Author Topic: Mode c and no squawk  (Read 4400 times)

SGS66

Mode c and no squawk
« on: February 24, 2019, 05:10:40 pm »
Hi,
Can someone please explain why the 192.168.1.1 Traffic page cannot show the SQ for mode C aircraft ?
Also how is the reg of these aircraft sometimes shown in the righthand column along with ft? and pwr?
the latter two items (ft and pwr) usually looking like junk.

Sorry if I have missed the explanation of this elsewhere.

Phil

exfirepro

Re: Mode c and no squawk
« Reply #1 on: February 24, 2019, 11:00:04 pm »
Hi Phil,

This is an extremely complex issue, but I will try to give as non- technical an explanation as I can.

Firstly, you need to be aware that the PAW Traffic Screen is not simply a list of aircraft present, but a database of information which has been received and is currently being processed by the unit at any point in time. Think of it as a ‘worksheet’ which PAW is using to assess threat levels and determine which information needs to be passed to the Audio System and Nav System Screen(s) to Alert the Pilot.

Unlike ADSB and Mode S signals - where we can use the transmitted Hex Code to collate data packets and link them so as to subsequently identify individual aircraft and report their height, registration and threat level, determining and reporting on pure Mode C transponders is extremely difficult because they don’t transmit a ‘Hex’ code or any other easily traceable identifier.

In the early days, all we could in effect do with Mode C was report the presence and altitude of the strongest ‘non-ADSB or Mode S’ signal (i.e. which didn’t have an associated Hex Code) as ‘Mode C’.

Fortunately, Lee has since done a considerable amount of work on the detection algorithms so that they now compare additional information including received signal strength of each data packet with other data packets which do include Hex IDs, so that we can eliminate ‘false positive’ Mode C reports where they can be positively and accurately aligned to a known Mode S or ADSB Aircraft already within the database. This is the function of the ‘ft’ and ‘pwr’ columns. If these compare directly (or within a determined accuracy range) with for example a known ADSB or Mode S Aircraft already in the table, the ‘Mode C’ entry is then associated with the registration of the known aircraft and if the association continues for a defined period, the previously attributed ‘Mode C’ entry is disregarded and is not passed as a ‘Mode C’ Alert. All very clever stuff!

Hope this helps.

Best Regards

Peter

SGS66

Re: Mode c and no squawk
« Reply #2 on: February 25, 2019, 09:43:31 pm »
Peter,
Thanks for your reply.
To simplify my problem here is an example. A radar controller tells a plane with a mode c transponder to squawk say 3666. When the pilot applies this code to the transponder the squawk appears on a radar screen as I understand it.
Surely then even mode c transponders must transmit this code, and possibly it could be a unique number which no other
aircraft is squawking. It is worth keeping this info and even reporting it so a pilot can infer that the threat aircraft might or might not be on the same frequency.
Phil

exfirepro

Re: Mode c and no squawk
« Reply #3 on: February 26, 2019, 11:49:19 am »
Peter,
Thanks for your reply.
To simplify my problem here is an example. A radar controller tells a plane with a mode c transponder to squawk say 3666. When the pilot applies this code to the transponder the squawk appears on a radar screen as I understand it.
Surely then even mode c transponders must transmit this code, and possibly it could be a unique number which no other
aircraft is squawking. It is worth keeping this info and even reporting it so a pilot can infer that the threat aircraft might or might not be on the same frequency.
Phil

Hi again Phil,

You are correct about Mode C Aircraft transmitting their Squawk in response to interrogation by ATC RADAR, however the problem lies in the phrase  ’... and possibly it could be a unique number which no other aircraft is squawking.’

Unfortunately in practice, in the GA world this only occurs when the aircraft concerned is transitting, or about to transit, controlled airspace and has been allocated a unique Squawk for the transit by the local ATC. During the transit, we of course already know what frequency it will be on and ATC will ensure effective separation.

In the vast majority of cases outside controlled airspace, GA aircraft will be transmitting a ‘Common Squawk’ such as 7000, the local ‘Listening Squawk’ (again common to all ‘listening’ aircraft) or a generic Squawk used by for example Scottish or London Information. Also, the digital code which produces the Squawk is in many cases exactly the same as that for the equivalent altitude. The code for ‘7000’ for example is exactly the same as that for ‘7000 ft’. As I have said previously, with Mode C there is no Hex ID or other reliable marker available to allow us to easily collate received data to an individual aircraft or easily separate between a Squawk and an Altitude transmission. In order to make this possible at all, Lee has had to develop extremely complicated detection and comparison algorithms. In some cases this comes down to monitoring the signal codes over a period as the altitude will vary over time, whereas the Squawk will generally remain constant.

As I said previously, the lower section of the Traffic Screen is the area where all the recording and comparison to make this happen is displayed, which is why this section of the table is so fluid. As far as ‘users’ are concerned, this screen is part of the internal working of PilotAware. It is made available to users for background information only and should not be used for aircraft awareness.

These  detection and comparison algorithms have been even further refined in the newest software, which we have now been testing for some time and which should be due for release shortly. Believe me when I say that getting to this point with Mode C has been an extremely hard slog and is the result of over 2 years of ongoing development.

We are always open to ideas and suggestions for improvement.

Fly Safe!

Best Regards

Peter

SGS66

Re: Mode c and no squawk
« Reply #4 on: February 26, 2019, 06:03:08 pm »
Peter,
Thanks again for your response. It is dawning on me of course the size of the problem with mode C.
However and this maybe because you don't fly in the South East  there are a lot of GA aircraft getting a basic service in a very large G class airspace, the whole of the South East actually where aircraft taking a Basic service from Farnborough Radar have unique squawk codes allocated in G class airspace who are never going to go into controlled airspace.  So there are areas of the country were something easy can be done and the codes are specific to that LARS area. You do get aircraft flying in the South East with the VFR squawk code not taking a service never the less a mode C aircaft with a unique squawk code is surely trackable. Why not have a go with the codes Farnborough Radar West,East and North use that get handed out all day long in sequence.

Admin

Re: Mode c and no squawk
« Reply #5 on: February 26, 2019, 09:43:26 pm »
Hi Phil

The root problem is that it is impossible to associate the mode A and mode C responses being emitted from the same aircraft. Remember a radar head has a directional receiver, but PilotAware is omnidirectional. So effectively all we hear are a sequence of 12bit numbers, with no clue where they came from
So if you are squawking 1234 and your mode C is encoding altitude 2000, how can we associate 1234 with 2000, they are simply unrelated numbers broadcast over the frequency.

Thx
Lee

SGS66

Re: Mode c and no squawk
« Reply #6 on: February 27, 2019, 05:54:37 pm »
lee,

Thanks, I have a lot to learn but I have learnt something.

Glad I have bought PilotAware and looking forward to your location service for Mode S traffic

I am based at Shoreham and I have spoken to the local OGN station at Southwick about the OGN-R possibility.

mariko

Re: Mode c and no squawk
« Reply #7 on: March 11, 2019, 05:21:51 pm »
Hi
this can be useful to understand why it is not possible to associate a C code with an A code.

http://www.aeroelectric.com/articles/Altitude_Encoding/modec.htm
« Last Edit: March 12, 2019, 04:06:44 pm by mariko »
Ciao
  Mariko

SGS66

Re: Mode c and no squawk
« Reply #8 on: March 13, 2019, 11:18:43 am »
Mariko,

Thanks for posting the very interesting article, you did well to find it.

I can see there is nothing further to be done with traffic having a Mode C transponder.

I am looking forward to the new software giving the position of mode S traffic.

brgds Phil

Paul_Sengupta

Re: Mode c and no squawk
« Reply #9 on: April 29, 2019, 12:49:11 am »
One thing you can do is to try and persuade anyone you see with just a Mode C transponder to buy a PilotAware.  ;D