06-02-2013, 03:28 PM
VEHICLE INTEGRATED NAVIGATION SYSTEM
VEHICLE INTEGRATED NAVIGATION.pdf (Size: 1.22 MB / Downloads: 85)
Abstract
Fibersense Technology Corporation has developed an
integrated navigation system for accurate navigation in
urban environments. The first application of this system
will be to improve the navigation accuracy of the New
York City buses. The environment of New York City is
particularly severe, and is probably the most challenging
in the U.S.A. for GPS blockage and multi path. As part of
the Americans with disabilities act the NYC buses must
announce the location of the bus stop. Bus status is
available at each bus stop (e.g. the next M2 bus will arrive
in 6 minutes).
During periods of good GPS data the system calibrates
it’s sensors. This allows accurate navigation when the
GPS data is unavailable or when the data is badly
corrupted by multi-path signals. This also operates better
in situations when a small number of satellites are visible.
This functionality can be added with very little additional
cost compared with a system using only a GPS receiver.
Introduction
Use of GPS in cities such as New York is unreliable as a
result of signal blockage and reflections from buildings.
This environment has been named "Urban Canyon."
For many consumer applications reasonable navigation
data with breaks of up to 50% of the time may be
considered acceptable. Other applications cannot tolerate
GPS inaccuracy in the urban canyon environment.
Obvious applications that will require robust navigation
performance are:
Intelligent Integrated Navigation System
Fibersense's I2NS uses the selected GPS receiver (card)
coupled with a MEMS gyro, vehicle odometer and
proprietary software to overcome the urban canyon
environment.
Fibersense's proprietary software detects when the
receiver's output is being corrupted by bad line-of-sight
information (due to reflections or sources of interference
i.e. jamming) and corrects the data.
GPS receivers provide output performance indicators such
as HDOP the "Horizontal Dilution Of Precision". These
performance indicators are based on the geometry of the
satellites being tracked. In the urban canyon environment
the true line of sight is severely restricted and satellite
range is corrupted. The GPS receiver does not know
which satellites are valid and which have been corrupted
due to extreme multipath from strong reflections off
buildings etc.
Algorithm Development
The keys to the success of an integrated navigation
system for use in an urban canyon environment are the
algorithms. Two versions of the algorithms have been
developed, one version uses the generic NMEA interface
the other one uses the more GPS receiver proprietary
binary messages.
The version using the NMEA interface has the advantage
of being more easily adaptable to different GPS receivers.
In this mode, however, the raw range/range rate
measurements to the satellites are not available. The
navigation solution from the receiver (position and
velocity) must be used. This means that the data from and
individual satellite cannot be rejected.
When the proprietary binary messages from the GPS are
used the receiver's raw range/range rate date are available.
The integrated navigation system has more information
available to it than the GPS by itself. The navigation
information available from the odometer and gyro inputs
allows the algorithms to reject the data from individual
satellites. This increases the accuracy of the system in the
urban environment and also allows the system to degrade
more gracefully as the number observable “good”
satellites reduce.
A stand alone GPS receiver has to start making
assumptions once the number of available satellites
reduces below 4. Assumptions such as constant velocity
etc are not required with the integrated navigation system.
As the vehicle travels along inner city streets information
from different satellites will become available. Even in
heavily built up areas, cross streets are passed and for
brief periods unobstructed data from other satellites is
available. As the “inertial” solution can navigate
accurately for short periods of time/distance traveled, very
little information is lost even if all 4 satellites are not
processed at the same time.