Transmission Line Calc     
   iPhone app to analyze RF Transmission Lines     

Please be sure to read Part 1 and Part 2, if you’re new to this series of articles.

All aircraft contain a piece of avionics technology called a transponder. This contains a receiver, and a transmitter. When the signal from ground radar is received, the transponder transmits a short burst on 1090 MHz, encoded with information.

There are several possible replies from an aircraft transponder:

• Mode A replies with a target ID code
• Mode B replies with the barometric altitude of the plane
• Mode S, also called the Extended Squitter, is the one we’re interested in.

Mode S, also called ADS-B allows a variety of types of data to be sent from the transponder, including:

• ICAO aircraft code (the tail number of the plane can be obtained from this)
• Flight Number
• Altitude
• Location (Longitude and Latitude)
• Heading

There’s an online document called ADS-B for Dummies that goes through the various messages, and their format.

Since the RTL dongles can receive 1090 MHz at a wide bandwidth, it turns out to be possible to use them as low cost transponder decoders. Very low cost. You can pick them up for around $15 on eBay. Dedicated ADS-B receiver packages are more. Much more. As in hundreds of dollars.

There are quite a few packages out for the RTL dongles that decode ADS-B transmissions. For Windows, there’s ADSB#:

For linux and Mac OS X, there’s Dump1090

I compiled Dump1090 for Mac OS X, here is what the output looks like:

The columns across the screen:

• Hex – the ICAO code for the plane
• Flight – flight number
• Altitude – altitude in feet
• Speed – speed in mph
• Lat – latitude of position
• Lon – longitude of position
• Track – heading in degrees
• Messages – the number of messages from this plane that have been received
• Seen – how long ago (in seconds) since the last message from the plane, that is, how long since it has been last seen (or heard from)

I’ve since ported the Dump1090 code over to Cocoa on Mac OS X, resulting in Cocoa1090:

A beta version of Cocoa1090 can be downloaded here: http://www.blackcatsystems.com/software/cocoa1090.html

   GMDSS     
   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
cd rtl-sdr/
autoreconf -i
./configure
make
sudo make install
sudo ldconfig

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
solved that.

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:
$ ./rtl_tcp
Found 1 device(s).
Found Rafael Micro R820T tuner
Using ezcap USB 2.0 DVB-T/DAB/FM dongle
Tuned to 100000000 Hz.
listening...
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:
client accepted!
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.

   SelCall     
   Generate Selective Calling Tones     

You may have heard of the latest SDR craze to hit the radio hobby – the RTL based USB dongle TV tuners. These were originally made to receive and decode the European standard digital television broadcasts. An enterprising hobbyist discovered that they can be tuned throughout the VHF and UHF range, and that you can get at the raw sampled data from the onboard A/D converter (only 8 bit, however). This allows them to be used as a very inexpensive Software Defined Radio (SDR) for VHF and UHF. How inexpensive? Mine was $17 shipped, although you can find them for even less, if you’re willing to get them direct from China and wait a few weeks for delivery.

Here is what I got:

There’s the dongle itself, as well as the small (about 4″) antenna.

It’s interested to note that the enclosure actually says SDR on it, the word has apparently gotten out about the SDR applications for this dongle, and someone is private branding them.

Here’s what the inside looks like:

The USB connector is on the left, the MCX style RF connector is on the upper right.

There are control programs available for Windows, Mac OS X, and Linux. For software, first I decided to try rtl-sdr I copied the libraries to the specified locations, restarted, and was greeted with:

>:rtlsdr_osx cps$ 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
Failed to open rtlsdr device #0.


It’s possible this is an older version of the rtl-sdr package, that expects the E4000 tuner chip. (Although a less cryptic error message would sure be helpful)

Then I tried Cocoa Radio. It crashes on launch. So far open sores is zero for two.

So next I tried the Mac OS X port of gqrx. Much better! It came right up, and within a minute I was receiving FM broadcast stations. I have noticed that if I make a change to the sample rate, I need to quit and re-start the app before putting it into run mode, or it crashes.

The sensitivity is not bad, I was able to pick up stations about 50 or 60 miles away using the included tiny 4″ antenna, laying on my desk.

Below is a screenshot of gqrx running on the FM band, you can see three FM broadcast stations, at 97.9, 98.5 and 99.1 MHz, the latter is tuned in for demodulation.:

I was also able to pick up 2m packet radio transmissions on 144.39 MHz, and one of the NOAA weather radio stations, on 162.525 MHz.

There are many varieties of these TV tuner dongles out there, mostly the difference is the RF tuner chip used. Previously the E4000 tuner was the preferred one for SDR applications, as it had the widest tuning range, although with a gap in the middle. It apparently is no longer made and is difficult to find tuners that use it. Currently the R820T tuner chip seems to be the preferred one for SDR use, the tuning range is slightly less, but there is no gap. Some eBay vendors identify the chip used, many do not, but there are lists online of the various USB dongles by brand name and model number, with the tuner chip specified, such as here.

My next project was mounting the dongle in a small metal enclosure, with a different RF connector, so I can easily connect one of my existing outdoor antennas to it. Read all about it here.