Thursday, July 01, 2010

APRS on HF

A post about the French ban on D-Star on QRZ.com has predictably become a platform for everyone who dislikes D-Star to reiterate their views on why it should be banned altogether, not just in France. These reasons boil down either to its use of a proprietary codec or the fact that it involves linking radios using the internet.

I admit to toying with the idea of making a post on the lines of "what if internet linking of radios was banned worldwide? Wouldn't that be a real catalyst for innovation, to try to replicate internet facilities using entirely amateur radio RF based methods.?" But further thought as to what would be the effect on APRS if it were denied the use of the APRS-IS internet backbone made me realize that the effect would be catastrophic. The amount of traffic would be far too great to be carried over any HF network, while the number of hops needed using a VHF network to achieve worldwide coverage would also be far too many and far too slow. You might be able to do something using a sophisticated network of amateur satellites, but that would be far too expensive. While some uses of the internet do devalue the use of radio itself, in my opinion, many of them make possible things that could not otherwise be achieved. The internet is an integral part of APRS just as it is probably an integral part of whatever D-Star is meant to achieve.

Which led me to the question of what exactly is the point of APRS over HF radio? I understand the purpose of the VHF APRS infrastructure, which is to capture the messages from local APRS stations and pass them to APRS-IS. But given that it would be impractical for messages from one side of the world to be conveyed reliably to the other using RF, what is the point of APRS networks on HF?

I started off the day trying the latest version of Cross Country Wireless's APRS Messenger software. This is an interesting product in itself, in that it enables APRS traffic to use various PSK data modes, which are arguably more reliable on HF than the 300baud packet most people use. Unfortunately there are not many users. I switched to using 300baud packet and my screen quickly filled up with callsigns from all over Europe. But when I looked at aprs.fi to see its map of stations received by my station I saw only one.

The reason, I surmised (since I am far from being an expert on this) is that my client software, APRSIS32, is doing "the right thing" and not forwarding most of the messages I received as they had already propagated by the maximum number of hops. The one station whose messages I did pass on, F6KPH-4, had against it the note "Seriously bad path." Following this was the explanation: "This station is transmitting packets with a configured path of over 3 digipeaters. This causes serious congestion in the APRS network and errors when plotting the station's route on a map. Please consider using a path of WIDE1-1,WIDE2-1 or WIDE2-2, or even WIDE1-1,WIDE2-2 if you are moving very far away from an iGATE."

I don't really understand this WIDEx-x business, I just do what I am told. But I think I get the gist, which is that APRS messages sent over HF should be configured to take no more than three hops. If they don't reach a gateway by then, they will be lost, which is just tough luck, because the HF channels can't handle the congestion that would be caused by messages being rebroadcast more than three times.

Which brings me back to the question of what exactly is the point of APRS over HF for the average amateur? It is certainly interesting to see what you can hear using your own equipment. But if it is impractical for an RF based network to ensure that a message could get from Sydney, Australia to New York, New York without touching the internet, what useful purpose is achieved by transmitting and receiving APRS on the HF bands?
Post a Comment