What traffic should pilotaware detect at an airfield?

[jamespratt]

Hi,

I have used my pilotaware for a number of flights now and picked up quite a few contacts while airborne - mode S and bearingless mode C. However I was at the airfield today, with me on the ground with a number of aircraft in the circuit, but picking up very little in the way of mode C. I was picking up airliners from 150 miles away and one locally based mode S aircraft but not aircraft that I know would have been squawking  C or S my vicinity. My settings were set to the recommended ones in the manual.

Any thoughts? Antenna issue?

[exfirepro]

Hi James,

It could possibly be an antenna issue but unlikely unless you have them screened by aircraft bodywork. Remember reception at these frequencies is  generally 'line of sight' so if you are on the ground you can expect to see less.

CAT (commercial) ADSB is 'easy' as they are generally transmitting very high power, hence long range detection.

You say you have had '...quite a few contacts - mode S and bearingless mode C.' How are you differentiating these? Mode S and Mode C are BOTH bearingless and will both show on your display in the same manner.

You say the local aircraft would have been squawking C or S - but can you be CERTAIN of this? Remember a transponder will only squawk if it is being interrogated by a radar head at the time - often unlikely on the ground - and of course assuming it is turned 'on' in the first place, which they often aren't, especially on short local flights.

Can I ask what Nav System you are using and what Mode C/S Select and Range settings you are using please.

Also what was your location?

Regards

Peter

[Admin]

I suspect they are not being interrogated
Big downfall of mode s/c - requires interrogation to be visible

Thx
Lee

[jamespratt]

Settings were ModeC/S separation 2000ft, mode c/s detect medium range, Mode-CS beta (my aircraft has mode S). Using Skydemon.

In terms of location, I was outside the aircraft standing near the runway at Netherthorpe clear of buildings, in line of sight with aircraft in the circuit (800ft). I know they would be squawking mode S or C because they were either aircraft based in my hangar (at least two are mode S) or flying school aircraft (C). We are under the edge of Doncaster's zone, so all a/c will be squawking, even if staying in the circuit. It makes sense that transponders won't be interrogated at ground level, but should be at circuit height, given the fairly flat terrain and proximity to DN?

I was not just going on the skydemon display, but also looking at the traffic info on the traffic page. Out of about 5 a/c, only one generated a green/yellow/red ring with a/c reg and altitude displayed on SD (so mode S, and I pick up this a/c regularly when flying). If I can pick up this one from 200 feet and upward, why am I not picking up others? I was picking up other mode CS traffic from some distance away with signal strength of 10 and less, according to my traffic page.

Any explanations gratefully received!

[exfirepro]

Settings were ModeC/S separation 2000ft, mode c/s detect medium range, Mode-CS beta (my aircraft has mode S). Using Skydemon.

I was not just going on the skydemon display, but also looking at the traffic info on the traffic page. Out of about 5 a/c, only one generated a green/yellow/red ring with a/c reg and altitude displayed on SD (so mode S, and I pick up this a/c regularly when flying). If I can pick up this one from 200 feet and upward, why am I not picking up others? I was picking up other mode CS traffic from some distance away with signal strength of 10 and less, according to my traffic page.

Any explanations gratefully received!

Hi James,

Much clearer now. You say you were outside the aircraft - presumably with your PAW 'portable', or did you just have your tablet outside with you, with the PAW still fitted inside the aircraft? If your PAW was portable, where did you have the 1090MHz antenna? Could your body have been screening it from the circuiting aircraft?

Your PilotAware settings should certainly be fine for normal flying use and you presumably have 'Show Bearingless Targets' enabled in SkyDemon or you wouldn't see any bearingless target 'rings' on your display screen. I also understand your premise as to the transponders being 'on' in the circuit and that close to Doncaster it is likely that aircraft at circuit height will be interrogated fairly regularly.

There are still several issues here, however, which you need to understand.

Firstly, the 'Traffic Table' is in effect only a 'collection box' showing all data received and processed by PilotAware. This information is not automatically passed to the Nav system, but is first filtered and the results presented to your Nav system as follows.

Your own ADSB/P3i/Mode S transponder is filtered out using your ICAO Hex address, (though Lee's recent advice is that if you have the unit set to receive Beta Mode 'C', you now also need to run the '+Filter' option, even if your own transponder is Mode 'S').

All other received 'moving' ADSB and P3i targets, are then passed to the Nav system if they are within your chosen vertical filter range (set within SkyDemon) - and will be visible on your tablet if within range.

In the case of 'bearingless' mode 'C' or 'S' targets, although they show in the Traffic Table, only those with a signal strength above one of three preset trigger levels for each Detection 'Range' will be presented to the 'sort' filter, which then evaluates these signals, filters those outside your chosen Mode C/S altitude separation (set in PilotAware) and then sends only one of these at any given time which is deemed to be the greatest risk to your aircraft - to the Nav system to actuate the 'bearingless target' warning rings.

With your range setting at 'Medium', Mode C/S signals with a strength below 160 are not considered to present a significant risk to your aircraft and will not be passed to the Nav System, so your traffic signals of '10 or less' would certainly not appear on your display. This may account for the lack of alerts you are experiencing, though I would certainly expect aircraft in the circuit with transponders operating to activate alerts. This is in fact the main source of reported 'nuisance alerts' via the audio system. You could increase your range setting to 'Long Range' for testing - which uses trigger levels starting from 150, but from all the testing we have done so far, signals below this level have not been deemed to present a significant danger to your aircraft. Extending the Mode C/S Detection Range will give you more visible alerts on screen (but also more equivalent audio alerts).

It is therefore likely that the reasons for the aircraft not appearing on your screen are due to a combination of the above factors, but please keep us advised as there just might be a problem with your 1090MHz Antenna or SDR Dongle, though these have generally proved pretty reliable. I take it your unit is a Classic Kit - not self sourced/built?

It would be useful to post screen grabs of your traffic table if you can so we can see the sort of signal strengths you are receiving or we could ask Lee to have a look at your Track Logs if felt necessary.

Hope this helps. Please let us know how you get on and we will get to the bottom of things.

Regards

Peter
(Mode C/S Development Tester)
 

[jamespratt]

Thanks for the detailed reply. The PAW is assembled from all the bits on the hardware list, upgraded from the original Pi and with the new bridge. The PAW is portable (ie wasn't in the a/c, in my hand) with the antenna vertical and having a clear line of sight. Bearingless targets enabled on SD.

I understand a threshold is required to be exceeded to trigger an alert. However the a/c listed on the traffic page had a steady low signal and were presumably being detected some distance away than those I could see in the circuit (which I assume were not being listed/detected because there was no increase/decrease in strength). Is it possible that the antenna/DVT dongle could be faulty even if it picks up ADSB and strong mode S signals? I am on my second dongle - the copper wire broke off from the PCB inside the dongle, and the second is dodgy where the lead plugs into the dongle, but tends to be all or nothing (I asked about alternative DVT dongles on another thread).

I have a screenshot from when I was in the right hand seat of our a/c while we were in the circuit (G-CCSR). I thought it was taken at circuit height but altitude says 10 feet! (I think the QNH was reading about right on the configuration page). Can't remember the traffic in the circuit at the time, but there was at least one school Cessna on the ground and one in the circuit.

I will switch to the +filter option and next time I am at the airfield I will repeat the exercise more methodically with some screeenshots.

[Admin]

Hi James
the heights are relative, so 10ft would be correct.

Thx
Lee

[exfirepro]

Hi James,

The '10ft' is the relative altitude between your own transponder at the time it transmitted and your PAW at the time it received/decoded that transmission, not height above ground. This slight variation is quite normal in the Traffic Table. I'm a bit concerned about your recorded signal strengths though. They all (including the ADSB ones) seem very low to be 'out in the open' and so close to Doncaster, though might be genuine.

If the aircraft in circuit weren't being listed, this means they either weren't transmitting (perhaps not being 'pinged' despite your proximity to Doncaster), or there is a problem with your 1090MHz receiver. Unfortunately not easy to tell remotely. Please try to get more screen shots next time you are flying as this will help.

As you have 'home-built' it might be worth trying a different receiver, e.g. the one Paul suggested on the other thread (which I have tried briefly), but using a short USB extender such as

http://www.ebay.co.uk/itm/SHORT-12cm-USB-2-0-EXTENSION-EXTENDER-Cable-Lead-BLACK-/110724232305?hash=item19c7adb471:g:cF0AAMXQuCdTfcMS

...or even

http://www.ebay.co.uk/itm/30cm-Blue-Plastic-USB-2-0-Type-A-Female-to-Male-AF-AM-Extension-Cable-Cord-FK-/261808766295?hash=item3cf504bd57:g:f68AAOSwPhdVAJ8~

...so it doesn't obstruct the other USB ports.

You would of course need to source a suitable 1090MHz antenna to match the new connector.

Regards

Peter