Date: Thu, 12 Apr 2001 
From: Bob Bruninga, WB4APR 

To: TAPR APRS Special Interest Group 

Cc: amsat-bb@AMSAT.Org,

Subject: TX-Cycle Recommendation?



ANSWER: It depends!  Are you there or not??

Live keybopard operations, set one beacon a minute.  And message as 
needed.  Unattended, one (and only one) packet every 5 minutes.

The goal for live operations is to make contacts and to play radio.

The goal of unattended beacons via satellite has been to provide a 
remote position/status report once a day with good probability.



Some numbers.  Lets say a good pass lasts 8 mins.  Thats 480 secs, but

its half duplex so thats 240 secs of 100% channel capacity.  An ALOHA

channel falls apart above 18% loading (success fails for everyone), 
so only about 40 packets can be successful per pass.  This can equate 
to about 10 live chats (1/min) about 10 mobile travelers (evy 2 min)
and about 25 unattended stations with 5 minute beacons.



NOW THE GOOD PART.  If we keep to the above rates, we are actually over
loading the ALOHA channel by a factor of 3, but then the geometery of a
pass is quite non-linear.  70% of the users are 6 to 10 dB farther away
than the 30% closer under the spacecraft.  SO this can reduce collisions
via the well known 10 dB capture effect.

So, in the long run, after the initial excitement, I would hope that 
there will be 50 or so users actively operating in each footprint

on every pass. (PRIME-TIME and FIELD DAY excluded!)  Thus, you can count

on a good chance at pinging the ISS to show off HAM radio anytime,
anywhere, 
when it comes into view (if everyone else follows the ISS 
UtIquette)...



This is not to discourage people with more important DEMOS to increase

their rate until successful, because the value to HAM radio of someone

else's demo is far more important to us all than our individual 1 seconds...




Just a thought. I'd love to see a rigorous analysis.



de WB4APR, Bob