Hi Simon,

Good question - "medium" was a deliberately vague term as it will also depend on the camera and available data rates etc.

I was hoping we might be able to move on from analogue SSTV now.. we should be able to achieve much better quality images,,,as you suggest the challenge would be to do the digital encoding on board the spacecraft without using lots of power but, as mentioned, - if they can do it on a tiny payload hanging beneath a small helium balloon then it should be possible to do something similar on a spacecraft even a 1U cubesat!

73

Graham G3VZV

-----Original Message----- From: Simon HB9DRV Sent: Friday, September 07, 2012 2:13 PM To: 'Graham Shirville' ; 'AMSAT BB' Subject: [amsat-bb] Re: Fw: Digital Slow Scan in space

Graham,

What's 'medium resolution' ? Standard SSTV is not digital, it's analogous with the (now no longer used) analogue TV. There is true digital SSTV used on 80m and 40m and (I guess) other bands, from memory this is EasyPal; to implement this in a satellite would be a long and tedious task.

Implementing one of the analogue SSTV modes would be easier - much easier - but quality is nowhere as good.

Simon Brown, HB9DRV http://sdr-radio.com

-----Original Message----- From: amsat-bb-bounces@amsat.org [mailto:amsat-bb-bounces@amsat.org] On Behalf Of Graham Shirville

We are looking for a practical system to use on a 1U, or perhaps 2U, cubesat to take medium definition images and transmit them to the ground on VHF for educational outreach purposes.

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Hi both.

I believe the digital SSTV that is used on HF / VHF is in reality DRM, which is a forward error corrected digital communications standard (?) I suppose the real difficulty is implementing a method of reading an image file and then encoding it for transmission in a broadcast type format for general reception rather than one that requires handshaking or acknowledgments from the groundstation.

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.

Thanks

David

-----Original Message----- From: Simon HB9DRV simon@hb9drv.ch To: 'Graham Shirville' g.shirville@btinternet.com; 'AMSAT BB' amsat-bb@amsat.org Sent: Fri, 7 Sep 2012 21:51 Subject: [amsat-bb] Re: Fw: Digital Slow Scan in space

Well you should be able to do it on small ARM processor, but you've always got to take transmission time into account - those pesky satellites do tend to move about quite a bit :)

There's also the format used by the NOAA weather satellites - simple and works...

Simon Brown, HB9DRV http://sdr-radio.com

-----Original Message----- From: Graham Shirville [mailto:g.shirville@btinternet.com]

Good question - "medium" was a deliberately vague term as it will also depend on the camera and available data rates etc.

I was hoping we might be able to move on from analogue SSTV now.. we should be able to achieve much better quality images,,,as you suggest the challenge would be to do the digital encoding on board the spacecraft without using lots of power but, as mentioned, - if they can do it on a tiny payload hanging beneath a small helium balloon then it should be possible to do something similar on a spacecraft even a 1U cubesat!

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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 . m5aka@yahoo.co.uk wrote:

--- On Fri, 7/9/12, g0mrf@aol.com g0mrf@aol.com 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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