Does APRS have a limited capacity in terms of beacons? What happens when an APRS frequency experiences a large number of packets?


When two packets are transmitted simultaneously on the APRS frequency in a local area, it is unlikely that either will be understood. As the packet density in an area increases, the chances that two packets will be transmitted simultaneously increases. This issue is compounded by the presence of digipeaters which can unintentionally flood an area with packets from a small number of sources.

The question then isn't a matter of the number of beacons, but the frequency of packets in a local area. Given a local area of 2 people where each send a 1 second packet once in a 10 minute interval, there's a 1/600 chance you will overlap with the other station. Once you get 100 stations together, without getting too much into probability, you can expect to get an APRS packet out a little over once an hour.

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    $\begingroup$ It's important to note that the chances of a collision occurring is reduced by the APRS transmitter listening to the APRS frequency in your area. If it hears a transmission in progress, many systems will wait until the transmission is complete before it does its thing. $\endgroup$ – Evan Fosmark Nov 20 '13 at 21:17
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    $\begingroup$ @EvanFosmark though perhaps equally important to note that some but not all systems do this. Some systems just blindly transmit, but only the courteous systems detect the collisions and scale back their transmission rate. It only takes a few clueless operators to mess it up for everyone. $\endgroup$ – Phil Frost - W8II Nov 21 '13 at 15:21
  • $\begingroup$ If you want to hear rather too many packets, try APRS at Hamvention. It's clear that not all the packets are getting through in this video: youtube.com/watch?v=XAWgyOcFcNk $\endgroup$ – scruss Nov 24 '13 at 19:06

In areas with a lot of APRS traffic, when you as a receiver are seeing a lot of collisions, one way to reduce the problem is to reduce your receiver sensitivity. You make your area of reception smaller, and thus see fewer beacons. As the spectrum grows crowded and people do this the problem will manage itself to some degree. Of course you reduce your area, but as long as you and others intelligently repeat the messages (via radio or internet) then everything will still get through.

In the future I expect some to start using sector antennas, and possibly MIMO technologies to resolve the issue. It's not trivial to separate simultaneous transmissions, but it is possible. You can start experimenting with SDR, multiple antennas, and not only separate different simultaneous transmissions, but also provide some amount of positioning information.

At the moment most don't view it as a big problem. Transmissions are often rather redundant and repeated frequently enough with enough random jitter that even blind beacons will get their message out more often than not, and if you wait you'll eventually get the information you're looking for. Certainly it would be good for those in congested areas to use more advanced beacons that pay attention to traffic and avoid stepping on other transmissions.

If you're trying to collect every 10 minute temperature reading from a remote station, though, and the station doesn't provide history or redundancy, then APRS is not the right solution. It's really a "best effort" but not guaranteed service, and should be treated as such.

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    $\begingroup$ I like this point. I think it's also important to limit your transmit power for the same reason (to limit transmit distance). I have seen people using 10, 25 or even more watts, which seems like way overkill in most cases. APRS is often repeated, in fact I run an APRS digi-gate to the internet. So it's usually not that important to long distance transmission or reception coverage. I would agree, in the area I live, you can occasionally hear signals on top of each other. $\endgroup$ – Scott Szretter Jan 24 '14 at 0:18
  • $\begingroup$ Another option is to switch from 1200 baud to 9600 baud, which helps reduce collisions by shortening the transmit time. The catch is, everyone has to use the same speed. I've heard some folks propose defining a UHF-APRS frequency for this, but there's no standard. $\endgroup$ – Trevor Johns Apr 23 '15 at 9:31

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