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:
(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
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,
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
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
Channel #1 (visible) of a representative capture from this antenna:
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:
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:
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.
Corgan Enterprises LLC