Global Positioning System Reference
In-Depth Information
an approximate or previous position can be used to extract the corresponding eleva-
tion from a DTM, DEM, or other source of elevation data. If the terrain elevation
varies greatly in the vicinity of the position, iteration may be necessary. Using a
DEM for this purpose may be easier from a computational perspective, since it
would involve a simple value lookup and interpolation based on the coordinates;
however, a large amount of storage would be required for the DEM. A DTM that
has the elevation data organized into vectors would use less storage but would
require more complicated computations to determine the elevation at a specific
point. Elevation data can also be integrated into digital road maps as attribute data,
which would simplify elevation lookup and keep the storage requirements lower.
Terrain elevation data has not yet been used widely to augment GPS; this will likely
change in navigation and driver safety systems once elevation information is
integrated into digital road maps.
Map calibration is very similar to the process of using GPS data to calibrate iner-
tial and other DR sensors. For example, with the same set of conditions that support
the heading fix generation, the constant road heading may be used to calibrate a
low-cost gyro or magnetic compass. Since the road heading is constant, the gyro read-
ing is then a direct measure of its bias. Another example is when the vehicle makes a
turn at an intersection, the change in heading between the inbound segment and the
outbound segment can be used to calibrate a heading sensor. With the current perfor-
mance of GPS, map calibration of sensors is less common than it once was.
9.3.2.3 GPS
As mentioned in Section 9.3.1, the discontinuance of SA enabled commercial use of
GPS at close to full accuracy, except for the inability to remove the majority of the
ionospheric delay (awaiting availability of the second civilian frequency). Previous
sections in the text have identified and discussed the major sources of GPS errors,
both in the measured pseudoranges and delta ranges or Doppler measurements, and
the determined positions and velocities. Of interest here are the sources of error in
the GPS-determined speed and heading and sources of error that may be unique to
the automotive environment. GPS-determined speed and heading are most useful in
calibrating automotive sensors that are typically sources of speed and heading infor-
mation: this direct comparison enables rapid calibration of sensor errors when GPS
is accurate. For errors that are small relative to the vehicle speed, the error in the
GPS-determined speed and heading can be expressed as:
(
)
δ
vvvvvv
nn
=
δ
+
δ
(9.16)
e
e
(
)
δ
H v vv vv v
n
=
δ
−
δ
2
(9.17)
n
e
e
n
where:
v
e
are the north and east velocity error components
v
n
and
v
e
are the north and east velocity components
δ
δ
v
n
and
δ
v
are the heading and speed errors, respectively
v
is the vehicle speed in a horizontal plane
H
and
δ
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