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6 Answers

Is a transponder of any value if it isn’t linked to an encoding altimeter?

Asked by: 3072 views , , , ,
Aircraft Systems, Airspace, Light Sport Aircraft

We were/are considering buying installing a mode-C transponder on our little ELSA. For three reasons: We might want to fly into some class-C airspace, we might want to use flight following, and (because we're fabric covered and don't create a great radar target) would like to be more visible to ATC and other aircraft with radar or ADS-B in.

We just discovered to our dismay that even if we buy a mode-C transponder it won't operate in mode-C (transmitt altitude information) unless it is getting altitude information from an altimeter that is an "encoding altimeter." We're 99% sure our basic steam-gauge altimeter has no such feature.

If we buy and install a mode-C transponder and don't have an encoding altimeter connected to it it then, if I understand right, it can operate in a mode (mode-A?) that would give position info, make us more visible, send our squwak code, but not altitude info. Apparently that would not be sufficient to allow operating in class-C airspace, but would it allow using flight following?

In short, would it be of much use?

And does anyone know if it's possible for a transponder to get altitude info out of our GPS (which reads altitude very accurately)?


Thanks for any replies


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6 Answers

  1. John D. Collins on Nov 17, 2012

    I doubt your panel altimeter is an encoding one, they are very expensive, like on the order of $10,000. Most folk use what is called a blind encoder that hooks to your static system and is wired to power and ground and the transponder. It provides a digital form of pressure altitude to the transponder. The cost of a blind encoder runs from under $200 (parallel interface) to just under $600 (serial interface). Most transponders will support a parallel interface although a few require a serial interface. Your avionics shop will tell you what your options are. I would suggest you buy and install a blind encoder with your transponder. In 2020, you will have to install an ADSB Out system in addition to your transponder in order to fly into Class C, B, airspace and above 10,000 feet AGL.

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  2. Alex on Nov 20, 2012

    Thank much for your very useful reply., John.

    By the time I read it I had already found out about blind encoders (obviously, as you suggest, the way to go if/when we get a transponder), but wasn’t, until you mentioned it, aware of the fact that some are for paralell and some for serial interfacing.

    The blind encoder raises and interesting question:
    Unlike my panel steam gauge altimeter, they are set for barometric pressure of 29.92 inches. No manual input to correct for outside pressure not being that. Between that and temperature induced errors wouldn’t be surprised if a blind encoder sometimes reports altitude 400 feet or more off actual altitude. In crowded airspace or complex airspace errors that big would seem unacceptable.
    So my question is when ATC (or aircraft with avanced avionics that also pick up info from others’ transponders) see the altitude report from the transponder connected to that blind encoder, does it have some way to automatically correct for at least the errors from the location pressure deviating from 29.92 …. or any way to get closer to the true MSL altitude?

    Yes, I know about the 2020 requirement for ADS-B out.
    That’s one reason we may hold off on getting a transponder:
    Our avionics guy suggest that as 2020 get closer many avionics manufactuers will be offering transponders with ADS-B out (and maybe ADS-B in) built in. Less cost and less installation hassles than separate units.
    On the other hand, desire to fly in class-C airspace isn’t super high on the reasons we’re looking at transponders…. we can live without that in our style of flying. More interested in being more visible to others (our ragwing doesn’t make a great radar target and in being able to request flight following.
    And rarely fly over 10,000, our aircraft is usually flown by sport pilots, who with only minor exceptions, are not permitted to fly above 10,000 anyway.

    Not to diminish the value of ADS-B-in, but might more look forward to having ADS-B-in. Be nice to have more info about other aircraft nearby which we don’t yet have visual or radio contact with.

    Still curious whether one can get flight following with a transponder that isn’t sending altitude info (whether because it’s older, never had altitude, or altitude sending is turned off. Any idea?


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  3. John D. Collins on Nov 21, 2012


    I would recommend you install a transponder with a compatible blind encoder. Transponders will still be required after 2020 to enter the same airspace in addition to requiring ADS-B Out. It is possible that some future transponder design will incorporate ADS-B out, but I doubt this will be more cost effective. Today, a used transponder can be purchased for $1,000 +/- and installed with an encoder for about twice that figure. Since your aircraft doesn’t show up well as a primary target, it is likely that you will not be pointed out to other aircraft. A mid air collision ruins the day for both aircraft involved.

    You can receive flight following service without an encoder attached to your transponder, but when you are not receiving radar services, ATC will see you and call you as traffic to other aircraft with something like this: “Cessna 12345, traffic twelve O’clock, opposite direction, altitude unknown”. If you have an encoder, they will say “Cessna 12345, traffic twelve O’clock, opposite direction, 6,500 feet, altitude unconfirmed”. If you are receiving radar services from ATC, as is the traffic, then ATC will tell the traffic: “Cessna 12345, traffic twelve O’clock, opposite direction, 6,500 feet, a Piper Cub” and then for you “Piper 45678, traffic 12 O’clock, opposite direction, 6000 feet, a Cessna 172″.

    You ask why the blind encoder is fixed at an altimeter setting of 29.92 and provides pressure altitude? This is intentional and has the great advantage of putting all aircraft on the same reference. If an aircraft has a traffic system, it will compare your pressure altitude with its own pressure altitude and display your aircraft height relative to its own. This could not be done if the altimeter setting could be changed as the other aircraft would have no way of knowing what altimeter setting you have set or miss set in your aircraft. Even if your altimeter was set properly it could be based on a reference a hundred miles away and the two altimeter settings could be based on settings 200 miles from each other. The ATC computer knows all of the altimeter settings nearby your position and will correct your transponder transmitted pressure altitude to a corrected MSL value for display to the controller. Every time you obtain ATC radar services, you will always be asked to confirm your altitude based on your steam gage. If your altitude disagrees by more than 200 feet from the value displayed by ATC on their radar, ATC will tell you “Stop squawk altitude”. Remember that altimetry is based on a model of the atmosphere, which rarely matches. As you are further from the ground reference for the altimeter setting, both in altitude and distance, the true altitude error increases. Also, altimeters are not compensated for temperature. These errors accumulate and can be well over 500 feet. That is one reason that all aircraft at or above 18,000 MSL on a standard pressure day, set their steam gages to a common setting of 29.92 and we call the altitude a flight level, meaning that we are flying at a constant pressure altitude and not a MSL altitude.

    Also, for IFR flight, FAR 91.411 requires that the steam gage when set to 29.92 and compared verses the transponder output and can’t deviate more than 125 feet. This is called the correspondence test and it wouldn’t be required for your VFR use, but is still a good idea.

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  4. Alex on Nov 21, 2012

    Much thanks again, John, for that lucid explaination of why the encoders are all set to 29.92 and how that’s handled. Like many, but not every, standard aviation policy/regulation, it makes great sense that we’re all so-to-speak “on the same page” as to what altitudes our transponders are reporting about our alititude separation or lack of separation.

    You wrote —
    “It is possible that some future transponder design will incorporate ADS-B out, but I doubt this will be more cost effective. ”

    Apparently some are already available:
    “New AXP340 Mode-S Transponder with ADS-B Out.”

    If nothing else if a combo Mode-C plus ADS-B Out unit would likely be more cost effective in that we’d be paying our avionics guy for one installation session, not two. And in our little ELSA our panel is VERY cramped for space. We’re already constrained on transponder choices as unless we can live with a box on top of the dash we’ll have to use one of the few 2.25 inch transponders or at most one of the small form factor ones. Not many of those on the used market. So in our case if we can stuff both functions in one box that’s an advantage.

    At the risk of changing the topic:
    You wrote, in part —
    “Remember that altimetry is based on a model of the atmosphere, which rarely matches. As you are further from the ground reference for the altimeter setting, both in altitude and distance, the true altitude error increases. Also, altimeters are not compensated for temperature. These errors accumulate and can be well over 500 feet.”

    I appreciate your confirming that impression I already had: I’m sure not an atmosphere scientist, but as a physics major I had strongly suspected the reports of barometric pressure from a some-times distant ground station when modeled to a different altitude, and temperature change errors on our aneroid altimeters had to have large errors; and had already observed errors approaching 500 feet when the steam gage altimeter was nominally correctly set to nearest currently officially reported barometric pressure .
    Add to the error source you mention the fact that the barometric pressure being broadcast may be a reading taken 20 minutes before a rapid change too place, the local small airport barometers you and I set our altimeters to may be a little off, then if we blindly trusted our altimeters you and I could believe we’re separated by 1000 feet of altitude and be much closer. There have been times when flying into litte unattended airstrips despite having set the altimeter to the nearest available most current barometric pressure if I’d not had eyes and my GPS altitude to over-rule the steam gauge altimeter I’d have been flying in the pattern 400+ off pattern altitude.

    In short, I trust the GPS altitude report as a reference to set the steam gage altimeter to far more than the barometric pressure report (if they dissagree, and they often do.) Officially a basic consumer GPS without WAAS or the widely available DGPS enhancement of the statellite signal, “has an altitude accuracy of … +/- 75ft” with a … 95% confidence.”
    In practice, when I compare what my GPS reports for altiude with known exact altitudes (surveyed runway altitude) I have never seen it off by more than 50 feet…. usually less.

    And further as a practical matter I will, if using the GPS as the calibration, check and notice if it’s significantly off reset it far more often than listening or calling for a baromatic pressure report.

    Have I made a credible case for setting the altimeter to the GPS altitude over entering a barometric pressure report? Or did I miss something?

    My two cents on that topic. 😉

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  5. hpux735 on Dec 04, 2012

    In regards to GPS altitude versus barometric altitude, the Cessna Skycatcher that I fly has a Garmin G300, which will display a “set baro” annunciation if the baro alt differs from the gps alt by more than a set amount. It’s a pretty nice feature.

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  6. ayaz khattak on Nov 26, 2013

    if the tranponder showing less altitude then enncoing altimeter on ground how can we adjust elevation height on transponder . plz send the adjustment procedure of altitude in tranponder

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