Local APRS thoughts

  Previously, I wrote about my initial experiences getting in on the RF side of APRS. It's been interesting to watch the network traffic and try to figure out N-paths and raw packets. I still haven't tried playing with the messaging capabilities, though I think that it has significant potential for SkyWarn and events such as marathons. 

  In an on-air discussion with K9EQ (Chris), he compared the 144.390 frequency to the old Channel 19 of the (11M) Citizen's Band. It's an apt description in terms of activity and traffic, but Channel 19 never had the capability of relaying signals beyond simplex range. In a way, the 144.390 frequency is the linked repeater network that we've studiously managed to avoid bringing into fruition in the metro area. The difference is that it all operates on simplex, and that means that we have to be careful about using that shared resource. 

  Chris is excited about the potential of System Fusion (C4FM) for UHF APRS. If the GPS and APRS functions are activated on a C4FM HT or mobile radio, the location of the radio is transmitted using the same format as other APRS packets. The location and callsign of these radios can be viewed by a NC, for example, and the packets can be gated to APRS-IS (the internet side of APRS) in the same way as VHF packets are. Obviously, these packets wouldn't be "seen" on the VHF RF network, but they wouldn't contribute to congestion on 144.390, either. 

  Just in my few weeks of watching the local network traffic, it's clear that a few simple steps could considerably reduce this congestion:

• Reducing the frequency of packet transmissions for fixed stations. There's no reason to send the location of your home every six minutes over RF. Every 30 minutes is quite adequate. This ties up the main digipeaters for no constructive end. 
• Using APRS-IS in some cases. I spoke with AE0F (Bill), who uses his Android phone and the cell data network to send packets directly to the internet side with the APRSDroid app. You can say that this isn't "real" amateur radio, but it's certainly considerate of "real" operators. Maybe those fixed stations (homes) could give the RF network a break once in a while and use the internet, too. 
• Adjusting your path to a more considerate range. If you're running 25W from your home and are a few miles from an I-Gate, maybe WIDE1-1 is all that you need for an N-path. Judging from my mobile experience using a 5W HT, "WIDE1-1,WIDE2-1" is adequate for driving around the metro. 
• We still could use a couple more RX-only I-Gates in the north and NE areas of the metro, such as Anoka, Coon Rapids, Forest Lake, or Ham Lake. This would mean that the metro I-Gates and digipeaters wouldn't be handling packets from the northern rural areas. 

  Let's look at how digipeaters handle N-paths. The path that you transmit with your location packet specifies a certain number of "hops" needed to reach a gateway. If you aren't interested in reaching a gateway (I-Gate) and having your packet gated to the internet, then you really don't need too many "hops" just to let people on RF hear you. Repeaters, for example, use a path that guarantees their packets won't be digipeated. There's no point in letting operators outside of your signal area know about your repeater, after all. The path is altered every time that a digipeater handles it, and one hop is "used up". Otherwise, your packets could be carried all the way to the coasts, and congestion would be unbearable.

  In the case of an N-path "WIDE1-1,WIDE-2-2", the first digipeater re-transmits the path as "WIDE1*,WIDE2-2", indicating that the first ("fill-in") hop is "used up". The second digipeater then re-transmits your packet with "WIDE1*,WIDE2-1" as the path -- with one of the WIDE2-2 "hops now being used up. The third digipeater then sends a path of "WIDE1*,WIDE2*", with the asterisks indicating that all of your hops have been used up. An I-Gate will still gate your packet at this point, but it won't be re-transmitted any further.

Realistically, your packet is probably being heard by K0YTH-13, WB0ZKB-5, KF0ZH or WB0MPE-10 if you're in the greater metro. Those digipeaters will easily "feed" one of the I-Gates directly without any further "hops" being needed. The odds are pretty good that you don't even need a digipeater at all, considering the number and variety of gateways that we have set up already. K0YTH-13, for example, has three I-Gates within a 10 mile radius of it. KF0ZH has two I-Gates within ten miles. And gateways such as N0AGI and W0YC-5 have pretty good sites and "ears" just on their own. 

  In my case, with an HT/stock duck indoors in Plymouth, my packets need the K0YTH-13 digipeater in Robbinsdale to pass the signal on to one of the I-Gates. That's one "hop". Now think about your much more powerful mobile with a gain antenna outdoors. How many "hops" do you think you will need to reach an I-Gate driving around the metro?

  Now, I'll concede that APRS packets don't have the range, on their own, that FM voice would have. I've read estimates of about one-half to one-third of the expected range of FM voice, because digital is an "all or nothing" proposition and there's no error correction or "ack" function with APRS packets. But there's a relatively simple way for you confirm your range. Go to aprs.fi and enter your callsign (with SSID, e.g. KD0TLS-7) in the "Track callsign" box and click "Search". A box will appear next to your last position on the map. Click "info" on the top bar of that box. You'll be taken to a page that shows things such as your last path. Up at the top, you can click "raw" to see the raw packets from your station. You can see the N-paths of your transmissions. 

  I'll examine one of my packets, and you can see how it's done. Originally, I used "WIDE1-1,WIDE2-1" as an N-path:

2015-02-08 08:36:54 PST: KD0TLS-7>APDR12,K0YTH-13,WIDE1*,WIDE2-1,qAR,N9MEC:=4502.05N\09325.01WE/A=000780 monitoring on 444.800+ 

First, we see the date and time (in Pacific Time). Okay. Cutting that away, we have: 

KD0TLS-7>APDR12,K0YTH-13,WIDE1*,WIDE2-1,qAR,N9MEC:=4502.05N\09325.01WE/A=000780 monitoring on 444.800+

"APDR12" is the software used. In this case, it's DroidAPRS version 12. No mystery there. Cutting that away, we have: 

KD0TLS-7>,K0YTH-13,WIDE1*,WIDE2-1,qAR,N9MEC:=4502.05N\09325.01WE/A=000780 monitoring on 444.800+ 

"monitoring on 444.800+" is my status message. It has nothing to do with the path, but it's transmitted along with it. Everything after the "=" sign is my GPS co-ordinates. Nothing to do with the path, so let's cut these two things away: 

KD0TLS-7>,K0YTH-13,WIDE1*,WIDE2-1,qAR,N9MEC 

This shows that K0YTH-13 passed my packet to (the I-Gate) N9MEC. "qAR" shows that the packet was gated to APRS-IS (the internet). K0YTH-13 changed "WIDE1-1" to "WIDE1*", showing that my first hop was "used up" when it went to N9MEC. I still had one more "hop" left from the "WIDE2-1" path. If another digipeater was needed, the path would change to "WIDE1*,WIDE2*", and no other digipeaters would touch it if the packet hadn't found an I-Gate by then. 

  So, if you examine the raw packets from your RF station, be it mobile or fixed, you will see how many "hops" your packet needed. It may be that you can safely get by with a default "WIDE1-1,WIDE2-1" path, even out in rural areas. It may be that your home station could do quite well with a "WIDE2-1" path, and it won't need to be sent up to Duluth or down to Mankato. Of course, if you know that you were transmitting your location on a certain day and no raw packets show up, you will know that you were out of range of an I-Gate. Then you can bump up your path to a "WIDE1-1,WIDE2-2". 

  If we use a little consideration, we can make this shared resource run efficiently. I'd also suggest that we take measures to reduce congestion in the cases of weather emergencies or public service events (e.g. marathons). 

  APRS is great, and has a lot of uses. If you're working DX, for example, someone can see your location on a map and measure the distance easily. They can also determine your grid square, if you don't happen to know it. And mobile operators can see the beacon messages from repeaters as they drive through unfamiliar areas, and find out the PL tone, offset, and even when nets are scheduled. You can find out which repeater or simplex frequency an operator is hanging out on by looking at their status messages (e.g. "monitoring on 444.800+"). And you can send short messages, either through the internet side or the RF network, to an operator. 

  We have a pretty robust network, and I really appreciate its existence and maintenance.