free-outernet

Free Software Outernet receiver

This is a Free Software (Open Source) receiver for Outernet designed to work with the gr-outernet GNUradio Outernet receiver modem. The key parts of the official Outernet receiver software, ondd and sdr100 are closed-source, distributed as binaries only. This project aims to provide a Free Software alternative to ondd, while gr-outernet can be used a a substitute for sdr100.

With free-outernet and gr-outernet you can receive the files that are broadcast by Outernet file service and the time packets that are broadcast by the Outernet time service.

All this software is the result of a reverse engineering effort, since Outernet does not publish any documentation for the protocols they use.

You can use free-outernet.py to receive UDP packets in real-time from gr-outernet or with a KISS file recorded previously by gr-outernet. free-outernet.py recovers the files that are transmitted by Outernet and prints the time packets. It also prints some interesting debug info.

Things that are not implemented/supported yet:

• LDPC decoding (see below)
• X.509 signature checking. The file announcements of the Outernet file service are signed with the Outernet X.509 certificate to prevent spoofing. This will not be implemented, as I have no interest in checking the signature. Perhaps some Amateur Radio operators or other people whish to use the Outernet protocols to exchange files, so it does not make sense for me to require that file announcements are signed.
• Some weird Outernet frames which I do not know what they do. They do not seem to do something very important, though.
• Using the time packets to set the system time. I do not know how useful it is for most people. ondd does it, because it is designed to run standanlone without internet conetivity. If you have Internet connectivity it is much better to use NTP. If you do not have Internet connectivity and/or your need precise timing, it is better to use GPS. The Outernet time service only has a resolution of 1 second and it does not seem to account for round trip time to geostationary orbit (around 200ms).
• Automatic decompression of received files. Most (perhaps all) files sent by the Outernet file service are .tar.bz2 files. ondd extracts the files automatically. This is easy to implement, but it is low on my priority list, as it is very easy to do this manually.

Sample KISS files

You can use some sample KISS files for testing.

What is this LDPC decoding thing?

LDPC is the only important thing that ondd does and that is not implemented in free-outernet. It is a Forward Error Correction code designed to permit to recover a complete file even if you have some missing blocks which could not be received. Without LDPC decoding, you need to receive all the blocks of the file or you cannot recover the file.

If you have a good uninterrupted signal, there is no reason why you should fail to receive some of the blocks, so you will have all the file blocks when the file transmission ends and LDPC decoding is not necessary. This has being tested with real world recordings.

However, if your signal is not very good or objects get in the way of your receiver ocassionally, you will have a few blocks missing. ondd is able to use LDPC decoding to recover the whole file, while free-outernet is unable to do so.

Of course, LDPC or some other Forward Error Correction is a very good idea to have on a file broadcast service such as this. What I mean is that it is possible to have a fully functional receiver without LDPC decoding, and this is what free-outernet does.

The problem with LDPC codes is that there are many different LDPC codes (and Outernet uses several different codes depending on file size and so on). Also, implementing an LDPC decoder is not trivial. Unless I manage to find an open-source library that implements the LDPC codes used on Outernet or I get some help from somebody with more experience with LDPC codes, implementing LDPC decoding in free-outernet is low on my priority list.

For now, free-outernet prints some debug information about the LDPC codes to help anyone interested in this get started in trying to reverse engineer the LDPC codes used in Outernet.