extrapolation further, for example to four days, increases the 
error to about three miles, with the error occurring mostly along 
the satellite track. 
All the above methods for estimating the error of 
tracking by TRANSIT techniques still leave a small uncertainty 
about the absolute precision Since they are only self-consistency 
checks and it is possible to imagine some form of consistent bias 
that cancels out when you "''close the loop.'"' For example, all the 
consistency checks above could remain valid for a system that 
consistently tracks a phantom point five miles ahead of the real 
satellite on the same orbit. This would not matter for navigation 
but would introduce real trouble when one attempted to use the 
results to deduce geodetic consequences. Fortunately a completely 
independent check has been made possible by the availability of 
optical sighting data of reasonable precision reduced by Smith- 
sonian. The data for sightings of TRANSIT 1-B from optical sta- 
tions in the United States agree with TRANSIT determination of the 
satellite position to about one-quarter maees Optical sightings 
elsewhere in the world disagree somewhat more (sometimes 1 to 
1-1/2 mile) because of uncertainties in the absolute location of 
these overseas sites relative to the center of the earth. 
It is seen that the precision of measurement now possible 
with TRANSIT techniques is quite good. However, both the determina- 
tion of orbit during a one day period and the ability to extrapolate 
the orbit for a day are presently limited (to a few tenths of a mile 
in each case) by inaccuracies in the present model of the force field 
(gravitational field, drag, etc.). On a world-wide basis there are 
further difficulties introduced by the unavailability of sufficiently 
accurate datum ties. To meet the ultimate program goals for TRANSIT 
thus requires considerable improvement in the present knowledge of 
these factors (roughly the shape and mass distribution of the earth). 
2 reGaied! "The Doppler Tracking of Project TRANSIT Satellites" 
Proceedings of the Institute of Radio Engineers, Sept. 1960. 
113 
