GNUradio OOT module for Outernet
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GNU Radio OOT module for Outernet

This repository contains a GNU Radio decoder for Outernet

The decoder receives the Outernet frames from the L-band signal. You can use free-outernet to recover files from the frames.

The most useful GRC flowgraph is examples/outernet-rtlsdr.grc. This flowgraph receives the Outernet L-band signal and saves the frame to a KISS file in /tmp/outernet.kiss. It also sends the frames by UDP. This will be used in a future version of free-outernet to do realtime file reception (for now it only works with the KISS file).

The flowgraph is preset for the I-4 F3 signal that is used in the Americas. If you are on another area, please change the variables freq and centre_freq in the flowgraph accordingly.

The flowgraph should start receiving frames automatically. The hex dump of the received frames will be printed in the console. In principle, there is no need for adjustment, but perhaps it is necesary to adjust the fine tuning frequency in the receiver GUI, since the crystal of the RTL-SDR may be a bit off (even if it is a good 0.5ppm TCXO).

The rest of the parameters are already adjusted for good results, but you can try to play with them to improve performance. You can monitor the signal quality by looking at the constellation plot in the BPSK tab. You aim for all the blue dots clustered around the points -1 and 1 (which represent the two symbols of the BPSK signal), with few points near 0 (which means ambiguities due to noise that will possibly result in bit errors). If you get improved results with some combination of parameters other than the stock one, please let me know.

The flowgraph needs gr-kiss.

Sample IQ recording

For testing, you can use a sample IQ recording by Scott K4KDR. You should open it with the flowgraph examples/outernet-recording-example.grc

Note about SWIG

The following problem happens to several people, especially in Ubuntu 16.04. The OOT compiles fine but the following error appears when running the flowgraph:

self.outernet_swap_ff_1 = outernet.swap_ff()
AttributeError: 'module' object has no attribute 'swap_ff'

The problem is that if SWIG is not installed, compilation will complain but it will not stop. This produces an OOT module which does not work. The solution is to install SWIG:

sudo apt install swig swig3.0

Then compile and install again.

This problem is not specific to this OOT module. It will happen with any GNU Radio OOT module which uses C++ blocks (most of them do), as SWIG is needed to access the C++ code from within the Python flowgraph.