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.
--Phil