1. A navigation apparatus comprising a digital processor and a memory that utilizes a first subset and a second subset of a set of measured quantities which do not vary by amounts that are improbable or impossible, the set of measured quantities being provided at periodic time intervals delta-time by an external source for determining the state of a platform on which the apparatus is mounted, the set of measured quantities being presumptively useful in determining platform state, the number of members of the first subset being equal to or greater than zero but less than the total number of members of the set of measured quantities, the members of the set of measured quantities not included in the first subset being subject to selection for the second subset by the apparatus in accordance with a predetermined set of selection rules.