[Discuss-gnuradio] Re: gr-noaa HRPT results

[Markus Kern]

That's awesome work you guys are doing Johnathan! Hopefully I will
soon have the time to build the neccessary hardware and give gr-noaa a
try.

Please keep up your great work!

All the best,
Markus


On 09.02.2010, 17:19 Johnathan Corgan <address@hidden> wrote:


> On Tue, 2010-02-09 at 16:15 +0100, Markus Kern wrote:

>> That looks impressive! Is there any documentation on the RF front end
>> you used? Can the DBSRX be used directly with a suitable antenna?

> Dave Hartzell, Mark Foster, and I have three different antennas that
> we've been experimenting with.  We're trying to see how well captures
> can be done without using a tracking mount, so they have been done by
> pointing each antenna at the nadir of a pass, and allowing the satellite
> to fly through the beam. All three use the same receive chain.

> The first antenna is a 10 ft. TVRO dish with a septum feed designed by
> Mark for amateur radio moon-bounce at 1296 MHz.  It still works at 1700
> MHz, though we suspect the polarization is compromised.

> The second antenna is a custom helical for 1700 MHz, also designed by
> Mark.  It has 15 turns with a circular one-lambda ground plane:

> http://corganenterprises.com/hrpt/data/helical-tripod.jpg

> (You can see part of the TVRO dish in the background).

> The third antenna is another custom helical, an improved version of the
> first.  It has 30 turns and uses not only a ground plane but adds a
> conical "reflector" on the outside of the ground plane that extends
> another lambda out at about 30 degrees.  I don't have a picture of this
> one yet.

> Post antenna, we are using a 1691/137 MHz downconverter/LNA with a 1 dB
> noise figure and 40 dB of gain, through coax into a TVRX board in a
> USRP1.  We've tried using a DBSRX directly without the downconverter,
> but ran into lots of intermodulation problems with other transmitters at
> the site.  A suitable bandpass filter would likely solve this.  Using
> the downconverter lets us have a longer run of coax to the receiver,
> however.

> The TVRO dish has a calculated gain of 29.5 dB, resulting in an overall
> G/T of the system of 9.6 dB/K.  This give us about 13 dB of link margin
> at 5 degrees elevation, and about 23 dB of margin at 70 degrees.  (The
> link margin calculations are missing quite a few items, so subtract 5-8
> dB or so from these numbers to be realistic.)  When pointed at nadir, we
> get zero bit error captures.  The very narrow beam width limits the time
> the spacecraft is in the beam to about 5-15 seconds of pass time,
> resulting in about 30-100 scan lines of telemetry.

> Our "best" image with this antenna, as processed through the HRPTreader
> rendering program:

> http://corganenterprises.com/hrpt/data/NOAA17-2009-269-66132978-fc.jpg

> The smaller helical has a calculated gain of 15 dB, or equivalent to
> about a 60 cm dish.  Using it we've successfully captured passes down to
> about 60 degrees max elevation.  The link budget shows about 8 dB in
> this scenario (at nadir), or a G/T of -5 dB/K.  The helical pattern beam
> width is wide enough to capture about 400-500 scan lines (about 1-1.5
> minutes of pass) with fairly decent SNR.  I'm quite impressed with the
> performance.

> Channel #1 (visible) of a representative capture from this antenna:

> http://corganenterprises.com/hrpt/data/NOAA18-2009-319-79054527-chan1.png

> The second helical is just now undergoing testing.  The calculated gain
> is about 21 dB, with a G/T at nadir of 1.5 dB/K.  I've gotten one
> capture of exceptional quality but only about 200-300 scan lines while
> the satellite passes through the narrower beam:

> http://corganenterprises.com/hrpt/data/NOAA18-2009-339-78435317-fc.jpg

> AVHRR is about 1.1 km per scan line, so these cheap (25 USD)
> non-tracking solutions give 200-500 km of imagery in the vertical
> dimension, which is still quite useful.

> The second helical and feed would likely close the link around 10
> degrees if we had a tracking mount, and would then get almost an entire
> pass of imagery.  It only weighs a couple pounds at most with
> LNB attached, so it should be possible to use an astronomy tracker, a
> security camera pan/tilt mount, or even a homebrew servo design.

> Martin Blaho in Europe has been testing gr-noaa with a full
> dish/tracking setup.  Here is a good example:

> http://martan.blaho.sweb.cz/HRPT-IMG/2010-01-12-1132-n18-rgb.with%
> 20cities.jpg

> With the weather and other priorities, we've haven't resumed work yet
> this year, but our next challenge is to add GOES whole earth image
> decoding to the software.

> Johnathan Corgan
> Corgan Enterprises LLC