REGISTRATION ACCURACY AND ATTITUDE ACCURACY 
Ai C. Fang 
National Aeronautics and Space Administration, Headquarters 
Washington, D.C. 20591 
ABSTRACT 
Since higher spatial resolution is desired by many applica 
tions disciplines coupled with registration in the sub-to- 
1 pixel range, improved registration accuracy has become 
necessary. The trend Is likely to continue, with image da 
ta resolution of 10 meters or less being a routine require 
ment before the end of this decade. This registration ac 
curacy can only be achieved by the combination of registra 
tion and attitude accuracy. This report describes the need 
for improving the attitude accuracy. The relationship be 
tween registration accuracy and attitude accuracy is shown, 
along with criteria for design consideration in establish 
ing adequate attitude accuracy. A precise on-board atti 
tude control system design is essential to provide the at 
titude accuracy needed to support the subpixel accuracy re 
quired for registration of higher resolution images. 
INTRODUCTION 
Registration accuracy is necessary for image analysis. In 
the registration process, mathematical transformations are 
used to map a digital image onto a reference image of the 
same scene. In this way multiple Images can be overlaid or 
compared directly for analysis. The reference image may be 
either a standard map or a previously obtained corrected 
image (temporal images). Multiple images are in registra 
tion when pixels in these images representing the same 
scene point coincide, and when one pair of coordinate 
values locates that pixel in each image. Determining the 
correct location of matching image points requires accurate 
measurements of the spacecraft attitude and position. For 
Landsat-D, the spacecraft position will be measured by the 
Global Positioning System (GPS) receiver to be flown on 
board. If the GPS performs as planned, accurate orbit in 
formation will be obtained, and position error caused by 
ephemeris uncertainty will be reduced. Consequently, 
spacecraft attitude is the remaining critical parameter in 
determining correct pixel location. Inaccurate attitude 
causes geometric error such as pixel location error in the 
image which results in subpixel registration difficulties. 
The pixel size, to provide the spatial resolutions needed, 
has been reduced from 80 meters for the Multi spectral Scan 
ner (MSS) to 30 meters for the Thematic Mapper (TM), and to 
10 meters or less for a future Multispectral Linear Array 
(MLA) type sensor. Subpixel accuracy for a resolution of 
10 meters or less will be a routine requirement for user 
communities before the end of this decade. Higher resolu-