248 
SYSTEM DESIGN CONSIDERATIONS 
From a technical point of view, adequate attitude accuracy- 
should be based on mission requirements and the sensor's 
resolution. Very often, because of technological or budg 
etary constraints, a compromise measure is necessary in 
stead of one that would satisfy registration requirements. 
Therefore, choosing a value of R to represent the specified 
ratio instead of the desired one may be of interest for 
system design purposes. In doing so, a consideration is 
also needed to treat all components of registration error 
as an ensemble with total error budgeted and to let R allow 
for it. In fact, when R is greater than 1, ground control 
points are needed for error correction. Therefore, a simu 
lation study may be needed to determine derived from 
equation (8) for a given value of R (R>1) such that the 
current processing techniques with available GCP can pro 
vide image registration with a minimum of errors. The GCP 
used in the simulation study will have to be accurate to 1- 
2 m from their correct locations on the map. Such a par 
ticular o m , if it exists, could be considered as an atti 
tude tolerance for improving the attitude system before new 
technology for attitude control system is developed. 
The limitations of current attitude control system are evi 
dent because of attitude error, which would be effectively 
removed by better system design. Registration accuracy of 
higher resolution data could be achieved if the attitude 
accuracy were 0.0001 degrees and the attitude stability 
were also improved to 10” 7 d eg/sec. The existing gyros 
cannot meet this performance goal. The current star track 
er is also limited in performance and generates errors when 
a star referenced inertial reference frame is converted to 
a rotating earth-centered reference using ephemeris data. 
Improvements in gyros, reaction wheels, and the star track 
er will improve attitude accuracy performance. However, 
technological improvements will be necessary to improve at 
titude system stability and to support the needed attitude 
accuracy if the improvements in the current attitude system 
cannot meet the future registration accuracy requirements. 
Technical studies including evaluation of the technical 
feasibility of achieving desired attitude accuracy versus 
preferred attitude accuracies and a tradeoff study on the 
cost of attitude system Improvements (or new technology de 
velopments) versus cost of ground processing would provide 
important information. These studies are beyond the scope 
of this report. 
CONCLUSION 
The improvement of attitude accuracy to a desired value of 
0.0001 degrees for sensor data with a smaller pixel is muoh 
needed. At the same time, attitude system design should be 
improved to provide adequate attitude stability of 10” 7 deg/ 
sec to achieve the desired attitude accuracy. These im 
provements are required to meet registration accuracy re 
quirements for higher resolution data. The savings on the 
ground processing for error corrections will pay back these 
improvements cost many times in the future.