3. EQUIPMENT AND INSTRUMENTS 



53 



AC and AC'=tan 20*x50cm 



Figure 11. — Construction of plate to test three-arm 

 plastic protractors. 



Figure 12.— Plotting Shoran (EPI, Raydist) 

 positions with Odessey protractor. 



Electronic Control Equipment 



3-23 Electronic Position Indicator (EPI). 



— The Electronic Position Indicator (Fig. 

 13) system was developed by the Coast and 

 Geodetic Survey after the close of World War 

 II. It was designed primarily to control off- 

 shore surveys which were formerly controlled 

 by Radio Acoustic Ranging, astronomic 

 sights, and dead reckoning, and to control 

 surveys beyond the limits of Shoran. 



Detailed instructions for use and mainte- 

 nance of EPI equipment are contained in 



Special Publication No. 265-A EPI Manual, 

 Mark III, Model 3. 



EPI is a pulse-type arcuate system which 

 requires mobile ship equipment capable of 

 transmitting pulses of electromagnetic en- 

 ergy and of measuring very small increments 

 of time (distance) ; and two fixed ground 

 stations which are capable of transmitting 

 similar pulses accurately synchronized with 

 those from the ship. The ship instrument 

 measures the time for the pulses to travel 

 from it to the ground station and return. 

 The time intervals are measured at a rate of 

 more than 20 times per second. For this 

 system, the velocity of propagation of elec- 

 tromagnetic waves through the atmosphere 

 is assumed to be 299,690 kilometers (186,- 

 218 statute miles) per second. Although it 

 is possible to fix the position of the ship at 

 any time, it is preferable to establish a defi- 

 nite time interval between fixes. 



3-24 EPI frequencies. — The pulse repeti- 

 tion rate is 41% per second with a pulse 

 length of about 60 microseconds. Radiated 

 power from the antenna is about 10 kilo- 

 watts. The receiver bandwidth is about 85 

 kilocycles. The frequencies used are 1850 and 

 1950 kilocycles and these frequencies are 

 shared with the standard Loran service. The 

 frequency used should not conflict with Loran 

 rates servicing the area. A Loran rate pre- 

 fixed by the numeral 1 is operating on 1950 

 kilocycles, and if prefixed by the numeral 2 

 is operating on 1850 kilocycles. Frequency 

 assignments should be requested from the 

 Washington Office. 



3-25 Range of EPI. — The minimum range 

 of EPI is about 10 miles, but EPI should 

 not be used at distances less than 15 miles. 

 Under low static conditions, dependable 

 measurements can be made to distances of 

 500 nautical miles, and to distances of 250 

 nautical miles when static is abnormal. Since 

 static is always worse at night, barring local 

 storms, it is customary to plan night opera- 

 tions in areas within these limits. 



3-26 Accuracy of EPI. — The systematic 

 errors of EPI equipment can be determined 

 by calibration tests and the observed dis- 



