5. HYDROGRAPHY 



195 



The beginning and ending of the line 

 should be accurately fixed in position with 

 reference to objects of known position. 

 Three-point sextant fixes, cross bearings, or 

 radar distances are acceptable in that order 

 of preference. 



5-128 Standard Loran. — When operating 

 in areas served by Standard Loran systems, 

 Loran lines of position should be observed 

 once each hour. At least three lines of posi- 

 tion should be determined when this is 

 possible. 



The range of Loran signals varies with 

 time of day, location of the observer, static, 

 amount of land between the observer and 

 the stations, and ionospheric conditions. 

 Signals may be received as ground waves 

 or sky waves. The maximum range of ground 

 waves is about 600 nautical miles during 

 the day and 500 nautical miles at night. 

 Usable sky waves may be received at a 

 range of 1,400 nautical miles under the 

 best possible conditions. As a general rule 

 dependable Loran fixes can be obtained at 

 distances somewhat more than half the above 

 stated maximums, and accuracy of positions 

 will decease as the distances from the sta- 

 tions increase. 



The observer must always match similar 

 waves, that is, ground waves or sky waves. 

 A ground wave should never be matched 

 with a sky wave. Only the first sky wave 

 (one-hop E) should be used, and no pulse 

 beyond this wave shall be used. The ground 

 wave always appears first and the one-hop 

 E wave second. If ground waves can be re- 

 ceived from both stations of a pair, they 

 should always be used. Sky waves are sub- 

 ject to variations in timing and changes of 

 shape known as splitting and fading, caused 

 by variations in the inosphere. 



The position of the observer is determined 

 by the intersection of two or more Loran 

 lines of position. The Loran readings shall 

 be recorded on Form 120 and a subscript "G" 

 indicating ground waves, or "S" indicating 

 sky waves shall be used in all cases. The 

 line of position shall be computed from the 

 data in the Loran Tables published by the 

 Hydrographic Office. A line of position shall 



be advanced or retired if necessary to reduce 

 all observations for a position to a specific 

 time. All Loran computations shall be 

 checked prior to submission with the records 

 for the cruise. 



5-129 Astronomic observations. — Astro- 

 nomic sights are used to fix the position of 

 a ship on track or cruise lines in a manner 

 similar to their use in surface navigation, 

 except that the sights are taken with greater 

 precision and care, and they are used more 

 accurately to control the line. The details of 

 observation, computation, and use of astro- 

 nomic sights are adequately described and 

 explained in standard works on navigation, 

 such as Bowditch (1958), Navigation and 

 Nautical Astronomy — Button (1957), and 

 Button's Navigation and Piloting — John C. 

 Hill, II (1958), and need not be repeated 

 here. The information in this manual is 

 limited to refinements of observation and the 

 best methods of using the data. 



Form 719, or 719a, Astronomic Sight for 

 Hydrographic Control, shall be used to re- 

 cord and compute all astronomic sights. 

 Form 719 is arranged for use of the cosine- 

 haversine formula, and Form 719a is de- 

 signed for use with the Tables of Computed 

 Altitude and Azimuth (H.O. Pub. No. 214). 

 Any kind of astronomic sight may be com- 

 puted on either form. For the most precise 

 work, all computations should be worked to 

 the nearest 0.1 second of time and to the 

 nearest 0.1 minute of arc. All computations 

 should be checked, the sheets bound in 

 chronological order, and shipped to the 

 Washington Office with the other survey 

 records. Smooth copies are not required. 



Astronomic observations should be made 

 with navigating sextants, read to the nearest 

 10 seconds, and the time of each observation 

 marked to the nearest 0.2 second using an 

 accurate stop watch. When sea and sky 

 conditions permit, each observer should 

 make a series of 6 measurements to the 

 star or other body being sighted upon. 

 Each observer should have his own recorder. 



Log readings or revolution counter read- 

 ings should be recorded at 10-minute inter- 

 vals during morning and evening star sight 



