station ranging is the elimination of blanking due to 
poor station geometry and increased precision due to re 
dundant measurements. By ranging to four stations, six 
independent nadir point solutions can be obtained. They 
are obtained by using the following ground station combi 
nations: (1-2), (2-3), (3-4), (4-1), (1-3), and (2-4). 
In the case of Figure 4, solutions from (1-3) and (2-4) 
would be thrown out due to bad geometry. The remaining 
four stations, however, have good geometry and can be used 
to determine the nadir point. Shiran range information 
can also be used to navigate the airplane along the pre 
scribed flight lines. 
Shiran employs an interrogator set which is installed 
in an aircraft and transponders which are placed on ground 
points. In operation, the interrogator set transmits an 
S-band CW signal which is received by all of the ground 
stations within range. Only one transponder, however, 
responds at any one time. In other words, the ground sta 
tions are sequentially interrogated. This is accomplished 
by applying a keying tone to the S-band carrier. The ground 
stations are coded so that they respond only when they re 
ceive their keying tone. In this way, 1) transponder 1 is 
keyed, 2) it responds at an offset frequency, 3) the inter 
rogator receives it, 4) it measures the range, and 5) the 
range is placed in memory channel #1. Transponder 2 is then 
9