Spurious contacts

[G-JBVP]

Have been getting spurious contacts showing aircraft Reg as G-OO2D or various last two letters G-OO?? With contact at same height and position as my aircraft with red warning circle.

Any ideas what's causing this?

[Ian Melville]

Can you post screenshots of your PAW home and config pages? Also, could you post a link to your log file? or send it to Lee.

[Admin]

I think you may mean C-xxxx
Where xxxx is an incrementing hex counter.
These are indicating Mode-C responses, what do you mean by spurious?

What qre your Mode C/S config settings ?

Thx
Lee

[G-JBVP]

Lee

Bŷ spurious I mean giving a warning of very close proximity red warning circle showing 0 feet height difference, looked like reg started with G but could be a C.
I will send you a screenshot of PAW & Config screen when I next fly.

Brian

[Ian Melville]

Brian, do you have a transponder? If so is it mode C or S ?

[G-JBVP]

Yes mode S I have setup our transponder code to be filtered out, perhaps should have mentioned using Skydemon

[Admin]

Hi Brian

Your Mode-S transponder will also reply to Mode-C interrogation
So you need to select the ModeCS+Filter option
Sounds like what you are reporting
Thx
Lee

[Ian Melville]

Why would that show G(or C)-00?? I was expecting Brian to say Mode C.

[G-JBVP]

Lee

ModeCS+filter option is selected.

Brian

[Ian Melville]

I would have expected you to have the error all the time, but just in case.. Can you confirm the hex code in the transponder is the same as set in PilotAware, which of course should match GINFO.

DId you send. Copy of the log to Lee?

[Admin]

Hi Brian,
If I can get the track, that would be useful

In the latest release, Mode-C contacts are reported (as they are received) as an incrementing count
so first Mode-C is assigned
C-0001

next
C-0002
etc...

This is because for Mode-C there is no ICAO code, but we need some identifier.
This identifier is then reused IF I think I receive a new Mode-C ping from the same transponder, this is done by a pretty complicated DSP algorithm that is looking at
time of arrival, altitude variation, signal power variation.

so if you see an ID of C-xxxx, this indicates a Mode-C response.

This is the probelm with Mode-C

PilotAware cannot know if the Mode-C response to an interrogation came from your transponder.
using the '+Filter' option, it tries to learn that a Mode-C response of high strength and similar height is probably the local transponder, so it reads the strength and assigns a hi power blocking filter, the filter recedes over time.

Where this does not work is when your transponder is NOT interrogated, and then suddenly receives an interrogation, at this point Pilotaware has not gone through the 'learning' phase, so reports the Mode C signal, it is subsequently filtered.

if over time you receive no more interrogations, the filter drops, you receive an interrogation, and again it is reported

Thx
Lee

[PaulSS]

using the '+Filter' option, it tries to learn that a Mode-C response of high strength and similar height is probably the local transponder, so it reads the strength and assigns a hi power blocking filter, the filter recedes over time.

Where this does not work is when your transponder is NOT interrogated, and then suddenly receives an interrogation, at this point Pilotaware has not gone through the 'learning' phase, so reports the Mode C signal, it is subsequently filtered.

if over time you receive no more interrogations, the filter drops, you receive an interrogation, and again it is reported

Just for my education, can I just confirm that PAW does not permanently remember what it has learnt about your own Mode C? Why is this? How long does it take before it forgets what it has learnt(which, of course, assumes your Mode C has not been interrogated in the meantime)? How long does it take to re-learn, or does it just start all over again each time it is started?

Just interested, is all (as they might say in the US of A) 

[Admin]

Just for my education, can I just confirm that PAW does not permanently remember what it has learnt about your own Mode C? Why is this? How long does it take before it forgets what it has learnt(which, of course, assumes your Mode C has not been interrogated in the meantime)? How long does it take to re-learn, or does it just start all over again each time it is started?

Just interested, is all (as they might say in the US of A) 

The 'learning' as I term it, consists of telling PilotAware it is situated close to a Mode-C transponder.
From this it deduces that the highest power Mode-C transmission, at the same height - must be coming from the local transponder.
when it receives a Mode-C Ping it sensitizes the filter based upon the received signal strength, the filter itself decays over time, I dont recall the exact number I think it is a minute, bear in mind your transponder will be interrogated every 2 seconds, so if your transponder is not interrogated for one minute the filter has effectively stopped working, which means the first time you get interrogated again, it will report your own transponder - then ignore it

So where this approach falls down is when you move in/out of areas where your transponder is interrogated, but in this case it should only report once.

There is no way around this, there is no way of ignoring your own Mode-C transponder, as there is no way of identifying it, with Mode-S its easy.

thx
Lee

[PaulSS]

Thanks, Lee, all very clever stuff 

I've got a Mode S transponder (still in the box, as is my PAW) so I'm obviously not going to be affected but it is interesting to see how you try to deal with the Mode C boxes. I can't wait to start playing with my gizmos but it's going to be some time yet 

[G-JBVP]

Lee

PDF's of various screenshots attached.

1 of 2 posts

Brian