The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences. Vol. XXXVII. Part B4. Beijing 2008 
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(a) Phobos’ leading side 
(b) Phobos’ trailing side 
Figure 5: Shape model from triangulation of the control 
point network. The x-axis (dot-dash) points 
towards Mars, the y-axis (long-dashed) points in 
the direction of motion and the z-axis marks the 
rotation axis. Note that the y-axis in Figure 5(a) 
points in the direction of motion and is in the 
negative y direction of the IAU frame. In Figure 
5(b) the positiv y-axis is displayed 
Object Pom! Accuraoes over varying Ubrabon Amplitudes 
— X-Coordmate 
YCoordinate 
¿Coordinale 
1.0 15 2,0 
Ubrawn Amplitude *n Deg 
Figure 6: Observed residuals of object points for varying forced 
libration amplitudes. The red vertical line indicates 
the observation by (Duxbury, 1991) , the green line 
indicates the minimum of SRC observations. 
points, indicate an amplitude of 1.2° for the force libration (see 
Figure 6). The observed amplitude differs slightly from the 
amplitude of 0.80 ±0.3° observed and computed by (Duxbury 
and Callahan, 1988, Duxbury, 1991), but is in well agreement 
with the amplitude of 1.2° derived from a topographic model 
(Borderies and Yoder, 1990). The topographic model of the 
later reference was approximated through a Kriging 
interpolation of the 98 control points from the 1989 control 
point network (Duxbury, 1989). A correction for the Stickney 
crater was made in the topography model. A deviation of the 
forced libration amplitude from previous observed amplitudes 
could have its cause in the mass distribution model. Current 
studies assume a homogeneous mass distribution for Phobos. 
Mass concentrations could significantly change inertia figures 
which can be related to the forced libration amplitude in a first 
approximation. Equation (1) shows this relation, where 0 A is the 
forced libration amplitude, e equals the excentricity of Phobos, 
and A < B < C are the moments of inertia. 
r= 
B-A 
C 
0 A = 
2e 
1 — y 
3 
(i) 
5. CONCLUSIONS AND FUTURE WORK 
We observed Phobos’ positions over a three year time span 
through astrometric measurements in SRC image data. A new 
reduction method was established which can be applied to any 
planetary body and may prove to be useful for analysis of future 
planetary data sets. The ongoing MEX mission with further 
close flyby maneuvers will make more observations available, 
adding constraints to the Phobos orbit models. 
The control point network was primarily used to observe the 
forced libration amplitude. Estimates of the volume and 
moment of inertia factors are yet to come, and may further 
constrain the bulk density and mass distribution of Phobos. 
Currently, studies of HRSC stereo data are under way to derive 
a global digital terrain model (DTM) in high resolution. A DTM 
derived from Viking orbiter images will be needed to fill the 
unobserved area on the anti-Mars side of Phobos.