When designing the periscopic sextant now coming into production it was realized that the conditions in the pressurized aircraft for which this sextant was designed differed sufficiently from those previously experienced to make it worth while reconsidering what methods would be most applicable to the new combination of pressurized aircraft and periscopic sextant.

The conditions differ from previous experience in providing (a) a higher speed; (b) very little, if any, direct view of the sky by the navigator.