Hi All,
SSDV indeed shows you the images "as they come in" Most High Altitude Balloons use RTTY with relatively slow baudrates. The balloons are limited to using FSK modems in the ISM bands, because of legal issues. Therefore, RTTY is the most feasable mode. Tim Zaman here in the Netherlands has run 1200bd RTTY before on a balloon. see http://www.timzaman.com/?page_id=1106&lang=en He used the faster baudrate to send bigger pictures in the same time and was running two baudrates alternating.
For satellites, you could run a more complex downlink off course, with less transmission time and FEC. 64QAM might work on bigger sats, but I think it is not feasible for a one or 2u satellite at the moment. I would place my bet on flying BPSK/QPSK for now, and more complex things later.
Needless to say that I agree with David about the handshaking and ACKs, but the balloons don't do that either. I do not know anything on HF SSTV, but I have seen SSDV working just fine.
Wouter PA3WEG
On Sat, Sep 8, 2012 at 9:54 AM, Trevor . [email protected] wrote:
--- On Fri, 7/9/12, [email protected] [email protected] wrote:
An essential feature would be that whenever a 'packet' of data is received, that data is displayed rather than waiting for the complete image to be received error free. I couldn't find much on DRM encoding but I think it was originally intended for digital voice in a narrow bandwidth so it sounds suitable for low power satellite use.
WinDRM (and DRMDV/FDMDV) were written by Cesco Lanza HB9TLK some info at http://n1su.com/windrm/download.html
To achieve a reasonable data rate it's usually run at 64QAM (with full SSB bandwidth). 64 QAM is less robust and requires a higher S/N.
I've no idea what the practical effects of a high Doppler shift would be, although a computerized station with frequent Doppler correction should track it okay.
Is the data rate on High Altitude Balloons about 300 bps ? I've never received any images from them but if 300 bps is correct then the images are either low res or take a long time to send ?
Analog SSTV is still going strong some 6 years after the introduction of Digital SSTV perhaps indicating that existing Digital SSTV approaches do not yet match the performance/robustness/ease of use of analog for HF working.
73 Trevor M5AKA
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