Yes, the transmission of APRS packets can be controlled by PTT or automatically by setting certain period of time between transmissions. It is up to APRS station owner how and when to transmit. The map will show only positions, which received by APRS network stations. Some times you are out of range ground repeaters but there are few APRS satellites, which able to pickup your transmission and re-transmit it to the network. This is very handy if you are in very remote area, where cell phones are useless.
In additional to position report, APRS protocol support sending short text messages between stations and to the regular e-mail.
Check this site: www.openaprs.net It provide a lot of useful tools like multiple stations tracking and ability to send message to the stations.
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