Avoiding harmful interference?

Howdy APRS friends,

In reading about APRS I thought, wow that's really cool! So I decided to study and get my amateur radio license. (actually passed if you can believe it

One of the things that became clear when studying is that hams are all about "polite" operation of the radio...that is they don't just blast out a signal if someone else is transmitting.

That got me thinking about how APRS works. Are there some options that will "listen" for others broadcasting and politely wait until they're done before transmitting packets? It'll be a while until the -7A is flying, but when I do get 'er airborne, I'd like to be a good amateur radio citizen as well as a good pilot.

Thanks,

Mike

mcencula said:

...Are there some options that will "listen" for others broadcasting and politely wait until they're done before transmitting packets?...

Click to expand...

Yes, that is a common way of doing it, but some argue it isn't helpful in this case.

mcencula said:

Howdy APRS friends,

In reading about APRS I thought, wow that's really cool! So I decided to study and get my amateur radio license. (actually passed if you can believe it

One of the things that became clear when studying is that hams are all about "polite" operation of the radio...that is they don't just blast out a signal if someone else is transmitting.

That got me thinking about how APRS works. Are there some options that will "listen" for others broadcasting and politely wait until they're done before transmitting packets? It'll be a while until the -7A is flying, but when I do get 'er airborne, I'd like to be a good amateur radio citizen as well as a good pilot.

Thanks,

Mike

Click to expand...

Congratulations on the Tech license, Mike!

The question you raised has been vigorously debated ever since the "blind" Micro-Trak trackers hit the market. Many old-time hams are vociferously opposed to transmitting blind, but blind APRS units are very common.

An argument has been presented that if an airborne tracker waited until it heard a clear frequency, it might hardly ever have a chance to transmit due to hearing every beacon within 500 miles. At least one aviation APRSer, however, has stated he uses a receiving APRS rig and it works fine.

So.....the choice is yours. As long as we use reasonable beacon intervals with short packets, it seems the airborne blind trackers are generally regarded as acceptable for the APRS net.

But.....I'm finding out hams are probably even more argumentative about their opinions than pilots! So we will most likely never achieve consensus on this matter.

Polite APRS use

Mike,

Imagine first, that 10 % of the people in a room were talking. Now we create a "rule" that says "no talking until everyone else shuts up". The natural result will be everyone trying to get a word in edgewise after waiting what, in their opinion, is a polite interval. This strategy applied to APRS will almost certainly result in more collisions of packets than random chance. To keep our room metaphor going, let us say that there are only two people in the room who are listening to the content of the talkers (everyone else is just listening for silence) These people will hear communications from people in only a few scenarios, given the cacophony of the room. First, there is the remote chance that one of them will hear something spoken by a single person when the room is otherwise quiet. Probability? Low. Second, the loudest person with the most powerful voice will override everyone else in the room, or third, a person will walk up and whisper in the ear of one of the two dedicated listeners. In the case of two speakers with identical volume, the listeners will most likely be able to hear (and process) the one saying the fewest words.

Altitude gives you a major advantage, but it is not as fool-proof as one might think. You are just in a bigger room of chatting people, closer than some, further than others, quieter than most.

I maintain that the key to polite operation is to use just enough power, use short packets (MIC-E) and send only as many packets as you need to communicate with the APRS network. For some people, in flat States, 300 mW's seems to be enough to work on the air AND on the ground, others need 10 Watts, some might need 100 ( That one will be for you Sam)

Best regards, (or "73" in hamspeak)

Allen R. Lord
VHS
KG6HXO

VHS said:

Mike,

Imagine first, that 10 % of the people in a room were talking. Now we create a "rule" that says "no talking until everyone else shuts up". The natural result will be everyone trying to get a word in edgewise after waiting what, in their opinion, is a polite interval. This strategy applied to APRS will almost certainly result in more collisions of packets than random chance.

Click to expand...

A similar strategy works with ethernet. Although in the case of ethernet it uses collision detection, both hosts can detect the collision, wait a random amount of time, then attempt to retransmit. This works well until you start getting into the higher end of bandwidth utilization when collisions start increasing and everything starts looking like a traffic jam.

This sort of thing would be difficult with a) a transciever, and the situation where two sending stations are not within range of each other while the receiver may hear them both.

Thanks for the opinions

First, I'm sorry it's taken me so long to respond...the remnants of hurricane Ike blew through Ohio and knocked our power out for a couple days. I can't imagine what those folks in Texas went (are still going) through.

Sam Buchanan said:

Congratulations on the Tech license, Mike!

Click to expand...

Thanks, Sam. Apparently all the planets were in alignment that day because I somehow managed to walk out with a general. Now if I only had a radio.

n5lp said:

Yes, that is a common way of doing it, but some argue it isn't helpful in this case.

Click to expand...

Sam Buchanan said:

So.....the choice is yours. As long as we use reasonable beacon intervals with short packets, it seems the airborne blind trackers are generally regarded as acceptable for the APRS net.

Click to expand...

VHS said:

I maintain that the key to polite operation is to use just enough power, use short packets (MIC-E) and send only as many packets as you need to communicate with the APRS network.

Click to expand...

BrentHumphreys said:

A similar strategy works with ethernet.

Click to expand...

After reading all your guys' comments, I think I'll pursue one of the blind trackers. Maybe one of the TinyTrack models.

Thanks again!