Pulse systems in the high- and medium -frequency range have advan- 

 tages because when short pulses are used the ground wave may be distinguished 

 even with sky wave interference. However, at frequencies below about 100 kc, 

 this advantage tends to disappear, as pulse generation and radiation become more 

 difficult and pulse transmission approaches continuous -wave transmission, where 

 phase -difference readings must be made. Figure 111-8 shows that high-accuracy 

 navigation systems dependent upon ground-wave transmission cannot operate ef- 

 fectively at ranges greater than a few hundred miles . Generally those systems 

 using high-precision cw phase measurement techniques are restricted to daylight 

 use to avoid sky-wave contamination. Combinations of pulse and continuous -wave 

 techniques are used for the long-range low-frequency navigation systems such as 

 Loran-C and Omega. 



F. ACCURACY 



The accuracy of a particular navigation system may be taken as a 

 measure of its performance. However, the statistical terminology for describ- 

 ing accuracy is not well -standardized and for many systems is not generally 

 available . In this report both the terms accuracy and precision are used in de- 

 scribing system performance . The accuracy of a system is taken to mean the 

 deviation of a value from the true value. The term precision indicates the sys- 

 tem's repeatability, expressed in suitable terms. 



The accuracy of a navigation fix is a function of the accuracy and sta- 

 bility of each line of position and the angle at which these lines intersect. This 

 stability is dependent upon a number of factors, many of which are variable with 

 respect to time. These factors are often classed as systematic and random 

 errors . 



1 ■ SYSTEMATIC ERRORS 



The errors grouped as systematic errors are those which are the re- 

 sult of system design or alignment. This class of errors is of constant value 

 and sign and adversely affects over-all system performance. Generally, these 

 errors can be calculated or measured and then allowed for in the design of the 

 equipment. In some cases they can be directly compensated for in the equipment's 

 alignment or adjustment . Some systematic errors are calibrated out in the charts 

 and tables provided. 



2 . RANDOM ERRORS 



The random errors cannot be so easily accounted for because they are 

 unpredictable and time -dependent and vary with changing radiation parameters. 



25 



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