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Re: Geosynchronous Satellites



This is true. The main reason for using SSB on HF is to reduce the
sensitivity to selective fading and minimize the bandwidth occupied. Neither
is a problem for satellite links. The peak power requirements for SSB (with
speech processing) and FM are about the same for the same. See page 58 of HF
Radio Systems and Circuits for a comparison.

AO-40 has a linear transponder for SSB because it was the best technical
solution at the time P3A was designed in 1977. RUDAK was placed on AO-40
(and AO-13) because it is recognized that digital methods will now a better
soltion.

John
KD6OZH

----- Original Message -----
From: "Phil Karn" <karn@ka9q.net>
To: <na9d@mindspring.com>
Cc: <amsat-bb@AMSAT.Org>
Sent: Sunday, 24 December 2000 07:18 UTC
Subject: Re: [amsat-bb] Geosynchronous Satellites


> Jon, I started to rebut your message point by point, but because of
> its high error density I thought it would be much clearer to just give
> the analysis from scratch.
>
> A NTSC video signal is 6 MHz wide, and it needs a SNR of at least 50
> dB in this bandwidth to provide studio quality. Transmitting this
> signal with SSB-AM implies a RF SNR of 50 dB in 6 MHz, or a C/No
> (carrier to noise spectral density ratio) of 50 + 10log10(6MHz) =
> 117.78 dB/Hz.
>
> Now let's look at FM. Standard analog satellite TV practice is
> wideband FM in a 36 MHz transponder bandwidth. The exact RF C/N
> required to provide good picture quality varies with the design of the
> qdemodulator, but it's around 12 dB. In a 36 MHz bandwidth, the C/No
> works out to 12 + 10log10(36MHz) = 87.56 dB/Hz. That's 30 dB less than
> SSB-AM!
>
> In other words, if a 10-watt transponder is enough for FM video, then
> it would need to run 10 *kilowatts* to provide the same picture
> quality with SSB-AM. Not bad for a modulation method that you're
> completely ruled out for satellite use, eh?
>
> So where did the truism come from, that FM is bad on amateur
> satellites? Simple, actually. It comes from the assumption that hams
> will tolerate poor audio SNRs. If you're willing to tolerate a average
> SNR of, say 10 dB in a 3 KHz bandwidth, then the average RF power is
> indeed less than the 10 dB or so C/N needed to quiet a 15 KHz NBFM
> detector. But as soon as you up the required audio SNR, even by a few
> dB, FM once again starts to look better. And digital voice (with good
> compression, FEC and modulation) is better still. That's why the
> cellular networks started with FM and are rapidly going digital.
>
> You might say that hams should just stick with SSB and accept a crappy
> 10 dB SNR. But that's much easier said than done. Human nature being
> what it is, every SSB operator is continually tempted to turn up the
> wick to improve the SNR and reduce operator fatigue. Because SSB is
> linear, every additional dB of audio SNR requires another dB of RF
> power. So on HF you end up with a lot of people running linears to get
> "armchair copy" in their ragchews. And the satellites have their power
> hogs trying to grab all of the transponder power for themselves.
>
> So it really seems to me that SSB's claimed power efficiency is
> largely illusory in practice. We might as well switch to nonlinear
> methods such as FM (or better yet, digital voice) that can give us the
> high audio SNRs we really want while using much less power than SSB.
>
> Phil
> ----
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