How to improve X-Plane and not get arrested

X-Plane supports connecting your favorite real-world EFB app like XAvion, WingXPro, ForeFlight and FlyQ. This way, you can use your iPad or tablet just like you would in a real cockpit – display enroute charts, approach plates, with optional weather and traffic overlays.

This worked extremely well with the XAvion app, a Laminar in-house product, but for 12.3 I wanted to improve support for other popular apps.

Foreflight showed me where I was, and also some traffic diamonds around me, but synthetic vision didn’t work, relative motion and tail numbers on the traffic didn’t work, there was no METAR and the NEXRAD didn’t show either.

Getting synthetic vision to work required sending the AHRS data in a way modern EFBs understand. Austin had coded this in 2013 when the ADS-B receiver of choice was a little plastic box called Clarity, that when you pushed the on-off button too hard just collapsed on itself. We’ve had several that lost their on-off button after you turned it off a bit too vigorously.

This was before the Appareo Stratus, uAvionics Sentry and other popular devices existed.

It’s been 12 years since then and X-Plane still pretended to be this falling apart, overheating plastic box… And Foreflight would flat out reject anything that X-Plane tried to tell it.

Luckily, it is easy nowadays to support ADS-B and GDL-90 thanks to an open-source project called Stratux!

I started by turning X-Plane into a simulated Stratux rather than a Clarity. Added the new AHRS messages, support for barometric pressure, etc.

Now, Synthetic Vision on the iPad was working! Traffic had relative motion!

But the weather was still stubbornly refusing to show up, even though all the weather packets were absolutely according to spec, and parsed to readable METARS, etc…

So something was still different.

Time to go wardriving!

I looked up the list of ADS-B towers: http://towers.stratux.me/

And glued my DIY Stratux into the window of my Tesla and went to see a tower!

These towers are optimized for sending data upwards into the sky, so you have to get rather close to them to pick up any signal on the ground.

I found one, parked my car, and started recording:

Unfortunately, the first tower I went to only rebroadcast traffic, but did not send any weather!

So, I had to drive to a different tower, until I found one that gave me the weather:

Picture me, parked outside the airport gate, with this thing in the window with wires coming off of it.

About 10 minutes in, some people from the local helicopter medical airlift service come out and politely ask me if I need help. I convinced them that I had no nefarious intent, was not spying, and merely taking advantage of the good reception under the tower…

Spoiler alert: I did not get arrested that day, despite having what looked like a makeshift detonator in the window.

With a few Megabytes of recorded FIS-B messages, I set up my remote office, and started to analyze what the difference was between the messages X-Plane sent and the messages I recorded from the tower:

Turns out, X-Plane didn’t fill in some header data containing a 80ns timestamp that shows when the packet was received from the tower (not the age of the weather report, simply when the receiver picked up the packet, measured using the GPS clock of the receiver).

Once I added this missing data, the weather is accepted as valid by my real EFB!

And boy does this work now! Look at all this data going into ForeFlight:

Now, you can tap an airport in ForeFlight, and it will show you the METAR of the actual weather in X-Plane at that airport, correctly indicating it came from ADS-B:

These two aircraft are AI aircraft, and the thunderstorm cloud north of Greensboro really was there on that day, but what you are seeing is the weather recorded on our server, replayed by X-Plane, and transmitted onto the iPad by X-Plane, showing what the weather looked like that afternoon!

And here you can see it works great with user-defined weather as well! Frankfurt did NOT have thunderstorms like that. This is simply me starting X-Plane in EDDF and setting the weather preset to nasty weather:

About Philipp Ringler

Flight instructor on sunny days, software developer on rainy days. Drinks Darjeeling while beating FAA database anomalies into submission.

View all posts by Philipp Ringler →