What a slant-delay value rests on beyond the product's stated grid.

The three fields are independent and can all be set on one value:

• held - the coverage miss a :hold coverage policy held the value through: :epoch_before_first_map, :epoch_after_last_map, :latitude_out_of_range or :longitude_out_of_range. nil inside coverage.

• degraded - a Sidereon.GNSS.Ionosphere.NodeGap naming every non-available node a :renormalize missing-node policy interpolated around. nil where every weighted node held a value.

• assumed_mapping - what the product declares, where the single-layer factor mapped a product declaring anything but COSZ: :no_mapping, :q_factor, :absent, or {:other, code} carrying the declared code's own text. nil where the product declares COSZ, which is the factor applied.

valid? is true when the value was neither held through a coverage miss nor degraded by a non-available node. An assumed mapping does not clear it: most published global products declare something other than COSZ, so a nominal result from the CODE, ESA, JPL or UPC grids would otherwise read as invalid. A caller that cares which factor was applied reads assumed_mapping.