detector extensions. Using staggered line arrays (see fig. 10), 
the following effects occur: 
- detector line length is halved 
- image field area is reduced to one quarter 
- focal length is halved 
- the optics need to be of high quality for twice as many line 
pairs per millimeter with respect to the line pairs per 
millimeter necessary for the pixel size. 
Staggered CCD-line arrays are used for instance in the SPOT- 
mission cameras. 
D IRR REED E eere] 
  
Focal length [m] 
e 
0 | 
0 10 20 30 40 
  
Detector element size [micrometer] 
Figure 9 Dependence between detector element size x 
and necessary focal length f for a given ground 
pixel size of X = 1 m from an orbit altitude of 
600 km 
pitch GSD 
+ 
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
Figure 10 Staggered linear detector array configuration 
3.3.3 Optics: The progress in production and test of optic 
systems enables now the utilization of highly efficient low-mass 
and low-volume optical telescopes for space applications. 
Examples are 
- Use of aspheric lenses in refractive telescopes 
- Use of folded arrangements for reflective telescopes (e. g. 
TMA) 
- Use of sophisticated catadioptric telescopes. 
Even if you can design a camera having weight compatible with 
a micro-satellite spacecraft, the volume of the lens system for 
high resolution space-borne imagers is a problem if you think of 
the restricted size envelope for piggy-back launch opportunities. 
The Technical University of Berlin currently performed a study 
concerning an interesting optics construction approach: the 
Dobson Space Telescope, DST, [6]. The core element of DST is 
a 20 " f/5 Newton telescope. The secondary mirror will be 
placed via four 2.1 m booms when the spacecraft is already in 
orbit. In order to fulfill micro satellite requirements it is folded 
to minimal space during the launch. This type of telescopes 
called truss design Dobson was originally invented by 
ambitious amateur astronomers. To increase the resolution for 
  
    
   
     
   
    
    
    
     
   
    
     
     
   
   
   
  
   
   
  
   
  
  
  
   
  
  
    
   
    
    
  
   
   
    
   
    
    
    
     
    
International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part Bl. Istanbul 2004 
remote sensing purposes, a “Barlow lens” with a factor of 2.5 
pushes the focal ratio up to f/12.5 which assures maximum 
possible magnification and a ground pixel size of about Im 
from a 700 km orbit. 
3.4 Pointing accuracy and stability 
There are many activities going on to develop and test 
instruments, actuators, and algorithms to control the pointing 
with high accuracy. The obtained accuracies are between 
arcseconds and fractions of degrees. For mapping of the Earth's 
surface, deviations from the necessary precisions can be 
corrected using precise ground control points. The pointing 
stability is of more importance in order to maintain the ground 
sample distance and the image quality. From an orbit altitude of 
600 km, a GSD of 1 m equals an IFOV of 1.7 grad or 
approximately 1/3 of an arcsec. During the dwell time, the drift 
shall be less than 20 % of the [FOV resulting in a drift rate of 
about 2.4 mrad/s or 8 arcmin/s in order to stay in the limit for 
minimal degradation of the MTF due to drift effects. When 
using the TDI principle to improve the SNR, for a 32 step TDI 
the tolerable drift rate becomes even 75urad/s or about 15 
arcsec/s! 
3.5 Data volume and transmission 
Data rate is a very important parameter for imagers on small 
satellites. Most small satellites use X-Band transmitters 
allowing about 100 MBit/s. The ground station contact time 
from LEO 1s about 10 minutes resulting in roughly 60 GBit to 
be transmitted. If no compression is applied, a quadratic image 
of 87 kByte x 87 kByte can be transmitted during the ground 
station contact time. Whatever is used, the store & dump mode 
or the real-time mode, careful planning of the orbit activities is 
of high importance to make most use of this bottleneck. 
4. CURRENT AND PLANNED MISSIONS 
This paper showed the problems connected with high resolution 
topographic imaging. But it showed also the possibilities 
resulting from the immense improvements in many fields of 
technology. So it 1s not surprising that there are a good number 
of small satellites (total mass « 500 kg) with high-resolution 
instruments (€ 10 m GSD) in orbit or planned. Table 11 shows 
the missions which have no stereo capabilities. 
The suite of small satellite mission in orbit or planned for 
topographic mapping is smaller (see table 12). 
From the technology point of view small satellite missions for 
topographic mapping are feasible. Table 8 shows that even a 
GSD of 1 m is attacked. Once the performance concerning data 
quality for topographic mapping is proven, there is a chance to 
install mapping systems with a low cost space segment. When 
we restrict ourselves to civil applications, the market will show 
whether or not those systems can compete with SPOT-5 
topographic maps (GSD of 5 m). 
On the higher resolution side, those systems will compete with 
the standard aerial photography market. If for some reasons 
high-resolution maps with worldwide high repetition rates are 
required,the necessary coverage asks for many cost-effective 
systems. Then there is a high need to install more small 
satellites for topographic mapping. 
   
  
Internatic 
Missio! 
Earth O 
NASA 
PROBA 
X-SAT, 
EKOSA 
German 
Korea 
MAC/ 
Korea, | 
DST/ G 
  
Table 11 
Missior 
FROS-/ 
Rapid E 
| Diaman 
| TOPSA 
| EROS-I 
  
Table 12