Doppler curves Simply provides redundancy and a consequent increase 
in accuracy and that each Doppler curve, in principle, provides 
enough data to determine the orbit. 
To complete a description of the operation of the 
TRANSIT system, it is only necessary to add that the formidable 
communication problem of disseminating a description of the orbit 
to all potential users is solved by using the TRANSIT satellites 
as a special communication link for this purpose. Specifically, 
after the orbit is determined on the ground a parametric descrip- 
tion of this orbit is transmitted to the satellite, recorded in a 
memory System contained in the satellite and subsequently broadcast 
at regular (2 minute) intervals. Thus the user receives a descrip- 
tion of the orbit of the satellite Simultaneously with his measure- 
ment of the Doppler shift. By this means it is possible to improve 
the description of the orbit frequently; in fact, it is intended to 
insert a current or updated description every twelve hours. 
While the above description of the TRANSIT system sounds 
relatively simple, it can perhaps be imagined that there were a 
number of rather formidable technical difficulties to be overcome 
before a practical operating system could be designed in detail. 
It is the purpose of this article to indicate some of these problems 
and the progress that has been made toward their solution by the 
TRANSIT launchings to date. 
To the time of this writing there have been three TRANSIT 
satellites placed in orbit out of a total of five attempts. A few 
of the Significant parameters of these three satellites are displayed 
in Fig. 1. It will be noticed that, although we have been moderately 
fortunate in getting satellites placed into orbit, we have been less 
fortunate in the orbits achieved. None of the three orbits has very 
closely approached the circularity that was desired and two of the 
three have had perigee at so low an altitude that atmospheric drag 
very seriously affected the calculation of the orbit. The last 
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