APRS with FLdigi

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.

Unlike classic AX.25 packet, VARA packets are wrapped in multiple layers of forward error correction and are incredibly robust in the face of noise, QRM, VHF multi-path distortion, HF selective fading, etc. VARA HF can provide error-free copy on signals that literally can't be heard in the radio speaker.

[ 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 "RMS" radio mail system on HF - an increasing number of Winlink gateways are now supporting VARA, in addition to classic 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.

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!

Standard HF Frequencies for VARA

The VARA HF modem generates/receives a spread of audio tones about 500 Hz wide (slow modes) or 1000 Hz wide (fast mode)s centered on 1500 Hz.

Normally the HF APRS-over-VARA frequency is stated by citing the USB suppressed-carrier frequency a.k.a. the radio "dial frequency" (rather than the actual center of the transmitted signal, which will be 1500 Hz higher).

The common APRS-over-VARA frequencies are:

40 Meters: 7083.500 KHz USB
30 Meters: 10148.200 KHz USB

Note that this is USB on 40 meters -- not the customary LSB used on this band. This can be a hassle to select with some HF transceivers. Especially certain Yaesu models that force you into multiple levels of menus to select USB on 40 meters.

Usually the easiest solution is to program a memory channel to select "USB" or "DIG" (digital). "DIG" automatically selects USB regardless of band -AND- enables the rear-panel "data" port interface connector.

Normally, the 30 meter channel works best in the daytime, while the 40 meter channel is best at night for continental coverage of North America.

The VARA slow modes used by APRS can easily fit through a 500 Hz CW filter
-IF- if the passband tuning on an HF radio is shifted to from it's typical default 700 Hz center to a center of 1500 Hz.
Switch to the CW filter and then adjust the passband tuning while watching the VARA waterfall display. Adjust it until the background noise in the waterfall is centered between the red "goalposts" at 1500 Hz.

[The Yaesu FT-817/818 works perfectly for APRS-over-VARA and VarAC Chat with the optional 500 Hz Collins mechanical filter this way. The incredible weak-signal capability of VARA is a perfect match for this low-power radio and a temporary antenna.]

The VarAC Chat keyboard-to-keyboard chat program has it's own built-in CAT control functions that automatically set the radio to the agreed-upon international calling/beaconing frequency on each HF band. (On most of the HF bands, VarAC Chat is on xx.105 MHz USB.)

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" middle-man 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.)

Using VARA For APRS

Introduction

Standard HF Frequencies for VARA

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

Using VARA For APRS

Introduction

Standard HF Frequencies for VARA

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

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