SSI image data Navigation data 
{À 
[NU ® 
Image 
Orientation 
Measure tie points 
  
  
Convert pixel coordinates 
to image coordinates 
| 
| 
Combined bundle block adjustment 
  
  
  
  
  
  
  
  
SSI Ground Improved 
image data control exterior 
network otientation 
   
    
   
   
  
      
  
DTM 
Generation 
  
  
Determine 
conjugate points 
I 
Convert pixel coordinates 
to image coordinates 
  
  
  
  
  
  
Forward intersection 
Surface modelling 
Digital terrain 
model 
Figure 2: Data flow for photogrammetric processing 
measurement of tie points as well as the combined block 
adjustment and provides improved exterior orientation pa- 
rameters and a 3-D ground control network on Ida. For the 
acquisition of DTM primary data, digital image matching 
is applied leading to a large number of conjugate points in 
the images. With the help of improved exterior orienta- 
tion parameters the image coordinates of conjugate points 
can be transformed into ground coordinates using forward 
intersections. Finally a DTM is generated using a sphere 
as reference surface. 
2.2 Image Orientation 
The first photogrammetric processing step involves the de- 
termination of a couple of tie points in the images. Point 
identification and point measurement were carried out by a 
human operator. All points are related to topographic fea- 
tures (mostly craters) on Ida’s surface and are distributed 
uniformly on the ground surface in order to build up a 
dense global network. 
Altogether 96 points were measured interactively in 36 
images with a precision of 10 um (0.7 pixel), where the 
ground pixel size ranges from 25 m to 600 m. The point 
identification was impaired by the varying image scale and 
by the fact that the asteroid rotated on its spin axis 235? 
during the time period covered by the images. 
The following step is to convert the measured pixel coor- 
dinates into image coordinates taking the interior orienta- 
tion into account. To this end, a geometric star calibration 
was carried out by measuring star locations of the Pleiades 
star cluster in the images and solving for free parameters 
in a least-squares adjustment (Davies et al. 1994). Be- 
sides the 3 image orientation angles, the calibrated focal 
length c — 1500.467 mm and the radial distortion coef- 
ficient k = —0.00002498 mm”? were determined in the 
least-squares adjustment in the order of 0.1 pixel accu- 
racy. This simple model is sufficient for images taken by 
narrow-angle cameras like SSI. 
The pixel coordinates (row,col) can be transformed by 
rz = ps(col — 400.5) y = ps(row — 400.5) (1) 
ds = z(1+ kr’) uy = —y(1+ kr’) (2) 
with 
7? = 1° + y? (3) 
where ps is the pixel size (15.24 um) and (uz, uy) denotes 
the image coordinates. 
In the next processing step, 3D ground coordinates of the 
tie points are determined and the exterior orientation (po- 
sition and attitude) of all images is reconstructed in a 
bundle block adjustment. The block adjustment was per- 
formed in the Ida-fixed reference system, where the origin 
coincides with Ida's center of mass, the Z-axis is parallel 
to the spin axis, and the X-axis lies in the prime meridian 
plane defined by the crater Afon (Davies et al. 1995). The 
crater Afon was introduced as error-free ground control 
point (GCP) for the definition of the global datum. 
Additional observation equations are formulated for pre- 
processed position and attitude data, which have been de- 
rived from S-Band Doppler tracking data and star images 
respectively. The relative accuracy of the position (atti- 
tude) data is assumed to be 100 m (0.1°), whereas the 
absolute accuracy amounts to 5 km (0.2°). In order to 
incorporate these navigation data into the bundle adjust- 
ment, they are transformed from inertial space into the 
Ida-fixed non-inertial coordinate system. The rotational 
parameters of Ida are treated as constants using the val- 
ues of Davies et al. (1995). 
The following data were introduced as observations: 
e Image coordinates of 95 conjugate points and 1 GCP 
(c —10 um) 
e Object coordinates of 1 GCP (ox =0y=0z=10 m) 
e Position parameters (z^, y^, z^) for 36 images (0 = 
100 m relative, c —5 km absolute) 
e Attitude parameters ($, v, K) for 36 images (g=0.1° 
relative, g=0.2° absolute) 
The results of the bundle block adjustment are summä- 
rized in Table 1. After 6 iterations a Go of 7.8 um was 
622 
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B4. Vienna 1996 
  
  
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