Scintillation

FlightAware ADS-B Flight Tracking
1

A few minutes ago I was watching the website for my feeder (located in Austin, TX) and noticed a cluster of orange planes suddenly appear over Houston. I clicked on them to get a feel for their height, and they were all between 1600 and 1900 feet up. After 15 seconds or so, they went pale and then disappeared. A couple minutes later, they appeared again, briefly again.

I assume what I was seeing was scintillation. Does anyone know what specific atmospheric conditions would cause this?

Another possibility I considered was that the signals were reflected off a closer plane, but at -14.4 dBFS (they were all within a tenth of this), that seemed unlikely. I don’t think a reflection would be that bright for several planes – maybe for one.

Thoughts?

2

More likely to be TIS-B?

3

What is TIS-B? I guess it’s this: en.wikipedia.org/wiki/Traffic_i … n_service_–_broadcast but how is that a better explanation?

4

OK. I think I understand how it works now. Correct me if I’m wrong. A transmitter that is in range of the imaged plane sends data to another plane. That plane, which may be some distance, e.g., higher up, broadcasts the data about the imaged plane. So the suggestion is that I’m receiving that broadcast. Correct?

Is there anyway to distinguish these messages from the ADS-B messages (I guess with multilateration, you could)? Or are they essentially just spoofed messages?

5

TIS-B and ADS-R messages contain aircraft location information.

avidyne.com/landing/ads-bee/ … -ads-b.asp

What is TIS-B?

TIS-B stands for Traffic Information System – Broadcast and involves ADS-B ground stations sending Secondary Surveillance Radar (SSR) targets to aircraft with ADS-B In. TIS-B targets will be updated at least every 2 seconds on the surface, 6 seconds in the terminal area, and 12.1 seconds in the en-route airspace.

What is ADS-R?

ADS-R stands for Automatic Dependent Surveillance – Re-broadcast and involves ADS-B ground stations repeating ADS-B messages from one link (1090-ES or UAT) to the other link for aircraft with ADS-B In. ADS-R targets will be updated at least every 2 seconds on the surface, 5 seconds in the terminal area, and 10 seconds in the en-route airspace.

There is a hockey puck volume around an aircraft that is broadcasting ADS-B(1090 or 978) that TIS-B/ADS-R uses to decide what to broadcast(when in range). faa.gov/nextgen/equipadsb/benefits/

6

OK, but that doesn’t really address the issue. My receiver, which is running PiAware, is picking up the signals. It seems, from the materials you linked, that the broadcasts are originating from the ground near the target aircraft. Those ground-originating signals would be even harder for my receiver to pick up than signals from a low-flying aircraft. So, if the broadcasts are coming from the ground, that would seem not to be a good explanation unless the ground station were close to me, but it sounds like it is only used for ground stations relatively close to the target aircraft (KAUS wouldn’t be broadcasting for Houston traffic, for example). Am I missing something?

7

You’d think that only nearby stations would send the data but I have reports of data from the other side of the continent sometimes being seen…

8

Interesting. Getting back to what I mentioned earlier, if the same message type is being used as ADS-B out, which is what I think we’re mostly receiving, then we should be able to easily distinguish between self-reporting messages and rebroadcast messages from a ground station using multilateration. Right? MLAT would always show the location to be at/very near the transmitting station. If the message sent included position data far from the source of the signal, then it’s obviously retransmitted data.

9

In my plane, I’ve seen data from aircraft 1000 miles away - some kind of anomalously in the ground station rebroadcast.

FYI, the planes you saw may have been non Mode S (ie. Just Mode C) targets… Those would never show up on MLAT…

10

The various forms of tis-b and ads-r do identify themselves as such, but dump1090 does not do much with that data, about the only thing is to check the address type for non icao addresses and flag tis-b data as such in the json (I don’t think it changes the map display, though a non icao address will)

Using mlat to crosscheck the position given in ADS-B is an idea that’s been floating around for a while but there has never really been a compelling reason to do it. Without some external synchronisation you have to use ADS-B to synchronize anyway so it ends up comparing one aircraft with another.

11

So it is captured in the JSON files? I’ve captured the data for the period, so if I knew what to look for, I could confirm that’s what I was seeing. How is it marked in the JSON data?

12

tisb:[list of things derived from tisb]
Much like how the mlat field works.

13

So in this example:


{"hex":"aca01a",
"squawk":"2117",
"lat":27.909091,
"lon":-97.695500,
"nucp":0,
"seen_pos":11.6,
"altitude":41000,
"vert_rate":0,
"track":45,
"speed":405,
"mlat":"lat","lon","track","speed","vert_rate"],
"tisb":"lat","lon","track","speed","vert_rate"],
"messages":926,
"seen":0.2,
"rssi":-5.6}

latitude, longitude, track, speed, and vertical rate were all derived from both multilateration and tisb? At a quick glance, I see records with tsib but no mlat but I haven’t found one with mlat but not tsib.

14

This is because mlat uses the TIS-B formats to send positions.

15

I figured. Thanks for the confirmation.

16

Curious about the ranges of the various message types, I created ranges for three categories of messages:

  • TISB messages without MLAT
  • ADSB messages without MLAT OR TISB
  • MLAT messages

I show them on this google map.

17

What ground station(s) do you pick up, ExCalbr? There’s one toward Georgetown I assume you can receive.

I only get uplinks very rarely when atmospheric conditions are juuust right… usually from San Marcos or sometimes Schulenberg.

18

How do I know what ground stations I pick up? I can watch planes landing at KAUS, so I assume I pick up KAUS, so that’s probably the most significant one. So far the only aircraft that I’ve picked up that broadcast ground that I’ve noticed are at KAUS.

19

If you run dump1090 without --quiet or --interactive it will show details for each message; an uplink site ID message may look something like:

UPLINK:
Site Latitude: +29.7025 (possibly invalid)
Site Longitude: -96.9007 (possibly invalid)
UTC coupled: yes
Slot ID: 0
TIS-B Site ID: 12

I’ve only done it for UAT from dump978’s output, that’s easy to filter uplinks for further inspection. Not sure if there’s a good way to filter out uplinks for 1090 without logging hundreds of messages per second!

20

UAT does report the site location like that, but there’s no equivalent 1090MHz message.

That said, it’s somewhat likely that 978MHz ground stations also operate as 1090MHz ground stations, so maybe if you look around for the UAT ones you can work out what you have a line of sight to. IIRC there are a few lists of UAT ground sites floating around on the internet (they are easy to find since they broadcast their locations and often 978-in hardware will show where you’re receiving data from; also, big antennas show up well on satellite photos if you want to doublecheck!)