a. Shipboard Equipment 



TWo distances measured simultaneously are required for a fix. The 

 transmitter aboard ship alternately transmits a pulse on two frequencies, one 

 for each ground station . The alternation rate between ground stations is about 

 10/second. The ground stations retransmit to the ship on a third frequency. 

 The ship's receiver indicator provides simultaneous and continuous indications 

 of the distances between the ship and the two ground stations. 



The indicator unit contains circuitry for measuring the time of a 

 round-trip pulse and for displaying the transmitted and received pulses on the 

 circular sweep of a cathode-ray tube. The transmitted pulse appears as a fixed 

 marker pulse, and the received pulses appear at points on the circular sweep 

 dependent upon the distances from transmitter to receiver. 



A phase -shifting method of pulse positioning is used as the measure 

 of the distance between the ship and the ground station. The pulses are matched 

 visually and read on a scale and vernier which are mechanically connected to 

 the phase shifter. As the ship moves away from the ground station, the phase 

 is advanced to permit coincidence of transmitted and received pulses. A phase 

 advance of 360° corresponds to a distance between transmitter and ground sta- 

 tion of 100 miles. Thus, the phase advance vernier repeats itself each 100 

 miles. It is, therefore, necessary that the observer kxiow the ship's position 

 within 100 miles or so, in order to fix its position accurately. 



The trace shown on the cathode -ray tube screen is circular, with a 

 reference pip at the top. The two shore station pips show elsewhere on the 

 trace. In making distance measurements, one aligns the two distance pips with 

 the marker pip. On the one-mile scale, for example, a circular sweep length 

 of about 6 inches corresponds to a distance of 1 statute mile. A resolution of 

 between 0.01 and 0.001 mile is possible using the scale and vernier. Indicator 

 range scales are 1, 10, and 100 miles. 



b. Ground Station Equipment 



The ground station equipment is designed to handle up to 20 interroga- 

 tions simultaneously and is, therefore, somewhat larger than the ship equip- 

 ment. Crystals at both ground stations are used to provide a continuous mutual 

 check on operations . These monitor crystals operate at a frequency of 93 . 109 

 kc, a cycle of which corresponds to a 1-mile loop time. Frequencies correspond- 

 ing to 1, 10, and 100 miles are also derived from this crystal . 



157 



artbtir 31.1ittlc,2lnr. 



