Mode 'S' - Confused

[TotalBeginner]

One of our club aircraft has recently been fitted with a GTX330 mode 'S' transponder.

I've been studying the manual and I must say, I'm slightly confused.

Quote: "In ALT mode the transponder replies to identification and altitude interrogations as indicated by the reply symbol. Replies to altitude interrogations include the standard pressure altitude recieved from an external altitude source which is NOT adjusted for barometric pressure".

So lets say I'm flying under the London TMA at 2400 (base 2500) on the LDN QNH 1002. the transponder is displaying PRESSURE ALT FL028 on it's display.

Will my returns indicate that I am flying at 2800ft, well above the lower limit of the LTMA???

[Fuji Abound]

Yes.

(but the controller will know the transponder reports flight level).

Mode S has added nothing knew in this regard, what is knew is your callsign is also transmitted (probably so they can charge you for use of the air in due course or more easily prosecute you when you infringe controlled airspace). Of course the good news is I have yet to find a single unit in the UK with mode S capability and I have also seen the view expressed that if (when) we all have mode S the functionality will be turned off by controllers to prevent radar clutter. Ah well I know there was a good reason for fitting one

[Warped Factor]

The transponder sends back level information using 1013mb as a datum.

The ATC radar processors have the local QNH information fed in and the software does the calculation and will show your level as an altitude (two digits) if you're flying below the TA in the area or as a flight level (three digits) if you're flying above the TA on the radar display.

Fuji,

We (LTCC) will be using Mode S derived info from the Autumn.

WF.

[nipper1]

Warped Factor - Will you actually be looking at the Mode S returns from all us little guys or will the 'S' for Selective mean we are selected 'OFF'?

I'm about to pay up for a mode S unit but it does seem a bit odd to be paying for something where the primary objective seems to be to implement a system where we can be ignored by ATC.

Or perhaps I am just being cynical.

[TotalBeginner]

Many thanks for your replies!

Makes much more sense now

[bookworm]

I'm about to pay up for a mode S unit but it does seem a bit odd to be paying for something where the primary objective seems to be to implement a system where we can be ignored by ATC.

There's a widely held misconception about the S for Selective bit of Mode S.

It's Selective at the very lowest level of the protocol. With Mode A/C, every transponder replies to every interrogation, leading to a lot of congestion. With Mode S, the transponders reply selectively to the interrogation, but the radar still tracks every transponder, effectively all the time, because there are still many replies from each in each second.

Just as with Mode A/C based SSR, it will undoubtedly be possible for ATC to filter out certain code categories on displays, but that is not what the Selective is about.

[Sir George Cayley]

Get hold of PF Mag

Last months Poplier Flying Mag from the PFA contained an article giving an excellent resume of Mode S and everything about it.

I'll see if I can post it but meantime try and get one


Sir G

[Warped Factor]

nipper1,

Pre and post Mode S, if you're flying along squawking 7000, we could choose to filter out the numbers from showing on the display but not the primary and secondary radar return.

Post Mode S we should also be able to "tag" you and see your unique Mode S id.

WF.

[IO540]

Out of interest, will ATCUs have a link to the database of Mode S codes, providing them with an instant lookup of the aircraft identification?

Since this information will be available anyway (for airspace busts etc) having it in real time would save a lot of chatter on the radio.

[Pierre Argh]

Mode S - Future ATC Systems

To all you luddites... Despite all the fears about clutter on ATC screens and controllers turning off Mode S, it is being considered as a standalone future ATC system... and I believe is either in use (or on trial) in Austria and Australia.... where the mountains in the case of the former and vast outback in the case of the latter makes normal radar systems difficult if not impossible. The idea of Primary Radar may(?) one day seem as obsolete as using morse code to transmit radio messages.

[bookworm]

Out of interest, will ATCUs have a link to the database of Mode S codes, providing them with an instant lookup of the aircraft identification?

That shouldn't be necessary. To use a computer analogy, Mode S equipped aircraft have both a hardware address like a MAC address, and a user configurable callsign like an IP address that can be set before each flight.

The database you describe links the reg to the hardware address. But Mode S transponders also transmit the callsign as one of the downloadable parameters, just like a computer puts the source address in each packet -- for most GA aircraft it can be stored in flash memory and never changed as it's the registration. Commercial crews will have to set it for their callsign (or have a clever FMS, I guess).

[tmmorris]

Pierre Argh - isn't it Alaska where mode S is making life a lot easier? Poor radar cover but they can get a radar service with mode S (and ADS-B, which I think is the clever bit).

Tim

[bookworm]

isn't it Alaska where mode S is making life a lot easier

Well actually not quite. ADS-B just requires a datalink. The FAA has introduced plans for ADS-B using both the "Mode S extended squitter" (i.e. let's shove some more bits into a Mode S reply) and the Universal Access Transceiver, which is a low cost datalink designed for aircraft that don't need Mode S for other purposes. The Capstone (Alaska) trial ADS-B is based on UAT, not mode S.

http://www.alaska.faa.gov/capstone/avionics.htm

[nipper1]

Warped Factor

Thanks for the info. I am always happy to try and understand how the 'other half' lives. And no Pierre Argh I am not a Luddite.

In fact my thoughts are now turning to how the system can actually be made to work for us small GA guys.

So for example (clean sheet thinking) if we shut down all the LARS (and other 'ATC' outside controlled airspace) and started again, would it be possible to combine all the mode S returns into 'Area based' FIS? I.e. one defined person to speak to in any geographical area.

The big problem now with FIS/RIS etc is the endless 'contact unverified not working this frequency' nonsense we have to deal with.

If, for example, I fly from SAM to CPT (which I quite often do) I will probably speak to Solent Radar and Farnborough. Aircraft all around may be working Boscombe or Benson. I have pretty much b***er all idea what they are up to even if they are conscientiously speaking to a FIS/RIS.

To keep it simple for the sake of my argument, let's base the FIS/RIS areas on altimeter setting regions for the time being. A hypothetical flight from Southampton to Shoreham might then go something like this.

Take off – Southampton Tower.
Flight in Solent Class D – Solent Radar
Flight in the ‘Portland Area’ – Portland FIS/RIS
Flight in the ‘Chatham Area’ – Chatham FIS/RIS
Landing – Shoreham Tower

No messing about with Goodwood (provided I remain clear) or trying to get London information to help.

Many fewer frequency changes and the MASSIVE added bonus of knowing that everyone else out there (or at least their mode S returns) is visible to the guy or gal you are speaking to.

Is this just pie in the sky?

It seems to me that with modern computer and communications technology this is not beyond the wit of man. A better service using fewer people and costing less to run. Too good to be true?

[bookworm]

So for example (clean sheet thinking) if we shut down all the LARS (and other 'ATC' outside controlled airspace) and started again, would it be possible to combine all the mode S returns into 'Area based' FIS? I.e. one defined person to speak to in any geographical area.

I think you're missing the point a bit. You can do that quite happily with Mode A/C, if your ATS provider is prepared to pay enough ATCOs to offer services in the 'areas'. At the moment, they're not, because we're not prepared to pay for it.

What Mode S as delivered in the US enables is a cockpit display of traffic information. Radar derived positions are displayed directly to the pilot on their multi-function display. That takes the controller out of the loop completely, and therefore takes away most of the marginal cost.

Humans are good at using their brains to manage complex situations -- yes, even ATCOs can do that . They're less good at providing a full and accurate picture simply by giving a verbal description of what they see on a screen, and at turning a verbal description into a mental picture -- the communication of traffic position is much weaker by having two humans in the loop.

The way forward has to be to let ATC get on with what it's good at, managing traffic, and let avionics satisfy the conflict alerting function.

[FullyFlapped]

Bookworm, I quite agree wth your last paragraph.

Am I however right in thinking that over here, we won't be able to see returns from other mode S aircraft in our cockpits ?

FF

[2Donkeys]

The TIS system available in the US (which is what is being referred to here) is not currently planned for the UK - because NATS have not specified the necessary hardware/software on their radars. Aircraft equipped with (for example), a GARMIN GNS530 and GTX 330 Mode S transponder can quite happily pick up TIS when the enter US airspace.

On another point of detail - There is no requirement for a database to convert between Mode S Hex codes and US N-numbers. The Hex code is a straight encoding of the tail number. Oh the benefits of being "in" on the system early.

2D

[IO540]

2D

There are just 24 bits available; how do they encode a tail code such 1234AB in that?

[2Donkeys]

Rather easy.

The number of combinations available in 24 bits is 2^24-1 = 16777215

The number of combinations of nnnnAA is 6760000 (in reality this is a slight simplication in view of the availability of shorter registrations), but the maths does not pose a problem. Talk to an avionics man about how the tail number is programmed into a GTX330 if you want further evidence.

2D

[bookworm]

On another point of detail - There is no requirement for a database to convert between Mode S Hex codes and US N-numbers. The Hex code is a straight encoding of the tail number.

I don't think it works for other states. e.g.

40041A is G-BUSK
40041B is G-BNWA

must be pretty fancy coding. I'm not totally convinced about the N-reg version:

52146651 = N66656
52146714 = N66666
52146757 = N66676

Different gaps in Mode S. Something doesn't quite chirp true...