under almost
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Blended
<> navigation
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euvering and
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IMU and two
5. INTEGRATED INERTIAL/GPS vs. OTHER
ATTITUDE SENSORS
Compared with other approaches for attitude measurements,
the inertial/GPS integration approach used in POS/AV has
distinct advantages that combine to offer excellent
performance/price ratio from the medium-low to the highest
performance range:
Multiple-antenna GPS: Commercial systems are available
that provide attitude information by using four GPS
antennas. Although they can be fairly inexpensive, they
Airborne sensor on
stabilized platform
POS IMU
GPS antenna
LOS azimuth angle
LOS angle error
Computed LOS Sensor line-of-sight (LOS)
Computed GRP position <> True ground reference
GRP position error point (GRP) position
Figure 3. Airborne Geocoding of Survey Data
provide only limited accuracy. This accuracy can be
improved by increasing the spacing between the antennas.
Flexing of the hull the antennas are mounted on, however,
quickly degrades performance and spacing the antennas can
be impractical on an airborne platform. Further, the rms (1-
6) level of accuracy achieved with this approach does not
fully reflect the performance of the system. Unlike a well-
behaved gaussian error distribution where the maximum
errors are at about 3-56, the error statistics in a multiple
antenna system are not well behaved. As a result, maximum
attitude errors in a multi-antenna system can be many times
the rms level. The error statistics of an inertial/GPS system
on the other hand are relatively well approximated by a
gaussian distribution and predictable from the rms level.
Inertial Navigation Systems (INS): These are usually based
on high-quality ring-laser gyros (RLG) with low drift rates
and high-quality accelerometers to provide the highest level
of performance. INSs can be operated unaided, but do require a
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International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B3. Vienna 1996
fairly extensive period of ground alignment. Although INSs
have come down in price, they are still quite expensive.
Because they are mostly developed for military applications
their interface is usually based on the 1553-bus and the NRE
costs associated with the development of a data acquisition
and control unit for it can be non-trivial. The size and weight
of an inertial unit, significantly higher than an IMU, can
also be a limitation and can restrict its ability to be mounted
on a small sensor. In some high-end applications, however,
only the accuracy of a ring laser gyro will meet the
requirements. For these applications, and since there are
currently no high-quality RLG-based IMUs, an INS must be
used. POS is currently being upgraded to provide an interface
to an INS and further enhance the INS’ performance by
allowing integration with differential GPS.
Attitude Heading Reference Systems (AHRS): An AHRS
consists of gyros, accelerometers, and a magnetic sensor
unit. The heading provided by an AHRS is magnetic heading
derived from measurements on the earth’s magnetic field. To
measure the platform’s heading the magnetic sensor must be
mounted on the platform and calibrated for the soft and hard
magnetic errors caused by the platform. Such a calibration
requires 3600 turns of the helicopter and it is not very
accurate. In order to provide true heading from magnetic
heading, a local magnetic field map is required to transform
magnetic to true heading. Besides the need for obtaining
such a map, local magnetic fields vary continuously and
further corrections based on time of year and day must be
applied. In general, an AHRS cannot be expected to provide
attitude accuracies better than about 1.00 rms.
Vertical and Directional Gyros: These provide only limited
capability and are suitable for applications where only
limited attitude information and accuracy is required.
Integrated Inertial/GPS: Inertial and GPS sensors have
complementary strengths and shortcomings. GPS provides
very good long term accuracy but in the short term suffers
from outages, multipath and noise. Inertial sensors have
excellent short-term characteristics but suffer from long-term
drift. By combining the two sensors, levels of performance
can be achieved that match or exceed those of much more
expensive INS systems. Integrated systems provide
excellent short-term dynamics greatly reducing GPS’s
multipath and outage problems, and have none of the long-
term drift problems associated with inertial sensors. Further,
they can automatically perform in-air heading alignment to
true North without the need for the ground alignment of INSs
or the turning of the aircraft required for AHRS.
Fully integrated inertial/GPS sensors such as POS/AV
provide the best performance of all position and attitude
sensors at a reasonable cost. Being small, modular and
lightweight they are suitable for virtually all applications
requiring accurate and reliable position and attitude
measurements.
6. AIRBORNE SURVEY APPLICATIONS
Airborne POS applications include film and digital frame
cameras, pushbroom-type multi-spectral scanners, scanning
lasers, shallow water bathymetry, SAR, airborne gravimetry
