Decoding TIS-B DF 18 CA 6

The "to-do" list for Virtual Radar Server is as long as my arm, but if you want to pile more work onto it then this is for you.
hikeofyourlife
Posts: 11
Joined: Mon Oct 27, 2014 4:23 pm

Decoding TIS-B DF 18 CA 6

Postby hikeofyourlife » Sat Jul 25, 2015 3:47 pm

I am using ModeSDeco2 to feed VRS and then pass through the data to ADSBScope and then back to VRS using Basestation format. Why am I doing this? Because VRS doesn't seem to decode the TIS-B messages I can pick up from San Francisco via knife-edge propagation over here in Reno on the other side of the Sierra Nevada. As you can see from the picture, VRS by itself misses all TIS-B from S.F. and only occasionally will it pick up an aircraft transmission there. The knife-edge effect from KSFO also happens on UAT 978 as you can see. Am I missing a setting in VRS that should be picking out the position messages via 1090 DF 18 CA 6?

Image

agw
Posts: 1896
Joined: Fri Feb 17, 2012 3:20 am

Re: Decoding TIS-B DF 18 CA 6

Postby agw » Tue Jul 28, 2015 1:02 am

From what I can find with a quick Google search it looks like TIS-B was being trialled here in the UK in 2012. I don't know if they're transmitting it around here yet, if not then it might be a bit tricky to test :)

Looking at the code it should be decoding DF18 CA6, when it sees those it should be extracting the aircraft address, extended squitter message and PI. If the PI isn't 0 once the parity has been taken off then I think VRS will ignore it, so maybe that might be why they're not showing up... but I'm just guessing.

When I get a chance I'll take a look and see whether there are any DF18 CA6 messages around here and, if there are, what's happening when VRS receives them.

thebaldgeek
Posts: 3
Joined: Tue Feb 24, 2015 3:06 am

Re: Decoding TIS-B DF 18 CA 6

Postby thebaldgeek » Tue Jul 28, 2015 2:55 am

Just today I added a feed from a friend that has TIS-B data.
The planes show up and then disappear from VRS. It's rather odd, not exactly sure what the cycle time is, but it seems like a pattern.
He is using a RadarBox and has a port forward for me (VRS) to pull the data from his box.

I am also looking for a way to either turn it on or turn it off, having it 'pulse' over the span of about a minute on and a minute off is pretty frustrating.

agw
Posts: 1896
Joined: Fri Feb 17, 2012 3:20 am

Re: Decoding TIS-B DF 18 CA 6

Postby agw » Tue Jul 28, 2015 12:47 pm

If the broadcast of TIS-B data for an aircraft was taking place periodically, and the interval between broadcasts was longer than the display timeout on the web site, then you'd get the effect you're seeing. The aircraft would appear, not move for 30 seconds (or whatever your display timeout is), then disappear for a bit before reappearing in a different position.

If that is what's happening then you can work around it by increasing the "Remove from display" value in Tools | Options | General so that the display timeout is longer than the interval between broadcasts.

thebaldgeek
Posts: 3
Joined: Tue Feb 24, 2015 3:06 am

Re: Decoding TIS-B DF 18 CA 6

Postby thebaldgeek » Thu Jul 30, 2015 4:41 pm

@awg Thanks for the idea to get the data to 'persist'. Spent some time testing different values, but could not get it working like we hoped.... There is something deeper going on with decoding the data is my suspicion at this stage.

@hikeofyourlife I tested your idea of using ADSBScope to 'pipe' the data into VRS but it did not show any difference.
Just one question. I am using ModeSMixer2 to bring a few feeds into one master feed, or that's the plan, my question is, what format data do you pipe into ADSBScope? I am bringing Beast as that is all ADSBScope seems to understand?
Thanks.

hikeofyourlife
Posts: 11
Joined: Mon Oct 27, 2014 4:23 pm

Re: Decoding TIS-B DF 18 CA 6

Postby hikeofyourlife » Sat Aug 01, 2015 4:37 pm

I'm using modesdeco2 to feed into VRS, which in turn feeds ADSBScope in "pass through" mode. ADSBScope in turn feeds back into VRS in Basestation MSG format. This seems to preserve the signal strength data. The Dump978 from a linux machine also is fed into VRS in BEAST format. UAT here is almost exclusively TIS-B and VRS seems to understand it just fine, this is why I'm confused as to why it doesn't understand it from the ADS-B side from modesdeco2 directly.

Vince

agw
Posts: 1896
Joined: Fri Feb 17, 2012 3:20 am

Re: Decoding TIS-B DF 18 CA 6

Postby agw » Sun Aug 02, 2015 3:07 pm

There's no field for signal strength in BaseStation format, you won't see a signal strength on that feed. VRS only picks up signal strength from Beast binary mode feeds as far as I can remember.

When you say that VRS is picking up the TIS-B on one feed but not the other, is the feed that works a BaseStation feed and the one that doesn't an AVR/Beast feed? Or are they both AVR/Beast?

hikeofyourlife
Posts: 11
Joined: Mon Oct 27, 2014 4:23 pm

Re: Decoding TIS-B DF 18 CA 6

Postby hikeofyourlife » Sun Aug 02, 2015 6:04 pm

Signal strengths are shown in VRS from the ADSBScope receiver

Here's the breakdown
ModeSDeco2 output --beast on port 47806

VRS input: 47806 AVR or Beast
Dump978 input: 30303 AVR or Beast

VRS output: 33003 passthrough

ADSBScope input: 33003 (binary RAW data client)
ADSBScope output: 31022 decoded data

VRS input: 31022 Basestation

If I turn off 31022 (ADSBScope), the decoding of TIS-B from San Francisco on 1090 doesn't occur (obvious when range map is turned off for that receiver)
(The below image doesn't contain any TIS-B 1090 BTW, I don't get very many, maybe a dozen a day and the tracking only lasts while the plane is in the Bay Area)
Fairly often I will pick up TIS-B on 1090 and the plane will show a track from KSFO to just outside the bay area, then the plane will drop off the radar for a few minutes and I'll reacquire it when it is directly visible here on the other side of the mountains but Mode S, so I'll show tracking for 30 minutes but the plane icon will still be sitting just outside San Francisco.

Image

agw
Posts: 1896
Joined: Fri Feb 17, 2012 3:20 am

Re: Decoding TIS-B DF 18 CA 6

Postby agw » Wed Aug 05, 2015 12:15 am

OK - the signal strength is just a hangover from when you were receiving the aircraft on the AVR/Beast feed. The site won't reset signal strengths back to null so you'll see the last signal strength from the Beast feed while the ADSBScope feed is showing the aircraft.

BaseStation isn't a raw message format. What happens with that format is something else (ADSBScope in this case) has decoded the aircraft's messages on VRS' behalf and is just telling VRS what to show. Hence why it looks like VRS is decoding TIS-B on one feed but not the other - it's actually ADSBScope that's decoding TIS-B and then just telling VRS the content of the messages, VRS doesn't know anything about where the values on the BaseStation feed came from.

I'll take a look into why TIS-B isn't being decoded by VRS on the raw format feeds.

hikeofyourlife
Posts: 11
Joined: Mon Oct 27, 2014 4:23 pm

Re: Decoding TIS-B DF 18 CA 6

Postby hikeofyourlife » Wed Aug 05, 2015 4:46 am

Ah, so VRS isn't completely discarding the frames, it's just not decoding the positions. OK! I hope you can find what's preventing it, I'd rather not have to run ADSBScope simultaneously if I don't need to. Only reason I noticed it was I was without internet for a few weeks last summer and ran ADSBScope since it had the map archived.

Vince


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