Using VARA For APRS

Introduction

The overwhelming majority of APRS activity has been sent over VHF FM radio networks using 1200-baud AFSK packet radio modems a.k.a. "TNCs".  For over 30 years a small percentage of APRS transmissions have also been made over long-range HF radio, from locations lacking 2-meter VHF activity.   HF-APRS has traditionally been done using 300-baud 200-hz shift FSK AX.25 packet, mainly on the 30-meter band.

Classic packet works well in the relatively benign environment of VHF FM. It is a terrible mode for HF. It has very poor immunity to noise, selective fading and interference from other signals. It is an even worse performer in the one-way  "one-to-everyone"  broadcast beaconing of APRS (rather than the two-way hand-shaking ACK/NAKs error-correction of traditional "connected" packet). 
    If an APRS packet is lost at the receiver, due to a noise burst or interference, there is no provision to request a retransmission (ARQ).  The receiver just has to wait until the sending station decides to send another beacon minutes later.

A few years ago, yet another HF radio data transmission format burst upon the amateur radio community.

Developed in Spain by EA5HVK, "VARA" is a very sophisticated DSP data modem that runs on sound cards connected to radios with the typical "sound card interface". It comes in two versions: One adapted for HF over SSB transceivers, and a separate version adapted for the greater audio bandwidth of FM transceivers on VHF/UHF.  Both versions are adaptive - they will automatically adjust modulation formats & transmission speeds upward or downward depending on the quality of the radio path and radio bandwidths in use. 

[ NOTE: VARA is only intended for Windows, although it has been run on Linux platforms,  including the Raspberry Pi,  via WINE. Because VARA is quite computationally intensive, it will only run properly on the latest and fastest versions of the Raspberry Pi.  ]

VARA is now widely-used as an alternative to the expensive SCS Pactor hardware modems used for the Winlink radio email system on HF - an increasing number of Winlink gateways are now supporting VARA, in addition to packet and Pactor.  VARA is also used for the VarAC keyboard-to-keyboard live chat program, as an alternative to modes like PSK31 or FT-8.

Both VARA versions achieve net data transmission rates comparable to expensive dedicated hardware modem devices such as the SCS Pactor modems, but entirely in software when properly interfaced to radios.
    For FM, this means using the dedicated data port connection of a radio normally used for "9600 Baud Packet" to achieve the highest speeds.  These charts of modulations vs speeds are taken from the VARA manuals.

Note that the free unregistered version of the VARA software only operates on the lower speeds. The "high speeds" are unlocked by registering the VARA modem for about USD $70. This is done by choosing "Help", "Buy A VARA License" from inside the running free version of the program.  Once registered, you can use as many copies of both the HF and FM modem as you want, as long as they all use the same callsign.

For HF APRS use, even the free unregistered version will work.

HF Speeds available in SSB bandwidth ("Standard") and 500 Hz CW bandwidth (Narrow).
Note that VARA can best a 1200 baud packet transmission on VHF-FM, using a 500 Hz CW filter on HF!

FM (VHF/UHF) Speeds
FM Wide requires data port input in 9600-baud packet mode.
FM Narrow will work through mic/speaker jack like classic 1200 baud packet.

Note that APRS beacons contain very few characters.  Speed is not the important attribute of a modem for APRS.  Robustness in noise and at very low signal levels is!

CLICK HERE for a report on the direct comparison of classic 300-baud HF AX.25 packet versus VARA for APRS on the 60-meter band. It shows the dramatic difference VARA can make on a naturally very noisy band.

Any APRS client application that can use a KISS-mode AX.25 packet TNC (radio modem) can, in principle, connect to VARA's KISS port instead.   This allows these programs to take advantage of the enormously superior weak-signal HF performance of VARA.
   (Conventional HF AX.25 packet requires a strong stable signal to work well; VARA can produce reliable results on signals buried in noise that you can literally not hear in the radio speaker!)

[ KISS (literally an acronym for "Keep It Simple Stupid") is a more-or-less standardized simplified interface between modems a.k.a. "TNCs", and packet-data-using applications like APRS clients. ]

There are at least three APRS clients (APRS-is32, YAAC and PinPoint) that can make a direct KISS-over-IP connection to VARA. However, some APRS applications, such as the venerable UIview32, can only connect to a KISS-mode modem over an RS-232 serial com port (either real or virtual) -- not an IP link.

Work-arounds exist for APRS programs that can only connect to TNCs or modems via a serial port.

The "Swiss Army Knife" of com port utilities, "VSPE" (Virtual Serial Ports Emulator),  can be configured as an  IP<-->serial  "bridge" between VARA's IP KISS port  and  com-port-using  APRS programs.

Configuring UIview or other
KISS-over Serial application
for use with VARA using VSPE

The open-source freeware tool "Com0com" and it's companion app "Com2TCP" are an alternative approach to bridging IP<-->serial is. They are described on this page about using FLdigi as an APRS modem. (FLdigi's KISS-over-IP interface can be bridged to KISS-over-serial applications in an identical manner to VARA.) 

    <http://wa8lmf.net/FLdigiAPRS/KISS-over-Serial.htm>