Observing the Busy Sky

Last month I showed this photo of the aircraft con trails seen early one morning:



That got me to thinking about where they were all going. There are various sites on the net where aircraft can be tracked in real time but me being me I wanted to be able to do the tracking myself. So, after a bit of research, I built up a list of what was needed:

A radio to receive the data transmitted by each aircraft.
An aerial to pick up the signals
Software to decode the signals and plot the aircraft tracks on screen.

Looking round, one of the cheapest receivers is a dongle which is designed to receive TV and radio broadcasts but which is able to be tuned from the computer. This is the one I ended up with. The Newsky DVD-T Stick RTL2832U / R820T SDR package:



It is small and plugs in a USB port. The main thing is to get one which is guaranteed to tune to the correct frequency for the aircraft data signals. 1090.000 MHz. I chose a supplier in the Channel Isles which lists on eBay and supplies it with a disc of programs and clear instructions to get started. An excellent buy for £16 including p&p.

Once it had arrived (in 24hrs from ordering) I set things up and then tried several aerials to see which worked best. First was a quick trial with one of my ham radio aerials which was already in place. Picked up signals and proved the idea would work but not very well. Next was an old discone:

Better but still a compromise so not good enough to pick up aircraft more than about 30 miles away.

Next was to build one using a length of copper earth wire stripped from some mains cable as detailed by G7RQG:

This one being tuned to the right frequency worked much better and I could receive aircraft to about 70 miles distance.

Then I spotted another design, a collinear made from lengths of satellite coax:

This is my final choice, for now. With that I can pick up aircraft up to 130 miles away in some directions.

For the software there is a special Windows driver to access and tune the receiver.
To grab the signals and interpret them I use the new free Beta version of RTL1090


To plot the aircraft on a map I ended up using the free program adsbScope:


The jagged shape drawn in red shows the maximum distance I have received aircraft. My position is dead centre on the map. How far in each direction depends on many factors. Buildings, large leafy trees, local hills and the altitude of aircraft can all limit the line of sight needed to pick up a clear signal.

A short video of the program working:



Video can be seen in 720p HD full screen. Solid lines are used while the aircraft is in reception range and dotted lines used once out of range to show the predicted route based on the last received information. I have the program set to erase aircraft when they haven't been received for more than 5 minutes. Aircraft can be clicked on to display information, type of aircraft, altitude, rate of ascent / descent. Also there is a split screen mode which shows that information next to the map.