Without GPS, the position error incurred in measuring the 
spacecraft ephemeris (10", after 2 days) is 500 m along 
track and 100 m cross track. When GPS is operational, the 
ephemeris errors will be reduced, and the error sources 
are indicated as follows: 
8 m 
Ephemeris 
Attitude 
Other 
Total (RSS) 
175 m 
36 m 
180 m 
where the total resultant error is shown by a root-sum- 
square (RSS) value. 
Eventually, the GPS will provide more accurate ephemeris 
information in the future» Attitude error becomes the major 
error source that requires attention. Without precise 
attitude data to indicate the pointing, ground data proc 
essing systems rely upon processing of ground control 
points (GCP) to correct geometric error in order to regis 
ter images. Registration accuracy then depends on the 
number and quality of GCP used and how well these control 
points are distributed. Increased numbers of well-dis 
tributed control points per image must be available to 
improve accuracy. Significant processing capability, human 
labor, and time are required for Identifying ground control 
points, selecting ground control features, and determining 
accurate coordinate values for each GCP. However, some 
desirable ground control features cannot be easily identi 
fied on an image. For instance, cloud coverage or snow may 
obscure the desired features. 
Inaccurate GCP locations, lack of well-distributed control 
points, or errors in ground control features can seriously 
constrain the processing systems. Current Landsat 2-3 
ground control data contain errors between 6 to 10 m and 
are insufficient to achieve subpixel registration for 10 m 
resolution images. In addition, the absence of GCP for 
remote and unmapped land areas has limited the registration 
since the multiple images cannot be corrected for pixel 
location errors. Therefore, ground control is inadequate 
for image registration in many areas and new methods of 
correcting pointing errors must be provided. 
One key in achieving the desired registration accuracy is 
to improve attitude accuracy by removing the source of 
large position error. Current methods of using GCP for 
processing data on the ground require computation, measure 
ments, estimations, correlation, etc., which induce numerous 
errors including GCP location error, correlation error, 
measurement error, computation error, etc.. These residual 
errors will eventually translate into a registration error 
with a resulting root-mean-square (RMS) value of about 30 
meters range even with perfect maps (reference 4, page 2- 
41). The resulting RMS exceeds the allowable limit for 
subpixel accuracy of TM or MLA data, and is very difficult 
to eliminate. Although improvements in ground processing 
techniques are anticipated, the image position error caused 
by an inadequate attitude system cannot be totally re