III. FUNDAMENTALS REVIEW 



A. GENERAL 



Most electronic navigation and precise ship -positioning systems de- 

 pend upon the measurement of the time it takes for radio frequency (rf) energy 

 to travel from the transmitter to the receiver . The transmitted energy must 

 be in the form of either short pulses or continuous waves (cw). Some systems 

 measure pulse -time difference; a second group of systems measures phase dif- 

 ferences, and a few systems use a combination of both techniques . 



While the accuracy of the distance measurement is primarily a func- 

 tion of the time measurement, the characteristic velocity of the rf energy must 

 also be known. For most distance measurements in ship -positioning or naviga- 

 tion applications, a standard value of propagation velocity is used. When meas- 

 urements must be extremely accurate, the atmospheric index of refraction must 

 be measured precisely so that the corrected velocity of transmission can be 

 calculated. The distance as measured by the product of the velocity of propaga- 

 tion and the time may not be the same as the geometric distance, and further 

 corrections for refraction and reflection may have to be made . 



Rf energy may be propagated between transmitter and receiver (1) by 

 direct path, (2) by reflection from the ionosphere, or (3) along the earth's sur- 

 face. When radio waves are refracted or reflected from the ionosphere, they 

 are called sky waves . Because of the instability of the ionospheric layer, cor- 

 rections for sky-wave paths must be computed but at best are only approxima- 

 tions . When sky-wave transmission can be predicted with greater accuracy and 

 reliable variations computed, they will be as valuable to long-range, precise 

 ship positioning as they are now to communications and navigation. For the pres- 

 ent, only the ground wave is used for the accurate ship positioning required for 

 hydrographic surveys. Sky-wave transmission is used at great ranges for navi- 

 gation purposes, but is less accurate and often is more of a hindrance than an aid. 



A wide range of frequencies has been found necessary to satisfy the 

 many precise ship -positioning and navigation requirements for accuracy and re- 

 peatability. Frequencies of 30 - 3000 mc and higher are most useful for line of 

 sight and where precision is required. Frequencies in the range of 300 kc to 

 3mc are used for systems operating out to distances of the order of 500 miles . 

 Frequencies from 10 kc to 300 kc are used for those navigation systems requiring 

 operation at ranges of several thousand miles . 



For some measurements, only a range (distance) is required. How- 

 ever, in most navigation and near -shore hydrographic work, the direction from 

 the transmitter to the receiver is also required, so that a position fix may be 



11 



arthur ai.ltittle.Ilnr. 



