Laymans comments on setup
All,
I've been a ham for too many years and active on APRS since pretty much its creation. I've run many a Digi so I've been keenly interested in the discussion here.
Note at this time, I'm not flying an APRS setup, and when I do it's normally centered around a Kenwood D-7.
But, I'm growing concerned at the setups that I'm seeing used and here's why.
An airplane will benefit hugely from it's advantage of ERP - this being a relatively small transmit power coupled with a usually high antenna and the advantage of complete LOS (line of sight) minus any obstructions between the aircraft and the receiving station.
However, if I read the setup info correctly, you guys are setting your non-smart transmit rate to 120 seconds!!!! that's 2 minutes by my book and I think thats too fast/frequent.
Remember the APRS network is a *relay* network and it's transmit rate is limited by bandwidth. This means that at 1200 baud, even short transmissions that have to be relayed between digi's take a second or 2 for each hop. And most Digis can hear one another and they are also listening to other terrestrial stations. They do have a small buffer whre they can "cache" heards until they are clear to transmit, but all of this delay makes short intervals "clog" up the system with lots of retries.
While that is part of the problem, the bigger problem is the ERP. Most APRS stations us whats called a back off timer, they try to transmit, if they don't detect that they were heard and relayed, they back off some interval and then retry. This usually continues until some threshold is set, then they give up and move on to the next posit. With short TX intervals, and the HIGH ERP that you have in the airplane setup, you are making it impossible for ground based stations to get their information into the network. Now multiply this factor by the LOS advantage of an airplane and you'll know exactly why the cellphone companies use technologies to block cellphones that are heard at multiple cell sites. Remember, whether the digi can decode your packet or not (you are right on the fringe of it's area), you still caused it to pause to listen and that paused everyone else around it.
I would strongly recommend that you set the non-smart beacon rate (Auto Transmit Rate) to at least 5 mins (300 seconds) and preferably 10mins (600 seconds) and allow the smart to beacon at something shorter, but only while maneuvering. There simply is no reason why a packet every 10 mins, when straight and level isn't appropriate. But then this is just my opinion, but I'm sure you'd be more friendly to the APRS network if you adjusted accordingly.
I would also adjust the slow and fast speeds in the Smart setup to account for what is slow and fast in an airplane and not a car, if you think about it, you are always using the Fast rate if it's set at 50mph during smart beaconing as likely you are going faster than 50mph.
Just my .02,
