Originally Posted by
FE Hoppy
Mode S enhanced squiter:
Mode S EHS (EnHanced Surveillance) isn't a squitter (ie sent, unsolicited, at regular intervals).
It's a response to an ATC interrogation (like Mode A/C), although some of the content (groundspeed, track, ACID, etc) duplicates what's in the ADS-B 1090 Extended Squitter (ES).