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a-value Same as Altitude Intercept. Called the "a-value," it is the difference, found in the last step of a sight reduction, between the calculated height (Hc) and the observed height (Ho) of a celestial body. It is used for plotting the LOP. It is labeled A (away) if Hc is greater than Ho, or T (toward) if Hc is less than Ho. In physical terms, "a" tells if we are closer or farther from the GP than the assumed position is. An a-value of 20' T, for example, means your true position is 20' (20 nmi) closer to the GP than the assumed position is. See also Geographical Position. Abbreviation:  a

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