Automatic Packet Reporting System Discussion


Postby VK7HDX » Sun Apr 22, 2007 7:51 am

I was wondering was anyone here @ the Forum using APRS system. I have 2 Amateurs friends (VK7JJ/VK7FLYN) travelling VK for the next 7 months and I can see where they are (depending on digi repeaters) and work out where to call them on HF (travelers net) or thur a local repeater (IRLP). APRS can be used of other reason as well example: stolen cars/you can be tracked if you are out riding your motorbike in the bush alone and if you are travelling in a strange town and you get lost and the other APRS opetators can see where direction you are going and tell you to head in a the right direction (very handy)

Its just another avenue of A/R. We (Amateurs) have so many options these days...Computers/digitals modes/satellites and other means of TX/RX. There are so many things to use and learn about...

Its a great hobby

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Re: APRS.....

Postby VK2OMD » Sun Apr 22, 2007 6:43 pm

VK7HDX wrote:... advantages of APRS...

Karl, last time I looked at APRS, it was choked with traffic from stations that never move. The bulk of the traffic was not even position information, but promotional content.

The effect is to reduce the probability of a position report being received by a gateway, or even a nearby digi. Since position reports are sent and not confirmed, interference on busy channels reduces the likelihood of success. The simple solution might seem to make more position reports, but that is self defeating, it increased the traffic and reduces the chances of success.

If you do a design exercise where the objective is to have a 90% chance of collecting position reports accurate to 1km for mobile stations travelling at up to 120kmph, you will find that a typical 1200bps channel supports very few stations, less in hilly terrain, even less when it digis everything just once, even less when it is choked with traffic from fixed stations.

If you allow for the likelihood that one mobile station cannot hear another, and may transmit over the top, blowing both position reports away, you will come to the view that the channel has to run at very very low utilisation to reduce the probability of these collisions. Nevertheless you will hear the channels running way over 50% busy, and I have observed then in excess of 80% busy. If you consider the service area of a digi, and estimate the probability that your mobile station cannot hear another station randomly located in the service area, lets say 90% for example, and the channel is 80% busy, a first approximation of the likelyhood that the digi will receive your position report is 100%-80%*90% or just under 30%. So, if you do 3 position reports every 60 seconds (ie one report every 20s), you should get a good report in every 1km at 100kmph.

Lets face it, APRS is one of those areas of amateur radio where it is more important to transmit than to receive, and it is effective at that.

If you want to join the APRS set, get a low powered transmitter, an ineffective antenna and a hopeless receiver (a handheld is just the worst) and put a station on air that can't be heard by other local stations (so causing collisions), and a receiver that never decodes packets due to IMD so wind the mute on real hard so that the system will transmit over the top of other traffic (more collisions = wasted channel bandwidth), and to top it off, do it from a fixed location so there is not real value to the traffic, but to put some icing on the cake, use inefficient encoding and add some low value text content such as a slogan promoting your station.

A bit cynical perhaps... but only a bit!

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