68 



HYDROGRAPHIC MANUAL 



Figure 20. — Typical record from Raydist brush recorder. 



The dials should be set to approximately 

 correct values first when approaching the 

 calibration point. The amount of the dial 

 reading errors should then be determined by 

 one of the above methods. If it is necessary 

 to reset the dials, a recalibration is required. 

 Since movement of the Ri dial will some- 

 times aflfect the R^ dial, the R, dial should 

 always be set first. Signals may be attenu- 

 ated when passing over land. Therefore the 

 calibrations should be made under conditions 

 which will prevail while hydrography is in 

 progress. 



When operating close to the Red station 

 the carrier signal from that station may be 

 so strong that it will feed through the filter 

 in the Green station circuit causing the au- 

 tomatic volume control to operate and blank 

 out the signal from the Green station. Under 

 such conditions the percentage of modula- 

 tion at the Red station must be reduced to 

 a level which will not cause interference in 

 the other circuit. The modulation at the Red 

 station can be increased in several steps as 

 the ship proceeds away from the station. 



3-59 Three-point fix calibrations. — When 



the three-point sextant fix method is used, 

 the fix should be as strong as possible and a 

 check angle observed (see 3-39). At least 

 three comparisons should be made after the 

 dials have been correctly set. The observed 

 dial readings are corrected for ship's head, 

 if necessary, the /?j dial reading is cor- 

 rected for placement of the Red station C-W 

 transmitter, if applicable, and the differences 

 between these values and the scaled values 

 represent dial corrections at that point. The 

 three comparisons should agree within a few 

 tenths of a lane. 



3-60 Buoy method of calibration. — Where 

 the area to be surveyed is distant from a 

 place where the three-point fix or other ac- 

 curate method of calibrating can be used, 

 and, if the depth of water permits, it is 

 advisable to establish one or more buoys near 

 by which can be used as check points or for 

 calibration if the occasion arises. The buoys 

 should be securely anchored with a scope of 

 about 1.5 to 1. The position of the anchor 

 must be determined. When calibrating at a 

 buoy, proper allowance must be made for 

 scope of the anchor cable and direction of 



