Some recorded I/Q traces using RTL-SDR and handlend antenna

[Alexei Colin]

Hi,

Here are some recorded traces of I/Q data via GNU Radio from RTL-SDR
with a a handheld 70cm Yaggi antenna, from East Coast USA during
predicted passes over last couple of days:
https://drive.google.com/open?id=13D7Xr2mRjBVIU4lmiTu9jkmyQY5tp7KL

The RTL-SDR is set to 437240 Hz and 256 kHz sampling rate.

Note that the correlator block in the decoder expects 64kHz sample
rate input, so the 256kHz recordings are downsampled by 4. (The 64kHz
is the data rate of the modulator in the CC430.)

There's that constant noise at 0 Hz from the SDR -- perhaps I should set
the SDR freq to the side and increase the sampling rate, and then
translate the 64kHz section around 437240 Hz. Maybe for next trace.


The spread-spectrum decoder is the one mildly forked from Zac's repo:
https://github.com/CMUAbstract/groundstation

To install on Arch Linux: cd archpkg && makepkg -si

On other systems, to install the gr-sprite module in a local folder:

$ cd gr-sprite
$ mkdir -p build/pkg && cd build
$ cmake .. -DCMAKE_INSTALL_PREFIX=$PWD/pkg && make && make install

Add path to ~/.gnuradio/config.conf:

[grc]
local_blocks_path=/path/to/gr-sprite/build/pkg/share/gnuradio/grc

Then run the decoder:

$ export LD_LIBRARY_PATH=$PWD/pkg/lib
$ export PYTHONPATH=$PWD/pkg/lib/python2.7/site-packages
$ ../SpriteDecoder.py --prnid0=X --prnid1=Y --infile data.rec

where X and Y are the PRNs assigned to your board, here's a list, not
sure how recent it is:
https://drive.google.com/open?id=1Egxp9LXg7zlFdr4CtmWQ2ThFdA2KfHuLuVnk3-zEEPk

Hex bytes should appear, but most will be decoded noise.

So far, I've seen a lot of noise bytes. One byte happens to matche our
board's beacon and another two bytes a valid checksum, but it's likely
to all be coincidental noise.

As far as I understand, I don't expect to see anything on a waterfall
plot because the SNR is very low, signal is likely below noise floor, so
you have to run this spread-spectrum decoder to check for signal.

-alexei

[Andy Thomas]

Thanks for this, the monitoring will be crucial in the next few days if we do not know when the Sprites are deployed. The last time deployment was by timer.

Could Zac please tell us if this list of Sprites is correct - are they all on board? Or if not, what is on board?

andy g0sfj