Is there a barrier to ensure that unlicensed third parties are unable to originate traffic into the amateur service via this gateway?
In the case of a message from SMS to APRS, the message only enters the amateur service when the iGate transmits it for local delivery to the destination station. Also, APRS is a data transmission, which means that it is permissible for the station to be automatically controlled.
There are barriers inherent in the way the system operates. First, the SMS number assigned to the gateway is unpublished. Only someone who has received an SMS message from the gateway would have the number. Having the number isn’t enough, however as you must know the message format and a valid callsign to send the message to. The gateway also only responds to messages addressed to a known destination callsign. In addition the message will only be gated to RF by an iGate which has recently heard the destination station. Otherwise, the message will never leave the APRS-IS network.
Examples:
A message sent without a valid callsign as the first word will be ignored (no response sent to the SMS user).
A message sent with a valid callsign that is unknown to the gateway will be ignored (you have to have used the gateway to receive from it).
A message sent to BLN or a group callsign will be ignored.
A message sent to a valid callsign that is inactive (not recently heard by an iGate) will be sent to APRS-IS, but never sent to RF.
The exception is when a conversation is active. If an APRS user has sent a message to an SMS device, replies from that device will be automatically addressed to the APRS user, the callsign does not need to be included. Conversations are kept for 24 hours after the last message was sent.
APRS users also have the option of enabling privacy mode. When privacy is enabled, the user will only receive messages from SMS numbers they have configured as aliases. This allows the APRS user to control inbound messages destined to them. With privacy enabled, all messages for that user from SMS will be ignored by the gateway unless they meet one of the following conditions:
- The source number has been configured with an alias by the APRS user
- The source number is associated with another APRS user (that user’s alias will appear as an alias in the message)
- The message is a reply in an active conversation initiated by the APRS user
The SMS gateway is analogous to an autopatch, but with text messages rather than voice.