[amsat-bb] The root of all the problems

Phil Karn karn at ka9q.net
Mon Jul 21 06:33:24 UTC 2014

On 07/20/2014 08:42 PM, Bryce Salmi wrote:

> Cubesats are standardizing AMSATs satellites and there's much much more to
> the satellite than simply the amateur radio mode used to communicate. If I
> do my job right, and others working on their Fox-1 subsystems do their jobs
> right too, you will never know it... it will be invisible to the average
> user.

This is absolutely true, especially for the power subsystem you're
working on. So it's vital that the payload use that power as efficiently
as possible.

The biggest load in a communications satellite (most spacecraft,
actually) is the downlink power amplifier, and that's why it's so
important that the downlink use the most power-efficient communications
mode available. And that's not FM, SSB or any other analog mode, not by
a long shot.

Here are some numbers for comparison. NBFM typically uses 15 kHz
bandwidth, and a SNR of at least 10-12 dB in that bandwidth is required
for acceptable performance. That's a P/No (power to noise spectral
density ratio) of about 53-54 dB-Hz.

A CODEC2 encoded voice signal at its highest rate (and quality) is 3200
bps. (Codec2 was specifically developed for ham radio digital voice by
VK5DGR, and outperforms many commercial voice codecs.)

The CCSDS-standard turbo FEC codes require an Eb/No (energy per bit to
noise spectral density ratio) of -0.25 to +1.5 dB, depending on code
rate and block size (the lower the code rate and the bigger the block
size, the lower the required Eb/No). This is remarkably close to the
Shannon limit.

Assuming an Eb/No of +1 dB and a CODEC2 data rate of 3200 bps, a single
voice signal therefore requires a P/No of 10*log10(3200) + 1 = 36 dB-Hz,
17-18 dB less than the FM voice signal.

That's a power savings of something like 60x, which in a power limited
system like a communications satellite turns directly into a capacity
increase of 60:1. That is, you could carry 60 digital voice signals with
the power required by a single FM voice signal. Or you could carry a
single digital voice signal with 1/60 of the power required for a single
FM voice signal.

This should give some clue as to why almost the entire (non-amateur)
world has gone digital.


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