If I send an SMS message via APRS to a mobile user, is that user able to legally respond to my message without having a HAM license?
Please note that the answer to this question will differ from jurisdiction to jurisdiction. The answer below applies to the United States, and similar rules apply in Canada. Please check your own jurisdiction’s regulations.
The relevant clauses are in part 97.115:
(a) An amateur station may transmit messages for a third party to:http://www.arrl.org/part-97-text
(1) Any station within the jurisdiction of the United States.
In the context of APRS with SMSGTE:
An amateur station (the iGate) may transmit messages (the SMS message) for a third party (the mobile user) to: Any station within the jurisdiction of the Unitied States (your RF station).
(c) No station may transmit third party communications while being automatically controlled except a station transmitting a RTTY or data emission.http://www.arrl.org/part-97-text
Technically, an iGate is an automatically controlled station, but since it’s transmitting a data emission, it is in compliance.