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 <[email protected]> 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 _______________________________________________ Discuss-gnuradio mailing list [email protected] http://lists.gnu.org/mailman/listinfo/discuss-gnuradio
