An Information-Based Validity Threshold for Orbital Estimators: When Angular Diversity Stops Supporting a Stated Tolerance

An estimator can continue returning a point value after the measurement geometry has stopped supporting the accuracy a mission requires. This note states a validity condition that detects that situation from a quantity the estimator already computes, and shows the angular diversity at which a stated tolerance ceases to be achievable. It accompanies a short article on treating abstention as a valid mission output.