and slave pulses and reads the time delay on a dial and digital counter. Two 

 such time -difference readings from two Loran pairs furnish the information re- 

 quired for a Loran position fix. Loran tables and charts must be used to estab- 

 lish a geographical position. 



3. ACCURACY 



The accuracy of a Loran-A fix is determined by the accuracy of the 

 individual lines of position and by their angles of intersection. The accuracy of 

 the individual lines of position is dependent upon the following: 



a . Synchronization of transmitters 



b. Operator skill in matching and identifying signals 



c . Uncertainty of sky-wave correction (when sky waves are used) 



d . Position of ship relative to transmitting stations 



e. Accuracy of tables and charts 



f. Timing and positional uncertainty. 



With a reasonable signal-to-noise ratio, an operator can be expected 

 to equalize the amplitude and match the leading edge of the received signals with- 

 in 1 microsecond of the correct value . 



Since the ionosphere does not maintain a constant height or angle with 

 respect to the earth, these corrections vary slightly from tabulated values. 

 Sky-wave reception is most reliable at distances greater than 800 miles from 

 the transmitter, where readings, with proper correction, are generally correct 

 to about ± 5 microseconds. At lesser distances, the uncertainty increases. 



The relative accuracy of Loran over the coverage area is apparent 

 from the spacing of the lines on the Loran charts . The most favorable position 

 is on the base line between the transmitters . ITie most unfavorable positions 

 are adjacent to the base line extensions, where the Loran line separations can 

 be several miles per microsecond. 



The accuracy of Loran tables and charts is of the order of a fraction 

 of a microsecond. Inaccuracies do occur in those areas where insufficient survey 

 data was available for positioning the transmitting stations. Where correction 

 data is available, this is included in charts provided with the Loran-A tables. 

 The timing uncertainty, in microseconds, results in a corresponding uncertainty 



64 



arthur ai.l.ittlci(nr. 



