a crude estimate is possible using the work of Wait and 

 Spies, ^'^ A delay of 10 to 20 cec may be expected. The 

 effect is neither confirmed nor rejected by the observed 

 phase differences because of the brief monitoring and also 

 because of the high scatter resulting from a poor signal-to- 

 noise ratio. 



The measurements at Den Helder include propaga- 

 tion directly over the Greenland ice cap and also propagation 

 at extreme northern latitudes. No attempt at analysis is 

 justified at the present time. In fact, the lane assignment 

 for this particular measurement is only tentative. 



Phase-difference measurements during the day are 

 Daytime more complicated to analyze than those at night, since 



propagation phase velocity varies with the solar zenith angle and exhibits 



related seasonal changes. In general, for normal illumina- 

 tion of the ionosphere, phase prediction is more easily 

 accomplished during the day than at night. In particular, 

 propagation during the day is less sensitive to geomagnetic 

 path orientation, latitude, and the effects of the second 

 waveguide mode. Prediction methods have been discussed 

 elsewhere and widely applied.*'^ The basic prediction 

 coefficients have been in use since December 1963. 



Day and night prediction coefficients have been 

 Computer combined in a computer program together with transition 



predictions criteria to predict the phase difference observed at any 



location at any time of day. While neither the daytime pre- 

 diction coefficients nor the transition criteria have been 

 changed since the inception of the computer program, changes 

 in the computation procedure at night were made early in 

 1965 and can affect the predictions at all times of day. Pre- 

 sent computer results therefore differ slightly from those 

 provided for navigational use during the early part of the 

 operation and those used in an earlier analysis.^ Also, an 

 offset erroneously present in the Criggion-Forestport cor- 

 rections has been corrected. Comparisons of observed 



19 



