How accurate is your Mode C?

[Warped Factor]

The radar processor has an appropriate local QNH input to it.

It then does the arithmetic automatically and shows the Mode C readout as an altitude below the TA and a flight level above. On our system that's by showing altitudes as two digits and flight levels as three.

[Mark1234]

Interesting stuff, hadn't ever considered this angle. I confess it's not at all uncommon for me to nip under airspace out here - I usually leave 100ft headroom, but it's technically legal to (and many do), fly right at the base.

And just in case, I wasn't suggesting going mode A only was a good idea, more curious how it's dealt with - I'm sure there must be a/c out there that have just mode A.. which it seems is in many ways, worse (for the controllers) than not having a transponder at all.

[Fuji Abound]

I wonder how many mode As are left.

On a typical three hour flight this week I reckon around 10% of the returns on TAS appeared to be mode A. Undoubtedly in the nicest way they are a nuisance. You inevitably have no idea whether they are above below or same level and often they seem to be the returns that come and go on TAS - I dont know why this should be but I assume it maybe because the signal generated is "weak" but does seem to be a characteristic of mode A returns. In consequence you are left wondering if it is spurious (although I have yet to have a TAS contact that has definitely proved to be spurious) and it takes longer to visually identify the target.

[LH2]

it's technically legal to (and many do), fly right at the base.

Is it? I cannot remember in which AIP I read that the upper airspace class applies., by way of example, if you had class C between A3000 and FL80, you could technically fly OCAS at FL80, but not at A3000. It might not have been the UK AIP, though, hence why I'm asking.

I'm sure there must be a/c out there that have just mode A.. which it seems is in many ways, worse (for the controllers) than not having a transponder at all.

Things is, someone who bothers to switch the transponder on is also likely to be on the radio, whereas that's perhaps not so common with non-transponding aircraft. So no I don't think it's worse.

[mm_flynn]

Originally Posted by Fuji Abound

...they seem to be the returns that come and go on TAS - I don't know why this should be but I assume it maybe because the signal generated is "weak" but does seem to be a characteristic of mode A returns.

There are almost no true Mode-A transponders installed. What we call Mode-A is actually a Mode-A/C transponder with the Mode-C frame empty (because Alt has not been selected or no altitude encoder is connected). As such, there should be no difference in the signal strength or quality.

However, I would bet most aircraft that do not use Mode-C are being operated at low level and hence will have intermittent contact with the interrogator, or will have a level of airframe blanking because the transponder antenna is on the bottom of the aircraft and you are looking down at them.

LH2,

I don't have the reference to hand, but believe that the general ICAO principle is that at airspace interfaces the less restrictive class applies. The UK has a special rule about flying exactly at the base (which broadly says you can do it non radio across an airway)

[Fuji Abound]

mm_flynn

Yes, thank you, that makes very good sense. Thinking about it invariably the transient targets I have spotted are invariably very low level.

Despite my earlier comments I guess I would prefer to see a mode "A" contact that nothing at all - although I suppose you never know anything about the others until you spot them or it is too late.

I am still fascinated by the targets that follow you! I have had a few that almost exactly parallel your course - sometimes gradually converging, sometimes not. Search as you will it also still fascinates me how difficult they can be to spot and the growing sense you feel that finally you have come across a phantom return. So far I cant say any ever have been phantom returns and it still never ceases to intrigue me when (if) you finally see the other aircraft. I have had a few occasions, including one this week, when you find the aircraft paralleling you was going to exactly the same destination (both outside CAS, non airways) albeit you were over 150 nm away from that destination at the time. This week the other eventually called up and it became obvious who he was, that he was no phantom and why he was paralleling us! We were faster and gradually pulled ahead, but even so I never saw the other aircraft until he eventually landed 15 minutes after us.

[Deeday]

Interesting discussion. To give some context to my original post, when departing to the East, from Redhill, we usually fly underneath Gatwick CTA (base 1500 ft), as close to it as practically possible (terrain elevation gets up to 800 ft, in some places).

The same "anti-Mode C" instructor I was talking about, today did something even odder: we kept the xpdr on Mode A for most of the flight (for the aforesaid reason) then, on our way back, when instructed by Farnborough to squawk 7000 and contact Redhill Tower, he switched it to ALT, just before descending and squeezing below the CTA. I really don't get that. As far as I'm concerned, I think I'm going to keep Mode C on all the time.
Looking back, I'm pretty sure I did all of my qualifying cross-country with just Mode A - an indeed I had a Robin flying right on top of me, tracking the same radial in the opposite direction, not so many hundreds of feet above (not the most welcome sight, on your QXC!)

[steveking]

For your own peace of mind and assuming you rent the same aircraft it may be worth getting a transponder readout check with farnborough to see what accuracy your transponder is giving. It'll give you a bit more of an idea how far to stay underneath.

It'd also be interesting if any ATC guys here would be able to tell us when an aircraft is considered to be infringing, is it when there transponder shows it or do they allow an extra 150ft on the transponder readout for any possible errors.

ie if under a 1500ft airspace would they wait untill the transponder clicked over to 1700ft before the alarm bells ring or is it the moment it goes onto 1600ft.

[Gargleblaster]

"How accurate is your Mode C? " I have no idea ! Why ? It's legally installed and inspected.

Is it a good idea to turn your car's headlights on just before or after sunset ? Yep it is.

So I always sqawk the mode that gives any interested party the most information about my whereabouts.

There are some confusing and contradicting statements in varous countries (including mine's) AIPs / VFGs re. transmitting lesser information, and I've chosen to ignore them. Some revolve around emergencies, and the last thing I'm going to fiddle with in such a situation nis my transponder, unless it's to change the second digit from 0 to 7.

[viva77]

From an ATC perspective an interesting thread and some very knowledgeable replies; I pretty much agree with what’s been said.
Mode C is an aid to your safety as well as others, I can’t see any reason for not having it switched on unless asked to deselect.
My answer to Steve King is that if an aircraft is indicating it is infringing, then we take the appropriate action. If 1 of our aircraft is going to come adjacent to it, we will take avoiding action to try to maintain a safe situation. If however there is no conflict with other traffic, I personally am pragmatic about it and accept the 200’ error, particularly if the mode C readout is not constant. I’ve got more important things in life to do than file paperwork! Do remember though that if you are transiting at the base of controlled airspace, any turbulence which causes a climb will put you into controlled airspace, and a TCAS equipped aircraft could easily get an RA from the climb.
Just for your interest, in my experience, when mode C is wrong it is completely wrong not just a little bit out. I’ve seen aircraft flying at minus 2000 feet, and in the circuit at 30000!

[Jumbo Driver]

Originally Posted by viva77

...Mode C is an aid to your safety as well as others, I can’t see any reason for not having it switched on unless asked to deselect.

I quite agree - in fact, UKAIP ENR 1.6.2 expects you so to do ...

ENR 1.6.2 — SSR OPERATING PROCEDURES

2.2 Mode A Conspicuity Code

2.2.1 When operating at and above FL 100 pilots shall select Mode A code 7000 and the Mode C pressure-altitude reporting mode of the transponder except:

(a) When receiving a service from an ATS Unit or Air Surveillance and Control System Unit which requires a different setting;
(b) when circumstances require the use of one of the Special Purpose Mode A codes or one of the other specific Mode A conspicuity codes assigned in accordance with the UK SSR Code Assignment Plan as detailed in the table at ENR 1-6-2-5 to ENR 1-6-2-10.

2.2.2 When operating a SSR transponder below FL 100 in airspace where the operation of SSR transponders is not mandatory, pilots should select Mode A code 7000 and the Mode C pressure-altitude reporting mode of the transponder except as in sub-paragraphs 2.2.1 (a) and 2.2.1 (b) above.

JD

[Mark1234]

LH2: Just FYI, in the Aus AIP, it definately says where two airspace classes meet / overlap, the less restrictive applies. Sorry, I can't be bothered to look it up!

In my mind it's a moot technicality, as it only matters over a distance of 1ft - how accurate is your altimeter, your flying, and how tall is the fin? Seems sensible to be a bit under, whether thats's 50, 100, or 1000ft..

Interestingly enough, was airborne on the weekend listening to Melb radar trying to get in touch with some chap, who turned out to be on another frequency, (relayed via another a/c). I assume he was transponding as they had a good fix on his location, but when they got hold of him he didn't know what the ident button was, and turned out to be in controlled airspace - by his own admitted altitude, no mode C.

[PD210]

Reading through this thread, replies largely talk about altitude accuracy. However, can anyone shed any light on a transponders accuracy laterally. i.e. putting an aircraft 2nm, 3nm etc from where they believe they are. I have looked through Bendix King web info but could not find anything. Does lateral accuracy depreciate the closer one is to terrain. Can surface structures cause multipath effects similar to GPS antenna locations?

Thanks

[BackPacker]

PD, a transponder (with the exception of extended mode-S I believe) does not emit any lateral data whatsoever.

It just responds to interrogation requests from either radar or TCAS units, and it's the radar or TCAS units which calculate the lateral position (or offset) based on propagation time, direction and such. Of course radar and TCAS will suffer from all sorts of errors, including surface errors but that's really not the transponders problem so you wouldn't find anything about it in the transponders documentation.

I believe PCAS (as in Zaon MRX/XRX or PowerFlarm), also uses signal strength to determine the distance. Reason of course is that PCAS doesn't send out its own interrogation signal so it can't use the timing information from the return signal.

The exception to this, as far as I know, is extended mode-S or whatever it's called. Such a reply can include an ADS-B signal that encodes the exact position. This position is taken from the FMS, who uses some sort of combination of INS and GPS to calculate the position.

[chevvron]

I have known a C172 with a slightly faulty transponder to consistently show its transpoder return about 8 miles displaced from it's primary blip, also many military aircraft have an ECM fit which does this intentionally.

[IO540]

However, can anyone shed any light on a transponders accuracy laterally. i.e. putting an aircraft 2nm, 3nm etc from where they believe they are.

It's down to the radar to place the target laterally.

Radar is not all that accurate - perhaps 1-2nm at 30nm. A lot of computer processing takes place on the data which makes it appear more precise but the basic system is not all that precise. Nowhere near as accurate as GPS for example.

I have known a C172 with a slightly faulty transponder to consistently show its transpoder return about 8 miles displaced from it's primary blip,

How does that work? Could it be caused by the transponder being way too slow to return the data?

[englishal]

I've had an altitude encoder fail and show me at 8,000' when I was at 2000' and had a nervous controller asking me "when are you going to decend?"....Actually that day I forgot to just select Mode A only on the return leg , and to the controller I was now at 11,000'

Funnily enough, no one queried the 11,000' all the time I was talking to ATC all the way along the south coast (I did tell them my alt) until on handoff from Southampton after a zone transit, I heard the controller tell another aircraft that there was "IFR traffic at 11,000 just been handed over from Southampton", which is when I remembered, told her and went back to Mode A.

[BackPacker]

How does that work? Could it be caused by the transponder being way too slow to return the data?

Or (but that's maybe less likely) too fast?

I haven't checked but I would assume that in the standard there's a fixed delay specified so that both very slow and very fast transponders will emit their response after the exact same delay. A delay specification "as soon as possible" would be different for each transponder type, particularly if there's some logic in the transponder that might need to pull a GPS location from the FMS, encode it in ADS-B and so on and so forth.

If that delay got misconfigured somehow, you'd either be too fast or too slow, leading to a mispositioning of your blip closer to, or farther away from the radar stations position. Particularly if the SSR return is not correlated with a primary return. Which, as I understand, is the default behaviour for civilian radar. And TCAS of course.

[mad_jock]

I once started showing -4000ft and got told to adopt the callsign submarine, witty chaps at Scottish.

[SNS3Guppy]

I believe a Mode C/TCAS altitude error almost caused a mid air between BA and Korean 10 years ago ..

I experienced a TCAS Resolution Advisory (RA) several days ago, in which I was required to deviate from my flight path in order to avert a collision. We were level, with the autopilot engaged at the time, when the other aircraft descended into our altitude and track. We got a "Descend! Descend!;" I disconnected the autopilot and manually descended. The other aircraft leveled and stopped their descent. We advised air traffic control, then returned to our assigned altitude once clear of the threat. The TCAS worked exactly as advertised.

That event occurred while in positive radar control, while actively talking to controllers, and taking vectors from the controller (meaning the controller has accepted responsibility for separation from other traffic). It occurred at night, in a busy area involving a lot of other TCAS targets; a lot of traffic. I had already ranged mine down to eliminate the high concentration of traffic displayed on my display. We had one TA, or traffic alert, and then less than a minute later, the RA. A resolution advisory is rare; one doesn't need to take an evasive action very often. Without that equipment and that warning, however, I very much doubt any of us would have seen the other aircraft.

Turning your transponder off, especially near busy airspace, is a really poor idea. So is skirting an area of controlled airspace so close that one is in danger of infringing on that airspace, if one isn't participating with the controller.

When I'm flying under IFR, I have no idea much of the time what airspaces I'm passing through. Under IFR, it's largely the same. I don't look, I don't care. I don't have access to that information, really, and I have no idea if I'm close to the bottom of the controlled airspace or going in and out of it. I'm flying a designated routing with a clearance. You might think that you're on the edge of he airspace and thus in good shape, and you may be very, very wrong. Consider several possible scenarios.

I have a traffic alert and respond by making a descent. I'm being operated near the "edge" of the controlled airspace. You're skirting the edge. You've turned off your transponder. I'm taking action to avoid someone who does have a transponder, and the message I get is "Descend! Descend!" If you had your transponder on, I'd have received a different warning, as my transponder TCAS would take into account the actions of your aircraft as well, in establishing the resolution advisory. Instead, your transponder squawking Mode A only, or turned off, my avionics don't "see" you, and we collide, during a legitimate avoidance maneuver.

ATC needs me to descend. They ask me to keep my speed up and give my best rate of descent. You're operating near the "edge" of the airspace, and think that means something to me or ATC. I deploy the speed brakes and pull the power to idle. I'm doing 250 knots and descend into you from behind, with a high rate of descent. I'm going faster than you think, descending into you at several thousand feet a minute. Even if the altitude to which I'm cleared is the bottom of the controlled airspace in that area, if you're right on the "edge," and if your altitude isn't quite correct, it doesn't take much error, especially with a closure rate of 250 knots or greater and several thousand feet per minute vertically, to wind up with a collision.

I'm departing. I'm heavy, so I'm not climbing as fast as the controller might like. I get a turn away from other traffic, and continue my climb. You're flying with your transponder off, or perhaps have it on, but elect to do so on the edge of a busy terminal area, thinking that it's a good idea so long as you're on the "edge." During departure, I'm busy getting cleaned up and configured for the enroute climb, and I don't have a really great view of other traffic, especially at our climb speeds which may be closer to 300 knots, even below 10,000 (minimum safe speed allows higher than 250). I may be on a vector, and I may pass in and out of controlled airspace during that climb. The controller may make a mistake. Why put yourself in the position of being in a traffic conflict in the first place; it's not safe for either one of us.

Those who would skirt airspace as closely as they can remind me somewhat of someone who might walk on the highway as close to the edge of the highway as possible. One might technically have stepped into traffic, but what if traffic drifts across the line, slightly? Doing the same thing in flight with altimetry systems that can be off slightly means that the analogy is a little more like walking along the highway blindfolded; you have no way of knowing exactly where the "edge" is. Give it a buffer. Stay on the sidewalk. Don't go near the edge. Playing near the edge is asking for trouble, whether it's performance, or airspace. I've spent much of my career in operations involving flight near the edge of performance, foreign airspace, and close to obstacles and terrain, and I'm paid in some of my employment to do just that. Are you? Then why go there?

We carry two transponders and encoding systems, and can switch between them. Some of our people will check them both after takeoff and have ATC give them an altitude check, especially before a long flight out of radar contact. We're checking our altitude, our tansponders, and our ability to remain where we say we are. Before going oceanic (class II navigation), we do a position check, verify our radios accuracy and make a comparison with our other nav systems, and do an altitude check. Additionally, there are actually systems in place geographically around Europe and on each side of the ocean which exist just to verify actual altitude vs. the broadcast (mode c) altitude. It's serious stuff.

Why not do the same thin when renting, then? Check your altitude, and get a confirmation from ATC; have a radar controller tell you what the radar controller is seeing.

Bear in mind that if your operating on the "edge" is close to a transition area where aircraft are operating on different altimeter settings, you may have some very significant indicated differences in altitude, yet be at the same height. Not a good idea. If you know the boundary for airspace is at 5,000, for example, why fly at that altitude? 6,000' works, as does 4,000.

If you're going to fly on the ragged edge of an airspace change, why not simply get a clearance in the controlled airspace? If you're not willing to do that (or able, circumstances depending), then you probably ought to give it a wider berth. Avoid the potential trouble.

There are places in the US where encroaching on certain airspace will earn you a fighter escort and a long talk with some very serious-looking people across a cold table in a quiet room with mirrors. Most of the violations of that type of airspace occur when people think they know where they are, or think they're just skirting the airspace, and they're not.

Radar is not all that accurate - perhaps 1-2nm at 30nm. A lot of computer processing takes place on the data which makes it appear more precise but the basic system is not all that precise. Nowhere near as accurate as GPS for example.

Depends on the radar. Radar is accurate enough to enable me to fly an instrument approach to low minimums using only the controllers voice and the radar display that's visible to the controller. Radar tends to be very accurate, especially terminal radar. It's not off by one or two miles; it shows traffic right where the traffic is. Radar separation is based on it being accurate.

Transponder returns are based on the transponder, but unless the transponder is set up using some advanced doppler shift software, the target is right where it appears. Radar information provided to a controller is filtered for local altimeter setting, and controllers can screen out traffic or highlight traffic, but air traffic control radar is quite accurate, and it's very accurate at distances much farther than 30 miles.

"How accurate is your Mode C? " I have no idea ! Why ? It's legally installed and inspected.

Is it a good idea to turn your car's headlights on just before or after sunset ? Yep it is.

So I always sqawk the mode that gives any interested party the most information about my whereabouts.

There are some confusing and contradicting statements in varous countries (including mine's) AIPs / VFGs re. transmitting lesser information, and I've chosen to ignore them. Some revolve around emergencies, and the last thing I'm going to fiddle with in such a situation nis my transponder, unless it's to change the second digit from 0 to 7.

You should know how accurate your mode C is reporting, and should check with ATC occasionally to find out.

You appear to be suggesting that you operate the transponder in Mode C all the time, but there are times and places to operate in Mode A only, or in Standby. When operating in formation, for example, one transponder will speak for the formation, with the others being either in standby or mode A. When on a busy surface area, some airports will require mode C operation for ground surveillance radar, while other places want the transponder in standby or off on the ground. Failure to abide local practice isn't giving extra useful information. It's causing a problem.

You could simply keep pushing the ident button all the time if you really want to stand out to controllers, but you're not helping anyone unless you're asked to squawk ident. Giving the maximum amount of information all the time can be taken to an extreme, and is not helpful.

I once got in a King Air 200 with an FAA examiner for a checkride. He saw the TCAS on board and groaned. As busy as the airspace was, he knew we'd be getting traffic alerts constantly. In such cases the TA's could actually cause a safety issue and at a minimum an unwanted distraction, and trying to avoid an aircraft being called out as a traffic alert could cause a collision hazard with another airplane. In that area, see and avoid was absolutely essential, and I kept my speed down and my head up until we were well clear of that airspace, accordingly.

However, can anyone shed any light on a transponders accuracy laterally. i.e. putting an aircraft 2nm, 3nm etc from where they believe they are.

Mode A and C transponder returns don't carry any information regarding lateral position. Radar determines where you are. The transponder simply makes you a little more visible and tells the controller who you are, and how high you are.

The same "anti-Mode C" instructor I was talking about, today did something even odder: we kept the xpdr on Mode A for most of the flight (for the aforesaid reason) then, on our way back, when instructed by Farnborough to squawk 7000 and contact Redhill Tower, he switched it to ALT, just before descending and squeezing below the CTA. I really don't get that.

Find a different instructor, preferably one who knows what he's doing.