There is a certain movement that advocates using a PL tone when using APRS. From what I have read about this, it doesn’t seem to be quite as fool-proof and user friendly as one would think. The idea of this set up is that when you are driving down the highway using APRS, you will then have a “reliable” way to contact another APRS user that you may see appear on your GPS screen. The commonly accepted tone for this is 100Hz. With this tone enable your radio will only “hear” transmissions that are transmitting this tone and will transmit this tone with each transmission. The theory is then, that if you can hear the other station’s APRS packets, you are close enough to have a voice conversation with them, can call them on the APRS frequency and arrange to move to a different channel (146.52 perhaps?). More information on this can be found at these two sites:
VoiceAlert - NWAPRS
http://www.ew.usna.edu/~bruninga/d700-faq.txt

While at first this sounds like a good idea, I’m not sure I know exactly how one would go about programming this system into their radio. If I set up the APRS side of my radio with the PL 100 so that it won’t hear a transmission that doesn’t have the tone, won’t I miss out on the packets being sent by everyone who is not using this set-up and all the digipeaters that don’t transmit the tone. If we just set the digipeaters to use the tone and make sure everyone is using it, it would defeat the purpose of the system because they would all be coming through anyway.

In order to use this system, I think one would need to have the APRS frequency programmed in to his or her radio twice. Once with the 100Hz tone on transmit only (not receive) so that their APRS transmissions would have the tone, but would they would not have to receive the tone in order for the radio to register the receipt of a data packet. You would then want to leave the volume turned down for this as it won’t be hearing anything new, you just changed how it transmits.

The second time the frequency would be programmed in to the user’s scan list and have the 100Hz tone on both transmit and receive. As the radio scans through, you will only hear transmissions on the frequency that have the 100Hz tone (and are then presumably in simplex range), if one of these is heard, grab the mike, press the key, and ask them to join you on a different frequency.

I’ve never used this type of set-up before, but I think that this is about how it would have be programmed to work as intended. I don’t think that it is very practical or necessary though. If you keep an eye on the GPS, you can pretty much know if there are other stations within simplex range that are running APRS. I personally program the call button for my 2 meter radios to 146.520 and 70 cm radios to 446.000 both of these being simplex with no tones. If everybody had these programmed in to their radios (it doesn’t have to be the call button, but somewhere in the memory channels) this would be a great use of the “national calling frequency” and I suspect a vast majority of hams do have these frequencies programmed in.

Not using the “VoiceAlert” setting/option would:
1. Leave less room for user error when setting up the radio for use with APRS (keep it simple)
2. Help prevent voice transmissions from interfering with APRS traffic or APRS traffic from interfering with voice communications
3. Free up another memory channel and have one less frequency to pass each time through the scan list
4. Make better use of the national calling frequencies and allow hams who do not use or know anything about APRS to join in the fun

Well, I might try it sometime, but I think I’ve pretty much talked myself out of using the “APRS Radar” at this point in time. If I’m mobile you can probably get in contact with me on 146.520.