[Date Prev][Date Next][Thread Prev][Thread Next] - [Date Index][Thread Index][Author Index]

Re: Uplink Frequencies

At 08:38 AM 9/18/98 -0700, Bob Hart wrote:
>I note that UO-22, KO-23, and KO-25  have two uplink frequencies
>listed in the various different references.
>The first question that comes to mind is, "Why?" 

It's a (partial) attempt to match the throughput on the uplink
and downlink.  On the downlink, the satellite is the only one
transmitting, so it can utilize the link with 100% efficiency.
On the uplink, many groundstations transmit more or less
randomly, with no coordination attempted.  This means that
part of the time, two or more stations will be transmitting (with
the probable outcome that none will get through) or no
station is transmitting.  It's easy to see that you can't get
anywhere near 100% efficiency under those conditions.

There's an often-quoted theoretical result that says the best
average throughput you can get on a random-access channel
(under a certain set of simple assumptions that is approximately
applicable here) is 18% of the channel capacity.  With that
sort of performance, you'd need between 5 and 6 uplinks to
equal the throughput on the downlink.

Luckily, the usage pattern for a store-and-forward pacsat is
not symmetrical.  Some of the information uploaded will be
downloaded more than once, so the bandwidth requirements
on the uplink are actually much lower than those of the
downlink.  Depending on what you believe about the desired
usage patterns, you (as a satellite designer) get to trade off
the cost of additional uplinks against the potential increased
throughput.  The AMSAT-NA designers who spec'd out the
AO-16 class of microsats chose 4 uplinks.

On AO-16, we did at one time have a theory about which
uplink should be used for what.  I don't know if there's
any such theory for the satellites you mentioned. I doubt it.
Just use whichever uplink seems to be working better for
you on any given pass.

73  -Paul

Via the amsat-bb mailing list at AMSAT.ORG courtesy of AMSAT-NA.
To unsubscribe, send "unsubscribe amsat-bb" to Majordomo@amsat.org