Mode C Indication

In everyone's observation, what is the typical difference between the
indicated altimeter reading with current pressure setting and the altitude
reported by mode c transponder. It seems mine always has a discrepancy of
100', sometimes 200' compared to what ATC shows.

Just wondering if it needs further investigation. Pitot static, altimeter
and transponder inspections are all current.

Thanks!
Jeff

Jeff wrote:

In everyone's observation, what is the typical difference between the
indicated altimeter reading with current pressure setting and the altitude
reported by mode c transponder. It seems mine always has a discrepancy of
100', sometimes 200' compared to what ATC shows.

As far as I understand, your mode C will be reporting pressure altitude to
ATC, and their computers will adjust that based on the altimeter setting in
use for the sector. So the difference between the mode C and indicated
altitude can be huge, but I don't think that's what you're asking.

What happens on the ground when you set your altimeter to the station
altimeter setting? How far off is it from the field elevation?


All the best,


David

"David Megginson" wrote in message
news
As far as I understand, your mode C will be reporting pressure altitude to
ATC, and their computers will adjust that based on the altimeter setting
in
use for the sector. So the difference between the mode C and indicated
altitude can be huge, but I don't think that's what you're asking.

You are correct that if you have a transponder which displays its reported
pressure altitude, that can be significantly different than field elevation.
Just turn your altimeter to 29.92 and note the difference between that
pressure altitude and the true altitude with a proper altimeter setting.

When ATC reports your Mode C altitude back to you, their computer has
adjusted the pressure altitude for the proper altimeter setting. The
allowed tolerance is 200 feet; if your altitude is off by more than this
then you should have your altimeter and static system checked.


--------------------
Richard Kaplan, CFII

www.flyimc.com

On Tue, 04 May 2004 16:15:26 GMT, "Jeff" wrote:

In everyone's observation, what is the typical difference between the
indicated altimeter reading with current pressure setting and the altitude
reported by mode c transponder. It seems mine always has a discrepancy of
100', sometimes 200' compared to what ATC shows.

Just wondering if it needs further investigation. Pitot static, altimeter
and transponder inspections are all current.

Thanks!
Jeff


Lot's of room for systemic errors here. Your encoder likely only has a
resolution of 100'. And ATC may not have given you the same altimeter
setting that their computer is using. Add in a 50' altimeter error in the
a/c, and you could easily have a 100' discrepancy.

If your altimeter has been certified for IFR, then you should know what the
error is there. Your encoder should also have been certified.

With my new CNX80, I also had installed an encoder with 10' resolution, and
the CNX 80 can read the output from that encoder. My panel altimeter, and
the output of the encoder, seem to agree once I take into account the
altimeter error from the last certification. Before, when I had the 100'
resolution encoder, I would occasionally be informed by ATC that I was off
by 100', and rarely that I was off by 200'. At the time, I might be off by
30 or 40 ft on my panel altimeter.


Ron (EPM) (N5843Q, Mooney M20E) (CP, ASEL, ASES, IA)

The least significant step that your encoder can report is
100' increments. If you are at 8350' per your correctly set
altimeter, the encoder/transponder is going to be transmitting
83 or 84. In an ideally calibrated encoder, it is adjusted
to switch at the 50' points, so while the aircraft is between
8250' and 8350', the encoder will report 83; while the aircraft
is between 8350 and 8450, it will report 84, and so on.
(Ignore all the blather about altimeter setting for a moment).

The encoder is typically an analog sensor going through an OpAmp
ampiflier, and then to an analog to digital converter. As with all
analog circuitry, it is subject to aging and drift. Over time, the
switching points can drift relative to the absolute altitude. The fix
is to adjust gain and zero pots in the encoder so that it tracks the
absolute altitude. At the same time, you also want your altimeter
to also track the absolute altitude. There is no provision for
"setting" the encoder to prevailing atmospheric pressure...

Because of the low resolution of the encoder, and potential discreptancy
of the encoder, the FAA treats what your encoder is reporting with a
grain of salt. They always ask "say altitude" so that they can
establish if your encoder is lying or not. They understand the
limitations of the system, so if you say 5700', and they look at the
scope and see 56, 57, or 58, they will be happy.

If they see 55 (or lower), or 59 (or higher), they will question if
you have your altimeter set correctly. If you confirm that you do,
then they will conclude that your encoder is outside the allowable
limits, and either tell you to stop alt sqwak, or tell you to get it
fixed.

Note that an incorrect altimeter setting, an inaccutate altimeter, or an
inaccurate encoder would all produce discreptancies. The only thing
you can do effect directly is the altimeter setting. Adjusting the
encoder can only be done during a static check. You could also fly
a hundred feet higher or lower than your assigned altitude to bias
the readings so that they agree... (a really bad idea). If your
altimeter is more than twenty years old, it needs overhaul and/or
replacement.

MikeM
Skylane '1MM
Pacer '00Z



Jeff wrote:
In everyone's observation, what is the typical difference between the
indicated altimeter reading with current pressure setting and the altitude
reported by mode c transponder. It seems mine always has a discrepancy of
100', sometimes 200' compared to what ATC shows.

Just wondering if it needs further investigation. Pitot static, altimeter
and transponder inspections are all current.

Thanks!
Jeff

Aside with the 29.92 altimeter setting, would temperature cause a difference?
I know that the encoders have a warm-up time while it's heating the sensor to a fixed
temp. Is that just for the electronics, or also for the static air? If so, then if
the static air/cabin temp is really cold, they could be different by a bunch, no? The
altimeter has temp error, whereas the encoder's kept constant.

The same argument as cold-weather minimums.

-Cory

Jeff wrote:
: In everyone's observation, what is the typical difference between the
: indicated altimeter reading with current pressure setting and the altitude
: reported by mode c transponder. It seems mine always has a discrepancy of
: 100', sometimes 200' compared to what ATC shows.

: Just wondering if it needs further investigation. Pitot static, altimeter
: and transponder inspections are all current.

: Thanks!
: Jeff



--
************************************************** ***********************
* The prime directive of Linux: *
* - learn what you don't know, *
* - teach what you do. *
* (Just my 20 USm$) *
************************************************** ***********************

The encoded altitude reported by Mode C is in 100' increments, so in the ideal
world the encoder resolution alone introduces a +/-50' error. Add on top of
that encoder calibration error, altimeter error, pressure differences between
your location and ATC, non-standard pressure lapse rate, temperature, etc, and
you can easily get a couple hundred feet difference. Fortunately, only the
altimeter and encoder errors are unique to your airplane, so any other airplane
near you will have the same systemic errors (outside the aircraft) as you do.

Ron Rosenfeld wrote:

On Tue, 04 May 2004 16:15:26 GMT, "Jeff" wrote:

In everyone's observation, what is the typical difference between the
indicated altimeter reading with current pressure setting and the altitude
reported by mode c transponder. It seems mine always has a discrepancy of
100', sometimes 200' compared to what ATC shows.

Just wondering if it needs further investigation. Pitot static, altimeter
and transponder inspections are all current.

Thanks!
Jeff


Lot's of room for systemic errors here. Your encoder likely only has a
resolution of 100'. And ATC may not have given you the same altimeter
setting that their computer is using. Add in a 50' altimeter error in the
a/c, and you could easily have a 100' discrepancy.

If your altimeter has been certified for IFR, then you should know what the
error is there. Your encoder should also have been certified.

With my new CNX80, I also had installed an encoder with 10' resolution, and
the CNX 80 can read the output from that encoder. My panel altimeter, and
the output of the encoder, seem to agree once I take into account the
altimeter error from the last certification. Before, when I had the 100'
resolution encoder, I would occasionally be informed by ATC that I was off
by 100', and rarely that I was off by 200'. At the time, I might be off by
30 or 40 ft on my panel altimeter.

Ron (EPM) (N5843Q, Mooney M20E) (CP, ASEL, ASES, IA)

--
--Ray Andraka, P.E.
President, the Andraka Consulting Group, Inc.
401/884-7930 Fax 401/884-7950
email
http://www.andraka.com

"They that give up essential liberty to obtain a little
temporary safety deserve neither liberty nor safety."
-Benjamin Franklin, 1759

Altimeter reading at field elevation with station altimeter setting is +/-
20-30', no problem there.

After rereading my first post, I think I misphrased the question. I'm not
necessicarily questioning the actual altitude (pressure) being brodcast by
the mode c transponder as much as i'm wondering about what ATC is telling me
they see.

For example: Depart local field, level off at two thousand five hundred. Fly
that altitude till clear of local airspace. Call up approach for flight
following and give the usual info. ATC calls back with squak code and
altimeter setting. ATC calls back November 1234 alpha radar contact 5 miles
south of brown vor two thousand six hundred (or seven hundred). That's the
descrepency I'm trying to get a baseline on.

Is it usual to have indicated differ from ATC reported by one to two hundred
feet?

Cheers!
Jeff


"David Megginson" wrote in message
news
Jeff wrote:

In everyone's observation, what is the typical difference between the
indicated altimeter reading with current pressure setting and the
altitude
reported by mode c transponder. It seems mine always has a discrepancy
of
100', sometimes 200' compared to what ATC shows.

As far as I understand, your mode C will be reporting pressure altitude to
ATC, and their computers will adjust that based on the altimeter setting
in
use for the sector. So the difference between the mode C and indicated
altitude can be huge, but I don't think that's what you're asking.

What happens on the ground when you set your altimeter to the station
altimeter setting? How far off is it from the field elevation?


All the best,


David

your altitude encoder may be off and need adjusting.

last year ATC was seeing me 200 ft lower then I was, I took it to the avionics
shop and he adjusted the altitude encoder, now ATC see's the same thing my
altimeter is saying.


Jeff wrote:

In everyone's observation, what is the typical difference between the
indicated altimeter reading with current pressure setting and the altitude
reported by mode c transponder. It seems mine always has a discrepancy of
100', sometimes 200' compared to what ATC shows.

Just wondering if it needs further investigation. Pitot static, altimeter
and transponder inspections are all current.

Thanks!
Jeff

Jeff wrote:

Is it usual to have indicated differ from ATC reported by one to two hundred
feet?

Not with my aircraft. Bendix-King transponder and AmerKing encoder, purchased 1995.

George Patterson
If you don't tell lies, you never have to remember what you said.