We shall try to define what the values of are from the 
subpixel registration standpoint. In order to satisfy the 
requirements of registration accuracy, the value of R must 
be less than or equal to 1. When R =1, we obtain 
4 = (K/l.4)(P/H) 
(9) 
4 = (K/l.4)(IFOV) 
(10) 
where IFOV represents the sensing instrument's angular in 
stantaneous field of view. The value of IFOV is 117.2 a 
rad for the MSS, 42.5# rad for the TM, and 14.2 Arad for 
the MLA, where 1 Arad is equal to 10' 6 radians and is also 
equivalent to 0.71 m at the altitude of 705.3 kilometers. 
Thus, equations (8) and (10) can be used as criteria for 
defining Based on equation (10), to meet the temporal 
registration accuracy requirement for the MLA data, is 
equal to 1.8 Arad which is approximately as follows« 
<4 = 0.0001 degrees (11) 
Hence an attitude accuracy of 0.0001 degrees is essential 
for achieving the subpixel registration accuracy of 10 m 
resolution without control points and without ground proc 
essing, provided other factors which would result in reg 
istration errors can be reduced to a prespecified toler 
ance small enough in comparison to this attitude tolerance. 
It is important that the other errors making up registra 
tion inaccuracies be corrected to support the attitude con 
straint. For instance, in the future when GPS is fully 
operational with planned 24 NAVSTARs in orbit, the ephem 
eris error can be reduced to a value less than the 8 m cur 
rently estimated. It is possible that the precise on-board 
knowledge of the spacecraft's position will be known. We 
assume that both the ephemeris and the attitude errors are 
reduced to 1.8 m respectively. Also assume that other er 
rors are minimized to about 1.6 m. This gives a total RSS 
of 3 m which means that an image of 10 m resolution will be 
brought into geodetic coordinates. 
This report does not cover intra-image distortion such as 
detector placement, scan variations, vibrations, etc., and 
those error sources not subject to control such as earth 
curvature, rotation, terrain effect, and so forth. These 
are assumed to be known and modeled for error minimization. 
With the potential advantage of the solid state sensors of 
the future, it is possible that errors in the sensors be 
controlled to meet higher accuracy specifications, provided 
the attitude sensor is relative to the instrument, not to 
the spacecraft body. 
It is difficult to minimize all error sources to negligible 
values, and ground processing for registration may still be 
needed. If this happens, the value of 0.0001 degrees is 
the required pointing knowledge. The result of such pre 
cise pointing will minimize GCP, reduce the complexity of 
ground processing, and make subpixel registration achiev 
able. As the stability is as important as the accuracy, 
reduction of attitude drift to lO 1 " 7 deg/sec is required.