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Stratux — An Open Source ADS-B Receiver
By Ryan Dewsbury, EAA 1117224
January 2019 - What is open source? In the simplest terms, it is like sharing a recipe. Once you have figured out how to do something, freely sharing how you did it can help others, and they can also go on to improve what you started.
Stratux is a recipe to build an ADS-B receiver for a lot less than buying a commercially produced unit. Because the software that makes it work is open it can be added to and changed to suit the user’s needs and is not limited to how a company thought it should function.
ADS-B is a topic that every pilot in the United States is aware of, and it is slowly trickling north. The original idea of ADS-B was for aircraft to broadcast their GPS location so that they can be seen by ATC without the need of costly radar installations. By moving the smarts into the aircraft, the ground stations can be much simpler and greater coverage is available. There is a cost to installing compliant hardware so the U.S. government decided to offer an incentive to the system. The ground stations broadcast weather data with no subscription cost. This is the ADS-B In to the transponder’s ADS-B Out. You do not need to be equipped with ADS-B Out to take advantage of ADS-B In weather and traffic data; you just need an appropriate receiver.
In the early days, the receivers were quite expensive and only talked to panel-mounted displays such as IFR GPS navigators. After a few years people began using tablets with electronic flight bag software (EFB) to display maps. Soon, manufacturers began making ADS-B receivers that sent data over Wi-Fi or Bluetooth so that these EFBs could show in-flight weather and traffic. These receivers were still priced pretty high at US$900-plus.
In 2015 a freshly instrument-rated pilot and radio enthusiast named Chris Young, EAA 1184340, decided to see if he could DIY one. It didn’t take very long before he had a working device using a cheap USB TV tuner and a small computer called a Raspberry Pi for less than US$100. He published the software on a repository site called GitHub and shared links to all the hardware required on a website called Reddit. All of the hardware was available for purchase on Amazon. It turns out there was some pent-up demand for a receiver in this price range. Very quickly other people jumped on board and collaborated to improve the software, and in a short time it went from a single-band receiver to dual band with GPS. Tuned antennas and higher quality GPS modules were sourced. A few experimental avionics manufacturers began supporting Stratux input via RS-232 so that the data could be displayed on an EFIS.
My contribution was to make 3D printable cases to hold all of the parts, and I rapidly made changes to the design to accommodate changes in hardware. Eventually people outside of the DIY community began to hear about Stratux and began making and selling ready-to-go models. With the increased demand, 3D printing was just a bit too slow so laser-cut acrylic cases became the norm. Eventually, laser cutting was also too slow, and it was decided to get the cases injection molded. Changes were made to one of my 3D printable designs, and 5,000 of them were ordered. They arrived a few weeks before Oshkosh 2017, and the first 1,000 were sold within a week and the rest shortly after. Around this time, an attitude and heading reference system (AHRS) was added so now Stratux could drive an artificial horizon to use as a backup.
Something that has really helped out is that the parts were sourced from Amazon so fast shipping is the norm. As people came up with better hardware they were able to quickly send it in to Amazon, which handles the distribution and sales. I started to import the parts and sell them via Amazon.ca as many of the parts were unavailable or were three to five times the price you would see them listed for on Amazon.com. I sell kits containing all the parts needed to build a unit as well as ready-to-go units and individual parts.
In a little more than two years, a small community of enthusiasts had created something that had a comparable feature set at less than one-third of the price of the competition. 2018 saw the big players in the market scrambling to add features and reduce the cost in order to compete.
Something else came out of the introduction of this hardware. Many of the commercially produced receivers would only work with associated software. This locked you into using a specific EFB. Stratux uses an open format for data called GDL90; this protocol is an open standard that was written by Garmin for the FAA (ironically Garmin does not use GDL90 — they use an encrypted version called GDL39). Using this format has allowed other smaller software companies to start supporting ADS-B weather and traffic data, and it is basically universal at this point.
That covers the U.S. side of things. Europe has three different formats for ADS-B data, and Stratux has been adapted to support all three. Canada is currently looking to do only 1090 ADS-B Out with no plans to broadcast weather data as the United States does. We have been working to build a DIY ground station that is affordable enough for a local group to install at a small airport. So far we are in the early days of testing hardware, but we hope that we will be able to have an affordable solution in the next year.
You can see what all the components of a Stratux kit look like here.