Just ran across this (from Instrument Procedures Handbook, FAA-H-8261-1,
page 4-6):
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The International Civil Aviation Organization (ICAO) definition of
established is considered as being within half full scale deflection
for the ILS and VOR, or within +/- 5 degrees of the required bearing
for the nondirectional radio beacon (NDB).
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Andrey


Jose wrote:
When within 10 degrees of the published [NDB approach] course.

Thanks. I take it the course is then 20 degrees wide, with some lesser
protected area on the outskirts.

Jose