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Serial No. 2631  Page 2.

REPORT ON
A METHOD OF
REDUCING SEXTANT OBSERVATIONS

OBJECT

If astronomical methods are ever to be used in aircraft navigation, it is essential that the time of reduction of observations be reduced to a minimum. At sea, twenty minutes to one half hour is the usual time required to work up a sight. On fast-moving aircraft, knowing where one was twenty minutes before is of little value.

A method of precomputation was evolved by the Instrument Branch of the Engineer-ing Division in the fall of 1922. It has been tested on several long cross-country flights and it has been found that the position line can be drawn in ½ minute after the sight is taken.

Charts, etc., were prepared for the World-Cruise to use in crossing the Pacific but, as those hops were flown in a dense fog, the sextant could not be used.

The basic idea of this method is due to G.W. Littlehales of U.S.N. H.O. His scheme, however, requires large maps and is inapplicable to strip maps which are so admirably suited to aircraft use.

CONCLUSION

This report is the first written description of a method of reducing sights, evolved by the Instrument Branch. By this method the position-line can be drawn within ½ minute after taking the sight. The operation consists of one simple interpolation and one subtraction.

Only the most elementary knowledge of astronomy is needed by the person using this method. All the astronomy necessary is embodied in this report.

This method assumes that before a flight there shall be available a person familiar with astronomy and astronomical calculations, who shall prepare the