Get pilots talking about ADS-B and they start griping about money. Forget the soon-to-be required ADS-B out systems; just getting free ADS-B weather and traffic costs between $500 and $1100. If you want it to work with ForeFlight Mobile, the most popular aviation app by far, you are limited to only one brand of receiver—Appareo’s Stratus.
Portable ADS-B In isn’t that complicated. It requires a 978 MHz receiver for weather and some traffic, a simple computer that converts those signals to a common protocol and transmits the results via Wi-Fi. You can pump up system performance with a second 1090 MHz traffic receiver, GPS position and AHRS.
So if you had the right shopping list, the software to put on the computer and a little technical savvy, you should be able make your own. And that’s exactly what the Stratux is.
We built one and discovered: Yes, you can build a working portable ADS-B receiver with parts ordered off Amazon. No, it’s not “just as good as an Appareo Stratus 2S.” And, yes, it will work with ForeFlight—at least for now.
We Can Do This
Chris Young is a programmer and pilot who loved the idea of ADS-B on his iPad, but didn’t feel the value justified the price. He’d heard other efforts to use a software-defined radio (SDR) to receive ADS-B didn’t work, but decided to try anyway. SDRs look like USB flash keys with an antenna and can receive a range of frequencies. They cost about $30.
Chris plugged one into a Raspberry Pi, which is a project computer about the size of a deck of cards. In a couple of days, he had ADS-B weather on his iPad, so he published the results to see if anyone could benefit. So many pilots expressed interest that Young created a sub-reddit that’s now the home of Stratux (https://www.reddit.com/r/stratux).
Users now have Stratux homebuilds flying in every configuration and regularly try out new equipment. They also support each other, recommend hardware choices and discuss additions to the software. Several have created 3D printed cases to stuff all the parts into so a Stratux is as convenient as a commercial unit. For a more utilitarian appearance, others just stick with rubber bands and duct tape.
Nuts, Bolts and Solder
The Reddit discussion includes instructions and a shopping list for building your Stratux. That list is already outdated, so read up before you buy. It also depends on how much work you’re planning on.
For example, if you just want to plug it all together and go, the $60 Raspberry Pi B+ Kit with a case and Wi-Fi included plus the newer Nano-SDRs ($40 each) are compact and easy. You also might want compact antennas ($10 for both) instead of the stock ones and their long cables.
But if you’re planning on building everything into a custom box, you could look at the new Pi Zero for a computer for only $5 and the $30 SDRs that you’ll remove from their stock cases anyway. The Pi Zero is new and we don’t know of a Stratux using it yet, but Chris Young says it should work fine.
The RY835AI GPS listed is a bare chip. We stuck it to the top of the Pi with double-sided tape, but you’ll want it inside some plastic case if you have concerns about damage. Connecting the GPS via USB is easy, but currently doesn’t support the AHRS. You’ll have to solder on jumper cables for that. You can wire all RY835AI connections (shown in the photo on page 20). If so, we recommend 20AWG jumpers for power, as the common 24AWG Pi jumpers barely flow enough juice. There’s some indication the RY835AI locks quicker and more reliably via USB, but we couldn’t show that definitively.
The Pi uses a USB power cable, which you can plug into a cigarette lighter converter or use a USB battery pack. Current flow from the battery packs can be an issue if you use two SDRs. Avoiding a USB hub helps. So does using a battery pack with two USB ports and a power cable that draws from two USB sources to the single power input on the Pi.
Getting Stratux onto the Pi requires downloading the image and putting it onto the mini-SD card the Pi uses as a “hard drive.” The Stratux software is self-launching and the Pi creates a Wi-Fi network called Stratux. Connect your iPad and you’re done. The aviation apps do the rest on their own. There’s also a web page you can access with your iPad to monitor the Stratux software directly. If you want to tweak the system and can type some Linux commands, there’s no stopping what you can do.
One oddity is the only shutdown is via the web interface (and only in the beta code as of this writing). You can just pull the power, but that has the potential to corrupt the SD card (many users report no issues).
The UPS system we found incorporates a shutdown button if you can do a little programing, and Reddit users have created custom systems. There’s also a way to protect the SD card to make it read-only if you know how to remote into the Pi and type a set of commands.
Enough Bang for Less Buck?
How well does it work? The answer is a big, “That depends.” The essential features are weather and traffic. Stratux’s SDRs and stock antennas simply aren’t as good as a dedicated system, and head-to-head with a Stratus 2, that’s exactly what we saw. The Stratux acquires ground stations at least as fast, or faster, and shows as many ground stations, or more.
That’s only a real issue if the Stratux shows zero towers, so this could be a deal-breaker if you’re in an area with poor ADS-B coverage or you fly something highly shielded, like many jets. It’s a non-issue if most of your flying happens in areas of good coverage. In addition, both systems report towers before they acquire actual METARs or NEXRAD frames. So long as both had towers, it was a toss-up which system showed fresh data first. Note that the newer Stratus 1S and 2S are supposedly more sensitive, so the difference could be more pronounced.
On the flip side, the Stratux lets you mount an antenna remotely if you want (something shops have been doing with other portable receivers), and several users have designed custom antennas. Antenna placement appears to be the biggest variable. Overall, Stratus wins on ADS-B reception with the units side-by-side—no matter what.
The Stratux was a bit better about receiving 1090 direct traffic. This is more important if you don’t have ADS-B Out on your aircraft. However, the Stratux showed some traffic up to 500 feet lower than the Stratus, or even the installed active TAS system. That’s presumably a bug that will be squashed, but it’s indicative of issues you may encounter with software that’s still in development.
The Stratus GPS locks faster that the GPS we used on our Stratux. The Stratux occasionally dropped GPS position for few minutes on one test flight. This code is still in development, so another point to Stratus, but that will likely be a wash soon.
The RY835AI chip we used in the Stratux also includes an AHRS, barometric sensor and magnetometer. However, it requires soldering at least two wires RY835AI to get that data. The AHRS data is strictly developmental and lacks filtering and damping required for a smooth, steady display. Heads-up view in ForeFlight and WingX works in the Stratux, so you can use the synthetic vision, but it’s rough. Clear point to Stratus, for now.
Battery life depends on what battery you choose, but we found it pretty easy to top Stratus’ claim of eight hours with the bigger battery. Young says the code is getting smarter about power use. A Stratus lets you run on ship’s power via USB and automatically switch over to batteries without interruption. We found a tiny backup power supply (UPS) for the Raspberry Pi, however, so a Stratux can do this, too. We’ll call power a tie.
When it comes to app compatibility, Stratux wins hands down. It works with ForeFlight, WingX, FlyQ and many others. Like ForeFlight or not, they own the lion’s share of the market. None of the commercial alternatives to a Stratus work with ForeFlight because the company won’t allow it; they own an interest in Stratus. Stratux bypasses this problem by using a GDL90 protocol ForeFlight does accept. ForeFlight thinks the Stratux is an installed FreeFlight ADS-B system.
ForeFlight could fight back and create a secret hardware handshake so the ForeFlight app knows this ADS-B isn’t coming from the FreeFlight system, rendering Stratux unusable, but hacking hackers is usually a losing battle. Our guess is ForeFlight will neither acknowledge nor deny the Stratux community. The number of subscribers they gain probably outweighs the loss of Stratus sales by an order of magnitude.
Stratux code is constantly being improved at the request of users. Young tells us that big on his improvement list are solid AHRS code, an open protocol for weather data, better power conservation, and even more app compatibility.
Software improvements are a double-edged blade. Sometimes you like them; sometimes you’re spewing language unfit for polite society. Overall, we like the idea of updatable and flexible code, especially where users have real input on development. Point to Stratux.
Like virtually every DIY project, if your sole motivation is cost, you should probably pony up for the off-the-shelf solution and be done with it. But if you like the idea of saving some bucks while tinkering, the Stratux could be for you.
Given some of the Stratux’s limitations and that it’s a community-powered project, we think the best deal is a simple, single-band Stratux. It’s a small investment and yields a great return.
The minimum Stratux would be a Pi Zero for $5, one SDR for $25, a Wi-Fi dongle for $10, a USB power supply and cable for $21. Download the Stratux image and tape the whole thing together.
That’ll get you just ADS-B weather and traffic on 978 (no GPS, AHRS or direct 1090 traffic) for $61, plus an hour of setup.
Even if you go all out and get the better Raspberry Pi B kit ($60), two Nano SDRs (two at $40), custom antennas ($10), GPS/AHRS ($40), a UPS power system for the Pi ($45), jumpers for wiring ($5) and a nice case ($30), that’s still only $220 in parts for an open source ADS-B in system with all the perks.
Compared to $549 for the cheaper Status 1S and $899 for the deluxe 2S model, $61-$220, plus some sweat equity sounds pretty good. Just know it’s not quite as polished an end result. Not yet, anyway. Give the Stratux community time and anything is possible.