The PCS contains all the data acquisition and processing
hardware, GPS receiver, data recorder and power supplies for
the PCS and the IMU. The PCS interfaces to a notebook PC
for control and monitoring functions. POS is nominally
configured with the Novatel Performance Series embedded
GPS card to provide a complete and self-contained position
and orientation solution. In many POS applications,
however, the target vehicle carries an existing GPS receiver
for navigation. POS can be configured to interface with a
number of external GPS receivers such as the Trimble 4000
and the Ashtec Z12. The position and attitude data from POS
is available in real-time over RS232, RS422, and ethernet
ports. An event marker port is also available for easy and
precise time-alignment with the sensor data. The real-time
data as well as raw data is also recorded for post-processing
on a built-in 8mm exabyte tape recorder. Using this data the
companion post-processing software ‘POSPROC’ generates
a more accurate position and orientation solution. POSPROC
runs on the Windows NT platform and is also available for
the Sun workstation environment.
3. AIDED INERTIAL NAVIGATION
Figure 2 shows the aided inertial navigation algorithm
implemented by POS. The term aided inertial navigation
defines a method of blending inertial data from
accelerometers and gyros with position and velocity data
from an aiding navigation sensor, in this case GPS or
differential GPS receiver. The algorithm combines inertial
navigation with GPS navigation and any other available
aiding sensor in an optimal manner to generate a blended
navigation solution that retains the best characteristics of
inertial and GPS navigation. The key components are an
inertial navigation component and an navigation error
regulation component.
initialization, leveling and heading alignment under almost
all normal dynamic conditions. Static initialization, as
would normally be required by an INS, is not required by the
POS. The POS error controller includes a coarse leveling
Strapdown Blended
navigator p» navigation
solution
Accelerometers
omm
Navigator
corrections
Ee,
Feedback Code
error controller 18 7-7]. Ievelina
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Filter
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Estimated
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NU,
Kalman
filter
GPS or
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GPS
Figure 2 : POS Aided Inertial Navigation Algorithm
loop that provides rapid coarse leveling of the strapdown
navigator mathematical platform. The POS Kalman filter
implements a modified y-angle error model for coarse
azimuth alignment, subsequent fine roll, pitch and heading
alignment and continuous error control.
4. PERFORMANCE
POS attitude performance is dependent on the quality of the
IMU, the quality of the GPS data, and whether real-time data
or post-processing is used. Typically, with a LN-200 IMU
and real-time differential GPS, pitch/roll of < 0.050 is
achieved. With a LR-86 IMU, carrier phase GPS, and data
post-processing, accuracies down to the 20arcsec level are
possible. Heading performance is further dependant on
maneuvering as well as latitude.
1. Following heading calibration maneuver.
TABLE 1. POS/AV Performance
Quantity Single
GPS
Position 75 m
Roll, Pitch 3 arcmin
Heading 20 arcmin!
Heal-Time Post-Processed
Differential Carrier -Phase
2-5 m 10-30cm
1 arcmin 0.5 arcmin
<10 arcmin! 5 arcmin!
The inertial navigation component comprises the IMU and a
full strapdown navigator that computes the IMU position,
velocity and 3-axis attitude. The error regulation component
comprises the Kalman filter and closed-loop error controller.
The Kalman filter estimates the strapdown navigator errors
and selected inertial sensor and GPS errors. The error
controller computes and applies resets to the strapdown
navigator integration process to achieve initial alignment
and subsequent continuous error regulation. When good
quality position and velocity aiding data are available, such
as from the GPS or differential GPS, the POS aided inertial
navigation algorithm provides
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B3. Vienna 1996
Typically, for latitudes below 700 and maneuvers every 10 to
15min heading accuracies of 10 to 20 arcmin can be achieved
in real-time. With more frequent maneuvering, and/or post-
processing heading accuracies of 4-5 arcmin are achievable.
If frequent maneuvering is not possible or operation at high
latitudes is required, a two-GPS antenna version of POS/AV is
available that eliminates the need for maneuvering and
brings heading performance at par with roll/pitch. Table I
summarizes POS performance for the LR-200 IMU and two
different GPS configurations.
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