FLdigi ("Free Light Digital Modes) is a free open-source computer program that uses a computer sound system as a DSP modem to send and receive text and computer data over two-way radios. It can generate a large variety of transmission modes that trade off various combinations of transmission speed, occupied bandwidth on the radio spectrum, and performance at very weak signal levels. The program is available in essentially identical versions for Windows, Linux and MacOS. It can be downloaded here:
Besides operating as a stand-alone program with it's own split-screen send/receive terminal windows, FLdigi can act as a modem to external programs using it's built-in KISS-over-IP interface.
Traditionally, APRS position reports and messages have been transmitted using classic AX.25 packet radio. While this works well on VHF & UHF frequencies with FM transceivers, AX.25 packet has been a rather poor performer on HF frequencies. Radio channels on HF frequencies are almost never as quiet as on VHF & UHF. Signals on HF bands are often plagued with constantly varying signal strengths, selective fading, random noise and static, inference from other stations, etc.
Some of the transmission modes/modulations offered by FLdigi offer vastly superior alternatives far better adapted to conditions on HF radio than 300-baud HF packet radio. Modes such as MFSK16 and Olivia include forward error correction, and can be reliably received at signal levels 10-15 dB weaker than conventional packet. These modes can provide reliable copy at signal levels that literally can not be heard in the radio speaker.
Any APRS client application that can use a KISS-mode AX.25 packet TNC (radio modem) can, in principle, connect to FLdigi's KISS port instead. This allows these programs to take advantage of superior weak-signal HF transmission modes.
However, most APRS applications expect to connect to a KISS-mode TNC via an RS-232 serial com port; not an IP link. There are at least three APRS clients (APRS-is32, YAAC and PinPoint) that can make a direct KISS-over-IP connection. These programs can directly connect to FLdigi's modems.
For APRS programs that can only connect to TNCs via a serial port, such as UIview, a work-around exists. Two open-source freeware utilities can be combined to create an RS-232<-->TCP/IP bridge that will allow "serial only" programs to talk to the FLdigi KISS-over-IP interface. More details below.