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12 Z. Popovic and R. Miucic
Positioning Requirements for Intelligent Transportation Systems
For sharing of position data to be effective in improving safety, it needs to be of
certain accuracy and reliability. There are some traffic scenarios in which road-level
positioning accuracy—knowing which road we are on—can be sufficient to provide
assessments needed for issuing warnings and corrective actions that can improve
safety. Examples include dealing with oncoming traffic in making unprotected turns
through intersections and slowing down for road conditions ahead (tight turns seen
in maps, icy spots reported by other vehicles, ...). Beyond that, there is a significant
portion of traffic scenarios in which reliably knowing positions of traffic participants
down to the lane of travel is needed (and sufficient) to prevent collisions or road
excursions. Among others these include: anticipating sudden traffic slow-downs
in our own lane, preventing unintended lane or road departures, and reacting to
traffic controls applicable only to our lane, where the latter can depend on similarly
accurate maps of intersection zones.
With the aim of quantifying the required positioning accuracy levels, road-level
accuracy values are derived from the need to distinguish between nearby or adjacent
roads that have similar directions of travel but are not physically or topologically
connected and thus can’t influence each other’s traffic. This distinction is needed
to prevent false warnings and reactions. In the most stringent and ambiguous
cases, for example adjacent lanes separated by a barrier, this requirement reduces
to the requirement of lane-level accuracy, but often 5 or 10 m of accuracy is
sufficient.
Setting a number for lane-level accuracy also does not have a definite anchor, but
1.5 m is suggested here because for a vehicle travelling down the middle of a 3 m
lane any larger error would place it in another (and thus wrong) lane. A 3 m lane,
which is an unusually narrow lane in the United State, is used here in order to cover
most of the lane width diversity: In the US, lanes with widths of 3 m or more are
used about 90% of the time [1].
The main source of positioning information in V2X systems, and the only source
of absolute positioning information, has been GPS (Global Positioning System,
maintained by the United States government) and similar systems maintained by
other governments, where any such system is known as a GNSS system (Global
Navigation Satellite System). GNSS can provide adequate positioning performance
for V2X in most cases, but to make a positioning system sufficiently robust for
safety applications it needs to be combined with other information, typically from
sensors that reliably relate a vehicle’s change of position over time (such as inertial
sensors), but can’t provide absolute position coordinates (which are required for
seamless sharing of location with other vehicles and for use of maps).
The aims of this chapter are to introduce: operation of GNSS systems while using
GPS as the main example (Section “GNSS Principles”), use of GNSS positioning
in V2X systems (Section “Basic GNSS Positioning in Cooperative Vehicles”),
GNSS sources of errors and ways of limiting those errors, including advanced
techniques for achieving centimeter-level accuracy (Section “GNSS Performance