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3.1.2 Absolute Pointing Correction: Unlike previous 
instruments on other planetary missions, the accuracy of the 
NAC pointing can be directly measured using the position of 
known locations on the target body. The absolute error of the 
NAC camera pointing was determined by deriving coordinates 
for the five retroreflectors on the Moon (three flown on Apollo 
missions, two flown on Soviet Lunokhod rovers), the true 
locations of which are known to sub-meter accuracy (Figure 4) 
[Murphy et al., 2010]. 
Figure 4. Distribution of the calculated retroreflectors locations 
before (red) and after (blue) the absolute point correction, 
compared to the actual retroreflector location (yellow) 
The five retroreflectors were located in 62 NAC frames, and the 
true instrument pointing was derived for each of those frames. 
These pointing values were then compared to numerous 
environmental factors. A strong correlation was found between 
spacecraft slew angle and absolute offset in the cross-track 
direction, with the caveat that the slew angle had a sign that 
indicates whether the camera was pointing to the east or west, 
regardless of spacecraft flight direction. Additionally, we found 
the absolute pointing error to correlate with the temperature of 
the LROC Sequence and Compressor System (SCS), which is 
mounted on the backside of the optical bench and not covered 
by the thermal blankets. The pointing error is smallest at higher 
SCS temperatures and more pronounced at lower SCS 
temperatures. We are currently investigating the thermal 
environment of key components of the spacecraft during the 62 
observations. 
  
3.1.3 Relative Offset Between NAC-L to NAC-R: The 
NAC-L and NAC-R were affixed to the spacecraft such that the 
cameras have a —135 pixel overlap in the cross-track direction 
and an offset of ~185 pixels in the down-track direction 
[Robinson et al., 2010]. Early in the mission, it was recognized 
that the offset between the two cameras was not fixed (Figure 
5). After analysing several thousand NAC pairs under all 
possible conditions it was determined that the amount of 
overlap varies in both directions (cross-track and down-track) 
with a strong correlation to the temperature of the spacecraft. 
  
Figure 5. The boundary (yellow arrows) between the left and 
right frames of a mosaicked NAC pair before (left) and after 
(right) the relative correction. 
A plausible mechanism for the time-varying relative offset is 
differential expansion of the mounting brackets or the spacecraft 
structure, as the spacecraft thermal environment changes. 
Thermistors mounted on the camera system and spacecraft were 
checked, and all available temperatures were strongly correlated 
to the relative offset between the NAC-L and NAC-R. SCS 
temperature was chosen as the correction parameter since it is 
included in the PDS header of each NAC image and is common 
to both cameras. 
In order for the NAC-L and NAC-R co-registration function 
(described in detail in Section 3.2.1) to work reliably, the input 
images were restricted to solar incidence angles «70? (avoiding 
large shadowed areas) (Figure 6 and 7). This restriction 
unfortunately results in very little data for SCS temperatures 
less than 2°C, and the data below 2° show no correlation 
between relative offset and temperature. In the current 
implementation, any image taken with an SCS temperature 
below 2°C was treated as though it had a temperature of 2°C, 
but it currently does not produce good results. Efforts are 
currently underway to make the correction better in this 
temperature range. 
«10° Down-Track Offset vs. SCS Temperature 
  
* + data 
2 
i j 3 seo fitted Curve 
Down-Track Offset (degrees) 
  
  
  
  
2 4 B 8 10 12 14 18 18 20 22 
SCS Temperature (?C) 
Figure 6. Best-fit curves for the down-track offset between the 
NAC-L to NAC-R. Blue dots are individual co-registration 
points within an image. Red line is the best-fit curve (2™ order 
Fourier series). Y-axis units are 10? degrees. 
481