iPhone/iPad/iPod app to decode GMDSS transmissions
Earlier, I wrote about the RTL2832U based USB TV tuner dongles that can be turned in an inexpensive Software Defined Radio (SDR). Please take a moment to read that for an overview of these insanely great (for the price) modules, if they’re new to you. I’ve since mounted the dongle in a small metal enclosure:
There were two reasons for this, first to reduce noise pickup, the second was to easily add an F style antenna connector.
Next, I wanted to try getting the rtl-sdr series of command line programs to run. I had tried a set of pre built binaries, but they didn’t work, so I decided to build it myself.
First I got the code from http://cgit.osmocom.org/cgit/rtl-sdr/
I followed the instructions from http://sdr.osmocom.org/trac/wiki/rtl-sdr
sudo make install
The first problem was after ./configure, namely:
configure: error: Package requirements (libusb-1.0 >= 1.0) were not met:
Turns out I had an ancient version of libusb.
sudo port install libusb
With the programs built, the next step was running rtl_test:
$ rtl_test -t
Found 1 device(s):
0: ezcap USB 2.0 DVB-T/DAB/FM dongle
Using device 0: ezcap USB 2.0 DVB-T/DAB/FM dongle
Found Rafael Micro R820T tuner
Supported gain values (29): 0.0 0.9 1.4 2.7 3.7 7.7 8.7 12.5 14.4 15.7 16.6 19.7 20.7 22.9 25.4 28.0 29.7 32.8 33.8 36.4 37.2 38.6 40.2 42.1 43.4 43.9 44.5 48.0 49.6
No E4000 tuner found, aborting.
So far so good.
Next I tried running rtl_fm, which lets you demodulate a FM signal. AM is supposedly also supported. I say supposedly because I could not get rtl_fm to work properly. It would run, and write demodulated sound data to a file, but playing it back always produced gibberish. Also, the files were way too large for the specified sample rate and length of time the program was running. The documentation for rtl_fm is sketchy, even by open sores standards. For example, the list of options includes:
[-s sample_rate (default: 24k)]
which naturally makes you suspect -s sets the sample rate. It does no such thing, it actually sets the IF bandwidth. Again, supposedly.
After several hours of trying to get rtl_fm to work properly, I threw in the towel, and moved on to rtl_tcp, which acts as a little TCP server, sending I/Q data to a connected client. I had much better luck here. Running the program produced the following:
Found 1 device(s).
Found Rafael Micro R820T tuner
Using ezcap USB 2.0 DVB-T/DAB/FM dongle
Tuned to 100000000 Hz.
Use the device argument 'rtl_tcp=127.0.0.1:1234' in OsmoSDR (gr-osmosdr) source
to receive samples in GRC and control rtl_tcp parameters (frequency, gain, ...).
I then connected to it via telnet in another console window:
$ telnet 127.0.0 1234
And the rtl_tcp server program responded with:
and proceeded to send I/Q data to my telnet session, which spewed it to the window. Mission accomplished.
Next I wrote a small program to open a connection to the rtl_tcp server, and grab all the received data, count the number of bytes per second, and display it once per second, as a quick and dirty test to see if everything was working OK. I got around 4M bytes per second, which is correctly for a 2 MHz sample rate (the data is 8 bit I/Q, so there are two bytes per sample).
Having accomplished this, the next step was to make some use of the data. I thought trying to decode and display ADS-B aircraft transponder messages on 1090 MHz would be fun. That is my next post.