Aircraft in close proximity but received by different Ground Stations

[Moffrestorer]

Hi Keith,

I frequently check glidertracker.org to see what is being received by PWKingsto OGN-R ground station.

Yesterday I saw tracks for two aircraft, one a glider and Flarm equipped, the other PAW equipped, at a similar height and in close proximity. Aha, at last, the PAW user should receive a rebroadcast of the glider location and relative height, I thought to myself. To my consternation I saw that the glider was being received by UKUSK Ground station even though PWKingstone is closer and in theory should be receiving higher signal strength (based on reception of the PAW signal), so NO rebroadcast as not working the same ground station!

I took the two screenshots below, showing the receive data for each aircraft. The glider has effectively passed my ground station and was consistently being picked up by UKUSK out to his Port side whereas the PAW aircraft was consistently being received by my station. In fact, the glider went out of reception for 1 min 46 secs so in reality would be slightly further West when their paths crossed but still didn’t appear to get picked up by the closer ground station. Being at the same location and height I don’t think the problem can be terrain shielding, I seem to be able to receive Flarm contacts OK, so wondered if this could be due to antenna shielding/masking problem with this glider or is there likely to be another explanation? I have noticed with the RAF Tutor aircraft, when in a turn, they frequently get received by OGN stations dependent on which direction they are pointing so perhaps this isn’t as silly as it sounds?

Chris
 sorry but unable to attach screenshots as files too large. Will post as soon as I’ve worked out how to do it or post JPEG which also gets thrown out!

[Moffrestorer]

Screenshots hopefully now attached. They are quite zoomed in; aircraft estimated to be about 1.5 Nm apart.

Thx,

Chris

[exfirepro]

Chris,

There are several reasons why Glidertracker would show the signal going to USK rather than Kingstone. With only a 10-25mW transmitter, antenna position is critical to FLARM transmission from aircraft. A poorly positioned antenna can easily result in transmissions becoming directional. In your screenshots, Kingstone appears to be directly behind the glider, which is often the worst direction of all for Flarm (or PAW) transmission as antennas are usually set up to prioritise the view to the front of the aircraft. It would also appear from OGN Range that UKUSK is a very well sited OGN receiver with (as reported on Grafana - http://grafana.glidernet.org/dashboard/db/receivers?orgId=1&from=now-60d&to=now&var-receiver=UKUSK ) ...slightly better sensitivity than Kingstone OGN. This combination could easily account for the glider showing as being received by UKUSK rather than Kingstone. Of course as USK doesn’t have OGN-Rebroadcast, it can’t receive or retransmit to the PAW aircraft.

That said, the report on glidertracker only means that USK was receiving the strongest signal from the glider at that time. I’m pretty sure it doesn’t mean that it wasn’t also being received (and therefore potentially rebroadcast) by other station(s) including Kingstone as well. To determine this, you would need to check your local PAW OGN-R logs via local ports 8080, 8081 and 8082, though IIRC these logs are not long term persistent, so need to be checked during or shortly after the event. (It’s a while since I have done this so will check tomorrow),

Regards

Peter

[Moffrestorer]

Thanks Peter,

I thought about the situation further after posting and came to a similar conclusion that the rebroadcast will be independent of what glidertracker is reporting at the time. I was actually away from home and watching this from Bristol so could not check the local logs.

It would be nice to be able to check that the rebroadcast is functioning OK, but short of going aloft myself with PAW and someone emitting Flarm also within my coverage area, this doesn’t seem possible. Given the reverse IFF way that the OGN-R is broadcast I wonder whether Lee may get be able to engineer in a log or counter to confirm if/when rebroadcast takes place to confirm all is well?

Thx again,

Chris

[Admin]

Hi Chris,

Are you aware you can monitor what is being captured and reboradcast from your groundstation ?
port 8082 shows a table of reception and rebroadcast

so whatever your ognpaw IP Address is

http://192.168.1.123:8082

or if your router supports zeroconf, you should be able to do

http://ognpaw.local:8082

Thx
Lee

[Moffrestorer]

Hi Lee,

I was aware that the 8082 page showed the reception table but hadn’t realised that this signifies a rebroadcast too. To be honest I haven’t checked it for sometime but tried before posting this thread and for some reason the PC wouldn’t connect (even though I’d bookmarked it in the browser). Will have to investigate. Trouble is that I think the table contents only show when live traffic received and does not save for later analysis, unless I’m mistaken?

Regards,

Chris

[Admin]

Hi Chris

Yes, live data only, another request has been made for logging information

Thx
Lee