between the transmitter and the receiver and hence is influenced 
both by the motion of the satellite in inertial space and by the 
motion of the receiving station as a point on a rotating earth. 
Because of the severe constraints imposed on the path of an earth 
satellite by Newton's laws and our reasonably complete knowledge 
of the forces acting, it is possible, simply from an accurate 
measurement of the Doppler shift at a ground station during the 
passage of a satellite within line-of-sight to do either of two 
things: (1) determine the orbit of the satellite if the position 
of the ground station is known, or (2) determine the location of 
the ground station if the orbit of the satellite is known. 
These two calculations are the basis of the TRANSIT 
System; the second calculation is performed by the navigator using 
a description of the orbit provided to him and a measurement of the 
Doppler shift which is made by his (navigating) equipment, the first 
calculation (or an elaboration of it) is used by the organization 
operating the system to determine the orbits for distribution to 
the users using Doppler measurements made at special ground stations 
at known locations. Of course the second calculation is vastly 
Simpler than the first and can be done with much more accuracy since 
only two variables (latitude and longitude) are required to specify 
the location of a ground station while the specification of a satel- 
lite orbit requires that at least six parameters be determined. 
Fortunately, the first calculation usually need not be done since it 
is generally possible, for tracking the satellite, to acquire more 
information than the Doppler data for a single pass at a Single 
ground station. In fact, the tracking of TRANSIT satellites is 
accomplished by the use of the Doppler data measured at a multi- 
plicity of ground stations and extending over a period of time (say 
12 hours) that includes a number of passes at each ground station. 
However, it is technically of interest that the multiplicity of 
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