3. EQUIPMENT AND INSTRUMENTS 



61 



be made of the correction with respect to 

 distance from the ground station. 



3-43 Shoran zero set. — Distances to 

 ground stations are obtained by matching 

 returning pulses with a reference pulse or 

 marker. The zero set is an adjustment which 

 changes the time position of the reference 

 marker, and has the effect of changing both 

 the rate and drift zero check values. The zero 

 set is thus used to make the zero checks 

 some convenient value or to reduce the sta- 

 tion corrections to a minimum. 



If a zero check reading (3-44) changes 

 more than 0.005 mile, from its normal value 

 either by a sudden jump or slow drift, it 

 should be considered as evidence of instabil- 

 ity in the ship set. The set should be in- 

 spected by a technician, a new calibration 

 should be made at the earliest convenient 

 time, and the zero set adjusted if necessary. 



3-44 Shoran zero cheek. — The zero 

 check measures the time relation between 

 the zero of the distance measuring scales 

 and the marker pulse. Since it is possible 

 for a drift to occur in the distance measur- 

 ing scales, the zero check is used to detect 

 any such shift. This is the method of ob- 

 serving the stability of the timing relation- 

 ship within the ship equipment. 



A zero check shall be observed and re- 

 corded immediately before and after a dis- 

 tance-correction calibration, at the beginning 

 of each day's work, and at least once each 

 hour while hydrography is in progress (see 

 5-42). 



3-45 Frequency comparison. — At the be- 

 ginning of each season and once every two 

 months thereafter, a WWV or WWVH fre- 

 quency comparison shall be made for all 

 CPN-2 and CPN-2A ground stations and 

 for all ship stations (CPN-3) that use 

 AVQ ground stations. It is good practice to 

 make these comparisons more frequently 

 than the above prescribed minimum. 



3-46 Shoran report. — In addition to the 

 continuous study of Shoran corrections car- 

 ried on during the progress of a survey, a 

 comprehensive analysis and compilation of 



calibrations must be made at the end of the 

 season or on completion of a project. Tables 

 of distance corrections shall be compiled for 

 correction of observed distances. A copy of 

 the table should be included in the descrip- 

 tive report to accompany each Shoran- con- 

 trolled survey (see 7-9) and reference shall 

 be made to the Special Report on Shoran 

 Corrections. 



The calibration sheet, details of computa- 

 tions, and similar data should not be for- 

 warded with the report. The methods used 

 should be explained and any abnormal cor- 

 rections specifically mentioned. In order 

 that the verifier may have a complete pic- 

 ture of the calibration program, the report 

 shall include a listing, by geographic co- 

 ordinates, of all calibration points used and 

 the dates when calibrations were made at 

 each point, (see 7-27). 



3-47 Precautions when using Shoran. — 



If properly used, Shoran provides an excellent 

 method of controlling hydrographic surveys. 

 The following is a partial list of precautions 

 which should be taken to avoid unnecessary 

 complexities in smooth plotting and verify- 

 ing a hydrographic survey : 



(a) Make frequent calibrations to deter- 

 mine distance corrections. 



(b) Establish check points for repeat ob- 

 servations. 



(c) Make calibrations over the full range 

 used. 



(d) If the signals pass over intervening 

 land they will be attenuated and special cali- 

 brations in such areas are required. 



(e) Avoid use of Shoran beyond limits 

 imposed by the constant K=1A2. 



(f) Watch for reflections when the signal 

 passes close to cliffs or bluffs. 



(g) Ground station personnel must be 

 alert and conscientious. Assign this duty 

 to the best qualified men available. 



(h) Voltage at all stations should be main- 

 tained at 115 volts. An appreciable drop in 

 voltage will decrease signal intensity and 

 increase distance corrections. 



(i) If reflectors are used at the ground 

 stations they should be properly pointed to 

 provide maximum signal intensity. 



