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Transponder properties

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Transponder properties

Postby CaptB » Tue Jun 06, 2017 3:10 pm

it seems that the wiki documented /instrumentation/transponder/inputs/knob-mode is a dummy propery which does not affect the transponder mode, while /instrumentation/transponder/inputs/mode does the job. Can anyone please confirm or explain the purpuse of /knob-mode then?
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Re: Transponder properties

Postby sanhozay » Tue Jun 06, 2017 6:16 pm

I'm pretty certain that knob-mode is not a dummy property. The knob-mode is driven by the radio dialog, for example:

https://sourceforge.net/p/flightgear/fg ... radios.xml

Looking at the source code, the knob-mode appears to be the "view" property that would be tied to the 3D model and the mode property appears to be the underlying "model" property that is driven from the knob-mode property through the instrument "controller". The "controller" also takes account of other factors, such as electrical supply and my reading of it is that an aircraft would be expected to manipulate knob-mode rather than mode directly.

What's your test case that leads you to believe knob-mode does nothing?

Which aircraft?

Does the transponder have power from some sort of electrical system?

Do you have a custom instrumentation.xml file?

Have you tried your test case in different aircraft (with a powered avionics bus)?
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Re: Transponder properties

Postby CaptB » Tue Jun 06, 2017 6:49 pm

@sanhozay

In order to make a video about ATC-Pie, I logged in as ATC and simultaneously tested many FGAddon aircraft including the C172, B747, 707, 757 and the radio dialog(F12) as well. You cannot have mode C or mode S without directly changing the values at /instrumentation/transponder/inputs/mode, which I confirmed with ATC-Pie. Operating the Transponder's 3D knob or the dialog pull down menu affects /instrumentation/transponder/inputs/knob-mode but that in turn does nothing.
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Re: Transponder properties

Postby sanhozay » Tue Jun 06, 2017 7:08 pm

I just tested it and you are right. I think it's a bug.

My test case was power up an aircraft (Beagle Pup, but that probably doesn't matter), switch the knob-mode to ALT and observe "instrumentation/transponder/inputs/mode" and the mode as shown in the radio dialog. Neither changed. So it looks like a local issue rather than a multiplayer one.

I can't be certain but I think this used to work. I vaguely remember playing around with this using OpenRadar.

This is latest from Git. What version of Flightgear are you running?

EDIT: Other properties, e.g. altitude, transmitted-id seem OK, in the property tree at least. I haven't checked over MP.
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Re: Transponder properties

Postby Johan G » Tue Jun 06, 2017 7:17 pm

This was also briefly touched in another topic just a few months ago: Transponder properties over MP
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Re: Transponder properties

Postby CaptB » Tue Jun 06, 2017 7:18 pm

@sanhozay: I am using 2017.2.1 latest stable from the webpage, windows build. I seem to recall a conversation with Michael(ATC-Pie) a good few moths ago where he mentioned the transponder was not working correctly when he was working on transponder modes I believe, so this might not be a new issue.
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Re: Transponder properties

Postby sanhozay » Tue Jun 06, 2017 8:19 pm

@Johan, thanks for that link.

I'll have a look at this tomorrow.

I note the comment at the top of transponder.cxx is:

Code: Select all
// Example invocation, in the instrumentation.xml file:
//      <transponder>
//        <name>encoder</name>
//        <number>0</number>
//        <mode>0</mode>  // Mode A = 0, Mode C = 1, Mode S = 2
//      </altimeter>

But the entry in generic-instrumentation.xml is:

Code: Select all
<transponder>
</transponder>

This may or may not be relevant.
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Re: Transponder properties

Postby jam007 » Wed Jun 07, 2017 6:45 am

This old topic might be of interest.
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Re: Transponder properties

Postby CaptB » Wed Jun 07, 2017 7:18 am

I can understand sloppy practices can happen in aircraft, but if all else fails, this should work out of the box from the menu's radio stack. Additionally, either I misunderstand the transponder's wiki, or there is no reference or a clear description of purpuse at all to /instrumentation/transponder/inputs/mode which is actually doing something.
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Re: Transponder properties

Postby sanhozay » Wed Jun 07, 2017 7:45 am

@jam007: Thanks for that link, very useful.

In addition, I've been reading the Wikipedia pages on transponder modes and also had a look at transponder manuals. I'm coming to the conclusion that the transponder instrument should not transmit its mode over MP, only the squawk code and altitude.

Let's fall back to real life for a moment. A radar station scans the airspace with a rotating beam. From the perspective of an aircraft, this looks like a periodic interrogation of its transponder. Depending on the capability of that transponder (mode) and its configuration (knob-mode), the transponder transmits a response that includes an identifying number (squawk code) and altitude. In addition, Mode-S transponders transmit additional identification information including a 24-bit serial number. The radar station combines what is a very simple response (ident + altitude) with the radar echo to display the position, ident and altitude of an aircraft on a radar screen.

In Flightgear, we don't have an independent radar simulation. We do it using multiplayer packets, which include lots of information that is not transmitted by a real transponder. The primary purpose of multiplayer is to render a third-party model in the simulator, so it includes all sorts of stuff like landing lights, gear position, etc.

For an ATC client, it has access to much more information than radar provides. If it wants to create a pure radar simulation, it should really just be looking at these items:

latitude/longitude (the radar echo)
ident (transponder squawk code)
altitude (transponder altitude)

I don't see that transponder mode should be transmitted over MP and I don't see that an aircraft needs to do anything with the mode other than set it to a fixed mode (usually S) that indicates its capability. The transponder instrument transmits ident and altitude based on the position of knob mode.

The bottom line is that I think the transponder instrument is doing a reasonable simulation. It doesn't deal with the mode S serial numbers and I'd question whether it should be transmitting ident in SBY mode but my feeling now is that it's working as it should. An ATC client has no business knowing the position of the transponder knob (knob-mode) or the capability (mode) of the transponder. The only information it should get from a transponder is the ident and altitude, which I think it does.

EDIT: I haven't tested whether ATC clients see ident and altitude correctly based on the (fixed) setting of mode and the cockpit setting of knob-mode. I am assuming that works.
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Re: Transponder properties

Postby CaptB » Wed Jun 07, 2017 8:25 am

@sanhozay:

As ATC we care about the data we recieve from the transponder in order to at least identify the radar blip , that should be either:

For Mode A: Assigned Squawk
For Mode C: Assigned Squawk+Altitude
For Mode S: Mode C data + Other

Let me state this clearly, on MP, whatever the value is set in /instrumentation/transponder/inputs/knob-mode, we do not see even the squawk.
Whatever the value is set in /instrumentation/transponder/inputs/knob-mode it will not affect what we see, regardless if you try to set it at the airplane's transponder, the menu or directly in the property browser, it also does not affect the value at /instrumentation/transponder/inputs/mode which in turn *does* transmit all relevant information as required over MP.

In other words I *do* get a positive identification and expected data on MP if /instrumentation/transponder/inputs/mode is set to:
2 = Mode A
3 = Mode C
4 = Mode S

Although the purpuse of the property seems to be clear, the wiki does not mention it at all and suggests a different property for mode operations which is not working. This is very confusing.
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Re: Transponder properties

Postby sanhozay » Wed Jun 07, 2017 8:37 am

CaptB wrote in Wed Jun 07, 2017 8:25 am:Let me state this clearly, on MP, whatever the value is set in /instrumentation/transponder/inputs/knob-mode, we do not see even the squawk.

That would be a problem. You should always see squawk (unless the transponder is off).

I *do* get a positive identification and expected data on MP if /instrumentation/transponder/inputs/mode is set to:
2 = Mode A
3 = Mode C
4 = Mode S

So is this a configuration issue in the aircraft? It needs to set one of those modes (or at least the default should be to set one of those). But not in response to the knob position, it should be a one-time setting.
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Re: Transponder properties

Postby CaptB » Wed Jun 07, 2017 8:46 am

sanhozay wrote in Wed Jun 07, 2017 8:37 am: So is this a configuration issue in the aircraft? It needs to set one of those modes (or at least the default should be to set one of those). But not in response to the knob position, it should be a one-time setting.


Not just the aircraft, it seems that everything that follows the wiki's guidelines and uses /knob-mode for transponder mode operations is affected, the menu's radio stack as well. It really looks like /knob-mode is a dummy property and /mode is the one that is showing correct behaviour. Maybe a typo somewhere where /knob-mode should initially be set by FG?
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Re: Transponder properties

Postby sanhozay » Wed Jun 07, 2017 8:49 am

Are you able to fire up an ATC client somewhere and we'll test it?
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Re: Transponder properties

Postby CaptB » Wed Jun 07, 2017 8:54 am

I was testing a moment ago, still online at EKCH
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