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 :confused:

[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 :D

[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]

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]

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 :ok:

[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...

[2Donkeys]

Whilst the country code-ranges are defined, each state has some lateral movement as to whether or not it opts to provide a direct lookup between its registrations and the range of the 24-bit address block allocated to it.

A number of countries have published algorithms or lookup tables covering the conversion. The UK is not amongst them. I have no idea how UK codes are allocated.

When a GTX330 is programmed on installation, the Hex number is the only external element which the installer is required to input. If hex code is within the US-allocated block, the transponder contains the algorithm necessary to set its own Tail Number. I don't have an electronic reference for that, but I do have the hard copy sitting in front of me.

2D

[2Donkeys]

A quick look through the FAA library reveals the way in which the codes are allocated.

Loosely, the method is this:

Starting at N1 (code 50000001) the Hex code climbs sequentially with the N Number

N number sequence is alphabetical

N1
N1A
N1AA
N1AB
..
N1BA
..
N1BZ
..
N1ZZ
N10
N10A
..
N10ZZ
etc

Registrations containing I and O as letters are missed from the sequence and Discrete codes as shown on the N-number database are of course Octal (perhaps the source of your problem, Bookworm), so that you wont find any 8s and 9s.

I hope this helps.

Nigel

[IO540]

When a GTX330 is programmed on installation, the Hex number is the only external element which the installer is required to input. If hex code is within the US-allocated block, the transponder contains the algorithm necessary to set its own Tail Number.

In that case, while ATC can get the tail number of an N-reg plane from the 24-bit return, how can they get the tail number of a non-N-reg plane?

Will every ATC radar outside the USA contain the algorithm which every possible country uses for the mapping of the 24-bit return to the tail number?

In any event, they won't get the aircraft details. The most they might get is the tail number. So, my original question is whether ATC (any ATC) is going to have a database lookup so they can see the aircraft details (or even just the aircraft type)

[2Donkeys]

how can they get the tail number of a non-N-reg plane?

This too is fairly straightforward.

In addition to being able to decode the Hex code (where possible), the Mode S protocol provides for a second crew-programmable ID field. In commercial use, this will normally contain the Flight number (BAW1234) which will be set from the FMC before each flight. This is distinct from the aircraft's Hex code

It is unclear to me what the Garmin 330 returns in this field, but I can find out by putting a test kit on mine. I would guess that in the case of an N-reg, the field defaults to the same value as the implied Hex value.

ATC systems will, according to the Eurocontrol standards documents, be capable of determining the flight ID through a variety of means (4096-code as now via lookup, Hex code, and Crew-programmed code.) ATC will define the priority to give to each of these mechanisms and the system will highlight any inconsistencies.

Some form of lookup will still be necessary though for comprehensive translation of transponder/callsign - but this is nothing new.

2D

[bookworm]

I hope this helps.

Help? How can it possibly help? It simply proves you right and me wrong. That doesn't help at all. ;)

[Pierre Argh]

Nipper 1... I guess anything's possible, you just need someone, who has the will, to find the money to go for such a reorganisation. Not too sure that in the big scheme of things LARS is too important an issue (personal opinion)... and the consensus seems to be that there would be resistance among the GA fraternity to having to pay a going rate for a LARS service so it is unlikely to be self funding.

(that's probably let another goat out!!!)

Sorry about the luddite comment, if that caused offence, it wasn't directed to you specifically though?

[IO540]

2D, my question remains unanswered. Will ATC have a database lookup?

You suggest that one might be necessary; that much I can see, since the transponder certainly won't return the data. By "nothing new" do you mean they already have a database lookup for aircraft details, given the callsign?

Come to think of it, U.S. radar units will need the same stuff, because plenty of non-N-reg planes fly to the USA.

It would be a good thing if ATC could see the aircraft type, weight, speed range, max POB.

[2Donkeys]

IO540

I thought I'd already answered your question.

ATC already have a lookup. Obviously system details vary by country. In principle though, your squawk code (we are talking about controlled IFR flights here) is cross-linked to your flightplan. Your aircraft and details can therefore, to varying extents, be looked up automatically and details provided to the controller. At its most basic, this functionality is used to provide code-callsign conversion on radar displays.

Under Mode S, in principle, IFR Flights will all squawk 1000 and the Mode S data will provide the link back to the flight plan.

Implementation details will doubtless vary by country. What is certain though, is that a lookup table will be required to tie one or both of the Mode S ID fields (Hex Code or Programmed ID) to the Flight Plan. If Hex Code is relied upon (predominantly GA I would guess), the lookup table will need to be quite comprehensive.

2D

[bookworm]

I don't think a "database lookup" is part of the plan, any more than you need a worldwide database of MAC addresses to surf the Net. The transponder will send the Flight ID, the radar system will display it.

[nipper1]

Pierre Argh - no offence taken.

I think you will find that the GA community 'pays' for every service it uses and much more besides, through fuel duty and VAT on Avgas, neither of which the airlines pay.

What I am trying to do (and seem to some extent to have succeeded) is to try and start a discussion on ways that we, the people who are paying for all those Mode S sets can actually gain some benefit from them.

The CAA is activly seeking input right now. See http://www.caa.co.uk/docs/7/DAP_ORA_ATSOCAS_Consultation.pdf if you want to have your say.

And bookworm, no I can't do it right now with mode A/C, no matter how many ATCOs there are. If I am working Solent and the other guy is working Farnborough (even though we are in the same piece of sky) all the information I get is 'contact in your twelve o'clock on reciprocal heading, height unverified not working this frequency". The Solent ATCO simply does not have the necessary information on his radar screen to help me. (Though perhaps I have misunderstood - maybe all those unverifieds are just aircraft with no Mode C?)

Now it may be that the new Mode S capable SSR they are currently installing for Solent Zone will include this capability in which case, non-too soon I say. Do you know about this Warped Factor?

2Donkeys gets close to the nub of the problem when he points out that NATS have not installed the equipment necessary to make the system really useful to us. No uplink, no cockpit information no ADSB etc. etc..

While here in the UK we are all bleating on about the cost of Mode S, in the USA pilots are willingly splashing the cash with their avionics shop to get the kit installed – even though it is not compulsory. Why? Because here the system is (rightly) perceived to have no real added value to the pilot in the (light) plane but is only there to serve ‘them’. This in turn has led to all kinds of conspiracy theories about what ‘they’ are up to. In the USA, the system has real, immediate and obvious value to the pilot so he spends his cash.

FAA 1: CAA 0

[PPRuNe Radar]

I think you will find that the GA community 'pays' for every service it uses and much more besides, through fuel duty and VAT on Avgas, neither of which the airlines pay.

A fallacy I am afraid. The CAA and Air Traffic Service providers have to self fund through direct user charges, they don't get any of the tax revenues generated.

From the CAA website:

The Civil Aviation Authority (CAA), which is a public corporation, was established by Parliament in 1972 as an independent specialist aviation regulator and provider of air traffic services.

Following the separation_of National Air Traffic Services from the CAA in 2001, the CAA is now the UK’s independent aviation regulator, with all civil aviation regulatory functions (economic regulation, airspace policy, safety regulation and consumer protection) integrated within a single specialist body.

The UK Government requires that the CAA’s costs are met entirely from its charges on those whom it regulates. Unlike many other countries, there is no direct Government funding of the CAA’s work.

[Fuji Abound]

"Because here the system is (rightly) perceived to have no real added value to the pilot"

Very well said.

Having fitted a mode S some while ago as I commented early I have yet to find a unit that can even tell me if it is functioning correctly. Maybe the benefits will come, but given the comments here I am not holding my breath.

[bar shaker]

I have a friend who is an International representaive on the EMF (European Microlight Federation). He is not from the UK.

Switzerland has already said that it will not make Mode S mandatory for rag and tube aircraft in 2008, my friend tells me that Germany and Italy are about to do the same. The reason is health grounds with regards to radiation in non metalic aircraft.

If 4000 UK aircraft get exempt, it rather puts us back where we started. Not that Mode S really moved us anywhere in the first place, without tens, if not hundreds of millions spent on secondary radar capable of using it.

PPRuNe Radar is right.

Gordon Brown trousers all the Avgas tax and still gets a 6% return on everything the CAA does. Those of you who are fans of Mr Brown will perfectly understand the concept of this not being a tax, but rather a return on investment.

That said, the SES draft document specifically says that \'when considering charging for services, member states must take into consideration the revunue raised from duty on fuel used for aviation.\'

[Warped Factor]

nipper1,

As PPRuNe Radar points out, nothing of the tax that you spend on avgas or anything else aviation related comes back to the CAA or NATS or any other UK ATC provider. They fund themselves through user charges.


Now it may be that the new Mode S capable SSR they are currently installing for Solent Zone will include this capability in which case, non-too soon I say. Do you know about this Warped Factor?

In a word, no. I know what is meant to be happening at the LTCC, nowhere else.

WF.