From: LARRY KLAES (ljk4_at_msn.com)
Date: Fri Jan 17 2003 - 20:07:11 PST
----- Original Message -----
From: Scot L. Stride
Sent: Friday, January 17, 2003 12:51 PM
Subject: Re: Fw: SETI public: New Active SETI experiment
Dear Larry,
This thread reminded me of when we wanted to get a signal from Mars
Pathfinder as it was going through it's EDL (Entry, Descent and Landing)
phase. A modulated telemetry signal would have been a poor choice because
the antenna would need to be omni-directional and there was so much
spacecraft motion that it would smear the spectrum and sidebands. To get
around this Gordon Wood (now retired) proposed to use CW semaphores to send
back the information. Basically the carrier was frequency hopped within a
narrow band. During a specific EDL event (like parachute deployment) , the
computer sent a command to the exciter to shift the carrier frequency by a
discrete amount. This idea worked very well at communicating to the ground
of what happened during EDL. We didn't have to get a receiver lock and
demodulate a telemetry signal. All that was needed was to look at a narrow
spectrum and count the frequencies being detected from a relatively strong
CW carrier. Gordon told me that with a stable enough USO, the exciter could
have output large numbers of discrete carriers within the passband of the
transmitter. In that way rudimentary data encoding could be done without
the need for carrier modulation. A set of 512 or 1024 discrete frequencies
could be transmitted serially and represent data. In some ways this sounds
like spread spectrum communications. What if ET sent us a signal that was
encoded using a frequency hopping carrier? Has anyone written about or
looked into this possibility? And, rather than transmit a modulated ASETI
signal having a suppressed carrier, might we also consider using a
frequency hopping method to increase the signal energy?
Cheers,
Scot Stride
*************************************
At 09:37 AM 1/17/2003 -0500, LARRY KLAES wrote:
>
>
>----- Original Message -----
>From: Yvan Dutil
>Sent: Thursday, January 16, 2003 8:46 PM
>To: Stephane_Dumas_at_sympatico.ca
>Cc: public@setileague.org
>Subject: Re: SETI public: New Active SETI experiment
>
>
>
>
>David Woolley a écrit :
>
> > > http://www3.sympatico.ca/stephane_dumas/CETI/
> >
> > I've not looked at it from an ETI's point of view much yet, but from
> > a contemporary earthling's point of view the use of JPEG was a bad
> choice -
> > you only got away with it because you oversampled by a factor of 8, which
> > meant the Discrete Cosine Transform (the bit that gives the real
> compression
> > in JPEG) saw only all white or all black, so could do a lossless coding.
> > I would have used GIF or PNG and not oversampled (embed it in an image
> > element and let the browser scale it, saving transmission time and
> > avoiding a file that causes memory distress if you try to manipulate it).
> > The GIF is 20947 bytes and the PNG is 14809 bytes. The JPEG at this
> > scale was about 111K, showing how bad JPEG is for this sort of data.
> > (A PNG of the oversampled image is about 32K.)
>
>Actually the transmission itself will be in is plain FSK (FM bit shift).
>
>
>
>
>
> > As regards the ETI's view of the transmission, I wonder what you are
> > doing to ensure strong spectral components at the nominal carrier,
> > so they know that there is something to look at, and at the baud rate,
> > or half the baud rate, to improve their chances of recovering the
> > message.
>
>We have done an FFT on the message and there is plainty of energy in
>the carrier. Actully since most of the page is white, we strongly underused
>the bandwidth.
>
>
>
> > I also wondered whether it was a good idea to use variable width symbols,
> > with variable starting offsets, as knowing where to look for symbols ought
> > to improve the signal to noise ratio.
>
>This is only true if you have periodic symbol. In practice, whatever is the
>symbol
>the difficulty is the same. The smaller symbol we used for the number have a
>very
>similar error resistance level as the larger characters. I dont have the
>exact
>number
>in mind however.
>
>
>
> > The subscripts and superscripts are going to confuse an ETI that is
> > relying on all symbols being on a line. Consider, that at the limits of
> > usable signal to noise ratio, you will want to do an analogue match on the
> > complete symbol before trying to decide which symbol it is, so you may not
> > be able to see individual pixels.
>
>I dont think this is much an issue. Pickuing up symbol is essentially a
>matter
>of picking
>the black rectangle whatever is shape. Anyway, only with the correct shape
>you
>will
>achieved a good detection rate.
>
>The nice thing with our new computer program is we can change the set of
>character
>at will since it act like a font set. So we could out other restriction on
>the
>character design
>like noise resistance under resolution degradation. As I said before, I fully
>open to comment.
>
>Yvan
>
>Yvan
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