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HYDROGRAPHIC MANUAL 



various compass headings. The directivity 

 pattern for each ship installation should be 

 determined during the first calibration tests 

 each season and spot checks on the correction 

 curves should be made periodically. These 

 patterns are usually in the order of one or 

 two microseconds ; however, they may be 

 much larger. The system of grounded and 

 floating antennas should be the same for all 

 periods of calibration and operation, and all 

 antennas should be ungrounded if practicable. 



3-31 Ground station personnel and com- 

 munications. — The basic minimum staff of 

 an EPI ground station is: one electronic 

 technician and two station operators. The 

 staff should be increased to five men if the 

 station is to operate continuously on a 24- 

 hour basis. A cook should be detailed to the 

 party when the station is in a remote area 

 and other mess facilities are not available. 

 One man in the party should be qualified to 

 service and maintain the portable genera- 

 tors. The accuracy of EPI distance measure- 

 ments depends on the quality of the work 

 done at the shore stations as well as that 

 done by the observer at the ship station. The 

 men assigned to an EPI station should be 

 carefully selected from the best available 

 personnel. 



Reliable and continuous radio communica- 

 tions must be maintained between the ship 

 and the shore stations if the system is to 

 operate efficiently (see 2-26). Communica- 

 tion on voice modulation is satisfactory most 

 of the time; however, when the ship is op- 

 erating at a distance beyond the range of 

 the radiophone, CW communication will be 

 required. At least one person at the shore 

 station should be proficient in CW radio tele- 

 graph operations under such circumstances. 



3-32 EPI fixed positions. — Frequency of 

 positions will depend on the scale of the sur- 

 vey. When plotting at a scale of 1:100,000 

 fixes at intervals of 10 to 15 minutes will 

 suffice. When running closely-spaced lines 

 for development of banks or other features, 

 it may be advantageous to reduce this inter- 

 val. Distances are read from the counters 

 and range dials to the nearest 0.1 micro- 



second. The readings shall be recorded in 

 the sounding record and, for convenience of 

 the plotter, on a plotting abstract. The range 

 dials should not be moved until the position 

 has been plotted. 



EPI positions are plotted with reference 

 to distance circles drawn on the sheet in 

 accordance with 5-11 and Table 9. An 

 Odessey protractor of suitable scale is used 

 for plotting the positions (see 3-22). When 

 the system is operating under favorable con- 

 ditions, there should be little diflficulty in 

 sounding along a prearranged line. However, 

 at night, or near the limit of usable signals, 

 or when atmospheric conditions are unfavor- 

 able, some difficulty may be experienced. The 

 skywaves at night are particularly trouble- 

 some and great care is necessary to avoid 

 use of a skywave when matching pulses. 

 Errors of this kind are usually obvious and 

 shall be rejected in the record book. 



On some projects it is desirable to have 

 the ground stations serve more than one 

 ship. Half-hourly fixes can be obtained by 

 three ships if all ship and shore station 

 equipments are operating properly and each 

 ship turns its transmitter off as soon as a 

 fix is obtained. Two ships can operate on a 

 position interval as small as 10 minutes. 

 When multiple use of a shore station is re- 

 quired, a definite schedule is prearranged and 

 each ship will turn its transmitter on 5 min- 

 utes before time for a fix. This is to allow 

 suflflcient time for the shore station to syn- 

 chronize its equipment. The ship transmitter 

 must be turned off promptly after the fix 

 is recorded to avoid encroachment on the 

 time allotted another ship. 



When operating in this manner, it is es- 

 sential that all units have the same clock 

 time, the same transmitting frequency, and, 

 most important of all, the same pulse rate. 

 If the pulse rates are not exactly the same 

 on all ships, the shore stations will have dif- 

 ficulty getting in synchronization in the 

 short period of time available. In prepara- 

 tion for this kind of operation, the 100 KC 

 crystal in the controller-indicator aboard one 

 ship should be adjusted against WWV. The 

 other ships operating in the system will ad- 



