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Is it SSB or NFM or other type of modulation?

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APRS is a data transmission protocol and is independent of the underlying connection details. So there is no required modulation for the protocol.

That said, the common implementation of APRS is FM modulated 1200 baud AFSK in the 2m band. Several major vendors like Yaesu and Kenwood support the protocol with built in functionality for APRS.

There have also been implementations of the APRS protocol over AX.25 and PSK31 on HF frequencies as well.

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  • $\begingroup$ APRS is always layered on AX.25, and that on some modulation. $\endgroup$
    – Kevin Reid AG6YO
    Mar 26, 2014 at 23:30
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    $\begingroup$ @KevinReidAG6YO is that true even when it's going over the internet? $\endgroup$ Mar 27, 2014 at 18:57
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    $\begingroup$ @PhilFrost Okay, not always. But the current phrasing implies that it's not on AX.25 when it's on AFSK, which is false, and also implies that AX.25 is a modulation (by mentioning it adjacent to PSK31). $\endgroup$
    – Kevin Reid AG6YO
    Mar 27, 2014 at 19:03
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    $\begingroup$ PSK31 is not a modulation, it's a mode. PSK31 uses BPSK modulation over SSB. AX.25 is a data link layer protocol (OSI layer 2), so it's not a modulation either, but a framework for establishing higher level connections and forming the data frames. $\endgroup$
    – WPrecht
    Mar 28, 2014 at 13:05

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