systems. 
ith other 
specially 
sra, SAR 
ms. 
nding on 
prmance: 
ry (flight 
is (1) but 
. mission 
(ground 
a given 
ientation 
ins great 
the vast 
are taken 
eometric 
rily to be 
tency is 
ientation 
e 2.2.2). 
it (2.2.3) 
plication 
joints for 
ormation 
srence of 
'n errors 
data for 
of block 
then the 
accuracy 
Tr, needs 
| ground 
e object 
neasured 
ription is 
d from a 
GPS for 
"here are 
ility and 
b system 
andatory 
'ameras. 
> as with 
r data or 
| position 
brought 
'ensors is 
ake such 
eference 
datum 
ms. And 
gain. 
that the 
uld make 
certainly 
| cases of 
ever, 3 
considerations which have to be kept in mind: (1) A photo-block 
which refers entirely and in absolute terms on orientation sensor 
data has reliability zero. The orientation data reference cannot be 
checked at all, the resulting system relies on them absolutely. (2) 
As soon as a system depends on absolute data then also all other 
circumstances must meet absolute requirements. It means that all 
systematic errors (interior photo-orientation, camera-GPS 
synchronisation etc.) must be under absolute control and must be 
relied upon. This is quite a difference against conventional 
photogrammetry, the good results of which are only obtained 
because many errors are compensated or neutralized by certain 
degrees of freedom. (3) If aerial triangulation proper is abandoned 
some local homogenisation might still be applied. 
3. ACCURACY REQUIREMENTS FOR ORIENTATION 
DATA 
The above considerations mainly referred to the principal 
importance and application of orientation sensor data in relation 
with other imaging or non imaging sensors. It remains to consider 
the required accuracy. There is no general answer to that question. 
The accuracy requirements which orientation data shoud meet 
depend very much on the sensor combination and the envisaged 
application. 
Possibly the highest requirements are set by high precision 
photogrammetric application, in terms of aerial triangulation. 
Conventional high precision aerial triangulation, at any scale, 
achieves attitude accuracies for the images in the order of 3 resp. 5 
or 6 mgon (for K resp. À and ()) or about 0.05 - 0.1 mrad. It is said 
that (expensive) INS systems can achieve such accuracy results, 
required with regard to absolute reference, but applications in aerial 
photogrammetry are not known. Attitude accuracies in the order of 
2 - 0.2 mrad are easier to obtain. They are quite useful for 
strengthening combined aerial triangulation, and can suffice 
directly for the photo- or orthophoto-rectification. Also, they can 
provide initial approximations for other purposes (like automatic 
aerial triangulation). 
The requirements for GPS camera positioning depend on the 
photo-scale. Large scale aerial triangulation reaches standard errors 
of perspective centres of a bout 3 -4 cm. That would require GPS 
coordinate precision of 5 cm, or better (not in terms of absolute 
accuracy but of internal consistency only). The extension of such 
blocks is usually « 10 km. For smaller photo-scales the GPS 
precision requirements drop to more convenient magnitudes of 10 - 
20 cm, and can go to 0.5 m or more for medium and small scale 
topographic mapping. It can be said that the performance of 
kinematic GPS positioning is sufficient for application within the 
full range of photogrammetric photo-scales. If, however, the GPS 
positioning is to be absolute, then all systematic errors (GPS and 
images) must be kept to the same order of magnitude, which is a 
completely new situation for photogrammetry. 
The laser scanning systems on the other hand depend practically 
in absolute terms on the positioning and attitude data provided by 
GPS and INS. The requirements can be specified in general to be 
about 10 cm (or better) for positioning, and 0.1 mrad in attitude. 
Combined system calibration is essential. 
In the field of remote sensing the multi-spectral scanners also 
depend entirely on position and attitude data, unless merged in 
conventional ways with maps or previous scenes. The 
requirements are usually not as high as in photgogrammetry 
proper, as precise geometry is often secondary. The direct and 
accurate orientation of such imagery has given airborne remote 
sensing a great push, in terms of extended applications and much 
greater practical convenience. A special case is radar imagery 
9 
(SAR). Here again the dependency on orientation data is direct. 
And high accuracy INS systems, combined with kinematic GPS 
positioning, mark the performance. 
In general, the application of GPS positioning and attitude 
determination by INS in combination with various airborne 
imaging or non-imaging sensors has given all aerial information 
technology a great impetus. The potential of application has been 
greatly widened and the conditions of application have become 
much easier and more economic. Of vital importance is the fact 
that the orientation data are obtained digitally for direct computer 
processing. The implications have just started to be realized. It is a 
great challenge for the near future to exploit the potential of multi- 
sensor systems. 
As a last remark it is mentioned that the availability and the 
performance of the orientation sensors also have successfully 
widened applications in fields which previously have not been open 
to airborne methods, but which quickly gain importance. Examples 
are helicopters or low flying light aircraft, equipped with various 
also non-imaging sensors for sensing environmental pollution 
under and on the ground or in the air. For such applications the 
direct and documented positioning is essential. 
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B6. Vienna 1996