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Re: [tracker2] Dual Layer of APRS (was Re: Pre-Summer Tracker Test )

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  • Randy Love
    ... Curt, I guess to answer this in part, one has to realize that the APRS system is a dual-layer system -- an RF layer and the APRS-IS (internet) layer. Once
    Message 1 of 10 , Apr 29 5:59 AM
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      On Thu, Apr 29, 2010 at 8:30 AM, captcurt@... <captcurt@...> wrote:
       

      Thanks Keith and Lynn:

      I'll read that info

      If I set the 3 hop path, does it ALWAYS go three or will it stop as soon as it hits an I Gate?

      I thought it stops when it hits the IGate.

      Or, if it gets to multiples, and some are digi's, will they keep it going out to the path max?

      Curt



      Curt,

      I guess to answer this in part, one has to realize that the APRS system is a dual-layer system -- an RF layer and the APRS-IS (internet) layer.

      Once your packet gets into the APRS-IS via an Igate, any additional reports (duplicates) of that packet are removed by the APRS-IS core servers. This is the reason that you cannot see any additional path usage on the RF side once your packet is 'gated'.

      The RF layer, for the most part, is not aware of any packets already gated to the APRS-IS. Thus, if you have a 3 hop path, and you are in an area where the 3 hop path can be fulfilled, your packet will go thru 3 different hops. Please note, this does not necessarily mean 3 different digipeaters. This means 3 different relays of your packet expanding outward in all directions! Since your in Novi, I'll give a local example ( I'm up near Utica ). If you bring up aprs.fi and center it on Southfield, MI, then expand out the map to see Lansing to the west, you will see the following digi's that I will use as examples.

      W8FSM-3 in Southfield
      K1DE-5 near Milford ( W of FSM-3 )
      WF5X-13 in Utica ( NE of FSM-3 )
      AB8XL in Brandon ( N of FSM-3 )
      W8FSM-5 near Holly ( NW of FSM-3 )
      W8FSM-4 in Lansing
      VE3KCR in Chatham/Kent

      Now, you send a posit from Novi using a WIDE1-1,WIDE2-2 path in SE Michigan, you can have all the possible routes happen:
      W8FSM-3,WF5X-3,VE3KCR ( your packet made it to Central Southwest Ontario )
      W8FSM-3,AB8XL,W8FSM-5 ( your packet made it to Flint area )
      W8FSM-3,K1DE-5,W8FSM-4  ( your packet made it to Lansing area )

      Only the original relay thru W8FSM-3 a single hop. All the other packets ( all six of them! ) are relays of the original first digi. All of that coverage above happened with ONE posit. Now, if your path had only been WIDE1-1,WIDE2-1, you can remove the last three calls from the list, and you can see that you just halved the amount of traffic created on the RF part of the APRS system -- plus removing a large chunk of Michigan and Ontario that aren't in immediate need of seeing your position.

      Generally, along major interstate routes, you will never be outside of accessing an Igate for very long using a WIDE1-1,WIDE2-1 path.

      Hope the example helps.

      73,
      Randy
      WF5X



    • James Ewen
      ... And even that is not accurate. You are telling your packet to go three levels (hops) outwards from your location. If there are only 3 digipeaters that can
      Message 2 of 10 , Apr 29 11:53 AM
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        On Thu, Apr 29, 2010 at 6:59 AM, Randy Love <rlove31@...> wrote:

        > Thus, if you have a 3 hop path, and you are in an area where
        > the 3 hop path can be fulfilled, your packet will go thru 3
        > different hops. Please note, this does not necessarily mean
        > 3 different digipeaters. This means 3 different relays of your
        > packet expanding outward in all directions!

        And even that is not accurate. You are telling your packet to go three
        levels (hops) outwards from your location.

        If there are only 3 digipeaters that can be accessed in your area, you
        will get three hops. If there are more, you'll activate more.

        Let's say you are in an area where three digipeaters can hear you
        directly, you'll be digipeated by all three (first hop). If each of
        those are heard by 3 more unique digipeaters, you'll get digipeated by
        those (second hop). If each of those digipeaters can be heard by 3
        more unique digipeaters, you'll get digipeated by those (third hop).

        Start counting. 3 digipeaters on hop one, 9 on hop two, and 27 on hop
        three. That's 39 digipeaters total. Obviously this is a simplified
        mathematical abstraction, and probably doesn't exist anywhere. In some
        areas, you might only hit one digipeater on the first hop, or maybe 6
        or 7... Each digipeater can be heard by a variable number of other
        digipeaters, which may or may not overlap with other coverage areas.

        If you want to see what kind of impact you are having, use the aprs.fi
        info page for your local digipeater, and look at the section that has
        "Stations heard directly by (your local digi)" copy down all the
        callsigns of the stations listed there as a digipeater. Do the same
        for each of those stations, and then once more again. Now, if you're
        heard by more than one digipeater on the first hop, go back and start
        all over. You'll probably find many duplicates because the digipeaters
        usually make a mesh rather than straight lines of propagation, but one
        thing you'll find out in a hurry, is that the number of digipeaters
        add up in a big hurry.

        Then what you need to do, is to figure out just how far each of those
        digipeaters cover. You can get a bit of an idea of that area by
        clicking on the "show map" link on the "Stations heard directly" line
        of the digipeater's info page.

        You also have to remember that the information collected by aprs.fi is
        all filtered data. The collected information is only the first packet
        to make it to the APRS-IS stream, as all the others are discarded.
        This means that we are only looking at a small subset of the number of
        packets bouncing around the network.

        I've asked Hessu (webguru at aprs.fi) to create a routine to automate
        all of the above for you, so that people could simply click on a link
        to see "Just how bad am I hammering the network", but Hessu is
        concerned that instead of people using it to reduce their impact, some
        would go the other way, and deliberately try to make as large of a
        splot on the map as possible.

        James
        VE6SRV
      • Lynn W. Deffenbaugh (Mr)
        James, An excellent analysis for your home QTH, but decided impractical for a multi-day, over-the-road trip. Lynn (D) - KJ4ERJ
        Message 3 of 10 , Apr 29 12:35 PM
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          James,

          An excellent analysis for your home QTH, but decided impractical for a
          multi-day, over-the-road trip.

          Lynn (D) - KJ4ERJ

          James Ewen wrote:
          >
          > If you want to see what kind of impact you are having, use the aprs.fi
          > info page for your local digipeater, and look at the section that has
          > "Stations heard directly by (your local digi)" copy down all the
          > callsigns of the stations listed there as a digipeater. Do the same
          > for each of those stations, and then once more again. Now, if you're
          > heard by more than one digipeater on the first hop, go back and start
          > all over. You'll probably find many duplicates because the digipeaters
          > usually make a mesh rather than straight lines of propagation, but one
          > thing you'll find out in a hurry, is that the number of digipeaters
          > add up in a big hurry.
          >
          > James
          > VE6SRV
          >
          >
        • James Ewen
          On Thu, Apr 29, 2010 at 1:35 PM, Lynn W. Deffenbaugh (Mr) ... I don t see how the concept of multiple digipeaters acting upon the packet for up to n hops is
          Message 4 of 10 , Apr 29 4:55 PM
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            On Thu, Apr 29, 2010 at 1:35 PM, Lynn W. Deffenbaugh (Mr)
            <ldeffenb@...> wrote:

            > An excellent analysis for your home QTH, but decided impractical for a
            > multi-day, over-the-road trip.

            I don't see how the concept of multiple digipeaters acting upon the
            packet for up to "n" hops is any different for a single packet as
            opposed to an extended trip.

            Each location where a packet is initiated is unique, and one must
            analyze each independently. The simplest case being where one is in
            isolation, where no one hears you "braaap"... I spend a lot of time in
            areas like that. I can run a 7 hop path with no adverse impact.

            The issue is where there is an APRS network within earshot. With a
            single hop path, you will only activate digipeaters that can hear your
            station directly. At a minimum, this will be one station because we
            have put in the definition of being within range of a network. At a
            maximum, it is hard to say, that depends upon the density of the
            network, and the propagation characteristics of the initial station. A
            balloon borne station has the potential to hit dozens or digipeaters
            on its first hop.

            Every hop after that has the potential to hit between zero and an
            unknown number of previously unactivated digipeaters. Each time you
            ask for another hop, you are potentially impacting more users.

            I seriously think that an application that would allow the user to
            select a location on the map, and then observe the area impacted while
            adjusting the number of hops requested would be a good thing. This
            application would need to build a digipeater mesh of the affected area
            by keeping track of which digipeaters can hear which neighbors.

            If the user could select a path previously travelled, and run the
            simulator against each position report, showing the full area of
            impact, and not just the path travelled to the first i-gate, the user
            could get an idea of how much of an impact they are having on the
            network.

            James
            VE6SRV
          • James Ewen
            On Thu, Apr 29, 2010 at 1:35 PM, Lynn W. Deffenbaugh (Mr) ... Just to move it from theoretical into the real world... here s a quick peek into Curt s
            Message 5 of 10 , Apr 29 9:23 PM
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              On Thu, Apr 29, 2010 at 1:35 PM, Lynn W. Deffenbaugh (Mr)
              <ldeffenb@...> wrote:

              > An excellent analysis for your home QTH, but decided impractical for a
              > multi-day, over-the-road trip.

              Just to move it from theoretical into the real world... here's a quick
              peek into Curt's neighborhood.

              I was going to layer all the images one on top of the other, but after
              starting to compile a list of digipeaters I got tired, and realized
              that I don't have the time to find all digipeaters within 3 hops of
              Curt let alone capture and composite all the coverage areas. I gave up
              after just 2 hops. Also note that this is only a list of digipeaters
              using a green star icon, and does not include any other digipeating
              hardware that might be using a different icon.

              Stations that most likely can hear Curt from his last reported
              location based on the area these stations are reporting being able to
              hear:

              N8PVL-2 W8FSM-3 VE3WRC-1 VA3SWO-1 W8OAK-13 K1DE-5

              N8PVL-2 can communicate with:

              K8TIH K8UI-1 K1DE-5 W8FSM-3 W8FSM-5 VE3KCR

              W8FSM-3 can communicate with:

              VE3KSR W8FSM-5 N8TJG-10 VA3DVR K8TIH K8UI-1 K1DE-5 WF5X-13 WB8NLS
              AB8XL KB8SWR-5 KF8YK-1 W8LBZ-1 N8PVL-2 VA3SW0-1 WD8EJC-10 W8FSM-5
              K8TIH N3EOY-11 W8OAK-13 W8FSM-4 VE3KCR W8XR W8LBZ-1 N8PVL-2

              VE3WRC-1 can communicate with:

              K8TIH VA3SWO-1 W8OAK-13 W8LBZ-1

              VA3SWO-1 can communicate with:

              VA3ROG W8FSM-5 VA3DVR K8UI-6 K8UI-1 VE3WRC-1 VA3KMS VE3KCR W8FSM-3
              W8WE-1 KB8SWR-5 K8DAC-7

              W8OAK-13 can communicate with:

              W8FSM-5 K1DE-5 WF5X-13 AB8XL VE3WRC-1 W8FSM-3 N8ZSA-1 WB8NLS WD8DX-9 WC8EMA

              K1DE-5 can communicate with:

              VE3KSR W8FSM-5 N8TJG-10 K8TIH NK8X-10 K8UI-1 WF5X-13 WB8NLS AB8XL
              W8FSM-4 W8FSM-3 KB8SWR-5 W8DF-5 N8PVL-2 WD8EJC-10 VA3DVR N8OBU-5
              WF5X-13 W8OAK-13 VA3KMS VE3KCR KB8UIH-2 KB8VEE-1 W8XR KB8ZGL-7 W8LBZ-1
              K8DAC-7

              You'll notice that many callsigns are included in a number of lists.
              this is because the digipeaters create a web of interconnections with
              each other. Because we are using the new-Paradigm setttings, each
              station will only fire off once, and this reduces congestion from what
              it used to be, but you can still see that there are an awful lot of
              stations out there that will get fired off.

              Remember as well, I gave up after just 2 hops. I didn't extend out to
              the third layer, where there will be many more digipeaters activated.

              In my desktop example, I only used 3 digipeaters on the first hop,
              where in Curt's world, it's probably 5 digipeaters that could hear him
              direct. On the second hop, every one of those digipeaters could
              communicate with more than three neighbors, with some able to
              communicate with over 20 neighboring digipeaters.

              In the real world, collisions and other propagation anomalies will
              kill a number of the packets being pushed around, but due to the
              interwoven nature of the APRS network, each of the listed digipeaters
              will probably end up handling the packet sooner or later.

              I have always said that it would be interesting to be able to defeat
              the anti-dupe filter on the APRS-IS to see how many different copies
              of the packet you can see, and all the different routes that the
              packet traversed. At the very least it would be neat to get a tally of
              the number of duplicates that have been discarded per packet.

              I know that I can fire off a packet from here with a no hop path, and
              get gated to the internet 3 times. Add in a single named digipeater,
              and now there will be 6 copies gated to the APRS-IS. Make it a 2 hop
              path, with 2 digipeaters, and now there are a total of 9 copies gated.
              A 2 hop path with 3 digipeaters makes it 12 copies.

              How many i-gates are within 2 hops of Curt and all those digipeaters?
              How many copies do each of those i-gates hear? It boggles the mind!

              James
              VE6SRV
            • Lynn W. Deffenbaugh (Mr)
              But wasn t his original question asking for advice on a suitable path for a 15,000 mile, cross-country road trip? Hence my suggestion that an in-depth
              Message 6 of 10 , Apr 30 4:38 AM
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                But wasn't his original question asking for advice on a suitable path
                for a 15,000 mile, cross-country road trip? Hence my suggestion that an
                in-depth analysis wasn't easily applicable to the environment in
                question. There's no doubt (in my mind, at least) that the shorter
                WIDE1-1,WIDE2-1 path is good for his local area. The question was
                directed at what to use when driving out into the (relative) unknown.

                Lynn (D) - KJ4ERJ

                James Ewen wrote:
                > On Thu, Apr 29, 2010 at 1:35 PM, Lynn W. Deffenbaugh (Mr)
                > <ldeffenb@...> wrote:
                >
                >
                >> An excellent analysis for your home QTH, but decided impractical for a
                >> multi-day, over-the-road trip.
                >>
                >
                > Just to move it from theoretical into the real world... here's a quick
                > peek into Curt's neighborhood.
                >
                > How many i-gates are within 2 hops of Curt and all those digipeaters?
                > How many copies do each of those i-gates hear? It boggles the mind!
              • Randy Love
                Look, guys. I live in the SE Michigan APRS network. I know what the actual coverage and reliability are and the theoretical coverage are. The point of the
                Message 7 of 10 , Apr 30 5:20 AM
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                  Look, guys.

                  I live in the SE Michigan APRS network. I know what the actual coverage and reliability are and the theoretical coverage are.

                  The point of the example was to show the possible extend of a 3 hop path in this area from Curt's location in Novi and relate it to the APRS-IS gating that would happen -- *not* to expound upon every possible hop, skip and jump. This was just to give Curt a reference for how far his packets could easily go to give him a visual of how the RF will keep going, if infrastructure exists, compared to being Igated within a hop or two.

                  No one here argues that a WIDE1-1,WIDE2-1, or even just a WIDE2-1 in a well covered APRS area, is the preferred path.

                  Also, for the most part, a traveling APRS station going through remote and rural areas using a WIDE1-1,WIDE2-2 path will cause little detriment to a local network giving a properly set beacon rate or properly programmed smartBeaconing setup, and of course, that he is traveling through. Curt was asking for advise on what to use during his 15,000 mile trip. Since that trip of his doesn't involve extended stays in LA or on the East Cost, I believe that he will be just fine with a WIDE1-1,WIDE2-2 path. And, I admit, I use the same path when I travel from Michigan to Arkansas twice a year.

                  I'm really sorry that this example turned into debate.

                  Randy
                  WF5X

                  On Fri, Apr 30, 2010 at 7:38 AM, Lynn W. Deffenbaugh (Mr) <ldeffenb@...> wrote:
                   

                  But wasn't his original question asking for advice on a suitable path
                  for a 15,000 mile, cross-country road trip? Hence my suggestion that an
                  in-depth analysis wasn't easily applicable to the environment in
                  question. There's no doubt (in my mind, at least) that the shorter
                  WIDE1-1,WIDE2-1 path is good for his local area. The question was
                  directed at what to use when driving out into the (relative) unknown.



                  Lynn (D) - KJ4ERJ

                  James Ewen wrote:
                  > On Thu, Apr 29, 2010 at 1:35 PM, Lynn W. Deffenbaugh (Mr)
                  > <ldeffenb@...> wrote:
                  >
                  >
                  >> An excellent analysis for your home QTH, but decided impractical for a
                  >> multi-day, over-the-road trip.
                  >>
                  >
                  > Just to move it from theoretical into the real world... here's a quick
                  > peek into Curt's neighborhood.
                  >
                  > How many i-gates are within 2 hops of Curt and all those digipeaters?
                  > How many copies do each of those i-gates hear? It boggles the mind!

                • James Ewen
                  ... That s what I was doing with the list of digipeaters within 2 hops... ... Agreed, but adding an additional hop does add a significant load to the network.
                  Message 8 of 10 , Apr 30 6:35 AM
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                    On Fri, Apr 30, 2010 at 6:20 AM, Randy Love <rlove31@...> wrote:

                    > The point of the example was to show the possible extend of a 3 hop
                    > path in this area from Curt's location in Novi and relate it to the APRS-IS
                    > gating that would happen -- *not* to expound upon every possible hop,
                    > skip and jump. This was just to give Curt a reference for how far his
                    > packets could easily go to give him a visual of how the RF will keep
                    > going, if infrastructure exists, compared to being Igated within a hop or two.

                    That's what I was doing with the list of digipeaters within 2 hops...

                    > No one here argues that a WIDE1-1,WIDE2-1, or even just a
                    > WIDE2-1 in a well covered APRS area, is the preferred path.

                    Agreed, but adding an additional hop does add a significant load to
                    the network. It is a significant little bit of the concept.

                    > Also, for the most part, a traveling APRS station going
                    > through remote and rural areas using a WIDE1-1,WIDE2-2
                    > path will cause little detriment to a local network giving a
                    > properly set beacon rate or properly programmed
                    > smartBeaconing setup, and of course, that he is traveling
                    > through.

                    Yes, in a remote rural area, you can use really long paths with
                    little detriment, but in well covered areas, it can be a detriment.

                    > Curt was asking for advise on what to use during his 15,000 mile trip.

                    The usual recommended national path would be the best bet... wandering
                    off into the unknown and choosing a long path just in case is probably
                    not the best bet. No matter what, you're going to find holes in the
                    APRS network coverage.

                    > I believe that he will be just fine with a WIDE1-1,WIDE2-2 path.

                    Yes, he would be just fine... no one will die because of the extra
                    hop. I've been known to run that path or WIDE3-3 as well... I know my
                    local network, and just how much impact I will cause.

                    > I'm really sorry that this example turned into debate.

                    Not really a debate, but rather a discussion, and hopefully Curt is a
                    little more knowledgeable and is aware of just how far the packets can
                    go, and not just through that one digipeater and then getting
                    magically sucked into the APRS-IS vortex.

                    Because the APRS-IS dupe filters destroy so much of the evidence, it
                    is easy for people to misunderstand just how much of the network
                    resources they are affecting.

                    James
                    VE6SRV
                  • Lynn W. Deffenbaugh (Mr)
                    ... http://www.aprs.org/newN/APRSpaths.gif from: http://www.aprs.org/fix14439.html Lynn (D) - KJ4ERJ
                    Message 9 of 10 , Apr 30 6:57 AM
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                      James Ewen wrote:
                      > The usual recommended national path would be the best bet... wandering
                      > off into the unknown and choosing a long path just in case is probably
                      > not the best bet. No matter what, you're going to find holes in the
                      > APRS network coverage.
                      >

                      http://www.aprs.org/newN/APRSpaths.gif
                      from: http://www.aprs.org/fix14439.html

                      Lynn (D) - KJ4ERJ
                    • k7rbw
                      It s examples like this that illustrate some of the holes in APRS documentation, especially in the area of documentation targeted to the new user. I think
                      Message 10 of 10 , Apr 30 8:50 AM
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                        It's examples like this that illustrate some of the holes in APRS documentation, especially in the area of documentation targeted to the new user.

                        I think Lynn's observation of the two layers of APRS communications (RF and Internet) was both helpful and confusing at the same time. It was helpful in clarifying the two types of APRS uses (to send packets by RF to other HAMs and to get packets into the APRS-IS). At the same time, it was confusing in that it's challenging to try to optimize for one without sacrificing the other (at least insofar as I understand how things work).

                        For RF coverage (i.e. to be picked up by other RF stations), the goal is to go out as far as necessary to get picked up by someone else on the road. I understand the logic in the reference Lynn provided below however, I can also see why someone would want to "add a little extra" (i.e. use WIDE2-2 instead of WIDE2-1) "just to be on the safe side." (The APRS version of "adding a little just for grandma.") The problem is there's no easy way for someone to know if that's actually helping or hindering the situation, depending on how many digipeaters you actually hit as you travel. It would help if that resulted in your packets reaching some form of civilization or fellow traveller but it could hinder if the extra hops result in QRM so neither your packet or other colliding packets are heard.

                        For I-gating, all you need is to be heard by a station that's connected to the internet. Unfortunately, it's difficult for a traveler to know how many hops they'll need (or watts of TX power) to reach one. Of course if you want to reach both the internet AND other RF stations, optimizing for RF station coverage can result in lots of internet dupes (which is probably why the dupes are filtered).

                        Some ideas...

                        The path simulator. While I can understand how some HAMs might see a challenge in pinging as many stations as possible, I think the vast majority would rather be good citizens. The fact that any such slamming activity would be public (and very visible by using the same tool) should be enough for things to work themselves out naturally. Worst case, filter the offender's call sign from being digipeated. I don't think that'd be necessary (too often, anyway).

                        An I-gate database that you could download to your GPS as waypoints. Then, though profile switching you could expand or contract your path accordingly. I don't know, but you might even be able to make that a script. (e.g if there's an i-gate within X miles, use profile 1, else, use profile 2).

                        Some sort of feedback to the APRS mobile. This wouldn't work for the TX-only beacons, but for smarter mobile units like the Tracker2, set it up so that if the tracker hears it's call sign being repeated by a digipeater it knows it's packets are being received and rebroadcast so it switches to a short path (WIDE2-1, for example). If it doesn't, it bumps it up a notch to WIDE2-2. What would be even better would be some sort of flag in the digipeated packet that says it's already been sent to the Internet. That way: a) the sending station would know that it's packet had made it to the internet and b) other Igates that pick up the digipeated packet would also know.

                        Finally, on the QRM reduction wish list, would be some way to get feedback to use for adjusting the radio transmitter power. This, of course, requires a suitable interface on the radio to adjust TX power. What would happen, the APRS tracker would listen for its own packets being digipeated (as above) and then dial down the power as needed. There are a number of ways to figure out the best power: trial and error being the brute force approach, another might be to include a received signal strength value in the digipeated packet and the tracker could raise or lower the power of subsequent transmissions based on that value. If the tracker doesn't hear its packet, then it just uses the longest path and the most power. If it does, it trims things down as needed.

                        Anyway, these are just some casual thoughts for a Friday morning.

                        73

                        --bob
                        K7RBW

                        --- In tracker2@yahoogroups.com, "Lynn W. Deffenbaugh (Mr)" <ldeffenb@...> wrote:
                        >
                        > James Ewen wrote:
                        > > The usual recommended national path would be the best bet... wandering
                        > > off into the unknown and choosing a long path just in case is probably
                        > > not the best bet. No matter what, you're going to find holes in the
                        > > APRS network coverage.
                        > >
                        >
                        > http://www.aprs.org/newN/APRSpaths.gif
                        > from: http://www.aprs.org/fix14439.html
                        >
                        > Lynn (D) - KJ4ERJ
                        >
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