inner cities.
At present there are two types of system, one of which is
commercially available and the other which is under development.
Satellite systems such as the global positioning system (gps)
will be operating universally within the next few years whereas
radio based techniques, such as Datatrack, are already
commercially available.
Satellite systems tracking suffer from a couple of major
disadvantages. They do not work terribly well in built up areas
as at least four line of sight tracks to four satellites is
required. Also they are not so good at tracking vehicles which
are moving at speed because of the time involved in integrating
the weak signals.
The radio based system currently in use in the UK is
Datatrack, designed by a subsidiary of Securicor. It is already
available throughout England and Scotland and is already used by
large and small fleet operators.
But towards the end of last year, Datatrack's monopoly of the
UK avl market came under fire with the announcement of another
system that uses existing tv and radio stations as the
transmitter.
Datatrack depends on an infrastructure of 22 time synchronised
transmitters producing 130 to 150kHz signals on a time shared
basis. Each vehicle has its own locator unit which can be
mounted either in the boot or behind the driver's seat.
"The locator unit contains a microprocessor board, a low
frequency (lf) receiver, a power board, uhf receiver and a unique
personality'" said Tony Scorer, product manager for Datatrack.
The vehicle also has to be fitted with a small antenna, which
incorporates a duplexer so that it can be used in conjunction
with a car telephone. After picking up the lf signals from the
antenna, the locator unit compares the phase relationship between
the received signals and uses this information to generate the
position of the vehicle relative to the transmitter network.
This position is converted into the Ordanance Survey grid
references for the vehicle's position and then this information
is sent to a network of base stations via a uhf link. The base
stations are further linked to a control data gathering and
distribution system which transmits data via a modem link to the
customers premises.
The base stations are situated every 50km, allowing data
coverage over the same area as the navigation transmitting
stations. According to the company, a vehicle should never be
more than 25km from a base station and could, in theory, be
within range of a number of stations.
The fleet operator can then see the position of each vehicle
on a series of maps displayed on a pc graphics screen as well as
an indication of where they are heading.
Further details, such as the fact that the driver is stopping
for a tea break, can be input to the Datatrack system via a Z8
based status keypad mounted in the vehicle. Important
information about the load, perhaps the temperature of a
trailer, can be automatically transmitted to the customer's
offices at a rate of 3600 baud.
Datatrack was conceived as a way of tracking moving vehicles
but another system, which exists as a prototype was discovered by
accident by a Cambridge University radio astronomer, Dr Peter
Duffet Smith.
During a series of experiments involving a portable radio
telescope, Dr Duffett-Smith had to work out a way of calculating
its position within a couple of feet. He eventually hit on the
idea of using local radio transmitters as a reference, avoiding
the need for a dedicated transmitter network.
"I can now track my position in Cambridge to an accuracy of 5m
using BBC medium wave radio," he said.
By using vhf he claimed to have tracked the progress of a man
walking outside his laboratory to within 10cm, comparable to gps.
According to Dr Duffett-Smith the accuracy of his system,
dubbed Cursor, is dependent on the wavelength of the signals
used; accuracy is generally better than 1/30th of the shortest
wavelength received. So a user can simply select the frequency
band to determine the desired precision.
Errors can also be reduced because Cursor can, at present, use
up to eight channels simultaneously. As three are needed to set
the position, as with Datatrack, the extra allows a weighted
average to be taken. The extra channels also eliminate some of
the problems caused by distortion of the wave field in built up
areas and various atmospheric disturbances.
At the moment, Cursor is just a trial system. But the
university's intellectual spin off company, Lynxvale, and the
venture capitalists Cambridge Research and Innovation are
currently looking for a company to take up the system
commercially.
Although there is a lot of interest, no company has yet agreed
to take Cursor from the trial stage to a commercially available
alternative.
Datatrack has been to look at Cursor in operation but will not
be buying up the rights to the technology.
"It will take alot of time to make Cursor commercial," said
Robert Peel, Datatrack's marketing manager.
Instead Datatrack is looking at other areas in which to
broaden the use of the system. As Peel pointed out, the data
network of Datatrack is as important as its ability to locate
vehicles.
"We can send 2000 reports a minute; it can take 5s to send
data on Band III or cellular radio," said Peel. "There is a vast
untapped potential for Datatrack and it is too ambitious to take
it on as a company."
But in the short term we are likely to see miniaturised
versions of the locator unit, thanks to an asic that replaces 70
discretes. This will allow Datatrack to be used by people on the
move such as police officers or doctors on call.
Soon the idea of getting away from it all could be a thing of
the past.