Mode C verification
At the risk of sounding like a pedant, it's worth pointing out that the +/- 200 ft does not reflect the pilot's (+/-100 ft) or aircraft's (differs based on date of manufacture) altitude keeping ability and only reflects the effects of altimetry and surveillance system errors. I say this only because I've heard some pilots discuss their belief that +/- 200 ft for SSR Mode C verification equates to their 'wriggle room' around a cleared level.
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From ICAO doc 4444, as it may apply this way for other countries...
8.5.5.1 VERIFICATION OF LEVEL INFORMATION
8.5.5.1.1 The tolerance value used to determine that pressure-altitude-derived level information displayed to the controller is accurate shall be ±60 m (±200 ft) in RVSM airspace. In other airspace, it shall be ±90 m (±300 ft), except that the appropriate ATS authority may specify a smaller criterion, but not less than ±60 m (±200 ft), if this is found to be more practical. Geometric height information shall not be used for separation.
8.5.5.1.1 The tolerance value used to determine that pressure-altitude-derived level information displayed to the controller is accurate shall be ±60 m (±200 ft) in RVSM airspace. In other airspace, it shall be ±90 m (±300 ft), except that the appropriate ATS authority may specify a smaller criterion, but not less than ±60 m (±200 ft), if this is found to be more practical. Geometric height information shall not be used for separation.
