Unusual Glider Behaviour

Hi all,

I've had some unusual FLARM returns over Oxford pointed out to me. Four 'gliders' are reported to be hovering 200-300ft above the ground, travelling at 0kts. They show some unrealistic traces too, i.e. erratic movement over quite a small area. I've cross-checked them on Flightradar24 and there don't seem to be any returns so it appears that it isn't someone transmitting arbitrary FLARM packets over the air.

FLR7402C8 - ICAO 7402C8
FLR83303C - ICAO 83303C
FLR82149C - ICAO 82149C

One of these ICAOs is Jordanian, two Indian and one French. Does anyone know what might be happening here?


Lee Armstrong

Staff member
They are fed to us from the FLARM network so we are plotting them as shown.

The track being erratic is probably a bad GPS lock with it hunting around. Also the altitude "floating" is probably down to atmospheric pressure and the altitmeter sending the data is calibrated to 1013.2. We are seeing quite a low pressure here in the UK at the moment it seems too.
Cheers Lee - it's an interesting one. I guess one way this could happen is if someone left their transponder on. The odd thing are the callsigns/auxiliary info such as airline, as well as the ICAO sources. If I were to guess, I'd say that someone might be tampering with input into the FLARM network.
For clarity, and as an update, I spoke to Paweł from OGN, where FLARM data is sourced, and he said that the addresses shown are not ICAOs (and indeed the packets are not FLARM) but instead the packets are OGN so should not display like this.

He also mentioned that the position and pressure altitudes are correct, but these are test packets. Since the GPS antenna is inside the house, the GPS altitude may be a little off.

Lee Armstrong

Staff member
Thanks for the update. We are doing some work later in the year on FLARM data so hopefully will engineer them out. At the moment we take a very rudimentary feed and will be looking to use their more raw feed and will probably get that info from it.
Good afternoon, I can post now on the forum :)

The OGN is in process of integrating various sources of data from systems other than FLARM units and OGN trackers. You may see already some positions from PilotAware, SPOTs and FANET (paragliders). Please take care to process the information is our messages for not just address, but as well address-type, aircraft-type and protocol-type. This is important as it is fairly easy to become visible on our network, see the FB page, address is below.

The positions spoken here are test OGN Trackers, and they clearly indicate address-type as OGN and protocol-type as OGN-Tracker. Two of these have barometric sensors and thus send pressure altitude. The GPS coverage is not great as all the units are inside the house, just close to a window thus GPS altitude can vary by some tenths of meters. The pressure altitude should be far more stable and follow the pressure changes due to weather.

If in doubt please refer to our contact email: contact@glidernet.org, our Facebook page: https://www.facebook.com/OpenGliderNetwork/
You can check as well how our live page interprets the data: http://live/glidernet.org


Lee Armstrong

Staff member
Hello Pawel,

We don't get that information in the feed we are currently processing and this is one of the reasons we will switch later in the year to (I think) the APRS based one that includes all this extra info.

The feed we currently process is in an SBS 30003 format to look like aircraft tracks and so I presume strips a lot of the info.

OK, don't know where this feed comes from, but reading APRS data is relatively easy: just open TCP connection to aprs.glidernet.org on port 10152 and you get ASCII messages like:

OGN82149C>OGNTRK,qAS,OxBarton2:/114011h5145.96N/00111.50W'256/000/A=000266 !W34! id3782149C +000fpm -4.6rot FL009.84 48.8dB 0e -1.0kHz gps5x8
OGN83303C>OGNTRK,qAS,OxBarton2:/114011h5145.96N/00111.49W'000/000/A=000121 !W73! id0783303C +376fpm +0.0rot 48.8dB 0e -1.4kHz gps18x26
OGN395F39>OGNTRK,qAS,OxBarton2:/114012h5145.94N/00111.50W'000/000/A=000354 !W58! id07395F39 +020fpm +0.0rot FL009.78 39.5dB 0e +3.2kHz gps4x5

Let me know, if you need a C++ code which does exactly this.

Last edited: