I have always been intrigued by aircraft and have, since a very early age, had a passion for them. I am interested in all types of aircraft but my favourite type are military aircraft (fighters, bombers, helicopters, trainers - WW1, WW2, etc. If it flies, I’m interested. With the advent of cheap (very cheap) SDR receivers, there has been a rapid growth in the hobby of aircraft tracking. Using a simple, and pretty cheap setup you can monitor what is flying about in the skies above you and out to a couple of hundred miles or so. About a year ago I started playing about with this, and a few months back decided to become a full time reporting station (a ‘virtual radar’ station for want of a better description). A great number of aircraft are fitted with beacon transmitters that report their location and height/speed etc. every few seconds. These beacons are used by Air Traffic Controllers (ATC) to plot the movements of aircraft. Software has been developed that allows the casual user to also decode these beacons and plot the progress of aircraft on a map. It is expected the the vast majority of aircraft will be fitted with these beacons over the next few years. Not only can plot the journeys of Passenger jets, cargo planes, but you can also see private aircraft too, such as the small, single propeller driven planes that frequent our skies during the daytime. You can also plot helicopters and, on occasion, you may find the odd military aircraft show up. You may not get a position shown though as, for obvious reasons, the military are not too keen to announce their position for the world to see!
On-board many aircraft are units called ADS-B transponders. ADS-B is the acronym for Automatic Dependent Surveillance Broadcast, a system which lets pilots see the same view as the ground based contropllers. The pilots can see other aircraft in the sky, pinpoint hazards (such as weather and terrain) and more. Because ground stations are easy, and cheap to construct and deploy, ADS-B can cover places where old radar systems could not be built. Using satellite systems with ADS-B has improved safety, and saved money on fuel for operators as accuracy as vastly improved, plus more direct A-B routes can now be planned that would otherwise be out of standard radar coverage.
For us interested enthusiasts, an ADS-B system capable of tracking aircraft out to a range in excess of 200 miles is reasonably simple and cheap to do. My ADS-B system is running 24/7 feeding a site that correlates data from all like minded peoples receivers and displays that for us to see, it is also available to the general public so they can track their holiday flights etc.
To set up an ADS-B receiving system, you need the following: Some form of processing the received data (a PC/Mac or one of the small component type computers, such as Raspberry Pi (RPi) or Arduino. These are surprisingly cheap and surprisingly powerful little computers. My system runs an RPi 3B with the O/S etc on a micro SD card. Next you need something to receive ADS-B signals. ADS-B is transmitted on 1090MHz (or 1.09GHz, if you prefer) so a receiver capable of covering that frequency, with a wide enough IF bandwidth to allow the ADS-B signals through. There are many options, dependent on how serious you are, and your budget. Bargain basement end receiver costs about £10 (these are RTL based TV dongles that have a modified driver to allow the chips inside to function as an 8bit SDR). There are a few version of the RTL Dongle, as they are known, each with better/different features. Ideally you need one that has an SMA antenna socket (lower loss than the MCX types usually fitted), a high stability oscillator and some form of static protection. The RTL dongles top out at about £30, for the latest one. Next up would be the Airspy mini at around £80, which gives great performance, and is a superb VHF+ SDR receiver in its own right. Its big brother, the Airspy R2 offers similar performance but has added features, like a 10MHz clock input for frequency accuracy, Bias-T to power preamps, etc, through the feeder. The Airpsy R2 costs about £160. Then you start getting into the semi professional systems that cost from about £350 - £thousands. Names to look out for are: Beast; Puck; RadarScape; SBS1 and SBS3. AirNav radar boxes are often seen, but do not have a good reputation. I cannot comment as I have never used one, but the general consensus is that there are better options.
This is a link to one of the cheaper RTL dongles (£10), ideal to test the water to see if you want to get more serious about ADS-B or SDR in general.
Finally, you will need an antenna of some kind. Without an antenna you will have a receiver that picks up pretty much nothing. Antennas, like the receivers come in all shapes, sizes and prices. Because ADS-B is at such a high frequency, even antennas with a decent amount of gain are fairly small. The most important thing about an ADS-b antenna is to get it as high and in the clear as possible. Ideally the antenna would ‘see’ a clear path to the horizon for a full 360 degrees. In practise, few can manage that. Just make sure whatever antenna you choose is as high as you can (safely) get it. Also, the antenna needs to be fed with good quality, low loss coaxial feeder. Keep the coax run as short as possible as losses tend to be rather high at 1GHz, even with low loss cable.
Satellite quality cable should be the minimum to aim for. The feeder I use is called W-103, manufactured by Westlake, and has a loss of about 1.3dB per 10m at ADS-B frequencies. I have used about 4m of it, to keep the losses to a minimum. Don’t forget that the connectors, adapters and joints all add to the overall loss. Good quality connectors are a must and use as few as possible. Even a 3dB loss equates to losing half the signal! It’s not quite as dramatic as it sounds, but losses soon mount up and will reduce the number, and range of aircraft you are able to track. There is a wealth of information on the web, just remember to check multiple sources before committing to something as not everything you may read is true - and that goes for ADS-B too. My antenna is a pretty simple design, that gives a little gain over a half wave dipole, and rather more over a ‘J-pole’ design. The antenna is called a “2 element co-linear” and was purchased from an East European supplier via eBay for about £22. Below are images of the antenna - it is about 6.5 - 7m agl, supported on a fibreglass pole (an old fishing pole, minus a couple of end sections to give it more strength and less flex). The ADS-B receiver is mounted in the loft/attic and is a “FlightAware Prostick”, which is an RTL SDR dongle modified for improved ADS-B reception (it has an on-board pre-amplifier, sma antenna connector, temperature compensated crystal oscillator (TXCO) and other improvements. To further aid reception, I have an inline bandpass filter (BPF) to attenuate out of band signals that may interfere with the receiver and blocking weak ADS-B signals.