Dear people wot know stuff,
The SkyEcho2 thread on the Flyer Forum raised a question in my tiny brain and I'm sure someone here will know the answer. Gaz mentioned that Turweston had seen his ADSB on their OGN-R and told him they could see him. I think, following his response, that we're both on the same hymn sheet and believe that it was actually the PAW unit part of the OGN-R station that saw him but I just wanted to clarify a couple of points.
Now, I know an OGN-R ground station is based on the PAW box of goodies and like any PAW should be able to see PAW, ADSB and, I presume, Mode C/S. A ground station will also be receiving, as I understand it, Flarm data for the nearby area over the OGN via the Internet. My question is what does that OGN-R ground station re-broadcast to be picked up by nearby P3I capable receivers?
I cannot imagine it will re-broacast Mode C or, at the moment, Mode S as those are bearingless and only based on signal strength. However, the Mode S multilateration is a different animal and I understand the OGN-R stations will be used to re-broadcast Mode S (MLAT) once the system is up and running.
What about P3I and ADSB that the ground station can see? I would imagine that re-broadcasting ADSB wouldn't be too useful as it has such a strong signal that they'll be picked up by a PAW unit anyway but how about P3I that is detected, for instance, 20km south of the ground station and sent to a PAW unit in an aircraft 10km to the north that wouldn't be able to see the southerly traffic (perhaps)?
Thank you, wise ones.
Edit is my bold PR
Hi Paul,
A ‘slight’ inaccuracy in your description. The OGN that is rebroadcast
isn’t received from the OGN via the Internet as that would introduce latency into the system. The 868MHz data is received directly by a high gain colinear antenna, which collects ‘FLARM-type’ data from the surrounding area (in some cases up to a range of 120 miles depending on antenna/aircraft height and local topography - though this is exceptional and not necessary or desirable for rebroadcast purposes).
This data is passed to the OGN servers in the normal way under standard OGN protocols as part of our agreement with the OGN.
The P3i receiver normally sits quietly ‘listening’ for any nearby P3i contacts within its receive range* - which can be up to around 30Kms - again dependent on antenna/aircraft height and topography. (*This saves power and minimises radio spectrum ‘chaff’.) When a P3i equipped aircraft comes within range the station automatically starts to rebroadcast any
live position data which it is currently receiving from any FLARM (or similar) equipped aircraft
within a defined range of that P3i aircraft (hence virtually no latency whatsoever and minimal ‘on-screen clutter’).
Basic OGN-R stations
only work on FLARM/P3i frequencies at present. They do not receive or rebroadcast Mode C/S or ADSB as, being much higher power, there is no real need to do so.
The station at Turweston is different. It is a ‘PilotAware Radar’ station - designed for use at smaller GA airfields to give increased situational awareness of local traffic. These use a different setup, combining OGN-R with 1090MHz receive and displaying the combined results locally on a ‘Virtual Radar’ Screen - hence why it can ‘see’ Gaz’s ADSB signal.
The intention is to rebroadcast Mode S-MLAT from OGN-R Stations in a similar way to what we currently do with FLARM/OGN Traffic - i.e. on request from specific PAW-equipped aircraft, but in this case using data from our MLAT provider via the Internet and processing this locally to show aircraft positions on your screen via PilotAware as moving aircraft, with a varying diameter circle around each to show potential positional inaccuracy due to system latency - calculated from the data received from the original and ongoing transponder transmissions from each individual aircraft. All very clever stuff.
At present, this only works from specific test sites on PAW units running Engineering Software The ‘positional’ Mode S Aircraft can currently be displayed on the inbuilt PAW Radar screen or on EasyVFR, though the other Nav System providers are now aware of how it works and will hopefully take it on board.
We ran it for most of the LAA Rally weekend at Sywell and it was very impressive - especially during the busy Sunday afternoon (departure) ‘scramble’
Hope this helps clarify the situation.
Best Regards (as always)
Peter
