FlightAware Discussions

Is Police Data Being Filtered Out? Going dark?

Why does the PiAware not have any data on this ???
Piaware doesn’t show the position on the map and the distance.
FlightRadar24 shows it all.

Everything on my PiAware is in the green… so is it deliberately excluding this?

Flight aware honour requests for privacy. Other sites do not.
Do you really want people to know where PolAir is flying?

This comes off the antenna, so it appears to be filtered locally, before it hits Flightaware. That’s what I’m questioning.

To answer your question, yes. I want to know.

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To determine the location for that aircraft MLAT is needed.
So only the MLAT results you get back from FA are filtered on their side as MLAT positions are calculated on the server.

The data your receiver receives directly (ADS-B positions and for example the altitude for that helicopter) are not filtered.

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So basically, the PiAware is getting the data, the location is being sent back to Flightaware servers to plot on the map and for some reason they are not being shown.

So what’s the workaround ? Can the PiAware do these calculations itself? Or do we have to move off Flightaware ?

VicPol is trackable with MLAT.

The Skyaware map you provided shows VH-PVO as the only aircraft in view of your station and therefore you had no ADSB reference aircraft for you to partake in MLAT.

My understanding is you need to provide MLAT data to Flightaware (by having an ADSB aircraft in view of your station and three other) and then Flightaware will send MLAT location information back to you to display on your Skyaware map.

S

Not quite. You sent the ID and timing to the FA server which does the triangulation calculations and sends you back the result.
Only having a single location, your receiver doesn’t have enough information to make the calculation its self.

Wouldn’t other stations nearby pick it up?

The problem is that you and the other stations all need to also see at least one other aircraft transmitting ADSB to provide a common timing reference for the MLAT calculations.

Same deal - if there is no “reference aircraft”, then no nearby station will be providing the the two signals required for the calculation.
Also, I think you only get the MLAT positions for those you have contributed too
(I don’t run MLAT, so I haven’t followed it’s behaviour)

Sorry, should have added this link for a more detailed explanation:

https://flightaware.com/adsb/mlat/

Ok. So its possible that Flightaware isn’t filtering this. I guess on the Admins would know. Next time there is another aircraft around, I’ll take a look. Thank you.

Evidence of ViCPol helicopter Flightaware MLAT from this afternoon:

wiedehopf is correct; there is no filtering done to the data you receive locally:

You can easily verify that by e.g. stopping piaware or disconnecting/firewalling the Pi from the internet and looking at the data you see; a disconnected system like that can’t be doing any sort of centralized filtering. Or you could check the source code, if you have the background - dump1090 / skyaware / piaware are all open source.

For aircraft that are not transmitting an ADS-B position, you don’t have position information, only the basic data that the aircraft is transmitting (identity, squawk, altitude, etc) - this is the scenario you see in your skyaware screenshot.

By coordinating data from several receivers, the aircraft position can be found via multilateration. You can’t do this using only your own local data, you need at least 3-4 (and ideally many more) coordinated receivers spread out around the aircraft. If you are running piaware, then FlightAware servers do the coordination and calculation work and return position results when we are permitted to. The computed mlat positions are then sent to skyaware and combined with your locally received data. For aircraft that have requested blocking or state/military aircraft mlat positions may not be returned and you’ll only see the locally received data.

The mutlilateration client & server code I wrote some years ago is public and available on github if you want to set up your own multilateration system. This is not a simple task (and please don’t ask me for setup help) but there are cooperative systems out there based on this code. Data quality may vary…

All that said, VH-PVO does appear to be ADS-B equipped (e.g. see this tracklog) and does not appear to be on the blocklist so the more likely explanation is just that you haven’t heard a good ADS-B position, perhaps because of poor reception.

Another possibility is that the aircraft has turned off ADS-B and multilateration isn’t returning any results because your receiver is not synchronized (you need to see at least one ADS-B equipped aircraft to maintain synchronization with other nearby receivers for the purposes of mlat)

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@obj - another one of my annoying questions…

While I was trying to capture the VH-PVQ MLAT example above, I first saw it come up on my feeder as mode S (as expected). It remained in that state displaying just altitude for a few minutes, but then rather than switching to MLAT status, it went through an intermediate stage of displaying as mode S with altitude and speed for a couple of minutes. This could be normal - I’ve never observed the change from mode S to MLAT closely before.

Does the MLAT calculation get reliable speeds more easily than positions? And is that what I was seeing?

VH-PVO is currently being tracked with MLAT

Around the city and 'burbs they appear to be Mode S only but turn on ADS-B when doing rescues and searches etc.

In the original post there was only one aircraft VH-PVO. There was no ADS-B aircraft in range.

I think a better antenna would improve range and enable more MLAT aircraft and VicPol to be tracked.

S

Possibly you started hearing Comm-B responses that carry groundspeed. These are Mode S messages that carry additional information that SSRs can query for (in this case it’d be BDS5,0 “track and turn report” which can report roll angle, ground track and speed, track rate, and TAS)

The opposite really, mlat speeds are less reliable than mlat positions.

mlat results are only ever position+speed together, you won’t see one without the other.

That explains the behaviour that the OP saw then - Mode S only, and no mlat results because of a lack of ADS-B aircraft for synchronization.

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