GEOREFERENCING REQUIREMENTS FOR AIRBORNE PHOTOGRAMMETRY AND REMOTE SENSING 
Prof. em. Friedrich Ackermann 
Institute for Photogrammetry, University of Stuttgart 
Keplerstrasse 11, D 70174 Stuttgart 
Germany 
IUSM Working Group GPS 
KEY WORDS: Airborne sensor orientation, GPS, INS 
ABSTRACT 
Aur survey methods are generally moving towards multi-sensor systems, made possible by the recent technology development. Of particular 
importance is sensor orientation by GPS (position) and INS (attitude). Different levels of specifications are distinguished, reaching from the 
stabilisation and internal consistency of imaging or laser sensors to the demands on absolute orientation data for the purpose of direct 
georeferencing. Finally, the accuracy requirements of orientation data in combination with other sensors are reviewed. 
1. CONVENTIONAL PHOTOGRAMMETRY, A MONO- 
SENSOR SYSTEM 
Aerial photogrammetry has been a mono-sensor system, for a long 
time, being limited to aerial photography. Because of that it was 
not autonomous at all, had to rely on external ground control in 
order to solve for the free system parameters (contrary to terrestrial 
photogrammetry). The application of aerial surveys has adapted to 
those conditions by successfully developing operational standards. 
The established technique and performance of aerial triangulation 
in particular has provided system consistence, with clear command 
of accuracy, reliability and economy. 
Various auxiliary camera orientation data from additional airborne 
sensors have repeatedly been considered and applied in the past, in 
order to ease the generally costly dependence on ground control. 
The efforts had only temporary success and could not basically 
alter the general conditions. 
2. TOWARDS AUTONOMOUS MULTI-SENSOR 
SYSTEMS 
2.1 New Technology 
The great advances in technology of the last two decades have 
started to thoroughly alter the fundamental concept and structure 
of photogrammetry, amounting to truly revolutionary changes. 
Photogrammetry is moving towards multi-sensor systems for 
digital information acquisition, possibly becoming nearly 
autonomous, i.e. practically independent of external parameter 
fixing. In the course of this development photogrammetry and 
remote sensing merge together in terms of methods, techniques 
and extended applications. 
There are 2 groups of sensor development to be distinguished, as 
far as photogrammetry and remote sensing are concerned: 
(1) Sensors providing orientation data, especially GPS, INS, and 
laser or radar altimeter; 
(2) Sensors providing multi-data about the objects, especially Laser 
Scanner, SAR, MS Scanner, in combination with photographic 
sensors or not, which in turn may become digital or linear array 
cameras. 
We concentrate here on sensors which provide orientation data, 
i.e. on GPS and INS in particular. 
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2.2 High Precision Kinematic GPS Positioning of Airborne 
Sensors 
2.2.1 GPS flight navigation. First, it is to be mentioned that the 
C/A code pseudo-range positioning in real time has gained great 
influence on the navigation of air survey flights. It is practically 
standard, the advantages being evident. Survey flight navigation 
now goes by coordinates, not any more by visual navigation. As a 
result the photo-coverage or the flight lines of other sensors 
become extremely regular, with all subsequent advantages. 
2.2.2 Phase Observations, Signal Loss of Lock. The real 
fundamental contribution of GPS concerns the high precision 
kinematic sensor positioning, not necessarily in real time, by 
differential phase observations with regard to one or more GPS 
ground receiver stations on known points. Prime application, so 
far, has been camera air station positioning for aerial triangulation, 
in post-processing mode. 
The general conditions for the application of kinematic sensor 
positioning are well known. They are highly favourable in general 
terms, advocating wide application. However, there is the problem 
of signal loss of lock, especially during flight turns, because the 
lines of sight to satellites can be interrupted by wings and fuselage. 
Attempts to fly turns more carefully cannot sufficiently be relied 
upon, and are not acceptable in operational terms. Interruption of 
GPS signals destroys the previous ambiguity solutions. Practicable 
solutions to restore or re-establish ambiguity solutions were not 
available, for a long time. Instead, approximate solutions were 
applied, based on C/A code positioning, resulting in biased 
ambiguity solutions, as consequence of which the following parts 
of the GPS trajectory showed systematic errors, called GPS drift 
errors. Fortunately, the drift errors turned out to be about linear 
for short subsequent time intervals. 
On that basis successful applications of kinematic GPS sensor 
positioning were developed, especially for aerial triangulation. The 
systematic GPS drift erros would be assessed and corrected for by 
using external information, for instance via combined 
adjustment in combination with photo-blocks. That type of 
solution and application in aerial triangulation has worked very 
well in practice, although some ground control points were 
mandatory for a datum transformation, and additional cross strips 
had to be applied in order to avoid system singularities. The relative 
precision of GPS positioning has been more than sufficient for 
application in all photo-scales, reaching magnitudes of < 10 cm, 
even « 5 cm. The limitation of the method was, however, that no 
absolute GPS positioning could be obtained. In combination with 
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B6. Vienna 1996