The ground station transmits a series of monitor crystal frequency 

 pulses wliich the shipboard operator uses for adjusting the ship station oscilla- 

 tor. These pulses appear on the indicator circular sweep as stationary or 

 slowly travelling pulses. The operator adjusts the pulses to a stationary posi- 

 tion and thus calibrates his shipboard equipment. Tliese pulses appear only on 

 the time base during a calibration period as selected by a panel switch. 



The Operate -Monitor switch is also used to check the zero adjustment 

 of the ground station. Pulses originating at the ship station are received and 

 amplified at the ground station. The signal is divided and sent through two delay 

 lines, one of which is variable. By a matching process, the operator is able to 

 adjust the leading edge of the two pulses to coincide and thus adjusts the ground 

 station for the proper delay. 



The transmitter at the ground station is similar to, but larger than 

 the shipboard transmitter. It need be capable of transmitting on only one fre- 

 quency (290-320 megacycles) and must be capable of answering to the pulses 

 from as many as 20 ships . The transmitted peak power is over 15 kw. 



The ground station antenna system consists of two dipoles--one for 

 receiving and one for transmitting- -mounted on a single 50 ft mast. Reflectors 

 are used to give directivity and antenna gain. This system is necessary for 

 working at distances greater than 50 miles. 



Ground station power is normally supplied by two gasoline or diesel 

 engine driven generator sets . One set acts as a back-up for the other in case 

 of failure. The station requires 1500 watts of 400 to 2600 cycle il5-volt ac and 

 400 watts of 27 -volt dc . 



3 . ACCURACY AND RANGE 



Two types of errors are inherent in the Shoran system: systematic 

 and random . 



a. Systematic Errors 



The systematic errors are those which can be precisely determined 

 and include errors resulting from the difference between Shoran and map dis- 

 tance and from the assumed velocity of the radio wave. Because of atmosphere 

 refraction and the elevation height of the antennas, the Shoran path is not a 

 straight line, but approximates the arc of a circle. The elevation of the antenna 

 is the dominating factor. Usually with elevations less than 100 ft and distances 

 less than 100 miles the error is negligible. 



158 



artbttr Zn.HittlcJnr. 



