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