refraction corrected Doppler or vacuum Doppler that is actually 
used in the TRANSIT program for orbit determinations and precision 
navigation fixes. Actually, the generation of the vacuum Doppler 
from the individual Doppler shift at the two harmonically related 
frequencies is accomplished by a relatively simple analog computer’ 
which would yield a precisely correct answer if the 1/(#req.)” 
refraction law were precisely correct. 
Results to date in the TRANSIT program indicate that 
at mid-latitudes or lower the refraction correction based on the 
1/(freq.)* assumption is sufficiently accurate to meet the TRANSIT 
program goals even in times of magnetic storm activity. However, 
there is need to explore further the refraction effect at high 
latitudes during heavy auroral activity to determine if this 
correction remains sufficiently accurate in these most Severe con- 
ditions. Considerable effort in this area is planned for the next 
year. 
Sie Transmission of the Orbit Parameters 
In principle the transmission of the orbit parameters 
from the satellite poses a relatively easy communication problem 
and can be accomplished straightforwardly in a number of ways. 
Practically it is desirable to develop a means which poses the 
least increase in complexity or power on the part of the satellite 
equipment and the least special equipment for reception on the part 
of the navigator. To this end a system has been developed in which 
the orbit parameters are transmitted in binary notation (zeros and 
ones) coded as a phase modulation of the basic transmission which 
are used to generate the Doppler shift. A specific modulation (60° 
phase advance followed by a 60° phase retard) is used which does not 
interfere with the ability to measure the Doppler shift with preci- 
Sion. The use of this approach avoids the need for an extra trans- 
mitter in the satellite or an extra receiver in the ground equipment. 
1/ Weiffenbach, G.C., "Measurement of the Doppler Shift of Radio 
Transmissions from Satellites," Proceedings of the Institute 
of Radio Engineers, Vol. 48, No. 4, pp 750-754, April 1960 
98 
