Beginner question about rtlsdr

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Michel Pelletier pelletier.michel at gmail.com
Mon Oct 22 01:22:21 UTC 2012


On Sun, Oct 21, 2012 at 5:50 PM, Jay Salsburg <jsalsburg at bellsouth.net> wrote:
> Hello
>
> May I suggested a few things not related to the "Programming" issues but
> related to the Radio Astronomy subject? Just like retail, about "Location,
> Location, Location," Radio Astronomy's is about the "Antenna, Antenna,
> Antenna." One thing the TV Dongle offers for Radio Astronomers is the ease
> of placing the receiver (Dongle) on the Antenna. The USB must be extended
> from the Computer to the Antenna, instead of bringing the Signal from the
> Antenna to the Dongle. The advantage is lower noise and higher gain (without
> noise). There are many references on the Internet for extending USB, some
> for purchase, some DIY.

Thanks Jay, I'm personally fascinated by the scientific aspects of the
subject so I do appreciate your comment.  I'm not sure if you're
referring to the link I posted in this thread to a picture of my
telescope, but I did as you suggested an embedded the dongle directly
in the cantenna.  My extension is a simple usb cable as my computer is
just across the wall, although for my next antenna on the other side
of my property, I intend to use a small dlink access point device
flashed with openwrt and using usb over IP and power over ethernet to
extend the other data source to my computer.  This will give me a
single interferometric baseline to begin using packages like aipy.  I
have the parts, I'm still piecing together the receiver.  I live in
Western Oregon so weatherproofing is key. :)

FWIW the EOR array uses simple, omnidirectional crossed dipoles and Python:

http://eor.berkeley.edu/

That's a pretty gorgeous picture of Centaurus A for some simple
dipoles!  Here is an excellent presentation on the python technology
behind it:

http://setiathome.berkeley.edu/~aparsons/papers/2008-08-10_LFSW_AIPY_Presentation.pdf

there are some pretty pictures at the end.  All of the software parts
of the interferometry are basically done, what is needed is to vet the
possibility that something like an rtlsdr dongle and a simple antenna,
arrayed across an area, can produce data of sufficient quality to feed
into a package like aipy.  As roger-'s ecellent pyrtlsdr can already
capture dongle complex data directly into numpy arrays, it seems like
all the pieces are laying around, simply needing to be put together,
and I intend to try it.

As an update, thanks to Scott's great help I am currently dumping data
at 1420 Mhz center frequency into numpy arrays and averaging their
squares.  Here's the simple "core" of my loop:

while True:
    yield numpy.average(numpy.absolute(radio.read_samples(2**20)))

I'm accumulating one average per secondish and dumping them into a
data file that I am plotting on a simple graph.  Cassiopeia reaches
maximum at my local position around midnight, and the graph is
currently trending upward.  I'm really hoping that by morning I'll
have a nice curve that peaks at midnight and hopefully not a bunch of
random squiggles. :)

-Michel




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