60 



HYDROGRAPHIC MANUAL 



distances and observed Shoran distances are 

 the corrections to be applied to subsequent 

 Shoran readings at that distance. A zero 

 check shall be recorded immediately follow- 

 ing the calibration tests. 



The range and angle method is often used 

 and is somewhat more precise than the three- 

 point fix method. When this method is used, 

 the ship proceeds along a sensitive range at 

 slow speed until a predetermined sextant 

 angle to right or left of the range is closed. 

 Shoran distances are observed at that in- 

 stant and compared with distances to the 

 ground stations which have been computed 

 in advance. The differences between obser- 

 ved and computed distances are the cor- 

 rections to be applied for Shoran readings 

 at that distance and for a particular zero 

 check reading. 



It is sometimes possible to bring a launch 

 or small vessel alongside a pier, piling, bea- 

 con, or other point whose exact position is 

 known. The Shoran distances can be com- 

 pared and the correction determined. 



3-40 Distribution of calibrations. — If 



possible, calibrations shall be obtained near 

 minimum and near maximum ranges used 

 in the survey. Intermediate calibrations are 

 desirable to help define the correction curve. 

 Care should be exercised in selecting cali- 

 bration sites to avoid reflections from shore- 

 line or other objects. 



It is advantageous to have one or more 

 check points in the area at which daily com- 

 parisons may be made, as in the range sys- 

 tem of calibrating. Fixed objects such as 

 beacons or piling are particularly useful as 

 check points for launch hydrographic 

 parties. 



3-41 Statistical method of calibration. — 



When circumstances prevent calibrating from 

 near minimum to maximum ranges used, a 

 correction curve may be derived by compu- 

 tation as a last resort. An extensive study 

 of Shoran corrections has revealed that: 



(a) The Shoran distance-correction curve 

 is nearly linear except when close to or far 

 from the ground station. 



(b) The slope of the Shoran distance-cor- 



rection curve is nearly the same for all 

 Shoran instruments when antenna heights 

 are equal or nearly so. 



(c) For any ground station height within 

 the range normally used for hydrographic 

 surveys, the slope of the distance-correction 

 curve is most sensitive to the antenna height 

 on the mobile unit. 



The correction curves for various equip- 

 ments operating under identical conditions 

 will be nearly parallel but will seldom coin- 

 cide. The curves will have negative slope 

 and will increase negatively at a uniform 

 rate with increasing distance from the 

 ground station. The rate of increases is, in 

 some respects, a function of ship antenna 

 height. For purposes of deriving distance 

 correction curves, ship antenna heights may 

 be classified as low or high as follows : 



(a) Low ship antennas are those under 

 40 feet and the slope of the curve is 0.003 

 mile per mile (statute). 



(b) High ship antennas are those in ex- 

 cess of 40 feet and have a slope rate 0.0018 

 mile per mile. 



To derive a correction curve, it is neces- 

 sary to determine the correction at one or 

 more distances by methods previously de- 

 scribed. From these points the curve may 

 be extended as a straight line by using the 

 values in (a) or (b) above. To aid in 

 applying the correction when plotting, the 

 curve may be translated to a tabular form. 

 Values in the table should be distances from 

 the ground station corresponding to a cor- 

 rection change of 0.005 statute mile. 



3-42 Recording calibrations. — All Shoran 

 calibrations shall be recorded in the sound- 

 ing record for the survey, or in a special 

 record reserved for that purpose. The record 

 shall include the number identifying each 

 ship or ground station equipment involved 

 in the calibration (see 5-91). 



When a number of correction measure- 

 ments are made at the same location, a plot 

 of the correction against time should be 

 made to detect any indication of equipment 

 degradation. When measurements over a 

 wide distance range are made, a plot should 



