B. ELECTRONIC POSITION INDICATOR (EPI) 



GENERAL 



The EPI electronic method of position control for use in hydrographic 

 surveys was developed by the U.S. Coast & Geodetic Survey in the late 1940's. 

 It was designed to complement the near-shore Shoran equipment (see Sec- 

 tion VI E) and has a useful range of 200-500 miles, depending upon atmospheric 

 conditions at the time of measurement. It operates at a frequency of 1 .85 mc 

 and is principally used for the control of a hydrographic survey on a scale of 

 1 : 100,000 and smaller. 



Like Shoran, EPI is a pulse phase -measuring system which requires 

 a minimum of two shore stations positioned at the ends of a fixed base line . In 

 place of the automatic transponder-type shore stations found in Shoran, the 

 EPI ship-positioning system uses interrogation and reply. It is a (rho-rho) 

 system which measures the distance from the ship to each ground station . 

 Periodic pulses are transmitted from the ship. The shore station operator ob- 

 serves the ship signals and his own shore station signal on a cathode-ray-tube 

 display. He manually makes proper adjustment so that the two signals coin- 

 cide. Delays of known size are incorporated in the circuits to prevent confusing 

 one ground station with the other. Aboard ship, the signal pips are viewed on a 

 similar cathode-ray tube and manually matched to the stationary ship signal 

 pulse. The distance is displayed in digital form in units corresponding to 

 microseconds of time. One unit on the dial equals about 150 meters, or 492 

 feet. Position fixes accurate from about 50 feet to 200 feet can be expected at 

 ranges of 200-250 miles. 



2. DESCRIPTION 



The equipment consists of a shipborne transmitter which transmits 

 precisely timed pulses toward the two shore stations positioned at either end of 

 a base line. These stations, in turn, retransmit similar pulses accurately 

 synchronized with those from the ship. A shipboard receiver-indicator unit 

 (Figure V-4) receives the two ground station pulses and displays them simul- 

 taneously at a rate of about 20 times per second. Distance measurements to 

 each ground station are then made by the shipboard operator. 



Because of the low transmission frequency of 1850 kc the rise time of 

 the pulse at the receiver is slow, i.e., 8-12 microseconds. For this reason, 

 the received pulse is not used to retrigger the shore transmitter as in Shoran, 

 because large errors would result if the pulse were noise-modulated, A syn- 

 chronizing method is used in EPI which precisely sets the time at which the 



110 



Arthur ZD.ItittlfJnr. 



