Mode S
 

Following on from a discussion I was having with another controller the other day, how common are Mode S transponders in the cockpit these days? And some say they are limited up to the numeral 7 as we are now, but also that they can use 8 and 9. Anybody out there care to end the suspense?

Gonzo.

Hi Gonzo,

Every modern airliner has a Mode S transponder nowadays. It's been a while since I studied the thing, but essentially it's a Mode C with datalink capability. It can send data like aircraft type and can communicate with other Mode S transponders in case of TCAS conflicts (ie. one plane will make the decision to advise a descent while the other will then advise a climb so that the pilots don't do the same thing and still collide).
Still the 0-7 digits though.

Full Mode S requires a Mode S transponder in the a/c and Mode S sensors and automation on the ground. The Mode S sennsor on the ground has a dozon or so transmission types to work with ATCRBS transponders, mode S transponders and military stuff. The thing that is special about Mode S is that interogations can be made selectively (where the "S" come from) to individual aircraft so you don't have to spend your time interrogating and getting rid of replies from all the airplanes you don't want to see to get at the one you want.

The data link part is kind of true. there are specifications for a reply that has more than the Mode C data that can be used as a data link, but isn't right now in any consistent fashion. One of the things this data link could be used for is for Automatic dependent Surveillance where the aircraft knows where it is and tells everyone else where it is. this is NOT TCAS, and in fact is independent of TCAS and doesn't interoperate with TCAS.

Anyone for dueling standards?

Thanks for the replies, so I can still make fun of you guys when a pilot reads back a squawk containing 8 or 9 then? Hehehehehe!

IZ,
When you say 'every modern' airliner, basically glass cockpit/FMC/FMS a/c? Just trying to get an idea of how long they've been in your flightdecks.

RATBOY, looking in the TCAS 2000 manual of our aircraft (B757), it says that it does work closely together with the Mode S transponder!

A definition of Mode S transponder from my manual:

Mode S
A type of Secondary Surveillance Radar transmission that permits direct interrogation of the transponder form the ground or from the air. Each Mode S transponder equipped aircraft has a unique ICAO registered address by which the transponder may be selectively interrogated. The mode also supports limited, two-way data communication between the aircraft and the interrogating station.
Mode S is the means through which one TCAS 2000 equipped aircraft coordinates avoidance strategies with another TCAS 2000 equipped aicraft.
---

I am not 100% sure but the way I read it, every plane that's TCAS equipped (be it version I, II or 2000) has a Mode S transponder. You're able to see that on the flightplan right, Gonzo?

Thanks Iz,

Now, I've not seen a proper flightplan for quite a while! I'l have to remember to have a look next time I'm at work.

Gonzo.

Well I believe ALL passenger airliners, even commuters are required to have TCAS so I guess they'll have Mode S, as well as modern jets (corporate jets, large transports).

Iz---
Wrong.

Thanks 411A. Enlighten us please! Just trying to help here.

Not all aircraft with TCAS have mode S transponders. Mode S is not required for TCAS to operate properly.

Requirements for TCAS equipage in U.S. is for all Part 121 (airlines and larger commuter aircraft) carrying pax for $. Ther e are some requirements for freighters too, I believe. TCAS uses the transponder provided information (including Mode C if the a/c provides it) but TCAS II Version 7 (the current one) has things in it to make use of all kinds of transponders, not just Mode S, including some older transponders that have some quirky logic in them.

Mode S has been in airliner and higher end GA aircraft in the U.S. for over 10 years. The reason for equipage is requirements in some terminal areas because the "regular" transponders couldn't handle the number of target replies in the time available (gross oversimplification provided gratis here)

The ground sensor for Mode S will take inputs from all kinds of transponders. There is a data link feature in the Mode S that was on it's way to being used in the U.S. a few years ago but the cost of building a system using it and getting everybody to equip and who knows what other factors was apparantly a "too hard" but the capability is still there. There are some people looking to use this spectrum and data link for surveillance and communications but there are some competing methods and it is not completely certain which data link will be used. Check out ICAO SICAS Pannel and Eurocontrol web sites.

Gonzo,

The transponders have 8 digits, 0-7, the corresponds to the following in binary
0-000
1-001
2-010
3-011
....
7-111

ie say when you set 7700, you are transmitting 111111000000 to the radar site.

TCAS I provides traffic advisories (TA) and proximity warning of nearby traffic to assist the pilot in the visual acquisition of intruder aircraft. TCAS I is mandated for use in the by a number of regulatory authorities for turbine-powered, passenger- carrying aircraft having more than 10 and less than 31 seats. TCAS I is also used by a number of general aviation fixed and rotary wing aircraft.

TCAS II provides traffic advisories and resolution advisories (RA), i.e., recommended escape maneuvers, in the vertical dimension to either increase or maintain the existing vertical separation between aircraft. Airline aircraft, including regional airline aircraft with more than 30 seats, and general aviation turbine-powered aircraft use TCAS II equipment.

The TCAS concept uses the same radar beacon transponders installed on aircraft to operate with ATC ground-based radars. The level of protection provided by TCAS equipment depends on the type of transponder the target aircraft is carrying. i.e if a 747-400 with TCAS II (with mode S transponder) comes near a general aviation aircraft with a mode A transponder (no altitude return), the 747-400 will only be able to get the range, and bearing of the general aviation aircraft, but no altitude return, and no vertical seperation.

The latest changes to TCAS II, collectively known as TCAS II Version 7,were certified in early 2000 and are now being implemented by the industry. TCAS II Version 7 is the only version of TCAS II that complies with the ICAO Standards and Recommended Practices (SARPs) for ACAS II. As such, Version 7 is currently being mandated for carriage in certain countries or regions, e.g., Europe, Australia, and India, and has been mandated for carriage in 2003 by the International Civil Aviation Organization (ICAO).

A Mode S transponder is required to be installed and operational for TCAS II to be operational. If the Mode S transponder fails, the TCAS Performance Monitor will detect this failure and automatically place TCAS into Standby. The Mode S transponder performs the normal functions to support the ground-based ATC system and can work with either an ATCRBS or a Mode S ground sensor. The Mode S transponder is also used to provide air-to-air data exchange between TCAS-equipped aircraft so that coordinated, complementary RAs can be issued when required.

Because of the selective address feature of the Mode S system, TCAS surveillance of Mode S equipped aircraft is relatively straightforward. TCAS listens for the spontaneous transmissions, or squitters, that are generated once per second by the Mode S transponder. Among other information, the squitter contains the unique Mode S address of the sending aircraft.

Following the receipt and decoding of a squitter message, TCAS sends a Mode S interrogation to the Mode S address contained in the squitter. The Mode S transponder replies to this interrogation and the reply information is used by TCAS to determine the range, bearing, and altitude of the Mode S aircraft.

:rolleyes:

Thanks for that. I was wondering about the use of the numerals 8 and 9 because when I'm doing Delivery I sometimes get a pilot giving a wrong readback of a squawk, which sometimes contains an 8 or a 9!

So am I correct in thinking that generally if an aircraft has TCAS II then it has Mode S?

On another but related subject, if you're on approach at, say, 3000 feet, and a helicopter crosses underneath you squawking, but no Mode C, you only get a 'Traffic Traffic' call, correct? And what is your procedure then? Look for the traffic and ask ATC it's level?

Gonzo.

Every aircraft that has TCAS II installed now, has Mode S and I'm sure about that one.
;)

Indeed, you're correct in your scenario. If there's no altitude reporting on the target, you only get a TA (Traffic Advisory) with the Traffic call. Look out and ask ATC.
If you get an RA (Resolution Advisory), the aircraft will issue you a vertical pitch command in order to stay clear of the traffic and the command will be announced (eg. "Climb", "Descend now", "Increase vertical speed"). That command must be followed unless you're absolutely sure that the other traffic is no factor.

Thanks Iz,

I ask because at Heathrow there is a low-level helicopter route that crosses underneath the 27R and 27L ILS. The route has a maximum level, which is separted from the traffic above, but it will be on another frequency, and often not squawking mode C. this usually results in a polite enquiry ;) about 'traffic on TCAS' which means replying with 'the traffic is following a published low-level helicopter route, promulgated at not above 1200 feet'. Then I have to phone down to the controller working the helicopter and get a confirmation of the helo's level and ask if it can squawk C.

Thanks again for your input.

Gonzo.

[This message has been edited by Gonzo (edited 05 May 2001).]

Who designed the airspace where 2 ILS approaches to a major airport have a low level route under the ILS approach, I presume inside the outer marker, and allow people to fly on this low level route without any altitude reporting?

Sounds a little more sporty and open to error than is desirable.

I believe (without checking the charts) that the nearest they get to the airfield is 8 miles, max alt of 750ft.

Gonzo.

Gonzo,

When I did my radio-telephony practical exam, BZF, (LBA), reading back a squawk code containing 8 or 9 would have been an immediate fail! :-) ...

Zeke, could you expand on your binary logic to give the whole table up to the numbers you quote, please? It was a long time ago, and I'm a tad lost....

Thanks,

Alien

320Driver,

LOL, How about sneezing on the RT? I've done that before......!

Gonzo.

Alien Shores,

A not to in depth discussion on the operation of transponders can be found on this AvWeb Transponder Basics link

A total of 4096 transponder codes are available under the present arrangement of 0-7, these are decimal numbers that are just converted to binary (ie 0 is converted to 000, 7 is converted to 111) so the pulses that are sent back to the ATC radar antenna at 1090 MHz in 20.3 microsecond blocks contain these transponder code binary pulses and the encoded altitude which is also a binary number encoded into a bunch of 0's and 1's.

:rolleyes:

While speaking of Mode S.
Anyone having the status regarding Eurocontrol (ICAO ?) introduction of Elementary and later Enhanced Surveillance functions to the Mode S ? Ref the addition of DAPs (Down Link Aircraft Parameters).

I’m not fully updated on this, but my understanding is Elementary Surveillance includes, among other things, the requirement of the Mode S to automatic report aircraft identification.

stab trim

Try here for info from the horses mouth.

Eurocontrol Mode S Information

------------------
10 West
UK ATC'er
[email protected]

[This message has been edited by 10W (edited 18 May 2001).]

Thank you 10W. Very informative site.

I think a computer nerd might argue that a squawk code is not a decimal number, it is an octal (base 8) number.

The reason for it (as Zeke points out)is that octal numbers convert exactly to binary, whereas decimal numbers do not.

8^4=4096
2^12=4096

You've wandered a bit off the thread here but just to answer 411A's comment that not all TCAS equipped aircraft have mode 'S' transponders.

TCAS equipped aircraft have mode "S" Transponders, that's how TCAS works. The TCAS creates a 'protected area' around the aircraft by interrogating nearby transponders operating in modes A, C or S. The TCAS antennae are direction sensitive but are accurate to only about 10 degrees or so. In combination with the range data obtained from the transponder replies the TCAS Processor derives a closure rate, and determines the threat level. If the TCAS determines that a threat exists, it tries contacting the target aircraft via the mode 'S' transponder data link. If the target is also TCAS equipped the two TCAS cooperate via mode 'S' to calculate a joint avoidance manouvre with one TCAS advising a climb or hold and the other a descend or hold manouvre. If the target aircraft has only mode A/C then a cooperative response is not possible and TCAS assumes that the target aircraft will continue its present flight path. TCAS then computes an avoidance manouvre based on no cooperation from the target.

To emphasise. By definition, ALL aircraft that have TCAS have mode 'S' transponders.

**********************************
Through difficulties to the cinema

Blacksheep---
You may be interested in knowing that our company aircraft (Lockheed 1329) has a mode "C" transponder fitted and also has TCAS I. Your statement about mode "S" transponder requirements is correct regarding TCAS II.

Blacksheep,

I have to concur with 411A, for TCAS I you do not need a transponder at all, only the threat aircraft does, the TCAS I system can have its own transmitter/receiver independent of the transponder.

TCAS I will happily work even if the threat aircraft only has a Mode A transponder. TCAS I provides a flight deck display of traffic in the vicinity and, if appropriate, alerts the pilot that an aircraft presents a potential threat by generating a Traffic Advisory (TA).

TCAS II will provide TAs and, if the system calculates a risk of collision with an intruder aircraft, which is equipped with an altitude reporting transponder or TCAS, recommend avoidance maneuvers, in the vertical plane, to the pilot by generating a Resolution Advisory (RA). TCAS II you can still get an RA if the threat aircraft only has Mode C (for the altitude encoding). For a co-ordinated RA, both aircraft need Mode S for the inter aircraft data link.

TCAS II equipped aircraft use the Mode S transponder connected to the top and bottom antennas to do the transmitting and receiving.

:rolleyes:

I stand corrected. You are right about TCAS I.

But TCAS I? That should have gone out with the ark. Even TCAS II with v7.0 software is only a short term interim solution to the full intent of European ACAS requirements. In US airspace TCAS II with v7.0 software is the current standard for heavy public transport machines. It ought to be standard for all aircraft everywhere but of course, safety has a price that politicians are not willing to pay. Cheap travel for the masses is far more important. :rolleyes:

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Through difficulties to the cinema

Blacksheep,

As I said on the previous page to this thread, TCAS I is mandated for use by a number of regulatory authorities for turbine-powered, passenger- carrying aircraft having more than 10 and less than 31 seats. i.e. turbopros of the Jetstream 31/32, B1900, Metro size.

:rolleyes: