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Most airborne electronic navigational components are evaluated in the manufacturers' laboratories. Due to present methods of procurement and the problems involved in marrying these components together, the testing of navigation systems frequently culminates in an operational evaluation. Such evaluations ordinarily reduce to a comparison of the aircraft's actual position with that indicated by the system under test. This article describes two methods of determining the aircraft's position; visual observation and photooptical observation. The problems involved and the general magnitudes of the errors which may be expected are discussed. Conclusions are made regarding these two types of observations.