Mode S question

M

Meanee

Line Up and Wait
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Meanee
Since my FBO has lack of planes, I am training for Diamond DA40. Since it has a required minimum, and somewhat high price, nobody books it. I have 1 hour in it, and I LOVE it. But I do have a question.

On the MFD, I can see traffic information. CFI who is doing the checkout with me said "I really have no idea how it shows traffic, but it does", when I asked what is the source of the traffic. After digging through few manuals, technical info, I discovered that the transponder is a Mode S, and does have capability to detect other traffic.

So, here's a question. How reliable is it? Does it show any aircraft that squawking anything, or does it show aircraft with other Mode S transponder? How accurate is the info? How many targets can it see at one time?

I know that I still have to see and avoid, but since I have another tool available to me, I want to make sure I know of it's capabilities and deficiencies.
 
The mode S transponder is most likely a Garmin GTX33. It supports a traffic feature provided by the mode S ground radar system (ASR 7, 8, or 9) called TIS. There are approximately 110 sites that use these radar systems and the service is generally available out to about 60 NM from the radar site. Mode S transponder technology provides a data link capability to the ground radar site. The radar uses 1030 MHz to interrogate your transponder, and if your aircraft is equipped with mode S, there are available bits to define up to 2 traffic reports for aircraft in your vicinity in each message. As the ground station sweeps by the azimuth of your aircraft, it can reliably interrogate your aircraft position about 4 times, so up to 8 targets can be sent to you. The targets must be within 7 NM and +/- 3500 feet of your position. Traffic is shown as a diamond, either open or filled in. The open means it is not a current threat, filled in means it could be a threat, and it switches to a yellow circle if the traffic gets very close to you in altitude and distance.

Because TIS is only provided at some locations and within a 60 NM radius, if you fly outside the radar coverage of the site, you will hear a message "Traffic Unavailable". See http://www8.garmin.com/aviation/tis.jsp for a map of where the sites are located.
 
John Collins said:
The mode S transponder is most likely a Garmin GTX33. It supports a traffic feature provided by the mode S ground radar system (ASR 7, 8, or 9) called TIS. There are approximately 110 sites that use these radar systems and the service is generally available out to about 60 NM from the radar site. Mode S transponder technology provides a data link capability to the ground radar site. The radar uses 1030 MHz to interrogate your transponder, and if your aircraft is equipped with mode S, there are available bits to define up to 2 traffic reports for aircraft in your vicinity in each message. As the ground station sweeps by the azimuth of your aircraft, it can reliably interrogate your aircraft position about 4 times, so up to 8 targets can be sent to you. The targets must be within 7 NM and +/- 3500 feet of your position. Traffic is shown as a diamond, either open or filled in. The open means it is not a current threat, filled in means it could be a threat, and it switches to a yellow circle if the traffic gets very close to you in altitude and distance.

Because TIS is only provided at some locations and within a 60 NM radius, if you fly outside the radar coverage of the site, you will hear a message "Traffic Unavailable". See http://www8.garmin.com/aviation/tis.jsp for a map of where the sites are located.
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:yeahthat: And just to add a bit. You have to be around the stations to get traffic information so it is not always available depending on where you are in the U.S. Don't be confused with TCAS either. (Traffic Collision and Avoidance system) TCAS is on board equipment that talks between aircraft and can actually give RA or (Resolution Advisories) to the pilots.
 
GTX33 if it's headless (I've got one of those). GTX330 if it's panel mounted.
The description of the mode S TIS is correct. Here in the eastern seaboard, I get coverage about 70% of the time or so. I also have a Xaon XRX on the glareshield which passively listens for other transponders.

Full up TCAS listens for both transponders (mode C and S) as well as communicating via the mode S datalink with other TCAS equipped plane. It can issue an RA any time targets are close, it coordinates RAs with two equipped planes. I've triggered RA's in airliners after C90 put them in a direct path with me (I had a visual, but approach didn't call out the traffic to either of us until after the MD80 descended initiated an RA-induced descent and reported it to them).
 
Great info, thanks!

So what I took from it, if there's no target on the MFD, it does not mean that nothing is there. If there's a target on MFD, it means that it's there.
 
Well, almost. You're right that if there's nothing there there still could be traffic around you. But the reverse can also happen -- you can have traffic show up that isn't real. This happens mainly if you're maneuvering the plane. You'll be making steep turns or something, and you know visually the area is clear but a plane will pop up on the display very close to you. It's actually your own plane showing up as traffic because you're moving around a lot and your position is changing too much for the system to figure out that the return is really you. Not a big deal, just be ready for it when it happens.
 
Well, thanks for that warning. We didn't receive any types of false positives when I did airwork (Steep turns, GRMs) but at least knowing that, I won't crap my pants if I see something like it.
 
Yes, the TIS phantoms aren't necessarily caused by you manouvering. It's also caused by the radar sites overlapping. Going from CJR over to OKV there's a spot everytime I fly through I get a spurious target at my altitude and location.

One really annoying feature of the MX20 TIS implementation is that the targets that are drawn on the map are not erased when the TIS is lost and there's no indication on the custom map that TIS is no longer available. You just get this target flying in formation with you at his relative altitude and position when TIS was lost. You have to go over to the TRAFFIC screen and explicitly turn TIS on and off to get it to go away.

Oddly enough over on the 480, it not only tells me if the TIS is operating, but it coasts the targets for a few seconds but then erases them when the TIS is lost.
 
Meanee said:
Well, thanks for that warning. We didn't receive any types of false positives when I did airwork (Steep turns, GRMs) but at least knowing that, I won't crap my pants if I see something like it.
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The other time I notice an occasional false traffic alert is right after climbing up into radar coverage in the pattern. Once it establishes good radar contact, it goes away and fortunately it only happens once in a while, but it does wake you up.
 
Every once in a while I like to decode some acronyms for the uninitiated.
TIS - Traffic Information System. (John described this)
RA - Resolution Advisory or recommended/required action (Matt already mentioned this)
C90 - Chicago approach control
GRMs - Ground Reference Manuevers
MFD - Multi Function Display
 
If the CFI does not know how it works, or understands TIS, it's time to find another instructor?
 
BillTIZ said:
If the CFI does not know how it works, or understands TIS, it's time to find another instructor?
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That is a little harsh. There are a lot of instructors who don't naturally come across this sort of equipment. I could probably ask several questions here on the technical details of TIS that most on the forum would find difficult to answer. How are the dirction vectors determined and what is precision of the relative angle, what increments are the distance and altitude along with its resolution. How long after the TIS is not detected does it take to get the Traffic not available message, what is the stay alive for? And so on.
 
John Collins said:
That is a little harsh.
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I thought so, too, when I first read it. But, although the delivery sounds kind of harsh, the message isn't. Yes, there are a lot of CFI's out there, me included, that don't know a lot of that stuff. And personally, I would encourage someone that came to me with those kinds of questions to go to someone who knew more. It isn't really a reflection on me, or others like me. It is an admission that I don't know it all and will recommend someone else.
 
Meanee said:
Great info, thanks!

So what I took from it, if there's no target on the MFD, it does not mean that nothing is there. If there's a target on MFD, it means that it's there.
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Almost. It means the target was there at some time in the recent past - there's a time lag for the radar to acquire the target in a sweep, for the computers to process it, and for the system to rebroadcast it. For slow targets it's usually not a significant discrepancy, but for fast targets it can be. And as noted earler, sometimes if you're manuevering the target may be YOU (or where you were earlier).
 
BillTIZ said:
If the CFI does not know how it works, or understands TIS, it's time to find another instructor?
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At least he admitted he didn't know instead of making up some BS answer. I find this to be so aggravating. If you don't know say so. Just because you are the CFI doesn't mean you know everything. To finish it of he should have and could have said "Let me find out for you!"
 
flyingron said:
Yep, I guess we use too many TLA's
(Three Letter Abbreviations).
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And LFLAs (Longer Four Letter Acronyms). Actually, they're efficient tools, as long as one knows the code, as many of us do, and oftentimes someone who doesn't know it will actually be brave and ask. But the occasional RAK (Random Act of Kindness) is sometimes appreciated.

And, to bring this back to the original topic, or at least the title, what does the "S" in "Mode S" signify? Is it just an arbitrary code, or does it stand for something? And the obvious related question is what about the "C" in "Mode C?"
 
gprellwitz said:
And LFLAs (Longer Four Letter Acronyms). Actually, they're efficient tools, as long as one knows the code, as many of us do, and oftentimes someone who doesn't know it will actually be brave and ask. But the occasional RAK (Random Act of Kindness) is sometimes appreciated.

And, to bring this back to the original topic, or at least the title, what does the "S" in "Mode S" signify? Is it just an arbitrary code, or does it stand for something? And the obvious related question is what about the "C" in "Mode C?"
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The "S" stands for select and is descriptive of the mode S transponder can be individually addressed or "selected". The Mode A and C relate to the four types of transponder interrogations, A, B, C, and D. A is used to request the 4096 transponder code be returned in the reply. C is used to request the pressure altitude be returned in the reply. Modes B and D are not used. The interrogation type is determined by the timing between of two pulses P1 and P3. For mode A, the timing is 8 microseconds and for mode C the timing is 21 microseconds. P2 is the second pulse and is used for side lobe suppression and for mode S fake out of non mode S transponders. P2 is transmitted from a non directional antenna and is deliberately weaker than P1, whereas the other pulses P1 and P3 are transmitted from the rotating antenna. Regular transponders key off of the relative magnitude of the P1 and P2 pulses. If P1 is stronger than P2 when the transponder receives them, then the transponder assumes it is being interrogated by the rotating antenna. Some energy will always leak out of the rotating antenna approximately 90 degrees to the direction the antenna is pointed, but it will be weaker than the signal that is in the direction the antenna is pointed. You don't want a transponder to reply unless the antenna is pointed directly at the aircraft, so if P1 is similar in strength as P2, it is likely that the side lobe energy is generating the P1, so the transponder checks for this condition and if it finds it, it suppresses any activity for a short period of time. Mode S uses the side lobe suppression logic of the regular transponder, by faking a P2 the same strength as P1, causing any regular transponder to shut up for a little while. Then the mode S system uses this time to select a single mode S transponder by its 24 bit address and exchange up to 112 bits of information both directions, while the regular transponders sleep from the fake out.
 
John Collins said:
The "S" stands for select and is descriptive of the mode S transponder can be individually addressed or "selected". The Mode A and C relate to the four types of transponder interrogations, A, B, C, and D. A is used to request the 4096 transponder code be returned in the reply. C is used to request the pressure altitude be returned in the reply. Modes B and D are not used. The interrogation type is determined by the timing between of two pulses P1 and P3. For mode A, the timing is 8 microseconds and for mode C the timing is 21 microseconds. P2 is the second pulse and is used for side lobe suppression and for mode S fake out of non mode S transponders. P2 is transmitted from a non directional antenna and is deliberately weaker than P1, whereas the other pulses P1 and P3 are transmitted from the rotating antenna. Regular transponders key off of the relative magnitude of the P1 and P2 pulses. If P1 is stronger than P2 when the transponder receives them, then the transponder assumes it is being interrogated by the rotating antenna. Some energy will always leak out of the rotating antenna approximately 90 degrees to the direction the antenna is pointed, but it will be weaker than the signal that is in the direction the antenna is pointed. You don't want a transponder to reply unless the antenna is pointed directly at the aircraft, so if P1 is similar in strength as P2, it is likely that the side lobe energy is generating the P1, so the transponder checks for this condition and if it finds it, it suppresses any activity for a short period of time. Mode S uses the side lobe suppression logic of the regular transponder, by faking a P2 the same strength as P1, causing any regular transponder to shut up for a little while. Then the mode S system uses this time to select a single mode S transponder by its 24 bit address and exchange up to 112 bits of information both directions, while the regular transponders sleep from the fake out.
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Thanks! Interesting way to add the addressable functionality.
 
John Collins said:
It was done this way to make these two otherwise incompatible systems share the same ether.
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Except for the one transponder that actually implemented the mode A/C suppression timing with enough accuracy that any mode S interrogations caused the transponder to not respond at all.
 
flyingron said:
Except for the one transponder that actually implemented the mode A/C suppression timing with enough accuracy that any mode S interrogations caused the transponder to not respond at all.
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IIRC, the affected transponder had an AD issued to resolve the issue.
 
I nominate this thread for a sticky! Some great descriptions in here by knowledgeable folk!
 
Still great info. I've read up on that and none of the technical documentation contains any summary that clear and concise. Thanks for sharing your brain, Scott.
 
Great thread. One comment though that has been insinuated but not stated. TIS does not replace see and avoid. TIS is an useful adjuvant to see and avoid and is quite helpful, but we still need to be looking outside for that traffic.

I know we all know this but I felt a need to say it anyhow.

Doug
 
scottd said:
Except for the fact that TIS is eventually going away to be someday replaced by ADS-B. It's like nominating a thread that discusses NDB approaches. :sad:
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What? You mean there is no sticky for NDB approaches. Well then I want to nominate it then! :thumbsup:
 
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