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The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences. Vol. XXXVII. Part B4. Beijing 2008 
Figure 2: Eauidistant man of current surface coverage of SRC images on Phobos. 
4. CONTROL POINT NETWORK 
4.1 Duxbury and Callahan control point network 
The Viking orbiters were the first to obtain complete ground 
coverage of Phobos with excellent phase angle coverage. Pixel 
resolutions of the images are in the order of 200 m or better. 
The first global control point network consisted of 98 points 
(Duxbury and Callahan, 1989b), mostly craters. It was later 
extended to over 315 control points. The measurements 
included relatively large craters covering several pixels. 
Uncertainties of the 3D-coordinates of the points ranged from 
±74 m to ±900 m (Duxbury, 1991). 
4.2 HRSC and SRC image data 
The resolution of HRSC images from a range of 2000 km is 
approx. 80 m. Hence, though the built-in stereo capability seem 
to make the HRSC attractive for control point network analysis, 
only a limited number of HRSC images from more or less 10 
flyby maneuvers would be suitable for an in depth control point 
network analysis. 
For SRC on the other hand, with its large focal length of 988.5 
mm, the ground pixel size is smaller by a factor of approx. 4.3 
over that of HRSC. Pixel resolutions of SRC images for flybys 
range from 100 m/pxl to 5m/pxl. The back draw in case of SRC 
image data is, that SRC is a single frame camera, pointed to a 
fixed point in the stellar sky during one flyby. Thus, the same 
surface area needs to be observed during different flybys with 
different viewing angles to be useful for photogrammetric 
techniques. The establishment of an independent control point 
network for Phobos requires a global image coverage. HRSC 
and SRC images cover approx. 74% of Phobos’ surface in 
stereo (cf. Figure 2). Viking orbiter images were incorporated 
into the image database to fill the gap on the anti-Mars side of 
Phobos. Fortunately very high resolution images, of the missing 
area, are available in the Viking data set. 
4.3 Object point determination 
We measured line/sample coordinates of control points in 53 
SRC images and 16 Viking orbiter images. Contrary to 
Duxbury and Callahan (1989b) our control points are defined as 
the centers of a crater on the crater floor. Since image 
resolutions allow us to observe very small surface features - 
even small features within large craters - we assume that the 
surface features are shallow and represent a mean surface to 
sufficient approximation. A total of 660 points were observed 
3845 times with a minimum of 2 observations and a maximum 
of 14 observations, but on average 6 observations per point in 
both image data sets. 
Initially the two sets of observations - SRC images and Viking 
orbiter images - were processed separately to control the 
predicted orientation information. Nominal navigation data for 
the SRC images could directly be used to determine object 
point coordinates of the control points in a least-squares bundle 
block adjustment. For a few orbits, normalized residual values 
of the camera orientation data indicated that uncertainties of the 
position and pointing observations were larger than expected. 
An adopted weighting scheme for observed camera orientations 
was applied to reduce the relevance of uncertain observations 
on the adjustment. Estimating that image coordinates were 
observed with a 1 pixel uncertainty, mean object point 
accuracies Ox, Gy, Oz of 39.6 m, 34.6 m, and 36.1 m, 
respectively, were computed, for control points measured in 
SRC images. 
SRC 
Viking orbiter 
No. of points obs. 
2953 
871 
Umax [m] 
92.1 
164.2 
Omin [m] 
32.7 
91.7 
Omean [m] 
36.8 
100.9 
Combined Adjustment of SRC and Viking 
No. of points obs. 
3841 
Omax [m] 
62.5 
Omin fm] 
4.4 
Omean [m] 
15.9 
Table 3: Object point accuracies for the different bundle block 
adjustment models.