Xastir—Open-Source Client for the Automatic Packet Reporting System

What do search and rescue, Amateur Radio and Linux have in common?
Linux APRS Bits

Linux is special and not just for the reasons most of you already know. Linux was the first operating system to have the Amateur Radio AX.25 packet protocol actually built in. Currently, it's a pluggable kernel module, but in the early days, we had to compile custom kernels to enable it.

AX.25 is based on ITU-T X.25, with a few additions that let us store and forward or digipeat (digitally repeat) a packet through multiple stations on RF (radio frequencies). This allows us to create a network from multiple radio stations on the same frequency. To the computer, an AX.25 port looks like any other networking port, so you must guard against sending out broadcast packets from some common Linux applications that can trash our low bit-rate radio channels. Firewalls or specific configuration of each errant dæmon are the fixes.

Once you set up an AX.25 interface, you can share it across multiple applications, run special digipeater or server software, or run multiple protocols across it. See the AX.25 HOWTO on the Xastir Wiki (see Resources) or the older AX.25 HOWTO for setup details.

Note: a simpler method for APRS only is to use the Serial KISS TNC interface within Xastir to talk directly to a serial port, USB→Serial adapter, or soundmodem terminal node controller (TNC). This method does not let you share the TNC across multiple applications, however.

For the TNC, there are several options. Software only (mostly?) options include the soundmodem driver on Linux/Solaris or AGWPE on Windows. See the soundmodem HOWTO on the Xastir Wiki for details (see Resources).

Most operators prefer a hardware TNC over the software solutions above. We recommend KISS mode for long-term reliability. The alternate TNC mode, command mode, was designed for human interaction and, thus, is prone to timing errors and commands getting lost or misinterpreted. KISS is a much simpler protocol; the computer does most of the protocol processing work instead of the TNC.

Internet APRS Bits

An entire tiered network of servers moves packets around the Internet and connects the various radio networks together, called the APRS-IS (APRS Internet Services). With proper gateways, two APRS stations anywhere in the world can exchange two-way messages. Alternative networks exist for Citizen's Weather (CWOP) data and for emergency-minded or weather-minded folks who wish to see more objects (Firenet.us). Firenet is one of Curt's interests; he contributes earthquake, weather, fire and river gauge scripts, also plotting positions of steam-train excursions.

Home stations can have a radio interface, can connect as clients of APRS-IS or Firenet, and optionally may choose to be a one-way or two-way igate (Internet gateway) between the Internet and radio. One-way igates (RF→INET) are welcome any time, but two-way igates are best coordinated so that they don't compete for limited airtime in one area. The typical two-way igate passes only APRS messages and ACKs/NAKs for stations it has heard recently on the local frequency. Mobile travelers can send and receive messages from their current locations, as the igates will route the messages appropriately. Xastir can function as such an igate, but if you need more advanced igate functions, other applications may be better suited.

Other important pieces of the infrastructure are Internet databases with Web front ends. See the Resources section of this article to find out more, specifically the info.aprs.net link, and click on APRS Internet System—it'll be well worth your time.

Xastir Major Features

Compared to other APRS clients, Xastir really excels in the map department with 125 types of maps, including topo maps, street maps, aerial photographs, weather alerts and radar images. It can use on-line or local map sources, or combinations of both. Map stackups can be custom-designed by the user in nearly any combination.

We do not support or condone use of proprietary map formats, but there are plenty of free maps from which to choose. You also can grab screen captures of maps you own and easily georeference them for use with Xastir. For those coming from the Windows side, UI-View maps can be converted with our inf2geo.pl script for use with Xastir. This helps users outside North America immensely, as they often lack access to the free maps we enjoy here.

For contiguous topographic maps from USGS DRG maps, Xastir topo code crops the white borders and stretches the image into a rectangular shape while drawing. Maps are stitched together into a contiguous view.

Figure 2. Xastir Topo Maps, Simulated SAR Mission, Mt. Pilchuck (Maps Courtesy of USGS)

Xastir supports direct or network connection to weather stations, GPSes and TNCs. Several coordinate grids/systems are supported: UTM/UPS, MGRS, USNG, Maidenhead grid and multiple latitude/longitude formats. A Coordinate Calculator is included that converts between many of these formats.

Menus can appear in any of seven real languages and five “not so real” languages. Curt took a bit of heat from other developers for adding those last five “languages”, but you know beer-fueled coding sometimes yields interesting results! All kidding aside, he was asked to support Talk Like a Pirate Day (September 19th every year). He found pirate-speak REGEX plus several other “languages”, so he added five in total. He's proud to be a supporter of Talk Like A Pirate Day, the only international holiday created from a sports injury!

Dig in to the menus, and you'll find some SAR-specific features, as well as features useful for public-service event tracking. Xastir is used for SAR, marathons, bicycle, motorcycle and car events.

Though Xastir and APRS are not well suited or intended for this application, one ham, when his vehicle was stolen, was able to tell police the car's current location, direction and speed. It does take some work though to explain to the police how you know this.