KD0TLS: General-target APRS messaging

  APRS allows an operator to send "broadcast" messages over RF or the -IS (Internet Service). In some parts of the world, such as Australia, these general messages are sent over the internet to be re-transmitted by amateur radio operators, and non-hams "listen" to these messages in more remote areas (e.g. Western Australia). These messages run from agricultural (frost, sheep, crop disease) to marine warnings (high winds, storms). Strictly speaking, this would be a more appropriate use of the "bulletin" or "announcement" functions of APRS. Amateurs in Spain also frequently use APRS bulletins to send weather warnings over wide areas.

 You can see all of the current bulletins and announcements on this aprs.fi page. You will also find some APRS services listed there, such as satellite pass predictions or weather forecasts for your location. 

 When I learned about the ways amateurs in other countries use APRS general messaging, I wanted to try it out here. My primary interest was in how these messages would be propagated. How far could these messages reach? Who would see them? Would they be gated, or just digipeated? The answers surprised me, and I slowly discovered that a lot of amateurs weren't familiar with these tools. I had to explore on my own, and some aspects are still a mystery to me. 

 There are four ways to send these messages, depending on your purpose and the nature of your message. All four can be used by any station that can send APRS messages, though bulletins would require some kind of specialized capability to keep re-transmitting them on a regular schedule. 

 None of these four types of messages will allow an "ack" -- an acknowledgement of reception. It should be clear that this would clog the network with dozens (or hundreds) of "ack" messages. However, without an "ack", gateways have no idea if the message was ever received. Any of these messages can, therefore, be re-sent three or four times by a gateway that believes it has failed to reach the recipient in its previous transmissions. In the Big Picture, this is a feature rather than a bug. APRS messaging in general is designed to be robust, rather than efficient. At some point, I'll go into some depth on how messages are handled on the network. For the moment, though, my point is that seeing a general-target APRS message transmitted multiple times is not a case of some operator sending multiple copies of the same message.

 I should probably note one very important limitation of these messages: they are only available to APRS stations that are "messaging-capable". This excludes a lot of trackers, most digipeaters, and even some gateways. You can create a filter on aprs.fi to show only stations that are messaging-capable. Some software, such as APRSIS32, has native filtering to show "messageable" stations as well.

Here is a screenshot of the current metro message-capable world:

metro-area message-capable stations 12/12

 Also note that some of these message-capable stations are unattended and will never see a message, such as W0PZT-1 or W0YC-5. Digipeaters and gateways that aren't messaging-capable will still handle messages; either gating them, re-transmitting them to RF-based operators, or simply digipeating them as "normal" packets would be handled. You just can't send messages directed to a specific station that isn't "messaging-capable". 

 I'm still not clear as to how the -IS propagates these general-target messages. The RF side is pretty straightforward: the message is received by anyone within simplex range of the station, and within the range of any digipeaters passing that message on. I suspect that the range of an internet-based message is determined by the filter radius of the receiving station. Any -IS station must set up a radius in which packets received will be shown. Otherwise, you would see all of the traffic from the entire planet at once, like trying to drink out of a firehose. But it also seems possible to me that some gateways could transmit an -IS general-target message over RF to reach RF-based operators.

Let's look at the four ways to send general-target messages:

ALL: 

 Sending a message to "ALL" (rather than typing in a specific callsign and SSID) sends a message to all amateur radio operators currently on APRS -- but only those with messaging capability, of course. Messages sent to ALL will be gated and digipeated just like any other traffic. Gated messages will be stored on aprs.fi for two weeks. In a way, it's like calling CQ. In another way, it could easily be abused. I've avoided the use of "ALL" in my explorations, though I can see obvious uses for EmComm-related purposes. For example, imagine sending a message noting an HF frequency for a disaster-related net, or even a SkyWarn activation.

QST: 

  Much like "ALL", this sends a message to all amateur radio operators that are capable of receiving messages. The difference is that "QST" has a specific meaning in amateur radio: information of interest to all amateur stations. I've used "QST" to send HF propagation reports at irregular intervals for a few months now, and I've closely watched how these messages are handled by the network. Frankly, I had expected that at least one curmudgeon would get upset by my use of this tool, but no one has expressed their aggravation to me yet. Some find it to be (somewhat) interesting information, while others are curious how this tool works. I've found that most APRS operators are kind of hazy on the issue of APRS messaging, and receiving an APRS message is intriguing -- even if it is just a report on the current MUF. Again, a QST is sent by an APRS client substituting QST for the recipient callsign/SSID. All gated QST messages are stored on aprs.fi for two weeks. 

BULLETIN: 

 Bulletins are slightly more complicated. Some APRS clients have a specific bulletin capability, repeating a bulletin at some set interval. There are also specific bulletin "groups" to target bulletins at only a specific group of operators. Examples would be "PROP" (propagation), "LOCAL" (local-interest only), "MUF" (current MUF), or "SYDWX" (Sydney weather). You can send a LOCAL bulletin, for example, by designating the recipient as "BLN0LOCAL". 

 Bulletins are intended to be repeated until they are no longer relevant, such as a tornado warning. But they can certainly be "one-time" affairs. This is the primary distinction between a "bulletin" and an "announcement" -- in other words, not much of a distinction at all. 

 While bulletins can be filtered by the recipient, they rarely are. A bulletin sent to "BLN0LOCAL" will generally go to the same stations as one sent to "BLN0" or "BLN1" -- in other words, all stations capable of receiving messages.

 There are some fairly outdated rules for bulletins, which are widely ignored today. For example, "BLN0" -- BLN with a zero at the end -- is supposed to be for one-time messages. Some clients, such as the aprs.fi app, only allow "BLN0" messages to be sent.

 Bulletins can be multi-line. There might a be reason to send more information than can be expressed in a single line of text. You would then increment each line, like BLN1, BLN2, BLN3, etc. Here's an example:

 Again, if you are sending a single-line bulletin, there's no distinction between using "BLN0" compared to "BLN4".

 A typical bulletin can be a severe weather warning, road closures, propagation reports, etc. Mostly, though, it's more mundane. Is your Field Day event, Hamfest, club meeting, new repeater, new gateway, etc. more properly a "bulletin" or an "announcement"? Who cares?

ANNOUNCEMENT: 

 Much like a bulletin, an announcement goes to all hams capable of receiving APRS messages. The difference is that an announcement is sent to BLN plus a letter, rather than a number in the case of bulletins. Sending an APRS message to "BLNA", instead of a callsign/SSID, will send an announcement to all amateur operators capable of receiving APRS messages. Multi-line announcements are incremented like bulletins: BLNA, BLNB, BLNC, etc. As with bulletins, you can send a single-line announcement to "BLNA" just as easily as to "BLNC". Announcements, like bulletins, are stored on aprs.fi for only 24 hours.

 W0ILO sends a multi-line announcement as follows:

Multi-line announcement

 Previously, I've sent an announcement using "BLNA" to advertise this blog. A few people far outside the local area have seen the announcement on aprs.fi and checked it out. I justify this by noting that the blog is specifically aimed at APRS operation. Still, I've stopped sending these announcements as it may be considered as something resembling spam. I'm unsure who would make the determination that an announcement for an APRS blog is acceptable, while an announcement for a car wash is not. I really don't care to find out. 

How far can you reach? 

 The answer depends on your N-path. If you are in the metro, you have several wide-coverage digipeaters available to you. A WIDE2-2 path will offer two hops. This means that, for example,  W0PZT-1 can re-transmit your message to another digipeater, who will re-transmit it again. W0PZT-1 has a large coverage area, and several smaller "dumb" digipeaters will re-transmit it as a matter of course. Reaching Aitkin, Faribault, Litchfield, Mankato, or eastern Wisconsin is easy on a single hop. Digipeaters in those areas will re-transmit your message to cover places like Willmar, Winona, Worthington, Duluth, Brainerd, etc. 

Two hops is a lot when you consider our local infrastructure. And, if you add a WIDE1-1, you can get into Canada, down into Iowa, or over to Madison, WI. This is probably further than you need, and a WIDE2-1 or even WIDE1-1 path will cover the metro and the surrounding areas. 

 My experience has been that a WIDE2-2 path will get you a response from an area including Little Falls, Willmar, Rochester, and Rice Lake in Wisconsin. While I haven't tried it, using a satellite digipeater can easily cover 700 miles.

 In practical terms, this means that even an HT within range of a big digipeater can reach outside the metro on one hop. And don't forget that a a lot of those smaller digipeaters (NOGEF-15, KE0CRR-10, K0KGK, and KC9NVV-3) will also be re-transmitting your message in their local areas.

 

 In summary, I think that having access to these general-target messages is something that's at least potentially beneficial to most ham radio operators. This could be something as simple as running APRSDroid on your smart-phone, and finding that someone has sent a message. Nearly all messaging-capable clients will inform you in some way that you have received a message, even if you aren't glued to the screen every moment. HTs and mobiles that have native APRS capability will also generate a tone when this kind of message comes in, and save it until the user clears it. 

 Obviously, there's potential for annoyance as well. You may very well believe that sending "Let's go Brandon" every day is cute, clever, and of interest to amateur operators everywhere. You may find out that it is not. You may make a special point of daring someone to stop you. Alternatively, you might send inane greetings like "Hello world" on a regular basis. In reality, all that you are doing is deprecating the value of APRS messaging in general.