Mode S transponder codes

[John Collins]

I have heard this before, both for 406 ELTs and 406 PLBs, but have never completely understood this. Do you know where this requirement comes from? Is it an FCC or FAA requirement? I wouldn't think the FAA would have anything to say about PLBs.

It is from the FCC. see below. See the registration requirements from this brochure: http://www.sarsat.noaa.gov/Registration Brochure.pdf

TITLE 47 - TELECOMMUNICATION

CHAPTER I - FEDERAL COMMUNICATIONS COMMISSION

SUBCHAPTER D - SAFETY AND SPECIAL RADIO SERVICES

PART 87 - AVIATION SERVICES

subpart f - AIRCRAFT STATIONS

87.199 - Special requirements for 406.0406.1 MHz ELTs.

(a) Except for the spurious emission limits specified in 87.139(h), 406.0406.1 MHz ELTs must meet all the technical and performance standards contained in the Radio Technical Commission for Aeronautics document titled Minimum Operational Performance Standards 406 MHz Emergency Locator Transmitters (ELT) Document No. RTCA/DO204 dated September 29, 1989. This RTCA document is incorporated by reference in accordance with 5 U.S.C. 552(a), and 1 CFR part 51. Copies of the document are available and may be obtained from the Radio Technical Commission of Aeronautics, One McPherson Square, 1425 K Street NW., Washington, DC 20005. The document is available for inspection at Commission headquarters at 445 12th Street, SW., Washington, DC 20554.

Copies may also be inspected at the Office of the Federal Register, 800 North Capital Street NW., suite 700, Washington, DC.

(b) The 406.0406.1 MHz ELT must contain as an integral part a homing beacon operating only on 121.500 MHz that meets all the requirements described in the RTCA Recommended Standards document described in paragraph (a) of this section. The 121.500 MHz homing beacon must have a continuous duty cycle that may be interrupted during the transmission of the 406.0406.1 MHz signal only.

(c) Prior to verification of a 406.0406.1 MHz ELT, the ELT must be certified by a test facility recognized by one of the COSPAS/SARSAT Partners that the equipment satisfies the design characteristics associated with the COSPAS/SARSAT document COSPAS/SARSAT 406 MHz Distress Beacon Type Approval Standard (C/S T.007). Additionally, an independent test facility must certify that the ELT complies with the electrical and environmental standards associated with the RTCA Recommended Standards.

(d) The procedures for verification are contained in subpart J of part 2 of this chapter.

(e) An identification code, issued by the National Oceanic and Atmospheric Administration (NOAA), the United States Program Manager for the 406.0406.1 MHz COSPAS/SARSAT satellite system, must be programmed in each ELT unit to establish a unique identification for each ELT station.

With each marketable ELT unit the manufacturer or grantee must include a postage pre-paid registration card printed with the ELT identification code addressed to: NOAA/SARSAT Beacon Registration, E/SP3, Federal Building 4, Room 3320, 5200 Auth Road, Suitland, MD 207464304. The registration card must request the owner's name, address, telephone, type of aircraft, alternate emergency contact, and other information as required by NOAA. The registration card must also contain information regarding the availability to register the ELT at NOAA's online Web-based registration database at: http://www.beaconregistration.noaa.gov. Further, the following statement must be included: WARNING Failure to register this ELT with NOAA before installation could result in a monetary forfeiture being issued to the owner.

(f) To enhance protection of life and property, it is mandatory that each 406.0406.1 MHz ELT must be registered with NOAA before installation and that information be kept up-to-date. In addition to the identification plate or label requirements contained in 2.925 and 2.926 of this chapter, each 406.0406.1 MHz ELT must be provided on the outside with a clearly discernable permanent plate or label containing the following statement: The owner of this 406.0406.1 MHz ELT must register the NOAA identification code contained on this label with the National Oceanic and Atmospheric Administration (NOAA), whose address is: NOAA/SARSAT Beacon Registration, E/SP3, Federal Building 4, Room 3320, 5200 Auth Road, Suitland, MD 207464304. Aircraft owners shall advise NOAA in writing upon change of aircraft or ELT ownership, or any other change in registration information. Fleet operators must notify NOAA upon transfer of ELT to another aircraft outside of the owner's control, or an other change in registration information. NOAA will provide registrants with proof of registration and change of registration postcards.

(g) For 406.0406.1 MHz ELTs whose identification code can be changed after manufacture, the identification code shown on the plant or label must be easily replaceable using commonly available tools.


​

 

[poadeleted20]

I'll respectfully disagree with you on this. If the owner knows there is a discrepancy with an installed piece of equipment then it is up to the owner to comply with 91.405.

Feigning ignorance on an installed piece of equipment or trying to blame the FAA does not absolve the owner from compliance.

...and, once again, how is it the FAA expects the owner to be aware of an issue which its own Airworthiness Inspectors apparently are not? Neither I nor anyone here except John seem to have been aware that the Mode S code is directly related to the N-number rather than the airframce by serial number. Mode S codes aren't even mentioned in either the job aid in FAA Order 8900.1 for issuing a replacement Airworthiness Certificate (including N-number changes), or the FAA's web page on how to change your N-number. Seems to me that the FAA should fix this rather than having its inspectors berate people and threaten enforcement action on something about which the FAA hasn't informed aircraft owners or even its own people responsible for the matter.

 

[RotorAndWing]

I have no disagreement with your statement. My post assumed the owner was not aware of a discrepancy and had not been informed of one.

You can argue that the owner should have been aware of the discrepancy, but prior to this discussion, where you aware of this? I suggest that the vast majority of pilots and IA's are totally unaware of the consequences I pointed out.

Yes I was aware of this. When I owned my last Cessna 337 I installed a Garmin 330. Since this was a new piece of equipment I made myself aware of all the operation requirements.

 

[John Collins]

I believe when you request an N number change, the FAA sends you a document with the mode S code that is associated with the N number. Someone who has gone thru this process can look at their documentation and determine what data is provided prior to the N number change.

 

[RotorAndWing]

Since you're not an Airworthiness Inspector, I guess you can't answer my question. Thanks again for your always helpful responses.

Your question was answered, however not to your desired outcome.

 

[poadeleted20]

I believe when you request an N number change, the FAA sends you a document with the mode S code that is associated with the N number. Someone who has gone thru this process can look at their documentation and determine what data is provided prior to the N number change.

I don't remember getting such a document, but that was five years ago. Perhaps it's different now.

 

[RotorAndWing]

...and, once again, how is it the FAA expects the owner to be aware of an issue which its own Airworthiness Inspectors apparently are not? Neither I nor anyone here except John seem to have been aware that the Mode S code is directly related to the N-number rather than the airframce by serial number. Mode S codes aren't even mentioned in either the job aid in FAA Order 8900.1 for issuing a replacement Airworthiness Certificate (including N-number changes), or the FAA's web page on how to change your N-number. Seems to me that the FAA should fix this rather than having its inspectors berate people and threaten enforcement action on something about which the FAA hasn't informed aircraft owners or even its own people responsible for the matter.

Wow, now that's over the top!

 

[John Collins]

I don't remember getting such a document, but that was five years ago. Perhaps it's different now.

There is an example of a filled out AC Form 8050-64, Assignment of Special Registration Numbers:

http://www.paglen.org/oddsnends/gitmoexpress.pdf

The ICAO aircraft address code is the transponder code represented as 8 octal digits. Note that it only applies to the requested N number, not the aircraft serial number. So for N44982 the octal code is 51267627. To convert it to hexadecimal, convert to 3 bit binary octets and then group into 4 bit Hexadecimal digits.

101 001 010 110 111 110 010 111

1010 0101 0110 1111 1001 0111

A56F97

 

[brcase]

Without responding to your purely inciteful statement, it seems to me that an FAA Inspector signing off the registration change would check this before signing the paperwork. And if an FAA Airworthiness Inspector doesn't know about this, it's quite absurd to expect an aircraft owner to know it, especially since it's not part of any FAA pilot publication of which I am aware that registration number changes affect Mode S codes. Perhaps you can point out where it says that in the AIM, FAR's, Pilot's Handbook of Aeronautical Knowledge, etc.

It would not be the 1st time an FAA inspector wasn't aware of the rules.

I have corrected my share of them.
Like the one who told us we couldn't carry passengers while towing gliders.
I am still trying to find the one that said our 1960 172 needed to have Red Fuel caps on it, I would like to know where he found that rule/requirement.

Brian

 

[poadeleted20]

There is an example of a filled out AC Form 8050-64, Assignment of Special Registration Numbers:

http://www.paglen.org/oddsnends/gitmoexpress.pdf

The ICAO aircraft address code is the transponder code represented as 8 octal digits. Note that it only applies to the requested N number, not the aircraft serial number. So for N44982 the octal code is 51267627. To convert it to hexadecimal, convert to 3 bit binary octets and then group into 4 bit Hexadecimal digits.

101 001 010 110 111 110 010 111

1010 0101 0110 1111 1001 0111

A56F97

Next time I'm out at the airport where those documents are, I'll have to look at mine. However, while one of the documents in the package you attached does have a Mode S code on it, there's nothing I see indicating that the code on that form is different from the old code. If the FAA wants this done right, they need to do a better job of informing both inspectors and owners on this matter.

BTW, after reviewing my records, my Mode S transponder was installed after the N-number change, and ATC has never said anything about it, so it appears the shop that did the work put in the correct code for the new N-number, and R&W will have to wait for some other reason to write me up for violating 91.405 (if that really is such a violation).

 

[John Collins]

BTW, after reviewing my records, my Mode S transponder was installed after the N-number change, and ATC has never said anything about it, so it appears the shop that did the work put in the correct code for the new N-number, and R&W will have to wait for some other reason to write me up for violating 91.405 (if that really is such a violation).

I have been told but have no way of confirming it that the ATC radar display doesn't show the mode S code or the N number from the code.

 

[poadeleted20]

I have been told but have no way of confirming it that the ATC radar display doesn't show the mode S code or the N number from the code.

I've heard the same. And if they did use the Mode S codes, the track swap which resulted in Phil Boyer's wife getting accused incorrectly of violating the then-DC ADIZ could not have occurred, since her aircraft was ADS-B equipped (the fact that led to the discovery of the FAA's error). Also, they wouldn't keep asking me for my full call sign.:wink2:

 

[flyingcheesehead]

In addition to the 24 bit ICAO address, a Flight ID is transmitted by the mode S transponder. It is the N number if it is fixed as would be the case for most of our GA aircraft. If there is a need to use a flight ID that can change each flight (for example in the case of an airliner flight number), then the 8 character flight ID is entered at startup and the transponder is configured accordingly.

Interesting. I wonder what's going to happen with the flights that are usually just an N number, but occasionally are something different. For example, "Skylane 271G" vs. "Angel Flight 71G".

 

[John Collins]

Interesting. I wonder what's going to happen with the flights that are usually just an N number, but occasionally are something different. For example, "Skylane 271G" vs. "Angel Flight 71G".

If the aircraft is regularly used for Angel flights, you might consider configuring the transponder for manual entry of the flight ID, although I am not convinced it makes a difference to ATC and I would not do it.

 

[flyingcheesehead]

If the aircraft is regularly used for Angel flights, you might consider configuring the transponder for manual entry of the flight ID, although I am not convinced it makes a difference to ATC and I would not do it.

I thought about that, but what a pain to have to enter the flight ID every single flight - Ugh, forget it!

 

[denverpilot]

Interesting. I wonder what's going to happen with the flights that are usually just an N number, but occasionally are something different. For example, "Skylane 271G" vs. "Angel Flight 71G".

The N-Number of the aircraft didn't change just because it's using a different *callsign*. A minor but important distinction.

Same thing with "CAP ####" callsigns, airline callsigns and even adding "LifeGuard" to your regular callsign or "Rescue" as appropriate.

What you are allowed to call yourself on the radio doesn't change the N-number painted on the tail. If Mode-S is N-number based, it doesn't need to change.

 

[denverpilot]

P.S. Callsigns can even be authorized by local FSDOs. The traffic reporters around here (what few of them are left) have a letter of authorization to use a special callsign when talking to DEN TRACON and/or ZDV and local Towers.

 

[flyingcheesehead]

The N-Number of the aircraft didn't change just because it's using a different *callsign*. A minor but important distinction.

Same thing with "CAP ####" callsigns, airline callsigns and even adding "LifeGuard" to your regular callsign or "Rescue" as appropriate.

What you are allowed to call yourself on the radio doesn't change the N-number painted on the tail. If Mode-S is N-number based, it doesn't need to change.

The N-number didn't change, and neither did the Mode S code - But if the airlines are going to use transponders that allow them to set a new flight ID on every flight, clearly someone's thinking that the Flight ID will be what's on the controller's scope at some point. So, what happens when I call in as "Angel Flight 71G" from an aircraft identifying itself as N271G? Just something to consider in the let's-do-everything-digitally-and-automatically age.

 

[denverpilot]

I see numerous possibilities for serious problems with direct human entry of such data. I suppose we pilots usually get our transponder codes right, but entering N numbers along with oddball codes for airlines, etc?

Of course I'm also the guy who says the fact that ADS-B is unencrypted with no authentication will mean that it will also be messed with by some hacker kiddie from the ground.

Anyone who's taken a basic CompSci course or worked in real-world networks can share lots of stories about unencrypted identification data getting jacked up. SNMP stated out w/o encryption, DNS got DNSSEC, SMTP got layered under SSL/TLS, HTTP got extended with SSL to HTTPS, ad nauseaum.

And the FAA is stuck in the 90's building unencrypted systems that'll be 15-20 years outdated the day they turn them on. Not smart. Not even best-practices for modern data networks.

Of course this is the same agency who built a network that could be brought down by one router in a closet in SLC crashing and not being able to find the guy on-call who had the keys to the closet.

Perhaps it bodes well that the FAA is going to need some real IT geeks and Management one of these days. Wonder when they'll start hiring and spending real money on doing it right?

Ooh maybe I just discovered the purpose of the Kent Shook EE degree?

 

[flyingcheesehead]

Of course I'm also the guy who says the fact that ADS-B is unencrypted with no authentication will mean that it will also be messed with by some hacker kiddie from the ground.

Anyone who's taken a basic CompSci course or worked in real-world networks can share lots of stories about unencrypted identification data getting jacked up.

Hell, ask the Air Force. The video feeds from the Predators are unencrypted.

Perhaps it bodes well that the FAA is going to need some real IT geeks and Management one of these days. Wonder when they'll start hiring and spending real money on doing it right?

Ooh maybe I just discovered the purpose of the Kent Shook EE degree?

Hahaa, maybe so!

 

[n12365]

I don't know. I have tried to obtain this information before, but was told that the algorithm was not in the public domain.

This is an old thread, but I figure this might be useful to anyone searching for how to convert N numbers to ICAO addresses. I am working on an ADS-B related project and needed to be able to verify your N number can be converted to your FAA assigned ICAO (Mode-S) address via an algorithm. Like John, I wasn't able to obtain the algorithm, only references to the existence of an algorithm. So I ended up looking at the FAA registry, which lists N numbers and ICAO addresses, and reverse engineered the algorithm. Attached is a simple write up of how to convert an N number to an ICAO address. It works for all of the entries currently in the FAA registry. Hopefully someone else will find this useful.

Ryan

 

[JimNtexas]

...and, once again, how is it the FAA expects the owner to be aware of an issue which its own Airworthiness Inspectors apparently are not?

.
The same way they expect the owners and pilots to aware of every letter of intrepretation on every bureaucrat's bookshelf in the country, as well as every remark in every NPRM.

ESP.

After all, all this has the 'force of law', right?

 

[propellerhead99]

Ryan - I tried to download the N number to/from ICAO algorithm with no joy. Is there a fix or could you send it to me?
Thanks.

 

[Jim_CAK]

I have been told but have no way of confirming it that the ATC radar display doesn't show the mode S code or the N number from the code.

On a tower tour at KCAK a few years ago, the controller, also a pilot, told me they had no way to display it.
I would be interested in hearing from the ATC guys on the board if there is some radar systems that can display it.

 

[dans2992]

ATC radar might not, but you can get it off FlightAware or FlightRadar24


Sent from my iPhone using Tapatalk

 

[kontiki]

Didn't really read the thread, apologies if this is a repeat.

Minimum Operational Performance Specification for a 1090 Mhz Mode S Transponder with ADS-B are contained in two RTCA Specifications. RTCA DO-181C defines a basic Mode S Transponder (up to and including Enhanced Transponder requirements required by Europe in 2000). The ADS-B format is defined in DO-260B (I believe C & B are the latest versions). There also some ADS-B performance standards, NIC, NAC and SIL, in the FARs too.

Now ICAO Annex 10 assigns the various countries a block of ICAO addresses. The coding algorithm for the US block is I believe defined in DO-181. Not all countries have the same number of bits available or use the same algorithms.

At one time (way back) the FAA distributed a Visual Basic program that looked up the ICAO address. I've never bothered to unwind the algorithm, it's weird. If I wanted to look one up, I'd use the FAA aircraft registry tool. It gives you the Octal and Hex version. The built in PC scientific calculator can convert either one into the binary version. see -> http://registry.faa.gov/aircraftinquiry/NNum_Inquiry.aspx

Just an edit, it wasn't an old visual basic program, it was a quick basic program from the days of the Microsoft DOS.

 

[kontiki]

Tom, here is the source file for that old FAA Quick Basic Program to convert a aircraft registration to the 24 bit ICAO code. I don't recall if it outputs binary, octal or hex.

You could probably somehow run it in the Microsoft command.com interpreter if you have Quick Basic (Q Basic). Just rename it from .txt to .bas.

From memory, Q Basic compiled text to an executable when you ran it.

About 15 years ago, I got it from someone who got it from someone else. I've used it 100s of times and at one point had to use it to produce a list of codes for an entire fleet of airplanes at an airline because the FAA inspector assigned to the airline didn't want to bother with OK city. That was when we had to post a letter to OK city for the data (ancient days). I've never seen it produce an incorrect conversion.

 

[Jo Kremsreiter]

The mode s code is the tail number of the aircraft - it is also known as the ICAO code. The tail number is converted to a digital number with 1's and 0's - this is a 24 bit number so there are 24 1's or 0's for example N1234 is
Hex: A061BB
Octal: 50060673
Binary: 101000000110000110111011 - Notice 24 bits..
1's and 0's are what computers use. The hex and octal eventually get converted to 1's and 0's in the end.
The mode S code changes when the tail number changes so your transponder, ELT, Satcom, FMS or any other system on board that has the ICAO code will have to have its settings changed to match the new tail number.