from anomalies in the index of refraction and are commonly formed in seashore 

 areas, where there may be large temperature and conductivity differentials, 

 often attributed to the movement of warm, dry air out over the water. 



Frequencies in the range of 100 - 10,000 mc are used for the measure- 

 ment of ranges to geodetic accuracies . A corrected velocity of transmission is 

 usually required for the most precise range determinations possible. 



E . RANGE CONSIDERATIONS 



Several factors have been shown to affect the useful range of a radio 

 signal. These are the shape and electrical conductivity of the earth, the state 

 of ionization and noise -producing qualities of the atmosphere, the bandwidth, and 

 the signal frequency . 



For ground -wave transmission, the ranges are essentially optical at 

 the shortest wave length and depend upon antenna height and line -of -sight trans - 

 mission. The effective operating range is greatest for the low and medium fre- 

 quencies and is dependent almost entirely upon wave length . At the longer wave 

 lengths effective antennas must be very large, and the radiation efficiency de- 

 creases to very low values, making it difficult to radiate large amounts of power. 



Certain sky-wave factors must also be considered where high-quality 

 ground wave transmission and reception is contemplated. At frequencies in the 

 range of 30 kc to 10 mc, long-range transmission may be expected. Useful 

 coverage is limited to a few miles during daylight and less than 2000 miles dur- 

 ing evening hours at frequencies near 1 mc . For frequencies less than about 

 0.1 mc the stronger sky-wave component tends to invalidate the longer ground - 

 wave ranges possible at these frequencies, and special circuit and statistical 

 techniques are required to achieve these long ranges . These effects are shown 

 in Figures III-7 and III-8. 



As indicated in Figure III -8, the continuous wave systems take advan- 

 tage of the characteristics of the predominating ground wave for ranges out to 

 400 - 500 miles . As distance from the transmitter increases, the ground -wave 

 field intensity drops off more rapidly than the sky wave, so that sky-wave inter- 

 ference tends to predominate at ranges of a few hundred miles . Because the 

 phase of the sky wave is random relative to the ground wave, the resultant signal 

 fades . Under these conditions the indicated phase is made random and the read- 

 ings bear little relation to the position of the navigator . The straight line of 

 Figure III -8 indicates the maximum useful range for pure ground waves . 



23 



artbur ai.littlcllnr. 



