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The accuracy of the GPS information depends
on how good the reception is. When the sig-
nals are strong and reception is good, GPS
can determine latitude, longitude and altitude
for accurate positioning in three dimensions.
But if signal quality is poor, only two dimen-
sions, latitude and longitude, can be obtained
and positioning errors are somewhat greater.
Positioning by dead reckoning
The 3D Hybrid Sensor in the navigation system
also calculates your position. The current loca-
tion is measured by detecting driving distance
with the speed pulse, the turning direction
with the Gyrosensor and inclination of the
road with the G sensor.
The 3D Hybrid Sensor can even calculate
changes of altitude, and corrects for discre-
pancies in the distance traveled caused by
driving along winding roads or up slopes.
Also, the navigation system learns the driving
conditions and stores information in the mem-
or y; thus, as you drive more, the precision of
the positioning becomes more accurate.
The method of positioning changes as follows
depending on whether the speed pulse of your
vehicle is detected or not:
3D hybrid mode
This is active when the speed pulse is de-
tected. Inclination of a street can be detected.
Simple hybrid mode
When the speed pulse is not detected, posi-
tioning is performed in this mode. Only hori-
zontal movement is detected, so it becomes
less accurate. Also, when the positioning by
GPS is not available, such as when your vehi-
cle enters a long tunnel, the discrepancy be-
tween your actual and calculated position may
become bigger.
p The speed pulse data comes from the
speed sensing circuit. The location of this
speed sensing circuit depends on your ve-
hicle model. In some cases, it is impossible
to make a connection to it, and in such a
case we recommend that ND-PG1 speed
pulse generator (sold separately) be used.
How do GPS and dead
reckoning work together?
For maximum accuracy, your navigation sys-
tem continually compares GPS data with your
estimated position as calculated from the data
of 3D Hybrid Sensor. However, if only the data
from the 3D Hybrid Sensor is available for a
long period, positioning errors are gradually
compounded until the estimated position be-
comes unreliable. For this reason, whenever
GPS signals are available, they are matched
with the data of the 3D Hybrid Sensor and
used to correct it for improved accuracy.
To ensure maximum accuracy, the dead reck-
oning system learns with experience. By com-
paring the position it estimates with your
actual position as obtained using GPS, it can
correct for various types of error, such as tire
wear and the rolling motion of your vehicle. As
you drive, the dead reckoning system gradu-
ally gathers more data, learns more, and the
accuracy of its estimates gradually increases.
So, after you have driven some distance, you
can expect your position as shown on the map
to show fewer errors.
p If you use chains on your wheels for winter
driving or put on the spare wheel, errors
may suddenly increase because of differ-
ence in wheel diameter. The system detects
the fact that the tire diameter has changed,
and automatically replaces the value for
calculating distance.
Appendix
En
192
Appendix