APRS on HF
Why use HF?
APRS operation on HF is highly attractive for people traveling "off the beaten track", as it provides a low cost way of keeping the rest of the world advised of their location, without relying on extensive VHF or UHF "hilltop" infrastructure.
HF APRS coverage is a National (actually Global) issue, as it is quite possible that the nearest APRS HF Gateway is outside the skip zone - your HF APRS packets might actually be picked up by a Gateway in a different state or country, and relayed into the worldwide APRS network from there!
HF Path Settings
The path of APRS v GATE, WIDE2-1 should get your position to a VHF IGate via a HF Gateway.
Although most HF Gateways are connected to the Internet, there are some gateways that offer a traditional RF gateway from HF to VHF.
Any HF to VHF Gateway the hears a beacon with GATE in the path will gate that beacon to VHF. The path that follows the GATE is the path the beacon will take on VHF, and must be adequate for the beacon to reach an IGate.
The dominant modulation scheme for HF APRS, is Frequency Shift Keying (FSK) with a frequency shift of 200Hz, and at the rate of 300 bits per second. (The same as ye olde HF Packet)
Note that, due to the NRZI nature of AX.25 packet radio, it doesn't matter which tone is the mark and which is the space; it's the transitions that matter. Importantly, this means that you can use an old ex-commercial HF transceiver which is only capable of USB operation, and still successfully operate on HF APRS. All you have to do is ensure your transmitted tones fall on the right frequencies (ie the suppressed carrier frequency for an USB transceiver will be lower than the displayed suppressed carrier frequency on an amateur LSB transceiver).
The exact two frequencies that you will transmit on are a function of your suppressed carrier frequency, whether you are running USB or LSB, and the audio tones generated by your TNC or modem.
Bell 103 based HF modems (such as the AM7910 used on VK6ZTN's Flash TNC) use two different sets of tones, depending on whether the modem is configured for "Originate" or "Answer" mode. The PK-232, and the Tigertronics TM-1, use different audio tones again. Overall, the dominant tone set seems to be 1600Hz & 1800Hz, followed by the 2110Hz & 2310Hz tones of the PK-232.