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ITU-R M.1371 encodes speed over ground in 0.1-knot units and course over ground in 0.1-degree units, reserving the top code of each field for "not available": SOG 1023 reaches us as 102.3 knots and COG 3600 as 360 degrees. Both are finite, so numberValue passed them through as ordinary readings and every consumer — vessel cards, the HUD, voice analyst queries — treated a missing measurement as a real one. Observed on a live AISStream feed of 3,062 vessels: exactly 3 records at 102.3 knots with no reading anywhere between 40 and 102.3, and 404 records at exactly 360 degrees. One Dutch rescue boat moored at its station carried both sentinels at once, so an analyst summary reported a maximum speed of 102.3 knots — faster than any vessel afloat. Filter both at the store, where AISStream messages are decoded, so the sentinels never enter the cache. Heading already had this treatment through normalizedHeading with its own separate sentinel of 511. Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>