photo-coverage that was acceptable, but other, non-imaging 
sensors would require absolute GPS data. 
2.2.3 Fast OTF ambiguity solutions. The recent hardware and 
software development for kinematic GPS positioning has started to 
basically alter the situation. Fast OTF (on the fly) ambiguity 
solutions are capable, in principle, to bridge signal interruptions and 
restore interrupted trajectories. That development is of greatest 
importance, also from a concept point of view, making combined 
sensor systems practically autonomous, at least with regard to the 
GPS reference system (WGS 84). There are, however, some 
problems implied which are being studied, at present. OTF 
ambiguity solutions can be subject to constant or systematic 
errors, depending on satellite constellations and distance to the 
GPS ground receiver station(s). With regard to that distance the 
reliability of the method is not yet safely established. 
With fast OTF ambiguity solutions the kinematic GPS positioning 
of airborne sensors is absolute, in principle, with regard to the GPS 
coordinate refernce system. This is a fundamental step in 
photogrammetry which cannot be overestimated. In combination 
with absolute GPS trajectories the integrated aerial triangulation 
(Le. orientation of photos and photo-blocks) can do without ground 
control at all, except for the GPS ground station(s). Experience and 
tests give reason, however, to issue a general warning: In an 
absolutely referenced system all systematic errors must be under 
strict control. This is particularly true also for systematic 
photogrammetric errors which did not show up in the conventional 
approach, being either not visible (at air stations) or compensated 
by ground control. The conclusion for multi-sensor systems must 
be that on-the-job system calibration becomes mandatory, unless 
some essential parameters are left free for subsequent fixing by 
external information, e.g. by ground control points. 
2.3 Sensor Attitude Data 
Sensor orientation includes attitude determination, in addition to 
positioning. The combination of GPS with aerial triangulation can 
do without attitude sensors, but in general they are needed for 
absolute sensor orientation. 
2.3.1 Multi-Antennae GPS. An interesting possibility to directly 
measure the attitude of an airborne sensor is the use of 3 or 4 
integrated GPS antennae on wings and fuselage of the aircraft and 
interferometric data processing. The special advantage is the direct 
relation to the GPS reference system. There are, however, various 
problems, also because the measurements refer to the carrier, not to 
the sensor itself. It implies integrated calibration and, in this case, 
also dynamic modelling of the system. The obtainable attitude 
accuracy seems to be in the order of 1 milli-radian, for the time 
being, which is not sufficient for high precision applications. 
2.3.2 Attitude determination by INS. 
Inertial systems have never been much applied in photogrammetry. 
Their positioning performance had too large drift errors, and has 
effectively been replaced by GPS positioning. The attitude 
determination by INS, however, is gaining ground. The internal 
precision of INS is very high. It can externally be maintained if drift 
errors are controlled in combination with GPS positioning. In that 
combination INS is also capable of helping bridge GPS signal 
interruptions. Laser scanning and SAR systems depend directly and 
in absolute terms on the accuracy of INS attitude data. It is again to 
be mentioned that inertial systems require thorough initialisation 
and total system calibration. 
8 
2.4 Application of Orientation Sensors 
Orientation sensors like GPS and INS are no stand alone systems. 
Their purpose and application is always in combination with other 
sensors which provide object information. These are especially 
imaging sensors (aerial camera, digital linear array camera, SAR 
and multispectral scanners), but also laser scanning systems. 
The different systems have different requirements, depending on 
the actual application. We may distinguish 3 levels of performance: 
(1) Orentation data for stabilizing only parts of a trajectory (flight 
lines, strips), without absolute reference. (2) The same as (1) but 
extended continuity and consistence for a whole flight mission 
(block), absolute reference still provided by other means (ground 
control) (3) The same as (2) but with reference to a given 
coordinate system. In that case we speak of absolute orientation 
data resp. of georeferencing. 
Photogrammetric block triangulation, for instance, gains great 
economic benefit from GPS camera positioning, by the vast 
reduction of ground control, whose accuracy functions are taken 
over by the GPS data. A photo-block has consistent geometric 
strength in itself. Therefore the GPS data need not necessarily to be 
absolute nor completely consistent. Stnpwise consistency is 
sufficient for still obtaining a block solution the absolute orientation 
still being provided by some few ground control points (see 2.2.2). 
If the complete GPS trajectory is continuous and consistent (2.2.3) 
the combined system is geometrically stronger, its application 
therefore preferable, still relying on some ground control points for 
datum transformation. Whether a separate datum transformation 
for georeference is required depends on the absolute reference of 
the GPS data and on condition that constant or other system errors 
are either not existent or have been calibrated. Attitude data for 
aerial photographs are not mandatory in the case of block 
adustment. If available and applied, however, they strengthen the 
total system and are advantageous even if of moderate accuracy 
only. The specific geometry of single strips, however, needs 
attitude information in any case, whether derived via ground 
control or by INS data. 
All other sensor systems mentioned which provide object 
information depend to a higher degree on directly measured 
orientation data, as without them no consistent object description is 
achievable, in different degrees. The terrain points derived from a 
laser scanner, for instance, depend absolutely on GPS for 
positioning and on INS for the attitude of the sensor. There are 
little possibilities to check or correct for geometric stability and 
datum of the resulting data sets. Instead, a on-the-job system 
calibration is essential. In a similar way attitude data are mandatory 
to obtain rigid image-geometry with digital linear array cameras. 
The function of GPS positioning thereafter is the same as with 
conventional photo-blocks. Also multi-spectral scanner data or 
SAR scanning depends in absolute terms on attitude and position 
data, taken during the flight, unless ground control can be brought 
in. 
The general tendency concerning position and attitude sensors is 
twofold. On the one hand high accuracy is essential to make such 
sensor data really useful. But increasingly also absolute reference 
is wanted, leaving open the necessity for overall datum 
orientation. In all cases we deal with multi-sensor systems. And 
the necessity for thorough system calibration is stressed again. 
Remark: It has been speculated, at various occasions, that the 
provision of position and attitude data by GPS and INS could make 
aerial triangulation in photogrammetry obsolete. This is certainly 
a valid consideration in general and may first be applied in cases of 
reduced accuracy requirements. There are, however, 3 
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B6. Vienna 1996 
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