need track replay Mode 4 replys only show with a radar contact.

[Hulkbust44]

If you have a friendly aircraft say 20nm away on radar and interrogate it(assuming your APX- is on), it will come back as friendly. If you were to then move the radar away or silence it to where no track exists, subsequent interrogations yield no reply whatsoever.

 

I was smoked by a friendly Hornet in my F-15 today because he didn't know I was there. Had this been working properly, he would've seen my aircraft when he interrogated the bandit across the circle from me and I might've made it back with those 7 air victories.

 

 

[BIGNEWY]

We would need a track replay to check the circumstances here and the setup of the hornet, it could easily be pilot error. 

[Phantom711]

Well…isn‘t this how IFF works? It identifies radar contacts. 
If that F-15 was STT on that Hostile then he would not get an ID on you. Not sure how it‘s like in TWS or LTWS, but then he still would have needed a radar contact/track file on you.

Edited June 22, 2022 by Phantom711

[Harker]

The CIT should receive replies (and even create CIT-only teackfiles, but let's not get into that) even without a radar contact. IF a radar trackfile exists, CIT data can be combined with it, via the MSI system and the ROE matrix, for HAFU classification. But a radar trackfile is not a prerequisite.

Look at how the F-16 and JF-17 do it in DCS, it's modeled there. They can interrogate the airspace in front of them and receive replies, even if they don't have a radar contact (the F-16 lacks the ability to combine a reply with a radar trackfile, but it can still receive it).

So, in the Hornet, if you run an interrogation and there is a friendly aircraft inside that space (with an enabled and correctly set up transponder), then you should receive a reply from it, even if you don't see it on radar or datalink.

[toilet2000]

On 6/22/2022 at 7:30 AM, Phantom711 said:

Well…isn‘t this how IFF works? It identifies radar contacts. 
If that F-15 was STT on that Hostile then he would not get an ID on you. Not sure how it‘s like in TWS or LTWS, but then he still would have needed a radar contact/track file on you.

 

Not at all.

An IFF interrogator is a radar itself (called secondary radar, basically what ATCs use) and will generate contacts that can be then combined through MSI with radar contacts to generate tracks and their related HAFUs.

As was discussed extensively before, IFF should be not only a contributor to the MSI picture, but actually a full-on sensor in itself. That's why you can steer and define IFF scan ranges on the AZ/EL page.

[Phantom711]

So what you guys are saying is, that there is a second radar in the nose of a fighter aircraft, which works completely independent from the fire control radar and it not only sends out the interrogation and receives the reply (if applicable) but it also detects bearing and range of those contacts (that reply positively)? 

[Tholozor]

Basically yes, the bird slicers forward of the canopy are the IFF antennas for the CIT.

[MrWolf]

On 6/23/2022 at 4:14 PM, toilet2000 said:

Not at all.

An IFF interrogator is a radar itself (called secondary radar, basically what ATCs use) and will generate contacts that can be then combined through MSI with radar contacts to generate tracks and their related HAFUs.

As was discussed extensively before, IFF should be not only a contributor to the MSI picture, but actually a full-on sensor in itself. That's why you can steer and define IFF scan ranges on the AZ/EL page.

 

On 6/23/2022 at 10:48 PM, Tholozor said:

Basically yes, the bird slicers forward of the canopy are the IFF antennas for the CIT.

 

On 6/23/2022 at 10:22 PM, Phantom711 said:

So what you guys are saying is, that there is a second radar in the nose of a fighter aircraft, which works completely independent from the fire control radar and it not only sends out the interrogation and receives the reply (if applicable) but it also detects bearing and range of those contacts (that reply positively)? 

 

You guys are confusing RADAR with antenna. While yes, the secondary RADAR is a different RADAR system from the primary one; because SSR transmits pulse encoded information the baseband system used for SSR it's different from a primary radar (and probably all the pass band processing is performed by the same system block that the PRI and SSR use); they both use the primary antenna. That's because SSR needs to know from which directions (which aircraft) are coming the "I'm friendly" responses (simplified).

To do that you need an antenna with narrow beam width (well actually is more complex, the antenna has to be capable of transmitting a narrow beam for the P1 and P3 pulses of an interrogation, but also omnidirectional for the P2 pulse of interrogation, also called control. Doing so only aircraft inside the interrogation beam treat the interrogation message as for them, the rest of the aircraft treat the interrogation message as not for them). The only antenna, capable of doing that, is the same one that the PRI radar uses.

Every slot (see radar image I attached) on the radar dish is an antenna. The use of all these small antennas, it's called an array of antennas. In order to modify the beam width, you can modify the phase of the EM waves that are transmitted or received from every small antenna, that is called phased array antenna. Doing that, you can also modify the electrical pointing of the antenna (the direction of the main lobe, simplified: the direction of the received and transmitted rays). So you could modify those phases in order to both scan the sky and also modify the beam of the array antenna. I legacy F/A-18 RADAR is PESA.

So you may be wondering, hey PESA sounds like AESA, I know AESA. So what is the difference with PESA?

Well the difference it's that in PESA antennas the all the antennas transmit the same signal. A better way to explaining it is that PESA antennas are "used like traditional antennas", with that i mean that once you have obtained the EM signal that you want to transmit or receive you connect it to the PESA antenna, but the system behind the antenna is the one that it's in charge of transmiting and receiving. On the other hand AESA systems use every single small antena (the slots on the RADAR dish) as a T/R system, EVERY SINGLE ONE, so basically an AESA system is a set of small antenna T/R (transmit/receive)  systems that work together and lend a result. On the other hand, PESA antennas are antennas formed up by small antennas but that are not independent.

At the end of the day, it means that AESA antennas can achieve narrower beam width than PESA antennas ( 1/2 narrower) , but most importantly AESA antennas (because each small antenna has its own RF frontend) have lower SNR constrains for detecting returns and overall are much more robust than PESA antennas (again with AESA antennas you have hundreds of receivers, one for every small antenna, VERY EXPENSIVE). AESA antennas also have a wider frequency range. Finally with AESA antennas you can perform electronically scanning much more easier than in PESA antennas (yes with PESA antennas you can electronically scan, moving the electrical pointing), but also perform much more complex beam forming than with PESA antennas. This means that AESA antennas should be able to perform much more complex tracking and guidance of targets, even with more than one target at a time.

 

IN CONCLUSION: If you move your Primary Radar antenna (the Antenna dish of small antennas), you are moving also your Secondary Radar antenna.

 

Case 1: You have turned off your Primary Radar (ONLY THE BASEBAND RELATED TO PRI RADAR, without turning off any component that may affect the use of the antenna dish) and you perform a IFF interrogation INSIDE THE SCANNING AREA OF YOUR ARRAY ANTENNA, you should receive an IFF response if there's  a friendly aircraft even when you don't have a target track.

 

Case 2: You have moved your array antenna dish (with the primary radar controls) in a way that there are no aircraft INSIDE THE SCANNING AREA OF YOUR ARRAY ANTENNA, then when you perform an IFF interrogation you SHOUDN'T RECEIVE ANY RESPONSE.

 

On 6/22/2022 at 11:19 AM, BIGNEWY said:

We would need a track replay to check the circumstances here and the setup of the hornet, it could easily be pilot error. 

 

If you are on Case 2, then it's well modeled.

Any doubts, check:

https://en.wikipedia.org/wiki/Secondary_surveillance_radar

https://en.wikipedia.org/wiki/Phased_array

https://en.wikipedia.org/wiki/Passive_electronically_scanned_array

https://en.wikipedia.org/wiki/Active_Electronically_Scanned_Array

 

 

 

 

 

 

 

 

 

[Phantom711]

@MrWolf

 

My question was intentionally a bit sarcastic trying to challange those that claim, that IFF is a "radar itself".

So for the Case 1 you are using as an example, how would the "system" know the range of that IFF reply? It wouldn`t I guess...

[Harker]

IFF gives a range estimate based on the time between the emission of the interrogation pulse and the receipt of the first framing pulse. Not as accurate as a primary radar, but oftentimes close enough to be correlated with a radar contact.

As for the APG-73, it's not a PESA, it's a normal mech radar and an upgrade to the older APG-65.

Also, the 742-100 clearly states that the CIT is a contributor to MSI and that IFF data is used to classify MSI trackfiles. It wouldn't be able to do that if it couldn't perform at least some basic ranging.

[MrWolf]

12 hours ago, Phantom711 said:

@MrWolf

 

My question was intentionally a bit sarcastic trying to challange those that claim, that IFF is a "radar itself".

So for the Case 1 you are using as an example, how would the "system" know the range of that IFF reply? It wouldn`t I guess...

As @Harker said, the range can be estimated from the response. This is because the "protocol" defines that the response start (pulse F1) has to be exactly 3 µs after the reception of the interrogation tail pulse P3. In practice, responses are transmitted 3µs +- 0.5µs tolerance (the time for processing the interrogation, detecting the interrogation).

The interrogator would receive the response 3µs +- 0.5µs + (2*R) /c_0) ( this is the propagation delay, notice that it's accounting for the path from INTR. -> AIRCRFT and then RECV -> AIRCRFT. So R = (c*t_delay)/2. For long distances the tolerance doesn't matter much, for close distances it does. Basically by calculating the total time it took to receive the response and subtracting the predefined separation between response and interrogation you can obtain the t_delay which is physically related to the distance traveled by the EM wave (the distance at which the aircraft is present, relative to you).

 

Also from my class notes, there are these modes:

Mode 1: Spacing between P1 and P3 (inter. pulses) 3µs, used for Identification, Military use.

Mode 2: Spacing between P1 and P3 (inter. pulses) 5µs, used for Identification, Military use.

Mode 3 or A: Spacing between P1 and P3 (inter. pulses) 8µs, used for Identification, Civilian/Military use.

Mode 4 or B: Spacing between P1 and P3 (inter. pulses) 17µs, not used (not explained why), Civilian use.

Mode 5 or C: Spacing between P1 and P3 (inter. pulses) 21µs, used for altitude data, Civilian use.

Mode 6 or D: Spacing between P1 and P3 (inter. pulses) 25µs, not used, Civilian use.

Mode S or Selective (Select): It is a more complex interrogation, it is compatible with Mode A and C interrogations (which means that it can change the P1 and P3 distances accordingly), it's a flexible interrogation. It differentiates from the other interrogations because it transmits encoded digital data on the interrogation using DPSK digital modulation (it can transmit different sizes of data 56 bits or 112 bits, depending on the symbol period and constellation size...). By doing that, it's able to interrogate specific aircraft because it can indicate to whom it's sent the interrogation inside of it. The response data codification it's the same used on all the other ones (well, the response it has always the same field for all the types of interrogations, but those fields have different meanings depending on the interrogation)

 

The response itself, it's codified by the presence or not of 12 pulses, this means 2^12 = 4096 different responses. It's a very primitive kind of digital modulation, even though it wasn't created as a digital encoding because IFF systems were created in WWII, and they didn't have digital transponders nor any ways of performing the modulations or Digital to analog conversions needed. So the response it's by nature analog, a set of 12 pulses which depending on their activations mean one message or another (with nowadays mind it can be seen as a digital encoding, but it's not intended).

 

My course notes are a bit old, so I guess that nowadays the responses would also transmit the information using digital modulations like DPSK. And because our current digital transceivers can transmit information very fast, the embedded digital information could be introduced between original design pulse gaps without breaking compatibility with old IFF systems.

Nowadays, IFF systems communicate information between them very similar to the way that your phone and your WIFI access point do. They use digital modulation to transmit digital data over the radio link very fast, very robust, with error correction (and detection), identification, tampering protection, etc.

 

Mode S is like a Swiss Army solution, and it can be used for all kind of interrogations (probably military have their own, idk). This is because S mode transmits digital data both for the interrogation and the response, by doing that you can transmit ANYTHING you want because you are transmitting anything that you can encode into binary data. This makes S mode versatile. S mode is like a protocol for transmitting digital data like Ethernet IEEE 802.3, or WIFI 802.11. With S mode, you could even ask for GPS position, armament on board, aircraft systems log, etc. Mode S messages of 56 bits codify 2^56 = 7.205759404×10¹⁶ possible messages and 2^112 = 5.192296859×10³³ (notice that this isn't how it would be used because there aren't all those different messages defined, here i'm just using the representation like we did with legacy one. In reality, you send digital information separating data into headers, fields, etc. Take a look at the attached link.)

 

You can also check, this link if you are interested.

 

https://military-history.fandom.com/wiki/Secondary_surveillance_radar

 

[Dragon1-1]

Just a note, Mode 4 is an encrypted military system which uses keys (you usually have two, A and B, this is used in case you need to be airborne at the code change time) for interrogation and response alike. Wrong interrogation code gets you no response, this was implemented because with older systems, if you interrogated them from two different directions, you could get a fairly accurate idea where the aircraft was, even if the response told you nothing about it. This could be used by the enemy for tracking the aircraft using the systems.

Also, Mode 1 and 2 were used (I'm not sure if they still are) for mission codes and finding a specific aircraft, respectively. A mission code could be used if you wanted, for instance, quickly locate the strikers you're escorting in order to join up on them. At fence in, both of those should be turned off (and Mode 3 as well). Only Mode 4 is used in combat.

[Hulkbust44]

On 6/22/2022 at 4:19 AM, BIGNEWY said:

We would need a track replay to check the circumstances here and the setup of the hornet, it could easily be pilot error. 

Trust me, I set up the Hornet correctly. It's the lack of IFF response modeling/this idea that IFF is only tied to the radar that is the problem.

[BIGNEWY]

3 hours ago, Hulkbust44 said:

Trust me, I set up the Hornet correctly. It's the lack of IFF response modeling/this idea that IFF is only tied to the radar that is the problem.

Its not about trust, if you want to report an issue please provide a track replay example and any evidence you have. 

[Phantom711]

@MrWolf

 

Thanks for your explanation. I didn‘t understand all the details, but I got the idea.

What made me think, that the range could not be calculated was because I thought one can not know how long it takes the responding aircraft to process and respond. But since that seems to be a defined amount of time it makes sense now!

[MrWolf]

On 9/8/2022 at 4:07 AM, Dragon1-1 said:

Just a note, Mode 4 is an encrypted military system which uses keys (you usually have two, A and B, this is used in case you need to be airborne at the code change time) for interrogation and response alike. Wrong interrogation code gets you no response, this was implemented because with older systems, if you interrogated them from two different directions, you could get a fairly accurate idea where the aircraft was, even if the response told you nothing about it. This could be used by the enemy for tracking the aircraft using the systems.

Also, Mode 1 and 2 were used (I'm not sure if they still are) for mission codes and finding a specific aircraft, respectively. A mission code could be used if you wanted, for instance, quickly locate the strikers you're escorting in order to join up on them. At fence in, both of those should be turned off (and Mode 3 as well). Only Mode 4 is used in combat.

Oh, this information is gold (thanks for this, because you made me look for the Military Modes in order to understand and explain better), now I see that my course notes that my professors gave have incorrectly nomenclature (The pulse distances are correct, but they mixed terms). SO the deal is civilian Modes follow alphabetical order, A, B, C, D..., S. Military Modes follow numerical order 1, 2, 3, 4, 5....

Civilian use:

Mode A: For aircraft identification.

Mode B: For identification, it seems that it can transmit more responses (more than 4096) than A. Not sure 100%, but it makes sense.

Mode C : For altitude interrogation. Responses have different meanings when decoded but are codified with 12 pulses as with Mode A. So they follow the same "Standard"

Mode D : Never used. No information found

Mode S : Stated on the previous message (long text).

 

Military use:

- Mode 1: For aircraft identification. The interrogation pulses are similar to the civilian ones, but shorter. The responses are codified only with 6 pulses, 2048 possible responses (2048 different identification for aircraft). Not compatible with civilian equipment. Use for identifying Mission Code.

- Mode 2: For aircraft identification. The interrogation pulses are similar to the civilian ones, same size (4096 diferent ident.). Usually used for carrier controlled approaches during to ships during inclement weather. Use to Identify the aircraft itself, I guess it would be the tail number or some identification number inside the Carrier Strike Group (thinking of F/A-18C on carrier), because the US has more than 4096 aircraft.

I guess that for inside friendly airspace, ATCs and other aircraft can fully identify Strike group and number plate just by asking for Mode 1 and Mode 2 ident.

Mode 3: For aircraft identification AND altitude interrogation. Basically, this mode is "the interface" between legacy (not Mode S) civilian Modes and military. It asks and understands interrogations and messages compatible with Mode A and C standards. So you basically can use Mode 3/A and Mode 3/C in order to work with civilian airspace controllers. In DCS we can see with the F/A-18C, for example, which "submode" of Mode 3 we are using.

 

- Mode 4: For IFF (NOT ONLY AIRCRAFT), ENCRYPTED. So I was curious of how this is works, I looked into the STANAG 4193 (listed as classified https://nso.nato.int/nso/nsdd/main/list-promulg , look for 4193) and AIMS DoD (https://studylib.net/doc/7683989/dod-international-aims-program-office---dod-aims) documentation public available, not very useful. Well, one thing that's clear is that there's a communications system which transmits the Mode 4 keys to all "allowed" Military NATO operators in the world. I suppose that in legacy times these Keys were broadcasted encrypted by radio stations and bases, for oceanic keys probably VLF bands. The first time entering the "community of devices that knew the keys" you probably needed a second party already inside it providing the current keys (since key are updated), and once you are in you can update your own keys. With the introduction of GPS those keys were probably transmitted inside navigation message carried Y-code (encrypted P code), and nowadays carried inside M-Codes (GPS) which they can carry more Military information and provide other capabilities related with DataLink, MIDS, etc.

It seems that Keys change depending on Area of Operations (Local time) and change once a day. One of these keys or some other number (code) it's used as the code to be used for the challenge.

Well imagine you know the keys (you are "inside the key community"), then the interrogation carries an encrypted challenge. In this case, for what I've collected for different wiki and public sources, the challenge consists of a known number (shared code) encrypted. If the interrogated aircraft posses the keys, it would decrypt the interrogation message correctly and check if the received code corresponds to the current shared code between friendlies. If the decrypted code (with the known key) matches the shared code between friendlies, the receiver would send a reply, otherwise it won't. By using this technique, the receiving aircraft ensures that he isn't responding (with a reply) to an enemy which could give your position and distance if the enemy triangulated your reply by receiving it from different locations, so it's verifying the authenticity of the interrogation.

Once the interrogation it's tested as friendly, the reply message it's created. The replay it's sent with a delay related to the encrypted challenge that the interrogator sent (so the receiver is verifying that he is friendly with this delay, see notes). The reply, it's a 3 pulse reply (I think these 3 pulses are used to correctly and precisely obtain the delay of the reply). If the interrogation receives the replay at a correct delay time, then he knows that you are friendly (from this it knows, bearing and range, and that you are friendly, as commented previously).

It's not clear if the reply message it's a standard 12 bit data codified on pulses encrypted or just 3 pulses at the specified delay, maybe both things. The first option would explain why the DCS F/A-18 contains an A on the mode 4 (stating Mode 4/A) and it would make sense because you also would obtain ID info from the aircraft. At the same time, the second option also makes sense. In the explanation, I have chosen option 2, but option 1 seems more useful. The thing is that on Wikipedia the information stated according to option 1 has 0 references.

NEVERMIND (FOUND ON EDIT): Mode 4 only identifies if interrogated system is friendly or not (NO IDENTIFICATION, numbers). Public military paper: https://www.globalsecurity.org/military/library/policy/navy/nrtc/14308_ch8.pdf. From this document it's worth noting :

 

"When a transponder on another craft receives a valid interrogation, it transmits (on 1,090 MHz) a response that designates the craft as friendly and may, depending on the system, also identify the craft."

 

also :

"Mode 4 provides crypto-secure identification of friendlies. Mode 4 interrogations are computer-encoded pulse trains, which consist of four “sync” pulses and possibly an ISLS pulse (if it is not transmitted in the antenna’s main lobe) followed by as many as 32 information pulses. Upon receipt of a valid mode 4 interrogation, the transponder computer processes the information “word” and generates a corresponding time-encoded three-pulse reply. The interrogator subsystem, in turn, receives the reply, converts it to one pulse, and time-decodes it for presentation on the indicators."

 

finally if there was any doubt:

"Mode 4 is used only to verify friendly status."

 

I may post a BUG thread, because DCS seems to implement Mode 4 as capable of transmiting Mode A (and posible Mode C too ) information. The A next to the Mode 4, means using Key A as interrogation encryption key (also implying using key B as reply encryption key).

 

Notes: The Keys used are probably 2 long prime numbers (look to RSA cryptography if you want to know why prime numbers are important, basically prime numbers are very hard to factorize). Then the shared code, gets applied the key as exponent, creating the encrypted challenge. Then the reply delay could be constructed using the challenge encrypted (using the same technique as before) with the second key (remember there are 2 keys). This way transmitter, it's verifying that he is friendly because it's sending a shared code encrypted with one of the keys, without compromising the shared code or the key to enemies. At the same time the receiver after verifying tx is frnd, it sends a reply that verifies that he is friendly because it's transmitting the encrypted message from the interrogator encrypted (again) with the second key, without compromising the shared code of the keys.

- Mode S: The same as for civilian aircraft.

- Mode 5: Modernization of IFF(Not only aircraft) ENCRYPTED. It carries information digital information, using digital modulation. In this case it uses spread spectrum techniques in order to avoid spoofing, detection, etc. With Spread Spectrum, you ensure that the enemy cannot know if you are transmitting and or at which frequencies you are transmitting. This makes it difficult for the enemy to jam your signals, try to spoof it with false signals, trying to triangulate positions. Without knowing the codification applied to the spread spectrum you cannot decode the digital modulation, and because they are spread signals their signal power level may be below the noise level (or very close, if you don't know the coding) which makes it physically impossible to detect that there's even a signal being transmitted. So basically Mode 5 uses encrypted digital messaging, which allows performing more complex interrogation, transmit more data (payload info, fuel, etc.) and overall provide variable capabilities, like Mode S does, but military focused of course. The encryption it's much more complex than with Mode 4 and probably more similar to "PC encryption" tools using AES-256 or AES-512 for example. This kind of modes (Mode S or Mode 5) you have to imagine as transmitting data over your Ethernet, but using other protocols and link layer kind of designs. It also can  ADS-B GPS position (as I said, data). I think in DCS it's not modeled and all information we get of friendlies (beyond that they are friendlies) it's obtained from Data Link or MIDS (MIDS is just Data Link NATO standard in order to standardize communication between different Data Links of every country as far as I know)

 

 

References:

https://en.wikipedia.org/wiki/Identification_friend_or_foe

https://en.wikipedia.org/wiki/Aviation_transponder_interrogation_modes

http://tscm.com/iff.pdf

https://aviation.stackexchange.com/questions/48050/what-is-mode-b-on-the-transponder

https://military-history.fandom.com/wiki/Identification_friend_or_foe

https://taskgroupwarrior.info/2020/iff-mode-1-and-mode-3/

https://www.globalsecurity.org/military/systems/aircraft/systems/mark-xii.htm

https://www.globalsecurity.org/military/library/policy/navy/nrtc/14308_ch8.pdf

 

Edited September 10, 2022 by MrWolf
Found public avaliable interesting material, added comments.

[MrWolf]

I've looked on how Deka implements IFF and it seems wrong. This is because:

For Modes 1, 2, 3: You only need a code FOR REPLY (Deka sets codes for the interrogator too from what I understood from a youtube video, that doesn't make any sense) since interrogations doesn't contain any information of the interrogator. So you would only need to configure the transponder (the system that replies to interrogations) reply squawk codes for Modes 1, 2, 3/A. From what I've read from general aviation forums, the civilian transponders when set ON mode only reply to Mode A type interrogation, and when set to ALT it replies to Mode A and Mode C interrogations (providing squawk code and altitude, 2 interrogations needed).

 Other things to mention:

Military aircraft should provide the same capabilities that civilian aircraft have with ON mode or ALT mode. In the case of F/A-18C DCS implementation, it shows that Mode 3 can be set to Mode 3/C (on the UFC). I guess that when the UFC shows Mode 3/C it's just plain Mode C. The ALT equivalent to military it's Mode 3 A/C: "Mode 3/C provides the aircraft's pressure altitude and is usually combined with Mode 3/A to provide a combination of a 4-digit octal code and altitude as Mode 3 A/C, often referred to as Mode A and C" (reference https://en.wikipedia.org/wiki/Aviation_transponder_interrogation_modes).

So it seems that DCS it's pretty much simulating it! Some aspects have to be polished, as I commented here. This is great because when we get the ATC improvement we would get the possibility to use them along the ATCs. And more importantly since DCS it's also implementing Mode 3 simulation we can have a simulation working between civilian and military, simulating air intercepts, hijacks with transponder emergency transponders, military aircraft working with civilian ATCs with Mode 3 settings, etc.

[Ourorborus]

Civil Mode C (often marked ALT) is the exact same system as Military Mode 3C. And is used for ATC.

Civil drops the "3" as they do not use Modes 1, 2, and 4. So until the advent of Mode S, all civil transponders only operated 3 and 3C. 3 (or 3A) and 3C are not different modes per say. They function identically and are compatible in all regards except for the addition of Pressure Altitude to 3C. (Note this is pressure altitude and is NOT subject to your QNH setting.)

From an operators perspective, 1,2,3 are functionally the same, you plug in an assigned code. The interrogator asks the question and gets the code in response. THATS IT!! The term IFF is a bit misleading as it ONLY idents Friend but not Foe.

Your 1 and 2 codes are assigned in the ATO. Your mode 3 is assigned on first contact with surveillance equipped ATC. (there also are generic codes for flight without a surveillance service).

If the interrogator is mounted as a steerable beam on a military platform, it is usually known as IFF.

If the interrogator has a regular sweep it is known as Secondary Surveillance RADAR (SSR).

Just like a primary RADAR, Azimuth is gained from the direction the sensor receives the response, and distance is gained from the response time.  As Secondary (IFF) is an active response, it has a greater range for a given power. SSR is preferred over Primary Surveillance Radar (PSR) in a cooperative environment (i.e. ATC) as it is more accurate, less prone to interference and gives an ident. Also most PSR do not give an altitude, and even 3D PSR altitudes are vastly less accurate than Mode 3C.

Mode four codes are assigned a bit more like crypto radio codes in that they are programmed into the system based on days and times etc. The Operator doesn't see the code. Mode 4 systems can also offer a little more functionality in interrogation than just the code response.

Mode S functions the same but there are more codes in the bin and it also responds with data from the aircraft's FMS. While not actively looked at by ATC, this can trigger alerts like the pilot has dialed a different level into the FMS than ATC has cleared them.

[MrWolf]

On 9/12/2022 at 12:17 AM, Ourorborus said:

Civil Mode C (often marked ALT) is the exact same system as Military Mode 3C. And is used for ATC.

Civil drops the "3" as they do not use Modes 1, 2, and 4. So until the advent of Mode S, all civil transponders only operated 3 and 3C. 3 (or 3A) and 3C are not different modes per say. They function identically and are compatible in all regards except for the addition of Pressure Altitude to 3C. (Note this is pressure altitude and is NOT subject to your QNH setting.)

From an operators perspective, 1,2,3 are functionally the same, you plug in an assigned code. The interrogator asks the question and gets the code in response. THATS IT!! The term IFF is a bit misleading as it ONLY idents Friend but not Foe.

Your 1 and 2 codes are assigned in the ATO. Your mode 3 is assigned on first contact with surveillance equipped ATC. (there also are generic codes for flight without a surveillance service).

If the interrogator is mounted as a steerable beam on a military platform, it is usually known as IFF.

If the interrogator has a regular sweep it is known as Secondary Surveillance RADAR (SSR).

Just like a primary RADAR, Azimuth is gained from the direction the sensor receives the response, and distance is gained from the response time.  As Secondary (IFF) is an active response, it has a greater range for a given power. SSR is preferred over Primary Surveillance Radar (PSR) in a cooperative environment (i.e. ATC) as it is more accurate, less prone to interference and gives an ident. Also most PSR do not give an altitude, and even 3D PSR altitudes are vastly less accurate than Mode 3C.

Mode four codes are assigned a bit more like crypto radio codes in that they are programmed into the system based on days and times etc. The Operator doesn't see the code. Mode 4 systems can also offer a little more functionality in interrogation than just the code response.

Mode S functions the same but there are more codes in the bin and it also responds with data from the aircraft's FMS. While not actively looked at by ATC, this can trigger alerts like the pilot has dialed a different level into the FMS than ATC has cleared them.

Thanks, this is a good summary of what i've explained on the previous messages (more detailled).

Just 2 comments:

 

Mode 4 only replies a set pulses at a given delay from which the interrgator can know if the target is frendly or not (this is the only information that we are sure we can trust since it's the only one comming from military source https://www.globalsecurity.org/military/library/policy/navy/nrtc/14308_ch8.pdf). No other information it's carried on the reception.

 

Mode S really doesn't function the same as the previous ones. It's a digital design. There are no "Codes" as in the sense every aircraft tune to a given code. Digital information it's sent using Mode S. The S interrogation and replies are created by selecting fields of information and the values. This different than with the other Modes. Message are generated similar to a packaging aproach like TCP/IP.

 

.

[Ourorborus]

I have deliberately avoided the technical side of pulses/modulation etc and limited to how the operator uses the equipment, but yes different modes transmit the data differently.

Mode 4 and S do have "codes" but yes they are different (i.e not a 4 digit number), and 100% transparent to the operator. i.e not set in cockpit.

Mode 4 (more correctly called a key) is input by the Comms guys in the same way crypto radio fills are coded. The operator (pilot) just turns it on and selects functionality.

Mode S uses a 24bit hex code. However this is assigned to aircraft (or operators) at registration and not assigned by ATC. The belief being there are more codes than aircraft. (side note, this same belief existed when Mode 3 was released). In practice though it is the aircraft callsign (Flight number/registration) in the datablock that is used. This does give a varied use case though, as an assigned mode 3 code is useless unless you know who it was assigned to by ATC, whereas you can get the callsign out of the Mode S. On the other hand Mode S has a lockout feature in that if you interrogate mode S, it will not respond to another interrogation for a set time. (a simplification but true). Limiting its use in TCAS/IFF type scenarios. I.E if you interrogated a mode S aircraft from your F18 (assuming you could) it might render that aircraft invisible to ATC for a period of time. There are exceptions to this that I wont go into.

 

In practice this means:

You set your Mode 3C to whatever ATC tells you to. You only disable Mode C (i.e. use Mode 3A) if ATC tells you your pressure altitude is not verified accurately (required precision is 200ft). You would turn this off at Fence in. And back on at Fenceout. (Side note: you almost never use the "ident button". It is only used if you are instructed "SQUAWK IDENT"  If you press it after dialing your assigned code, ATC will assume you are an idiot.)

Set Mode 1 and 2 as per ATO for your package. Leave on/ turn off at fence in according to SPINS direction.

Mode four, turn on, leave on.

That's it for transponder settings. Your onboard interrogator will use your Modes 1,2 and 4 "codes" to interrogate contacts. Always remember a Negative IFF (1,2 or 4) only means they are not friendly. It DOES NOT mean they are enemy.

 

Edited September 25, 2022 by Ourorborus

[Northstar98]

On 9/7/2022 at 2:00 AM, MrWolf said:

Every slot (see radar image I attached) on the radar dish is an antenna. The use of all these small antennas, it's called an array of antennas. In order to modify the beam width, you can modify the phase of the EM waves that are transmitted or received from every small antenna, that is called phased array antenna. Doing that, you can also modify the electrical pointing of the antenna (the direction of the main lobe, simplified: the direction of the received and transmitted rays). So you could modify those phases in order to both scan the sky and also modify the beam of the array antenna. I legacy F/A-18 RADAR is PESA.

The Hornet's radar is a planar array, slotted waveguide antenna, fed by a common feed (well, it does have the ability to adjust the shape of its beam from pencil to fan, perhaps by feeding different parts of the antenna, or rather by not feeding certain parts of the antenna (like the vertical ends)).

A phased array antenna has phase shifting elements that allow the different antenna elements to adjust and measure their phase. Through superposition and the resulting constructive and destructive interference it allows them to steer the beam purely by adjusting the respective phase shifters. They also allow for higher gains with greater sidelobe attenuation capability.

The Hornet doesn't have phase shifting elements and the slots all radiate at the exact same phase, meaning the Hornet cannot electronically steer its beam - meaning that beam steering is purely mechanical.

Essentially, what we've got is a different way of achieving pretty much the same thing that a parabolic reflector would do, just with cheaper, more effecient construction.

Edited September 27, 2022 by Northstar98
grammar

[CaptPickguard]

Please see attached two track replays. One in the Hornet, the other in the Viper for reference.

On 6/22/2022 at 8:47 AM, Harker said:

The CIT should receive replies (and even create CIT-only teackfiles, but let's not get into that) even without a radar contact. IF a radar trackfile exists, CIT data can be combined with it, via the MSI system and the ROE matrix, for HAFU classification. But a radar trackfile is not a prerequisite.

Look at how the F-16 and JF-17 do it in DCS, it's modeled there. They can interrogate the airspace in front of them and receive replies, even if they don't have a radar contact (the F-16 lacks the ability to combine a reply with a radar trackfile, but it can still receive it).

So, in the Hornet, if you run an interrogation and there is a friendly aircraft inside that space (with an enabled and correctly set up transponder), then you should receive a reply from it, even if you don't see it on radar or datalink.

In the two trackfiles, we can see what Harker has described above in action. The F-16, upon initiation of an IFF interrogation, shows CIT replies on the FCR with ranging information, while in the Hornet it seems they are incorrectly just painting the radar trackfiles green if they exist.

The F-16 modelled in DCS does not correlate IFF returns with radar trackfiles (this is accurate to my knowledge), but in the Hornet this correctly DOES occur and they do correlate. The bug, to be specific, is that this only happens when radar contribution is present, even though the information is there to feed to the MSI system for creation of a trackfile if neccessary.

IFF_Report_Viper_Example.trk IFF_Report_Hornet_Example.trk

[Frederf]

On 6/22/2022 at 8:47 AM, Harker said:

They can interrogate the airspace in front of them and receive replies, even if they don't have a radar contact

I would call that a "non-radar-correlated IFF return". I don't know how (what symbol exactly) it would be displayed on the radar format on the DDI but it makes perfect sense that such a return is able to be displayed.

[Harker]

On 10/18/2022 at 10:04 AM, CaptPickguard said:

... in the Hornet this correctly DOES occur and they do correlate. The bug, to be specific, is that this only happens when radar contribution is present, even though the information is there to feed to the MSI system for creation of a trackfile if neccessary.

Agreed, if a reply is available, it should be displayed at the correct spot, regardless if a trackfile is present or not. If a trackfile is present, the IFF reply should be added to the trackfile information. And yeah, IFF replies should also be able to generate CIT-only MSI trackfiles, but that's part of the larger MSI implementation.

Edited October 27, 2022 by Harker