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APRS Explained

I got asked a few questions about APRS, and figured that the topic would make a good post. This isn't everything there is to know about APRS (by a long shot), but it does explain things in (I hope) a way that can help clarify how various components in the system work...

At any rate, here were some of the questions that got asked:

Explor: I've been checking your post here turbodb and must say it's helping me get started in aprs. Very nice write up !

  1. I'm wondering what tablet you use ?
  2. I have a windows based laptop for programming my radio and wondering if I can use that ?
  3. Some tell me I needed a TNC , but ham outlet told me I don't because my Yeasue FTM 400 dxr has everything I need for aprs ?

I'm very new to ham (1 year) and I'm confused as heck as to what I need and don't need. Still going through growing pains , and don't have anyone around to help. Heck I went to a ham fest last year and asked around about aprs , people looked at me like I had two heads !

Basicly im trying to get aprs , maps, navigation , pretty much what you are doing.

Welcome! Let me start by saying that I'm no expert - but I'm happy to tell you the little bit that I know :)...

Easy question first: What tablet do I use?
I use a Nexus 7 (because I had one laying around; it was free).

There are better tablets to use. Mine only has 16GB of storage, and no SD card. For offline maps, you want a lot of storage, so I recommend something that's cheap, but with a micro SD card slot.

The only requirement is that it can communicate with the TNC via Bluetooth (if you're using a Mobilinkd, the only TNC I have experience with). That really means that it needs to be Android or Windows based (iOS doesn't support the necessary protocols over BT). I prefer Android since there are apps that work well on it (APRSDroid, Back Country Navigator). Windows should work too though, since it has APRSISCE/32.

Whew, even the easy question had a complicated answer.

What is APRS, and what's needed to "do it"?
OK, before I mention how to get APRS setup with maps/nav, it's important to realize that APRS and maps/nav are seperate things. You don't need maps/nav to "do APRS," and "doing APRS" doesn't get you maps/nav. Think of APRS info as data that can be overlaid on a map - just like an address, or GPS track might be overlaid on a map. Also - APRS does more than just position beaconing - it allows you to send/receive messages and data - but let's stick with the position stuff for now.

To "do APRS" via your ham radio you need three things:

  1. A GPS - APRS is about reporting your position, and so you need a GPS to get the lat/long coordinates of your current position
  2. A TNC - You need to convert the lat/long coordinates into sounds (tones) that get sent over the ham radio, and to convert the sounds you receive from others back into lat/long coordinates that your device can display. The TNC does these conversions for you (it's a modem).
  3. A Ham radio - Needed of course to send out the tones that the TNC generates to other ham radios/repeaters/iGates, etc.---
  4. Optionally, you can have a 4th thing - a map application that understands APRS data - where you can display the GPS/APRS information

These four things connect like this (either as seperate devices or internally in a single device)

Can I "do APRS" with just my ham radio (in this case an FTM-400XDR)?
Yes. But it may not be as useful as if you also run some mapping app that understands APRS.

Now, you're Yaesu FTM-400XDR has all three of the required things built into it - there's a GPS and TNC inside the control head, and of course it is a ham radio :). That's why Ham Outlet said you can "do APRS" with just your radio. Essentially, the picture becomes this:

The inclusion of the GPS and TNC is unlike most mobile ham radios, which require an external TNC and GPS - which is why Monte said you need a TNC. So, if what you want to do is send positional beacon information so that people can track you on [URL]http://aprs.fi[/URL], then all you need to do is configure your radio and you're set. I don't know how to configure your radio since I don't have one - but it looks like there's an entire manual (in addition to the normal manual) about how to do it. Of course, I can give you detailed instructions if you want to send me a 400XDR so I can figure it out! 🙂

The thing is, if you "do APRS" via just your radio, then you're limited to displaying that information in the way your radio displays it - which is on the control unit (likely as a list of beacons you've received, and your distance/direction to them) - as opposed to overlaid on a map. That's why we generally all use external TNCs and GPSs - because those allow us to do the map overlay.

How do I get the APRS data overlayed on a map?
As I mentioned, APRS data is (for the sake of this post) just position information from a GPS displayed on a map. So, that means you need a few things:

  1. APRS data for both your position and the position of others (think of this as GPS data)
  2. Offline map software that also knows how to consume the APRS data and overlay it on the map

Now, Yaesu, Kenwood, and the ham radio manufacturers generally want you to use their stuff, so it's not always easy to get the GPS/APRS data out of the radio (usually it's possible but it's so radio specific that you have to spend a lot of time in the guts). But, if you have an external TNC that can communicate with a device that has a built-in GPS, then things get "simpler" because you can use the ham radio as a "dumb transmitter" and do all the heavy lifting of APRS outside the radio. This is what we generally do. It makes the picture look like this:

Getting this setup is what I covered in my previous post on my Kenwood D710G, using a Mobilinkd TNC and an Android tablet (Nexus 7 in my case, but any tablet will work) running APRSDroid. The configuration for a Yaesu 400XDR is similar since you turn off all of it's APRS functionality and just use it as a "dumb transmitter" (I'm sure, but I've never done it myself - again feel free to PM me if you want to send me one and I'll be more than happy to try it! :)).

The tablet/laptop communicates GPS/position data to the Mobilinkd TNC via Bluetooth, which in turn communicates via a cable with the ham radio.

Can you do maps on your Windows laptop?
Yes, absolutely - but with caveats. To do it, you need to get an offline (assuming you go places without internet) APRS+map application that you can run on your laptop, and you need to be able to get your own GPS information (which usually means you'll need an external GPS that plugs into your laptop, since I've never seen a laptop with GPS). The only software I know of that can do offline APRS+map is APRSISCE/32. I've never used it, but there are a bunch of YouTube videos, and of course info on their site. Spend some time looking through it - you won't understand everything at the beginning, but it'll sink in over time.

Lastly - a bit of soapbox :sorry:
I think that there's a tendency to think about APRS or even just ham radios as "critical" - some thing that earns super-mega-expo-points. In my experience, that's not the case at all, and it's important to understand why you're doing it. If you regularly go out with others who have APRS capability, then it's a nice thing to have. In that case, you should emulate their setup, so that you can help each other troubleshoot/share parts/etc. - it's just another thing on your rig which you should think of as community property when you're out exploring. If you don't regularly go out with others who have APRS, then you should really think about why you want it. There are good reasons to have it - safety reasons even, but it's not necessary (there are other ways to overcome the safety reasons that are both safer and easier - e.g. a Garmin InReach)

Much more important IMO is having offline maps and GPS, which you can get working super easily on a cheap Android tablet with Back Country Navigator - I've explored for 18 years in my Tacoma, and that was by far the single biggest improvement in my experience. Being able to have a satalite map (vs. paper maps or just "directions") available offline, and even a pre-run GPS track telling you where to go - that's 1000x more valuable than APRS.

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Hope that all helps. For me, the ah-ha moment in understanding it all was realizing that I could think of the TNC like an old dial-up modem. All it does is convert digital<>analog, and send that converted data either to the ham radio for transmission or to the APRS+maps application for display.

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