the residual discrepancy between the observed average and 

 the computed value in centicycles (cec). Since the observa- 

 tional average from only a few nights of monitoring is not 

 Discrepancy of necessarily equal to a long-term mean at a particular site, 



night averages the rms residual discrepancy of 5.8 cec for the 30 site-LOP's, 



5.8 cec where the prediction theory is applicable, is not completely 



due to prediction error. Presently, the prediction theory is 

 invalid near transmitting stations where significant energy 

 may be propagated by the second wave-guide mode or incases 

 where one of the component propagation paths passes over 

 arctic regions of very poor conductivity."^ 



Although practical navigation was not recommended 

 using a station within 1000 n.mi. , for tabulation purposes 

 Perturbations only measurements with component propagation paths shorter 



over short paths than 700 n.mi. (1300 km) have been identified as "near. " A 



at night theoretical estimate of the effect of the second mode on the 



resultant phase over short propagation paths is shown in 

 figure 4. The curve was produced from the work of Wait and 

 Theoretical Spies ^ based on the isotropic model ionosphere. * The theo- 



perturbations retical work is important since it indicates peak deviations 



not large of on the order of one n.mi. or less. However, the work 



cannot yet be considered sufficiently reliable to warrant addi- 

 tional corrections. In particular, if the quasi-wave length 

 were only slightly off, corrections of the wrong sign might be 



*Specifically an ionospheric height of 90 km, gradient of 

 0,5 km and infinite surface conductivity were assumed. 

 The curves of Wait and Spies were then used to obtain the 

 relevant parameters for propagation at 10,2 kc/s, viz: 

 relative velocity, attenuation rate, and excitation. The 

 values obtained were, respectively, 1.0003; 1.6dB/Mm; 

 -1.3 dB at 6.3° for the first mode and 1.0303; 8.8 dB/Mm; 

 and 1.4 dB at 20^ for the second. 



16 



