by Darryl Phillips
"Radar Contact". What does it mean when the controller utters those words? Well, like most government gobbledegook, it usually does NOT mean radar contact.
Suppose you key your mike and ask "Is anybody out there?" And somebody answers "Yes, I am ten miles west." That is "RAdio Detection And Ranging all right, but it certainly is not radar. Bouncing a signal off the aircraft skin and picking up the echo is what radar is all about. Controllers call skin reflection "primary radar". And they don't use it much, because it shows too many cars and trucks.
The box we call a transponder is called a beacon by the feds. Air Traffic Control Radar Beacon Service, or ATCRBS. Pronounced AT-crabs. As the primary radar dish spins around, it carries another antenna for ATCRBS. Hundreds of ATCRBS interrogations are sent out each second on 1030 mhz. (The radar is on a much higher frequency.) In the plane, the transponder listens constantly on 1030. Each time it hears an interrogation, it issues a reply on 1090 mhz. If everything works right, that reply results in a blip on the controller's screen. Since it's a "radar" screen, the controller thinks he has radar contact! But it's really just one box asking if anybody is out there, and another box answering back. ATCRBS, not radar.
Incidentally, it's the 1090 mhz transponder reply that is picked up by the antenna inside the Airsport Altitude Alerter. It receives and decodes all the signals sent down to ATC.
Long ago there was just one type of interrogation. Mode A. Pilots call it squawk code. When the transponder receives a Mode A request, it sends out a string of pulses that carry the four digit code. Mode A interrogations are still the most common. And regardless of the code set by the pilot, the frequency is always 1090 mhz.
The second type of ATCRBS request is Mode C. When this interrogation is received, the transponder sends out a string of pulses that look very much like Mode A pulses. Indeed, it's often impossible to look at a reply and know if the information is Mode A or C. But the Mode C data isn't squawk, it's altitude. This data is supplied to the transponder by the altitude encoder. Mode C replies are on 1090 mhz too.
When ATCRBS sends out a Mode A interrogation, it assumes the reply is Mode A and interprets the reply as squawk code. If Mode C, the pulses are decoded as altitude.
Usually an aircraft is receiving interrogations from many radar sites. Civil ATC, military interrogators, Customs and DEA and more. Transponders don't know which way the request came from and they don't care. Transponders reply to everything, in all directions. With many interrogators and many aircraft, this single frequency can get very busy.
In Part 1 we discussed the basics of ATCRBS. Mode A is squawk code, and Mode C is altitude. Interrogations from ATC are on 1030 mhz, and transponders always reply on 1090 mhz.
On the same frequency we also find Mode S. "S" stands for select, meaning ATC can selectively interrogate any transponder it desires. That's a great idea, it was revolutionary when first proposed in the 1960s. Mode S was refined in the 1970s, the scheme was signed off in 1983, and more than a decade has passed since then. How's it coming?
All commercial carriers of 30 seats or more have Mode S installed, as do some smaller ones and a few general aviation aircraft. FAA is struggling to get it's first ground stations on line, without much success. So there aren't many ATC Mode S interrogations yet.
Mode S replies are different than A/C, but they are on the same frequency, 1090 mhz. The average ATCRBS reply contains around 3 or 4 microseconds of energy, Mode S contains either 30 or 58 uS! That means that each Mode S signal contains 10 to 20 times as much energy.
Since there aren't any "S" interrogations yet, why does it matter how long the reply is? If it weren't for TCAS, it wouldn't matter at all. But the airliners continually use this collision-avoidance equipment to interrogate other planes. The feds can't see Mode S, but other planes can. And do.
Suppose there are ten airliners within radio range of each other. Each sends out interrogations, and for each interrogation the other nine reply. Ten requests, 90 replies. Why is each plane sending a separate message to every other plane? Since the data (his altitude and ID) are the same for each recipient, why can't he just send it once? Those are good questions to ask the feds.
Ten planes are easy. But suppose there are a hundred. That is 9900 replies. Big long troublesome interference-producing replies. Repeated many times per second.
Plus, TCAS interrogates our transponders too! The requests are only for altitude, TCAS never asks for squawk code. So in addition to all the Mode S clamor on the frequency, there is also a tremendous amount of Mode C. These extra Mode C replies are how the Airsport Altitude Alerters can spot TCAS traffic. And of course there are still all the replies to ATC.
Like ripples in a pond, transponder signals radiate in all directions. Mode A/C replies are 3.3 NM long, when the last of the databurst leaves the plane the leading edge has rippled 3.3 miles in all directions. Mode S replies are as much as 20 miles long! Take an old sectional and draw a circle with a radius of 3.3 NM. Now draw another circle with 20 mile radius. That gives you a picture of the difference between ATCRBS and Mode S, and illustrates why we're seeing so much interference to our transponders.
"Turn your transponder OFF as you approach Lake Parker". That is in the official FAA Sun'n Fun arrival NOTAM. Too much traffic, all those airplanes will overload the radar, so turn it off.
Is that a joke? Isn't the whole purpose of Air Traffic Control to keep airplanes from running into each other? And doesn't the chance of collision increase when there are lots of airplanes?
Why do we have transponders anyway? They don't do anything to help the pilot, just take up valuable space on the panel, add weight and battery drain, increase antenna drag. It ain't for us, fellow pilots, the transponder is to help ATC do their job. And their job is to keep us from bumping. So why do they tell us to turn it off when it is needed the most?
Because they can't handle very many replies, that's why. The system overloads and goes haywire. Not at Lakeland, but at Orlando and Tampa where the airliners go. And ATC can't have that!
The problem isn't limited to Sun'n Fun and Oshkosh. Mark Twombly wrote in January AOPA PILOT about flow control through Jacksonville Center that kept aircraft separated 20 miles in trail because of frequency congestion. And this was because of some football game! What ATC didn't tell him was that it's not communications overload, it is overload of the 1090 mhz transponder frequency. Friends, if the capacity and safety of our airspace is upset by a ballgame, we're in deep ----, er, trouble.
General Accounting Office has documented instances of Tracons shutting down for as much as 15 minutes in rush hour traffic. FAA calls it computer capacity, and they're right as far as it goes. But the basic problem is that they cannot deal with all the TCAS and Mode S signals that have been piled on top of our transponder signals.
The most common symptom of overload is when ATC cannot see a particular plane. Maybe your plane. Sometimes they lose you altogether. Other times they get squawk, but no altitude. Or the reverse. And sometimes they get both, but altitude is inaccurate. This can lead to altitude violations, but usually ATC and the pilot work it out verbally. The problem comes when a VFR flight is well above the terminal area (or perhaps a military restricted area) and not talking to them, and ATC mistakenly decodes his altitude as within the regulated airspace. The unfortunate pilot did everything right, his equipment was performing properly, but he may get violated anyway.
Sometimes ATC can lose your transponder when they're not overloaded at all. It's called AGC, transponders are designed to reply to the closer interrogators and ignore the weaker ones. If you have several nearby TCAS planes constantly interrogating your transponder, it will sometimes refuse to cooperate with the much weaker ATC interrogation. Everything is working as designed, it just wasn't designed by people who fly.
OK, the transponder frequency is overloaded. We covered that in the past 3 columns. Now we get to the good stuff. What you can do when the controller says your transponder is faulty.
If ATC says they're not getting good transponder data, turn your DME off. The books don't tell you this, but DME and transponder operate on nearby frequencies and they cannot stand each other. So there is a "suppression" wire between them, if DME is busy the transponder has to wait, and vice versa. If you don't need DME, shut it down. (By the same token, if you're having trouble getting a DME fix, you might try putting the transponder on standby. Not legal, but it works sometimes.)
The next trick is to ask ATC to go to their backup equipment. Honestly, I don't know if they do it, or if they just pretend. But sometimes it succeeds. Partly it works because the controller now knows you're serious. And partly it works because anything that uses up time gives your signal a better chance of succeeding against the overload. So it's worth a try.
Another ploy is to request a different squawk. Maybe another bit pattern will get through, but it's probably just the extra controller time.
He will often ask you to recycle your transponder. Nobody knows what that means, just do it! Personally, I do nothing and it works. Whatever you do, don't turn the transponder all the way off. In most units there is a warm-up timer that begins at turn-on, operation can't begin until it times out, and you don't want that.
Most transponders have a "reply" light. If it's blinking you assume there are good replies. Not necessarily true. The light may say "reply", but in most units it is activated by an interrogation, not a reply. There could be an internal fault that prevents a reply, but the light will blink anyway. The answer here is a Transponder Monitor.
When ATC reports transponder difficulties, many pilots respond that they have an onboard Transponder Monitor, and it shows that the transponder is operating normally and is squawking the proper code and reporting such and such altitude. Under FAR 91.215(d), the controller can permit the flight to continue even though he isn't receiving the transponder signal.
Other tricks. Contact another facility. Talk to Center, for instance, even if you are VFR. Verify that your transponder looks OK to them. Then negotiate a handoff to the controller that wasn't getting your signal. Let Center argue with him on the landline!
Or wait until the last minute before calling approach. Signal strength is twice as strong at the Mode C veil as it was just 12 miles further out. Just don't penetrate without permission.
Or call from above. If you absolutely positively must get into the terminal area, fly over it and call from there. Again, signal strength is a weapon.
So far, this series has concentrated on the relationship between the transponder and Air Traffic Control, particularly the overload on the transponder frequency caused by Mode S and TCAS. But what about Mode C? What is it, how does it work?
A few years ago, most of us appended an encoder to our transponder, so it's easy to think that the altitude data is somehow appended to the transponder reply. But it doesn't work that way. When ATC wants squawk, they issue a Mode A interrogation, and the transponder sends out squawk code only. When they want altitude, they issue a Mode C request, and the data sent by the transponder contains altitude information only. Interrogations are repeated hundreds of times each second, typically A,A,C,A,A,C and so on.
Mode C data begins as static pressure. Airspeed, VSI, altimeter, and the encoder are all connected to the static system. Imagine an altimeter with no face or hands, just wires coming out. That is the encoder. Most modern encoders use an electronic pressure sensor, but some still use the same mechanical mechanism found in an altimeter.
We frequently reset the barometric pressure on the altimeter, but never reset the encoder. Why? All encoders are permanently set for standard pressure, 29.92". This is equally true of encoding altimeters, resetting baro only changes the display, it doesn't affect the electrical data. If this weren't so, ATC would need to know whether you are using an encoding altimeter or a blind encoder.
Imagine a huge cake, and a tiny insect burrowing through. The layers are vanilla, chocolate, strawberry, lemon, and so on. At each altitude request, the insect shouts "strawberry". There is no such thing as strawberry and a half. It's either strawberry, or it's chocolate, or whatever. The Mode C system works the same way, with layers approximately 100 ft thick. A Mode C interrogation will result in a report of, say, 5000 ft regardless of where you are in that layer.
Another analogy. A calendar shows what day it is. And we know each day is 24 hours. Does that mean ±12 hours? Well, at exactly noon it does, but a calendar cannot tell when it's noon. 24 hours isn't the same as ±12 hours, and likewise 100 ft. is not the same as ±50 ft. Forget plus and minus, it's either strawberry or it ain't.
If everything were perfect, and pressure was exactly 29.92", then the encoder would switch at the 50 ft points. As you climb through 1000 ft, for instance, it would read 1000 until you reach 1050, at which time it would change to chocolate!
But the world isn't perfect. One slice may be 120 ft thick, the next 90, and so on. Every encoder is different. And usually barometric pressure is not 29.92. At 29.97, for example, the report changes from strawberry to lemon at the exact altitude rather than 50 ft higher. As the plane flies along, the Mode C data is jumping up and down a hundred feet and that's what the controller sees.
In Part 5 we discussed Altitude Encoders, and the hundred foot increments. The altitude layers are like a huge cake. Vanilla, strawberry, chocolate layers. When an encoder reports altitude, it doesn't know where it is within the layer, it's strawberry until it changes to chocolate.
Consider this scenario: A pilot owns a beautiful aircraft and bases it at an outlying field near a major city. He strives to keep his airplane in perfect shape, he certainly wants everything to be legal. On a beautiful morning he goes out to the field, along with his instructor and mechanic and all the FAA inspectors and legal brass he can find. He asks them to find something illegal about the plane, or about the flight he is preparing to make. They swarm all over the plane and it's paperwork. And they check his papers too. After consultation, they all agree that both the pilot and the aircraft are legal and ready for the flight.
The pilot gets in, starts up, taxis out and takes off. He makes no mistakes at all. And when he lifts off the runway, he is instantly illegal!
How could this be? It is simple: He is flying within the Mode C veil, and his encoder isn't warmed up yet!
Warmed up? Most encoders have a pressure sensor housed in a tiny "oven", and until it reaches operating temperature the encoder output is inhibited. Which is good, if the data weren't turned off it would be erroneous, and that's both illegal and unsafe. (AR-500 owners, beware!) The pilot should be provided with an indication that the encoder is alive, but the manufacturer chose not to give him that information.
Some encoders take as long as 15 minutes to reach operating temperature. Most take 5 to 7 minutes. And the mechanical encoders and encoding altimeters have no warm-up time at all.
A Transponder Monitor is the best way for the pilot to see when his encoder comes alive.
What about accuracy? We must get correlation checks every 24 months, but what does that mean? Many shops only check 4 or 5 specific altitudes, they cannot take the time to check every 100 foot increment. It's usually not necessary. But some encoders can check good at certain spots, yet be off in between. Plus, the correlation check is done at just one temperature. What happpens when the weather gets hot or cold? And the correlation test is done on the ground with the engine off. Vibration prone intermittent connections between encoder and transponder often aren't found. A Transponder Monitor is the answer here, too. The pilot can see exactly what his equipment is reporting to ATC all the time.
A few encoders have ten foot increments. In the plane they do. But the Mode C world is made of 100 ft layers, and there is no place in the bit codes for anything finer. So, no matter what encoder you have, your transponder only broadcasts altitude data in hundred foot slices.
In the final installment, we'll wrap up by discussing reasons the pilot needs to see everything his equipment is telling ATC, and leave you with a puzzler.
The theme of this series of articles has been the transponder. As pilots, we give more attention to fancy GPSs and moving maps and such. The transponder is a rather ho-hum piece of equipment. Yet it is the link between the aviation community and the feds. What they see about us is what the transponder tells them. When it tells the truth we benefit. When it doesn't tell the truth we risk violations or worse.
And when the link overloads and the truth gets lost, or the link breaks down due to obsolete equipment and idiotic rules, aviation suffers needlessly.
We fly in two worlds at the same time. In the real world are the majestic towering CUs, fantastic sunsets never seen by surface-dwellers, and a thousand other experiences that mere mortals never enjoy. The real world contains embedded thunderstorms and uncharted towers and magneto failure, too. And we fly in the regulatory world, full of restrictions and FARs and thou-shalt-nots of every description. We must successfully navigate through both of these worlds simultaneously.
The transponder is sort of a bridge between the two worlds. That's why it's important to know what your transponder is reporting.
Here at Airsport, we know our Altitude Alerter / Transponder Monitors don't have the sex appeal of color moving maps, but they serve an important need. We're very proud of them. Our PRO model has been proven on the market for more than 3 years, they are flying on every continent. During that time we've listened to our customers and added many features, including baro pressure in millibars for Europe, and the ability to see the actual transponder pulses for avionics shops and the technically inclined. Two other models have been added, offering the features most pilots need at money-saving prices.
Call or write and we'll be pleased to provide information on the full range of AirSport's Altitude Alerter models.
I promised to finish the series with a puzzler. How about this one:
Imagine two identical aircraft. Not just similar, but aircraft absolutely identical in every respect. All instruments are calibrated precisely alike. No differences whatever. These planes are flown by pilots that are exact duplicates as well. They do everything precisely the same, in accord with common operating practice.
On the flight in question, we find these two planes flying along at the same speed, wingtip to wingtip. Same altitude, same heading, same temperature, same everything. And there has been no equipment failure, no damage, no ice, no problems. Got the picture?
But when we look inside, we find that the altimeter in one aircraft is reading hundreds of feet lower than the altimeter in the other plane. Yet when they land, both altimeters agree perfectly. How can this be?
Drop me a card or letter if you figure it out! Or contact me and I'll let you in on the answer. Fly safely!
AirSport Corporation
1100 West Cherokee
Sallisaw, OK 74955-4025
(800) 343-6690 or (918) 775-4010