Global Positioning System Reference
In-Depth Information
Two of the early experiments illustrate the two different approaches to
centrally managed dynamic route guidance. The Ali-Scout systems from Siemens
[2] used a network of local infrared traffic communications beacons that acted
rather like electronic sign posts. The other system was the System of Cellular
Radio for Traffic efficiency and Safety (SOCRATES) [3] and was a collaborative
project of the E.U. Framework research programs. It was based on cellular radio
and autonomous navigation. Both systems were before GPS was generally
available but their performance was in some ways better as GPS coverage was
never an issue. Other projects, such as PROMISE [4, 5], pioneered the portable
personal information terminal for travelers who were not driving.
Ali-Scout and SOCRATES used a combination of magnetic compasses,
vehicle odometers, and the all-important digital vector maps, which treated a route
as a set of vectors connecting a set of nodes.
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It is useful to look at these systems
in more detail, as the way they worked is now much closer to the way indoors
person-based systems would work and thus sets a framework of a universal
ubiquitous positioning approach to guidance.
The Ali-Scout system used modified traffic signals, which in addition to the
normal colored lights had extra dark lamps that emitted infrared data. At each
node, usually a city junction, the data message broadcast to any passing vehicle
was the vector map fragment to take the vehicle to the next node in the journey.
The vehicle communicated its destination to the beacon, which responded with the
appropriate map fragment personalized according to the journey destination. The
central traffic computer did not need to know every vehicle's journey, but worked
rather on a standard set of possible destinations. It was easy to collect vehicle
delays from the vehicles since they had followed a known route from the last
beacon and because the journey segments were timed. The central computer thus
continuously calculated the most efficient route and node scheme for an entire city
based on real-time measurements and any other a priori information.
The SOCRATES system did not need the map to be communicated since it
was carried in a digital database on a CD-ROM in the vehicle. Cellular radio with
an embedded data system was used to communicate road link impedances to the
vehicles from the central system. In turn, the central computer collected a global
view of congestion from the vehicles using the data link since each vehicle
measured link impedances locally. Routes could thus be calculated in the vehicle
autonomously, taking into account real-time traffic conditions.
The map included historic link impedances, but these were supplemented by
the real-time centrally calculated information that was regularly updated. By
adjusting the impedances, the central computer could influence the routes taken.
This system was more autonomous than the other, had more expensive in-vehicle
equipment, but did not need a beacon network that required expensive landline-
based data circuits.
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Chapter 8 explains the significance of digital vector mapping and a brief example is shown in the
Epilogue.
