Some years ago, I wrote a tutorial on APRS messaging, leaning heavily on the existing work of KJ4ERJ. If you don't understand how APRS messaging works, you might want to review that previous post from 9/22.
The messaging capability of the APRS network is a powerful and mostly-overlooked facet of APRS. The -IS (Internet Service) is leveraged to deliver messages globally. It provides, through gateways and digipeaters, the means to reach RF operators over a wide area with general-target messaging. But increasingly, operators are discovering that APRS messaging provides access to services, too.
These services include things like ANSRVR, which allows group chats. Whether it's a club, a CERT team, or just individuals sharing POTA spots, ANSRVR allows you to instantly create a group and have others join it. It also handles things like nets, such as APRSH and APRSTHURSDAY -- which is probably where most operators' familiarity with APRS messaging begins and ends.
But there are other services, like MPAD (and its variants). There are at least a few services providing weather info (current and forecasts), and things like an IRC chat room for amateurs. The SMS service allows an amateur operator to send or receive SMS texts to/from cell phones. Satellite pass predictions, local information, and social media crossovers are also available to hams using APRS messaging. It seems like a new service pops up every year, and these are usually Python-based applications run on -IS connected APRS stations.
RF-only messaging
Most messaging and services rely on the -IS to function. Yes, it's entirely possible to send an RF message to another RF station by way of simplex (if both are in range), using the NOGATE path. In the vast majority of cases, however, "RF to RF" is handled through a gateway or digipeater.
Suppose that you are in the metro using a 5w handheld as an APRS rig, and you want to send a message to another RF operator in Two Harbors (north of Duluth). You can do that. Your packet will be digipeated to one of our local gateways, or heard directly by an RX-only or TX gateway -- and there are several of these in the metro. The message will be carried by the -IS to the TX gateway in Knife River (KNFRVR) and transmitted over the air to your intended station. This assumes that the operator in Two Harbors has transmitted their location recently, and that KNFRVR has gated that location packet.
In this case, the APRS network has used the recent RF location packet of your recipient to determine that KNFRVR is likely to be able to reach your acquaintance in Two Harbors. It will use the nearest TX gateway to the recipient to transmit your message over RF.
The network will not, however, use the TX gateway in Fargo, Faribault, Winona, or St. Cloud (for example) to transmit your message to the RF operator in Two Harbors. Why not? Because there is no evidence that these stations are in range of the recipient in Two Harbors.
It's important to note that digipeaters will not perform this task. Since they aren't connected to the -IS, they will never get your packet sent to a metro gateway. Yes, your original RF message can be digipeated, but that only helps if that digipeater can reach a gateway. And yes, your message can digipeated after being transmitted by KNFRVR.
IF your RF-only recipient is not in range of a TX gateway, then you are probably out of luck. In that case, it does no good for your RF-only recipient to reach a TX gateway through a digipeater. Your message will certainly be sent over the -IS, but if your recipient isn't connected to the -IS, that's of no use.
Western Wisconsin
Okay... enough background. It should be clear that you need a TX gateway to send messages to operators over RF.
Western WI has very few TX gateways.
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| Arrows mark TX gateways |
As always, you can click on the image above to view a larger version. The map above shows only gateways. If there isn't an arrow pointing to a particular gateway, then it's an RX-only gateway (or it's not in western WI). Frankly, the Stevens Point gateway is stretching the boundaries of "western WI" as it is.
An RX-only gateway can take a received RF-based message and send it over the -IS to the appropriate TX gateway for RF transmission. Since the RX-only gateway has no RF transmission capability, it simply can't relay an RF-based message to a digipeater -- or even directly to an RF-only recipient located across the street from the gateway.
If an RF-only APRS operator isn't within simplex range of N9MEC-8, KB9WGP-10, or KB9TTQ-1, then they are very unlikely to receive any messages sent to them.
In theory, the massive digipeater network in WI could relay the original RF-based message sent by another operator, and that digipeated message could possibly be heard by the recipient over RF. But the digipeaters would just be handling the original message like any other location packet or beacon ID. And it would be limited by the number of "hops" in the original message's N-path. It's hardly a reliable solution, however.
Different models
As I've previously written, Wisconsin has a more organized model than MN. And that WI model has always relied almost exclusively on digipeaters to provide coverage.
Over the past few years, many gateways have popped up across WI, unsanctioned by WINET or WECOM. This isn't to imply that they are operating illegally or creating problems; it simply means that they are not part of the "old school vision" embraced by the big amateur radio organizations in Wisconsin. Going back just five years, the number of I-Gates in the state was in the single digits and almost all were in the eastern half of the state.
There are still factions in the packet community that see RF operation as the only 'legitimate' means, and they see any kind of -IS integration as 'cheating'. But the potential of APRS messaging and access to services is slowly undermining that "vision".
The dirty little secret of the old WINET/WECOM model is that it relied almost entirely on gateways in other states in order to work properly. Endlessly tossing packets across the state over RF might be nice if amateurs using APRS were sitting at a terminal watching RF traffic in real-time, but few participate in APRS that way. By incorporating the -IS through gateways, traffic can be stored, compiled, and examined -- while being available to operators across the globe. Packets that don't get gated just evaporate into the ether, like a tree falling in the forest that nobody sees.
The increase in RX-only gateways across Wisconsin largely solves that problem. Digipeated packets are now gated efficiently. Propagation can be evaluated in real-time. Coverage of individual stations can be assessed. And "enthusiastic individuals" can make a contribution to the wider network by setting up a gateway -- regardless of any "old school vision". But access to messaging isn't a feature of this newly-developed situation.
By contrast, MN has a fairly robust network of TX gateways around the state. Certainly, there are glaring gaps in that coverage, and all gateways aren't created equal. We also have our fair share of RX-only gateways, but these are more of a supplement to existing coverage, providing a path to gateway for digipeaters and boosting coverage for low-power stations in the area. None of this has been dictated or co-ordinated by a central organization. It's all "organic" growth spurred by individuals who see a need and step up to fill it. Nothing about Wisconsin precludes the same efforts from being successful.
Summary
Digipeaters are a poor solution for RF-based APRS messaging. If they succeed in providing that function, it's sheer luck. Placing even a modest TX gateway near a big digi can show dramatic results.
Perhaps the gateway last heard a mobile station 30 minutes earlier, and that mobile station is already out of range of the modest gateway. The network will still see that modest TX gateway as the best solution, and the message will be transmitted over RF by the modest gateway -- but it will be re-transmitted by the big digipeater to actually reach that mobile station that is now further away. Without that modest TX gateway to make the RF transmission of the message, none of this is viable.
And none of this involves massive towers, expensive gear, or extensive technical knowledge. It could be as basic as a 5w HT connected to a Microsat TNC running a 1/4-wave ground plane antenna on the roof of a rambler. As long as the modest TX gateway can reach the big digipeater, the message will probably reach the recipient, assuming the gateway heard the recipient station recently.
As far as RF-based messaging goes, a TX gateway is only as valuable as the range of its direct coverage (i.e. stations heard without being relayed by a digi). In this regard, "mega-stations" have an advantage. But positive results can easily be achieved with small-scale solutions, because APRS is a network.
