What type of modulation is commonly used for VHF packet radio transmissions?

VHF packet radio, which includes popular modes like the Automatic Packet Reporting System (APRS), is most commonly transmitted using Frequency Modulation (FM) or Phase Modulation (PM). The digital packet data is first converted into audio tones using Audio Frequency-Shift Keying (AFSK). These audio tones are then fed into the microphone jack or data port of a standard VHF FM transceiver. This approach is extremely popular because it allows amateur radio operators to use ubiquitous, inexpensive FM voice radios for digital data transmission. Modes like SSB and AM are typically reserved for HF bands, not standard VHF packet systems.