Before examining the above components, a brief description of the 

 instrument and methods used in vector data collection is warranted. 

 These data have been collected in large quantities in all the world's 

 oceans by the Naval Oceanographic Office's project I'lAGNET aircraft, and 

 are available for studies of this type. 



The component data used in this study were collected on track 15 

 (Fig. 2), and are shown in Figure 10. Basically, the instrument used 

 for magnetic vector measurement was a gimbal suspended flux-gate mag- 

 netometer mounted in a magnetically compensated aircraft. The instru- 

 ment measures F, inclination or dip (I), and magnetic heading (MH) 

 directly, with probable errors of + 7.5 nt in F; 3 minutes of arc in 

 I, and 5 minutes of arc in MH. Ordinarily, aircraft true heading (TH) 

 is determined for declination (D) measurements by celestial observa- 

 tions, averaged over 100 seconds; taken at 5 minute time intervals. 

 Unfortunately, during the transit of the Caroline Be sin the sky was 

 overcast and TH could not be determined by this method. An alternative 

 method had to be developed. 



It was decided to use the N-1 Gyroscopic Compass, corrected for 

 gyro error (Ge) to determine the TH. The Ge was found by using the 5 

 minute D measurements based on celestial observations made earlier and 

 later in the flight, which were assumed to be accurate, combined with 

 MH and gyrocompass headings (G) taken at the same time. Ge for several 

 observations is then determined by: Ge=(G-MH)-D (west D negative). The 

 mean Ge for several observations was found to be 6 30'W + 5'(s.d). TH 

 may then be determined by first visually "smoothing" the G analog trace. 



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