$PFLAA message in track file

[Stu B]

Is there a published definition of the $PFLAA message as recorded in the track files? In most instances, I believe the first 4 elements after $PFLAA are alert state, Metres north of me, meters east of me, and metres above me, but I see several aircraft which have a suspiciously round number (e.g. 1500, 3000, 4500) in the field that I thought was "Metres N of me) and the "Metres E of me" field is then in each case blank. These cases appear to be flagged to generate alerts by PAW. From the Planefinder.net web site, these appear to be form aircraft that had indeed been flying in my immediate vicinity but which were actually probably landed at the times these messages appear in the track file. I am wondering if perhaps ADS-B outputs a different message string after an aircraft has landed - if so, how is that handled in the track file?

[Ian Melville]

A bit of an obtuse reply because I cannot link the document. You need to search for

FLARM DATA PORT SPECIFICATION FTD-12

This document is available from FLARM

[Admin]

Hi Stu
The suspicious numbers of 1500, 3000, 4500 are Mode CS, bearingless targets.
These are purely for driving the nav display, the numbers are not accurate, merely intended to gauge the importance

If you are seeing these on fr24 or similar, they are most likely MLAT derived, not ADSB, not sure if there is a way to confirm that.

Finally, I am pleased that the mode CS detection is doing its job, as you identified these were 'in your vicinity'

Thx
Lee

[Stu B]

Thanks, very helpful. Having seen this helicopter identified with its registration in my track file analysis spreadsheet with these (to me) odd values, I looked it up on the net and Planefinder.net had tracked its ADSB output for a while, then the track they displayed stopped (positioned at Old Sarum) so I presume from that it landed there (I believe it is based there anyway). My PAW record was while I was on the ground about 3 miles from Old Sarum, and PAW did NOT have this ADSB track - but if the helicopter was hovering or perhaps even in the circuit at Old Sarum it  would not have been within "line of sight" from my location so no surprise that I did not pick up the ADSB I suppose. Then, at the exact time that Planefinder.net showed the ADSB signal ceasing, my PAW record started showing these Mode S records, which continued for a short while (less than a minute) then ceased. It seems that perhaps in the helicopter's shut-down sequence the ADSB-out feed was turned off before the transponder itself was powered off, and that the PAW was able to make some sense of the mode S signal even though its reception of the ADSB was too weak to make sense of?

I created my track file analysis spreadsheet about a year ago, probably before I had a PAW version recording Mode CS contacts so now I'll need to upgrade the spreadsheet to handle thees records properly, which with the info above I will be able to do.

BTW, somewhere in your documentation I found a sentence saying a Track File analysis spreadsheet was available to download form your web site, but I have not been able to find any links to it - is it in fact still available?

VBR

Stu

[Ian Melville]

Two things
Flight Radar 24 delays the display of aircraft. I think, but not sure that Planefinder does the same. Even if there was no deliberate delay, there will be latency between the receiving station and the server.

Both Panefinder and Flight Radar 24 can show the source of the data. on PF click additional tracking data in the LH side panel when the aircraft is selected. On FR24 select aircraft and in the LH side panel under 'Speed and Graph', there is a section called Radar. They will either show ADSB, MLAT or T-MLAT1

[Stu B]

I was looking at PAW track and Planefinder about a week after the flight, not in real time so even if Planefinder does inevitably have some latency, I presume the times labelled on the screen are correct?
Thanks for the point about more tracking info, indeed the data source is MLAT, not ADSB as I had assumed. But one odd thing - although the Planefinder plot stops, and the PAW trace starts when the helicopter is at Old Sarum, the last datapoint on Planefinder shows 71kt and 2,300'! Looks more like he either just got out of range of one of the multilateration sites, or someone turned their multilateration sensor off? But it makes it all the more strange that the PAW trace starts just as the Planefinder one ends! Could of course be coincidence, but seems a bit unlikely!

[Admin]

IIRC, mlat does not work at low altitude

[Ben Smith]

IIRC, mlat does not work at low altitude

MLAT can work down to ground level, however there are not many locations with a suitably high density of receivers that can see them this low (also at least one of them must be a FR24 supplied box), I have seen MLAT at farnborough on the runway as I have coverage to the ground there and there are several home brew stations close by.
Also re Ian’s post, FR24 doesn’t delay tracking, some of the US flights used to have a delay of several minutes as they used a FAA feed but the FAA stopped delaying that last year so all flights are now near real time (delay of only seconds rather than minutes), I have often watched an aircraft pass overhead while tracking it on FR24.