2-4-1
DIRECT PLATFORM ORIENTATION IN AERIAL AND LAND-BASED MAPPING PRACTICE
Dorota A. Grejner-Brzezinska, Charles K. Toth and Edward Oshel
Center for Mapping
The Ohio State University
1216 Kinnear Road, Columbus, OH 43212-1154
E-mail: dorota@cfm.ohio-state.edu
USA
Commission II, Working Group I
KEY WORDS: Direct Platform Orientation, Sensor Integration, Global Positioning System (GPS), Inertial navigation
system (INS), Charge-coupled device (CCD), Mobile mapping
ABSTRACT
Airborne integrated GPS/INS systems providing direct platform orientation are generating increased interest in the aerial
survey and remote sensing community. The primary driving force behind this process is a need to accommodate the new
spatial data sensors, such as LIDAR or SAR systems that do not offer any indirect methods for geo-referencing, or
multi/hyperspectral scanners, for which the indirect methods are very complicated. Another aspect is the cost reduction of
the traditional aerial mapping. The introduction of GPS data has already substantially decreased the need for control
points leading to the cost reduction even for surveys with conventional optical or CCD-based frame sensors. Even though
GPS/INS systems are rapidly reaching maturity and cost-effectiveness, they cannot, from the practical standpoint, fully
replace the aerial triangulation (if applicable) for at least two reasons. First, AT is needed in the areas where GPS signal
is subject to strong interference; second, it is indispensable for the calibration of integrated systems and quality control.
This paper reviews our recent aerial and land-based experiences with GPS/INS-derived direct orientation of a digital-
frame camera. Practical aspects of the system calibration, performance and operational issues are discussed.
1. INTRODUCTION
Direct determination of position, attitude and velocity
of an airborne platform can be achieved by inertial
navigation or multi-antenna GPS, or, for highest
accuracy, by integration of both systems in order to
utilize their complementary features. Many authors have
reported the use of the integrated systems, based on
strapdown IMU and differential GPS, for direct
orientation of various airborne imaging sensors (Kerr
III, 1994; Schwarz and Wei, 1994; Schwarz, 1995;
Lithopoulos et al., 1996; Skaloud et al., 1996;
Abdullah, 1997; Toth, 1997; Mostafa et al, 1998; Toth
and Grejner-Brzezinska, 1998). The traditional large-
format camera-based aerial surveys, however, still
depend primarily on indirect geo-referencing
accomplished by using ground control points and aerial
triangulation.
The introduction of GPS in aerial surveying has
substantially decreased the need for control points,
ideally requiring control points only from the block
corners. Nonetheless, the use of GPS did not change the
aerial triangulation process itself, which still requires
the same amount of measurements of tie/pass points,
and the adjustment process. The GPS/INS systems, with
all their attractive attributes including cost-effectiveness
primarily due to the reduced ground control
requirements and decreasing cost of the sensors, and
increasing accessibility, could be considered an
alternative for the costly aerial triangulation. In fact, in
many cases this is already an accepted alternative, as
aerial surveying companies tend to acquire GPS/INS for
their operations. On the other hand, the direct platform
orientation is limited primarily by the quality of the
calibration of the integrated system, including the
boresight calibration, the rigidity of the imaging
sensor/INS mount, the quality of the IMU sensor, and
the continuity of the lock to the GPS signal. Naturally,
one of the reasons for fusing GPS with the inertial
navigation sensors is to bridge the GPS losses of lock.
This is accomplished with different levels of accuracy,
depending on the quality of the IMU sensor and the gap
duration. The medium to high accuracy strapdown
systems, such as LN-100, combined with differential
GPS can allow for reliable bridging of the GPS gaps,
provided that the gap is relatively short. For such a
system, the positioning error growth is below 10 cm for
the horizontal components, and below 20 cm in the
vertical direction, after a 60-second loss of GPS lock
preceded by 900-second INS calibration, which still
enables instantaneous ambiguity recovery after the GPS
signal is recovered. If the local gravity signature is
known (deflections of the vertical), and implemented in
the strapdown solution, the horizontal error growth can
be further limited to below 4 cm (Grejner-Brzezinska
and Wang, 1998). During longer gaps, exceeding a few
minutes, the quality of the free navigation solution
decreases quite rapidly, providing positions and attitude
estimates that might not meet the accuracy requirements
for the most demanding applications. The extended
losses of GPS lock do not occur very frequently in the