As long as I can remember, I always been impressed to see an airplane take off. The biggest they are, the most impressive it is. I am not of those plane spotter whose parks beside the airport and pictures every plane coming or going. But I like to watch when I am around.
I live near the town airport, almost directly under the path planes uses to land and take off. It is not an international one, but it can be busy with medevac, regional, cargo & charter flights. So about 5 years ago, while waiting to board an aircraft to get to a remote job site, I discovered that some major flight tracking website were lending ADS-B receivers to whoever might wish to host them in locations where coverage is limited. In exchange to host this receiver, the host gets a free top membership of the website.
So while waiting, I reviewed the requirements. A 24/7 internet connection, proper location to put the antenna outside and with 360 degrees clear sky view and pictures taken from where the antenna would be installed. I knew my house could become a good location for one of those receiver. I took a look at the site and saw that there was no coverage over my region. So I wrote a line into my to do list to apply when I would be back home.
ADS-B, what is it ?
ADS-B stands for Automatic dependent surveillance – broadcast. To make it simple to understand, ADS-B is a protocol that sends data of the aircraft like GPS position, altitude, bearing, speed and more. It is «automatic» because the aircraft sends the signal without any intervention of the pilots. It is «dependent» because it requires the data from the aircraft other systems to get the right data. It «broadcast» because the signal is not intended only to ATC (Air Traffic Control).
Right now ADS-B is mainly used to send flight data from the aircraft to the ground ATC station. But more and more equipment are becoming available to aircraft owners to receive ADS-B signal while in flight. Using either portable receiver or compatible avionics, it is possible to see other aircraft in the sky and be safer while flying.
Is it even legal to receive ADS-B?
It all depends of the use one will make of the data. But because the signal is broadcasted and not encrypted, it can be received legally just like radio scanning where you can listen to various communications like emergency services. As long as the data are not used for wrong purpose, you can catch it.
So, back to 5 years ago… When I came back home, I went on the roof to take pictures from where I wanted to install the antenna. 35 foot above ground, 360 degree views of the sky, nothing to block the signal. Also took the GPS with me to get precise coordinates and elevation. Once back on the ground, I went in to fill the application form. I submitted it with the pictures and the coordinates. I expected a long wait before getting a reply, but less than 48 hours later, I got the reply. I was approved! Now I had to wait to receive the box and to install the receiver.
Powering it up
So few minutes after the box came in, I was already installing it. When the setup has been complete, I opened up my browser, pointed it to the website for whom I am hosting this receiver and then I powered the receiver up. It took few minutes to start seeing aircraft in the sky around the house.
After few days, I wanted to find out what my coverage was. A quick internet search brought me to download ADSBscope. I used this piece of software to plot the area I was receiving signal and see how far I was able to follow aircraft. To my surprise, it was over 225nm (~259miles / ~418km) in some area. This is a huge area compared to many other receivers. See the screenshot below for my coverage.
Until this year, I never really had something to do. However every year I go on the roof and check overall state of the antenna housing, bracket, coaxial cable…
This year during winter, my maximum coverage started to get lower and lower every month. After contacting the owner of the receiver, we concluded that the cable and the antenna are to be changed. So I went on the roof once I got replacement parts to change them. Everything got back to normal. Meanwhile, an idea started to grow…
What if I could get a better receiver?
While waiting for the spare parts, I thought that I might be able to assemble my own receiver and try to beat the coverage of the one I already host at home. So I ordered a Raspberry Pi, an SDR dongle, an antenna cable and an antenna.
When the order arrived, I had already found few website accepting raspberry pi feeder data including the website I was already hosting their feeder. But there was one that stand out, ADSB Exchange or ADSBx. Why? First there is no filtering, so all data are displayed. Second, this is a sharing site, so you can also retrieve data from other feeds as long as you share or you can also pay for those data. Third, when you setup their MLAT client, you get the results returned to you when your receiver help establish an MLAT position calculation.
Right now, while writing those lines, I get a great coverage from my raspberry pi receiver having the antenna in a window. In the next days, I will install this antenna beside the other one on the roof. I will then tweak receiver setting to get better coverage than the receiver I am hosting. Honestly, I think it will not be hard, see the screen shot below.
The light grey area is the professional receiver I host with the antenna on the roof of the house. The darker area is my raspberry pi with the antenna facing a south window. There is no clear view on the 3 other sides of the house. So the signal pass through the house. Once on the roof it should compare to the other receiver or offer better coverage.
Another challenge will be to bring 2 or 3 other receiver in the area to offer an MLAT coverage as a lot of aircraft here fly in mode S only.
Range rings are 25nm apart. Spikes are the results of meteorological reflections a phenomena well known from HAM (Amateur Radio).
I also have plan to make a mobile unit that will allows data collection out of home.
MLAT & Mode S
Some aircraft do not send ADS-B signal per choice or because their avionics is not up-to-date. So using Mode S signal, when 3 or more receivers receive the same signal and combine their data, it is possible to calculate approximate location of an aircraft using the Time Difference of Arrival or TDoA. We call it MLAT or multilateration. ADSBx host MLAT server that combine data from all feeders that share Mode S data and make calculation when possible. The results are then returned to the feeders that helped to make this calculation possible. It then allows the owners of those receivers to see one more plane on their scope. MLAT calculation precision depend on how many feeders are receiving the signal, the distance between them and the pattern of the receivers’ location on the ground.
For more details about MLAT, see Wikipedia article about multilateration.
What is the point to tracking aircraft?
At first it was useful to get information about the flights I was taking to get to work. My girlfriend was also able to monitor my landing time in a more precise way to come get me at the airport.
Second, I like to learn, so it helps me learn a ton of stuff about radio communications, frequencies, antennas, receiver hardware and more. This is one of the main reason that I look forward to pass my HAM radio license during winter.
To the top!
There are plane spotter that does it above everyone else. Using a short calculation between, the position of the aircraft, the altitude and the same for a camera, some spotters are able follow the plane while in-flight using a pan-and-tilt camera (see this youtube video). Some also broadcast the radio chatters over internet using a scanner… There is many other things possible into the world of plane spotting, including taking pictures and display them on specialized website.
My actual setup
With time, things evolve. First evolution I made is on the software. I stopped to use ADSBscope, at least as my main software. I now use Virtual Radar Server. It offers to display multiple receiver using one single web-based interface. So even away from home, I do not need complicated setup to access my receivers data. It also gives the advantage to check my 2 receivers and ADSBx feed with just 3 clicks.
For the raspberry pi, I flash the SD card with the latest raspbian distribution then I setup the whole software side using ADSB Receiver. It allows for a fast an easy setup of the receiver. It contains the setup process to create your own display portal, share your data with various website and install some accessory software to make special setup.
On the hardware side, I’ve started with what is considered as a professional receiver. Plug-and-play, ready to use out of the box. I still run it, it will be used as my reference for my raspberry pi.
For the raspberry pi, I bought 3 SDR dongle. For production I went with the FlightAware Pro Plus because of its integrated filter and amplifier, it gives the best coverage I’ve been able to get so far. It does get few more NM, not a big improvement, but whatever I can get, I will try to get it !
I also have an RTL-SDR.com V3 and a NooElec NESDR Mini 2. Both gives similar coverage. RTL-SDR V3 have one great advantage, it is the integrated software activated bias tee that allow to power an active antenna or an amplifier. So I actually use both dongles to search the radio spectrum around me and experiment.
l would like to host a receiver…
If you have the budget, make your own one. A raspberry pi, a SMA-male to N-Male cable, a FlightAware pro plus dongle and a FlightAware antenna. It will cost you below 200$cad on Amazon and you will be ready to go. It have the advantage to allows you to do whatever you wish with the receiver and the data. This way you can share with ADSBx, FlightAware, FlightRadar24, RadarBox24 and whatever other services that allows external feeders. You can also experiment as you wish. You can make your own antenna, try new software, change hardware…
If you don’t have the budget, most of the service named above accept application to host receiver in area where they have low to no coverage. If you wish to share the data with exchange websites to get data from outside your reception area, be sure to check the lending condition. Only a minority of those services allow the sharing of those data.
If are interested into joining or you are curious, please visit ADSB Exchange and register to the forum to find out a great community to help you!
Click here to continue your reading with my next article where I goes in details with my plane spotting gear.