Understanding APRS paths

APRS is most frequently used on radio frequency bands that have line-of-sight coverage.

To increase the range of APRS stations, special fixed stations known as "digipeaters" are set up to store and forward traffic they hear.

In traditional packet radio, one would have to specify the actual call signs of every digipeater needed to get your packets from your station to the intended receiving station. This is not how we do it on APRS, where we are transmitting packets to be received by anyone who is listening, and may be mobile so that different digipeaters hear us at any given time.

Instead, APRS digipeaters are set up with generic aliases, and APRS stations use these aliases instead of specific digipeater callsigns. Each repeater with that alias that hears our station will store what it hears and then retransmit it.

It is possible to set your station up to use multiple "hops" of digipeaters so your packets are heard over a wider area.

TL;DR --- just tell me what path to use

Unless you know what you're doing or are doing something very special, a path of "WIDE2-2" should work for most people.

But you should read on, this is important stuff and fundamental to the APRS system on RF.

Specifying paths

The path your packets will take is specified in the "UnProto Paths" setting in any radio interface you set up. The format is just a comma separated list of digipeater names or aliases.

Wide area digipeaters

Special digipeaters, intended to serve a wide area, are given the generic callsign WIDE, and are set up to allow a certain number of hops from one wide digipeater to the next.

Specifying the desired number of hops

The "WIDE" digipeater callsign is never used directly, but rather in the form "WIDEn-N", where n and N are the desired number of hops you wish to allow.

Each time a WIDE digipeater hears such a packet, it will decrement the "N" value, often insert its callsign in the path as a "used" hop, and then retransmit the packet with the modified setting. Once the "N" has gotten down to zero no wide digipeater will retransmit it.

Thus, a packet sent with an "UnProto" path of WIDE2-2 will be retransmitted to "WIDE2-1" by the first wide digipeaters that hear it, and by "WIDE2" by any digipeaters that hear the retransmission, and there it will stop.

How this works is illustrated in the sketch below.

Duplicate checking

Digipeaters have enough smarts to remember when they've already digipeated a packet with a particular payload very recently. When using this "WIDEn-N" method, this avoids having a digipeater transmit a packet, have it retransmitted by another digipeater, then heard by the first one and retransmitted again.

As an example, look back at the figure above. While it's not drawn in that figure, if DIGIA happens to hear DIGIC's retransmission, it won't retransmit it a second time because it's already seen that same packet. Similarly, if DIGC just happened to hear the initial transmission directly (not through any other digi) and transmitted it as a first hop, and then DIGIA and DIGIB heard DIGIC's retransmission, neither DIGIA nor DIGIB would retransmit it a second time, because they've already handled it.

It wasn't always this way

Many years ago, the WIDEn-N technique wasn't in use, and wide digis all just used the alias "WIDE" with no adornments, and "fill-in" digis (below) used to call themselves "RELAY". Users used to have paths like "RELAY,WIDE,WIDE,WIDE" for a three-hop path. But there was no duplicate checking for those types of paths, and you could listen on the APRS channel as every single digipeater in the area would ping-pong the same packet back and forth for as many hops as were in the path. And users back then loved to use paths with as many as five hops.

If you'd like to see how bad it got, take a look at Bob Bruninga's diatribe from 2005 and rejoice that this is not how we work anymore.

Local area "fill-in" digipeaters

In some areas, mobile stations can't easily get their packets to a wide area digipeater. Home stations can set themselves up as "fill-in" digipeaters by aliasing themselves to "WIDE1-1".

A mobile station in such an area would then set their path to "WIDE1-1,WIDE2-1", to specify that it should take at most two hops, and if it is heard by a local fill-in digipeater it should be repeated by that fill-in.

Because wide area digipeaters recognize that any "WIDEn-N" format is intended for them, if a wide digipeater happens to hear a packet with this path even without a fill-in's help, it will digipeat it whether it's the first or second to hear the packet.

If it's heard directly, then the wide will transmit it and a second wide might then retransmit it.

If it only hears it through a fill-in's retransmission, since the second component of the path looks like one that was intended to be two-hops but only has a "-1" at the end, the second digi to hear it will think it's already been digipeated once (it has!) and digipeat it, but the third one to hear it won't.

Long paths are bad for the APRS radio environment

While you might want everyone in the world to see your packets, the APRS channel has limited bandwidth and transmitting your packets over excessive numbers of wide hops is considered anti-social.

The recommended path setting for almost all APRS communities

A two-hop path is generally considered the considerate path for almost all casual APRS use.

However, if your area has an especially dense population of APRS stations, even a two-hop path might be excessive and could cause you to be visited by a feedline cutting party. See below, concerning the ALOHA circle.

If your area has very good WIDEn-N digipeaters

If your area is well covered by wide digipeaters and you really don't need local fill-in digipeaters to help you reach them, you can use the path

WIDE2-2

with perfect safety

If your area uses a lot of fill-in digis

If your local community has a lot of fill-in digipeaters and you actually need to use them to get to a WIDE, or if you are traveling and might be passing through such an area, you might want your path to be

WIDE1-1,WIDE2-1

NEVER use a digipeater path for airborne operations

If you are flying an APRS station on a balloon or aircraft, it is extremely antisocial for you to specify any digpeater path at all for your packets. The higher up you are, the more digipeaters can hear you directly, and even a one-hop path might mean that your packets are being retransmitted hundreds of miles and lighting up every digipeater in that area.

For special needs

If you are in an area where APRS activity is sparse, or if you are in a remote area and need your packets to get to a more populated area and it requires more than two hops to do so, it is not completely unacceptable to use a three-hop path, such as

WIDE3-3

or

WIDE1-1,WIDE3-2

Using a path longer than three hops is generally going to get you hated by your local hams. Xastir will even warn you that this is a bad path. The online APRS web site aprs.fi will flag your station as using a bad path, too.

The ALOHA circle and path selection

After Xastir been running for a while and receiving packets over the radio (not the internet), it can start computing something called the "ALOHA" distance. This is a crude estimate of how heavily loaded your local APRS RF environment is, and was considered by Bob Bruninga to be an essential feature for all APRS clients to implement (see his ALOHA Circle Concept page).

Under the View Menu you will find a menu entry "ALOHA statistics". If enough stations have been found to compute the ALOHA distance, this will be reported by this selection.

You can also look on your map screen for a large yellow circle (you'll have to be zoomed out far enough for it to fit on your screen) that represents the same distance from your station.

You should select a path that doesn't digipeat your packets any farther than that circle. You can determine how many hops you need to do that by looking at the digipeater icons inside that circle and seeing how many hops it would take for a packet to reach from the center of the circle (your station) to the digipeater closest to the edge of that circle.

This depends very much on how active the local APRS channel is. In dense areas, even two hops might be too much, and in APRS deserts you might need 3 for anyone to hear you at all.