Tragedy of the Commons
Anybody interested in a serious discussion about this needs to read APRS designer Bob Bruninga's site about the "Tragedy of the Commons"
http://www.ew.usna.edu/~bruninga/aprs/fix14439.html
<snip>Tragedy of the Commons: APRS suffers the classic fate of all limited resources (as well documented since the 1830's See Tragedy). Whenever there is a balance between individual interests and the common good, human nature guarantees the overloading and ultimate demise of the common resource. In this case, APRS throughput. This is easy to understand since the benefit of adding one more packet to the network always immediately benefits the SENDER, but the negative "cost" is spread over everyone else, and NOT the sender. There is no natural solution, other than the establishment of "Golden Rules" to live by for all concerned. <end snip>
Since the explosion in popularity of APRS several years ago, there has been a move afoot to reduce power transmission, number of packets sent, and to reduce the number of bounces allowed from one digipeater to another, as well as to increase the number of digipeaters available (similar to what has happened to cellular telephone networks 10 yrs ago to increase bandwidth). We need to be very careful, since as APRS users in aircraft we hit digipeaters and Igates for hundreds of miles, and have a much greater impact on the APRS system than ground-bound users.
Ron, could you design for the power supply but not install it? That way, you could try the low power installation in the area you fly and see whether coverage is sufficient. Although these are low power transmitters, they are very well tuned to the frequency (with a crystal IIRC) so ALL of the power gets tramsitted on exactly the frequency you want it on, and its not spread over a "band" of frequencies - thus they can transmit greater distances than higher power transmitters which are not tuned as well (according to a long discussion I once had with Byon Garabrant - somebody smarter than me please correct this if I'm wrong - heck I'm just an ME). And being that you have line of sight a long, long way from an aircraft, even at low altitude (in line with Sam Buchanan and Pete Howells' recent posts), I believe that you will have pretty darn good coverage with the lower power transmitter.
I understand the desire to have coverage in the mountains if you're in a SAR situation, but if you have a trail of breadcrumbs from higher altitude and then lose signal when you get below ridgeline, wouldn't that still be pretty good? In addition, even with a higher power transmitter I don't think it will help much being below ridgeline and out of line of sight of a digipeater or IGATE.
I have often thought that the ideal is a switch set up for "normal" and "emergency" modes - NORMAL mode consisting of smart beaconing at low power, and EMERGENCY mode being continuous transmission at high power with a mayday message encoded. If Byon and Allen were to design such a product, they would have a real winner for the airborne APRS tranmistter market. (I think that Sam B is considering something like this??)
There are a lot of people a lot smarter than me on APRS, this is just my $0.02 from having casually lurked on various APRS sites for the last 10 years or so and used APRS for some flying I've done in the 182