Mode C Basics
Correction for Station Pressure
Verifying a Mode C Readout
    Invalid Mode C
Cold Weather Altimeter Issues

Mode C Basics

The Mode C encoder in a transponder adds an altitude value as part of its reply to a radar (or other interrogations, like from TCAS from other aircraft). The altitude is determined based on the standard altimeter setting of 29.92 inches of mercury, regardless of what the aircraft altimeter is set to. The altitude reported is, therefore, the pressure altitude of the aircraft, not the actual altitude. This means that the aircraft always reports in flight levels, so transition from the Altimeter Setting Region to the Standard Pressure Region (in Canada and the US this is FL180) is transparent to the user, and the pilot doesn't reset his transponder when climbing or descending through it. Those who are a little more aware of the definition of the Standard Pressure Region in Canada will also know this includes oceanic airspace and the extreme northern portions of Canada. Radar coverage is scarce at best in these areas, so it is not an issue as far as the radars and Mode C replies go.

The logical question that comes to mind is, "doesn't it almost always report an invalid altitude?" The answer is, "yes." The altitude reported is never right unless the local station pressure (corrected for sea level) is 29.92. In fact, it's right even less often than this, since it is only right if ICAO standard atmospheric conditions exist. Mode C does not report what the pilot sees on his altimeter gauge, unless he has that set to 29.92 as well, and then it's just coincidence since the two are not connected. How does ATC get a valid altitude readout?

Correction for Station Pressure

ATC's radar data processors are fed with current altimeter setting information for any given point within radar coverage. These come from weather reporting stations around the region. A reply received from a transponder which contains Mode C information is corrected using the local altimeter setting before it is displayed on ATC's radar, and it corresponds with the old rule of 1,000 feet per inch of mercury. That is to say that an aircraft flying in a region where the local station pressure is 28.92 would be reporting an altitude that is 1,000 feet off what the aircraft is actually flying at. Now remember the other old rule of pressure changes with barometric altimeters: if it's set too low, it will read too low, and vice versa. In the above example, an aircraft flying an indicated altitude of 3,000 feet with his altimeter set to 28.92, his Mode C would report an altitude that is 1,000 feet too high, or 4,000 feet. The radar would correct this using the altimeter of 28.92 in the local weather report, and suddenly ATC sees the correct altitude of 3,000 feet.

What if the pilot has the wrong altimeter setting dialed in? This is actually a side benefit of this type of set up. It would seem best, at first thought, that the indicated altitude on a pilot's gauge should be transmitted to ATC's radar with the transponder reply. The way it is done currently allows an indirect backup. If the pilot in the above situation had inadvertently dialed up 29.42, a setting which would correspond to a 500 foot error, he would still be showing an altitude of 3,000 feet. Since he is set .5 inches too high, the gauge would show 500 feet higher than he actually is, so he would really be at 2,500 feet. Since the Mode C is reporting a value that is 1,000 feet too high in our example, it would be reporting a value of 3,500 feet. The radar would correct this based on local pressure setting and display 2,500 feet to the controller. When the pilot reports in at 3,000 feet, the controller would hear one number, but see a very different one. Now the controller must do something to see what's going on, and that's coming up in the next section.

There is one more point to be made here, and that involves incorrect information in the radar data processor. This is comparatively rare, since the data is obtained electronically and automatically, but pressure settings are often reported by humans. This means that an occasional error could work its way into a weather report, and therefore be picked up by the processor. This means an inappropriate conversion will be done, and erroneous data will be displayed. This ends up being another sort of "built in" back up, since the pilot may be flying the correct altitude on a correct altimeter setting. ATC will investigate his end when the pilot confirms the airplane's end of it, and ultimately the error will be discovered and corrected, if all goes well.

Verifying a Mode C Readout

A controller must verify a Mode C readout before he can use the altitude information on a radar display. Typically, this is done on radar identification, such as on departure or when the pilot is asked to squawk ident. For example, a pilot calls in:

Pilot: "Moncton Center, good morning, Air Canada one two three is off runway two niner out of one thousand eight hundred for five thousand."
ATC: "Air Canada one two three, Moncton Center, radar identified, cleared on course, climb to flight level two four zero."

There. Did you see that? The controller just verified the aircraft's Mode C readout. Oh, yeah. You couldn't see it in that crappy example. The controller sees the target on radar as the pilot calls in, and when the pilot reports his altitude, the controller sees that the displayed altitude is within 200 feet of the reported altitude, so it is considered validated. No further mention from ATC is required. Occasionally a pilot will not report his altitude on departure, and the reply from ATC might include, "... radar identified out of one thousand four hundred..." Another example could go something like this:

Pilot: "Halifax Terminal, Cessna Golf Romeo Uniform Bravo request flight following."
ATC: "Cessna Golf Romeo Uniform Bravo, Halifax Terminal, squawk ident."
followed by a flash on ATC's radar screen...
ATC: "Romeo Uniform Bravo, radar identified at four thousand eight hundred."
Pilot: "Romeo Uniform Bravo is actually at four thousand five hundred, sir."

This means the Mode C readout is different by 300 feet. And that means it is not useable. The Mode C must be within 200 feet of the reported altitude in order for ATC to use it. ATC can take steps to validate it, though, in such a situation. Controllers and those in the know should have noticed at least one thing missing in the above exchange: ATC didn't provide an altimeter setting to the aircraft on initial contact when the pilot called in requesting flight following. This is something that should have been done, since the aircraft is not in the "flight levels", and now we certainly have to, since the Mode C isn't right. The conversation continues where we just left off:

ATC: "Romeo Uniform Bravo, verify altitude. Halifax altimeter three zero two three."
Pilot: "Three zero two three, now showing four thousand seven hundred, descending to four thousand five hundred."
ATC: "Romeo Uniform Bravo, roger."

Now that's a little better. The readout differs only by 100 feet, so it is now validated and useable. If the pilot were to confirm his altitude as 4,500 but still show 4,800 on the radar, the Mode C is invalid and must not be used by ATC. ATC may ask the pilot to turn off his Mode C (Phraseology: "STOP ALTITUDE SQUAWK"). This means, "turn off the altitude reporting but leave the transponder on." Some transponders may not be able to turn off the Mode C without stopping replies to radars altogether. In such a case, ATC would rather see the target with an invalid Mode C than not see a target at all. This holds true for aircraft with TCAS, also, since TCAS can only see aircraft with functioning transponders, Mode C or not. For the sake of information, many transponders have the following settings; OFF, STBY (which equates to on, but not replying to radar or TCAS interrogations), ON (which means replying to interrogations but not sending altitude information), and ALT (which is the full meal deal).

Another side note here is that if ATC tries to confirm your altitude and you notice it's not what he just said it was, make sure he knows. Advise him at the earliest opportunity that your altitude isn't what he said and request the current altimeter setting if he doesn't offer it. If your Mode C doesn't work right, make sure someone knows so it can be fixed. Another pilot can't see you if he's looking in the wrong place. Whether he's looking at his TCAS for you, or ATC is trying to point you out to him on radar, either one has to know your altitude as well as your position.

Just how many Mode C readouts are invalid? Not that many. In the 12 years I've been in ATC at the time of writing this topic, I have seen only a handful. Take it another step. The vast majority of those that have been invalid in my experience have missed the mark by a good margin, too. I've seen a helicopter out the tower window barely a few hundred feet above the treetops (ground elevation between 300 and 500 feet ASL) with his Mode C reading 6,000 feet. The best one, though, showed a pilot who reported out of 11,400 climbing showing a Mode C of 114,200 which was steadily and slowly descending as he climbed. When he finally leveled off at FL330, his Mode C showed stable at FL932, or 93,200 feet. Obviously an error. When an invalid Mode C readout is mentioned, a pilot should consider it an important issue and so should service personnel on the ground.

Cold Weather Altimeter Issues

As mentioned in last week's topic, and alluded to at the beginning of this one, barometric altimeters aren't perfect. In fact, many errors can creep into readings, and one of the more significant ones is the famed Cold Weather Altimeter Error. One astute reader asked the question of whether this is included in the radar processor's calculation for display to ATC. The answer is, "no," it isn't. The Mode C readout will show what the altimeter is indicating if the altimeter is set to local station pressure and the radar processor also has the correct station pressure. So when a pilot applies a 200 foot correction factor to 3,000 feet, he will indicate 3,200, and the transponder will return a reading that will display 3,200 to the controller. He will, in actual fact, be flying at 3,000 feet as described last week, even though he indicates higher.

Once again, thanks for taking the time to read. My e-mail address, if you have any feedback or questions, is moxner@nbnet.nb.ca.