Say Altitude.

[Hankal]

Long CX IFR at 7000
Controller comes on "say Altitude" My response "7000" He says you show 300
feet lower and gives me the altimer reading. I confirm. He says "turn off you
mod C"
I comply.Look at my GPS which is at 7060.
I go down 50 feet the gps stays at 7060. I go up 100 feet the gps stays at
7060.
The controller comes back and says" I think this is due to a cold front in my
flight area.
15 miles further my GPS altitude goes to 7000. I queery ATC to turn on my mod
C. He concurs my altitude at 7000.
Request lower for decend for landing. The gps works fine now, ATC is happy.

HAS ANYONE ELSE EVER EXPERINCED THIS?

[Bob Gardner]

Many times. Glitches can occur with their software. My most common
experience was having one controller tell me to turn off the altitude
squawk, being handed off to another sector, asking the new controller if I
should try Mode C again and having no further problems. Don't sweat it. If
they don't like what they are seeing they will ask for verbal altitude
reports.

Bob Gardner

"Hankal" > wrote in message
...
> Long CX IFR at 7000
> Controller comes on "say Altitude" My response "7000" He says you show
300
> feet lower and gives me the altimer reading. I confirm. He says "turn off
you
> mod C"
> I comply.Look at my GPS which is at 7060.
> I go down 50 feet the gps stays at 7060. I go up 100 feet the gps stays at
> 7060.
> The controller comes back and says" I think this is due to a cold front in
my
> flight area.
> 15 miles further my GPS altitude goes to 7000. I queery ATC to turn on my
mod
> C. He concurs my altitude at 7000.
> Request lower for decend for landing. The gps works fine now, ATC is
happy.
>
> HAS ANYONE ELSE EVER EXPERINCED THIS?
>
>
>

[EDR]

In article >, Hankal
> wrote:

> I comply.Look at my GPS which is at 7060.
> I go down 50 feet the gps stays at 7060. I go up 100 feet the gps stays at
> 7060.

GPS altitude is only accurate to +/-150 feet.
It is a "rough" reference, not to be considered "precise".

[Nathan Young]

GPS is not nearly as accurate in the z dimension as in the horizontal
dimension. I often see mine (even with WAAS corrections) differ as
much as 3-400 feet from my altimeter.

On 05 Apr 2004 00:29:34 GMT, (Hankal) wrote:

>Long CX IFR at 7000
>Controller comes on "say Altitude" My response "7000" He says you show 300
>feet lower and gives me the altimer reading. I confirm. He says "turn off you
>mod C"
>I comply.Look at my GPS which is at 7060.
>I go down 50 feet the gps stays at 7060. I go up 100 feet the gps stays at
>7060.
>The controller comes back and says" I think this is due to a cold front in my
>flight area.
>15 miles further my GPS altitude goes to 7000. I queery ATC to turn on my mod
>C. He concurs my altitude at 7000.
>Request lower for decend for landing. The gps works fine now, ATC is happy.
>
>HAS ANYONE ELSE EVER EXPERINCED THIS?
>
>

[Kai Glaesner]

Nathan,

> GPS is not nearly as accurate in the z dimension as in the horizontal
> dimension. I often see mine (even with WAAS corrections) differ as
> much as 3-400 feet from my altimeter.

Reference for your altimeter is a pressure level (hence it's called altitude
and not height), reference for GPS height is an approximation of the earth
called the "geoid", usually in form of WGS-84. I'm not sure if WAAS takes
this into account. If not the difference is system immanent.

Regards

Kai Glaesner

[[email protected]]

You didn't specify if your GPS was connected to the altitude encoder or not.
If it's an IFR GPS, it'll be connected to the encoder, which only outputs the altitude
to the nearest 100 feet. Since you say it stayed at 7060 after minor altitude
changes, I'm guessing that it was connected... and the altimeter setting in the GPS
made the nice, round 100' increments "round" to 7060. Remember, the encoder only
outputs pressure altitude, which is then corrected to actual altitude on the ground
(or in an IFR GPS) by the equipment given the current altimeter setting. If it's
unusually differing temperatures where you are than where the altimeter is, the
plane's altimeter could be a bit different than the encoder's corrected output.

-Cory

Hankal > wrote:
: Long CX IFR at 7000
: Controller comes on "say Altitude" My response "7000" He says you show 300
: feet lower and gives me the altimer reading. I confirm. He says "turn off you
: mod C"
: I comply.Look at my GPS which is at 7060.
: I go down 50 feet the gps stays at 7060. I go up 100 feet the gps stays at
: 7060.
: The controller comes back and says" I think this is due to a cold front in my
: flight area.
: 15 miles further my GPS altitude goes to 7000. I queery ATC to turn on my mod
: C. He concurs my altitude at 7000.
: Request lower for decend for landing. The gps works fine now, ATC is happy.

: HAS ANYONE ELSE EVER EXPERINCED THIS?




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

[Hankal]

>You didn't specify if your GPS was connected to the altitude encoder or not.
>
>If it's an IFR GPS, it'll be connected to the encoder, which only outputs the
>altitude

My GPS is not connected.
Under most conditions it agrees with my
altimeter.
It was just strange that the GPS and ATC
did not not reflect the same as my altimeter.
Hank

[[email protected]]

OK... even still, though, the GPS isn't too accurate, and is based on absolute
altitude, not barometric altitude (mentioned previously). Also, the temperature
affects the altimeter... how high AGL were you? I read an interesting article (avweb,
I think) regarding temperature correction and obstacle clearance. When the altimeter
reading is down low and temps are cold, the higher you go the more error the altimeter
tells you. So, if the altimeter setting is down in the valley at 1000' MSL, but
you're shooting the approach with MEA at 10K', there can be issues with terrain
avoidance. Supposedly Canadian and AK folks get more indoctrination to this than
those in the lower '48.

-Cory

Hankal > wrote:
:>You didn't specify if your GPS was connected to the altitude encoder or not.
:>
:>If it's an IFR GPS, it'll be connected to the encoder, which only outputs the
:>altitude

: My GPS is not connected.
: Under most conditions it agrees with my
: altimeter.
: It was just strange that the GPS and ATC
: did not not reflect the same as my altimeter.
: Hank

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

[Bob Gardner]

You're right. AIM 1-1-20(a)(8) tells us that we should not use GPS altitude.
When WAAS is in common use this will probably change.

Bob Gardner

"EDR" > wrote in message
...
> In article >, Hankal
> > wrote:
>
> > I comply.Look at my GPS which is at 7060.
> > I go down 50 feet the gps stays at 7060. I go up 100 feet the gps stays
at
> > 7060.
>
> GPS altitude is only accurate to +/-150 feet.
> It is a "rough" reference, not to be considered "precise".

[Tarver Engineering]

"Bob Gardner" > wrote in message
news:R0hcc.78998$K91.171081@attbi_s02...
> You're right. AIM 1-1-20(a)(8) tells us that we should not use GPS
altitude.
> When WAAS is in common use this will probably change.

The altitude error is part of the WGS-84 model and is not a WAAS issue any
longer. The CNX-80 provides VNAV capability through the use of pressure
altitude from a transponder source and the pilot's baro correction input;
married to a partial TAWS data base. GPS/WAAS is not likely to become an
acceptable substitute for pressure altitude. Larger airplanes use air data
derived pressure altitude in conjunction with GPS for improved RNP
capability.

[Bob Gardner]

I think the whole WAAS thing is in flux. The AIM gets wordier and more
difficult to interpret with each change to its discussion of GPS. IMHO,
until there are a whole lot of WAAS capable boxes flying in the system we
will be flailing about in the dark.

Did you see the change to the AIM with regard to a reversed W on approach
plates? Doesn't create much confidence in the system.

Bob Gardner

"Tarver Engineering" > wrote in message
...
>
> "Bob Gardner" > wrote in message
> news:R0hcc.78998$K91.171081@attbi_s02...
> > You're right. AIM 1-1-20(a)(8) tells us that we should not use GPS
> altitude.
> > When WAAS is in common use this will probably change.
>
> The altitude error is part of the WGS-84 model and is not a WAAS issue any
> longer. The CNX-80 provides VNAV capability through the use of pressure
> altitude from a transponder source and the pilot's baro correction input;
> married to a partial TAWS data base. GPS/WAAS is not likely to become an
> acceptable substitute for pressure altitude. Larger airplanes use air
data
> derived pressure altitude in conjunction with GPS for improved RNP
> capability.
>
>

[Hankal]

>You're right. AIM 1-1-20(a)(8) tells us that we should not use GPS altitude.
>When WAAS is in common use this will probably change.

I was not using the GPS for altitude. Always use the altimeter
The altimeter was showing 7000.
ATC was showing 6700
Gps was 7060. However when I changed altitude the gps stayed frozen at 7060.
Hank

[William W. Plummer]

"Hankal" > wrote in message
...
> >You're right. AIM 1-1-20(a)(8) tells us that we should not use GPS
altitude.
> >When WAAS is in common use this will probably change.
>
> I was not using the GPS for altitude. Always use the altimeter
> The altimeter was showing 7000.
> ATC was showing 6700
> Gps was 7060. However when I changed altitude the gps stayed frozen at
7060.

Aren't controllers supposed to tell you to turn off your altimeter and go to
VFR when an error that big is detected?

[Ron Natalie]

"William W. Plummer" > wrote in message news:ctncc.82738
> > I was not using the GPS for altitude. Always use the altimeter
> > The altimeter was showing 7000.
> > ATC was showing 6700
> > Gps was 7060. However when I changed altitude the gps stayed frozen at
> 7060.
>
> Aren't controllers supposed to tell you to turn off your altimeter and go to
> VFR when an error that big is detected?
>
They can tell you to stop altitude squawk if they want. They have no business to
tell you to "go VFR"

[Tarver Engineering]

"Bob Gardner" > wrote in message
news:%Glcc.191812$_w.1879080@attbi_s53...
> I think the whole WAAS thing is in flux. The AIM gets wordier and more
> difficult to interpret with each change to its discussion of GPS. IMHO,
> until there are a whole lot of WAAS capable boxes flying in the system we
> will be flailing about in the dark.

I believe that VDL will be more of a factor moving forward than WAAS will
be. FAA has spent a lot of money on space based WAAS while being in
posession of a lot of underused VHF bandwidth.

> "Tarver Engineering" > wrote in message
> ...
> >
> > "Bob Gardner" > wrote in message
> > news:R0hcc.78998$K91.171081@attbi_s02...
> > > You're right. AIM 1-1-20(a)(8) tells us that we should not use GPS
> > altitude.
> > > When WAAS is in common use this will probably change.
> >
> > The altitude error is part of the WGS-84 model and is not a WAAS issue
any
> > longer. The CNX-80 provides VNAV capability through the use of pressure
> > altitude from a transponder source and the pilot's baro correction
input;
> > married to a partial TAWS data base. GPS/WAAS is not likely to become
an
> > acceptable substitute for pressure altitude. Larger airplanes use air
> data
> > derived pressure altitude in conjunction with GPS for improved RNP
> > capability.
> >
> >
>
>

[Rich]

I've never been told to turn off my altimeter!

Rich


William W. Plummer wrote:
> "Hankal" > wrote in message
> ...
>
>
> Aren't controllers supposed to tell you to turn off your altimeter and go to
> VFR when an error that big is detected?
>
>

[[email protected]]

Bob Gardner wrote:

> I think the whole WAAS thing is in flux. The AIM gets wordier and more
> difficult to interpret with each change to its discussion of GPS. IMHO,
> until there are a whole lot of WAAS capable boxes flying in the system we
> will be flailing about in the dark.

You're right on. And, in an effort to sell LPV (WAAS) minimums, the friendlies
"tweaked" the VNAV missed approach criteria so that VNAV minimums will increase,
all to make WAAS look better.

[[email protected]]

Ron Natalie wrote:

> >
> They can tell you to stop altitude squawk if they want. They have no business to
> tell you to "go VFR"

When was the last time you had a controller ask you to turn off your altimeter? ;-)

[[email protected]]

Rich wrote:

> I've never been told to turn off my altimeter!

I would if I could only find the switch.

[Hankal]

>Aren't controllers supposed to tell you to turn off your altimeter and go to
>VFR when an error that big is detected?
>

The controller requested I turn off mode C.
In 25 miles he asked to turn back on.
Everything went fine from there on.
I could not have accepted VFR since I was in IMC condition.

[Ron Natalie]

"Rich" > wrote in message ...
> I've never been told to turn off my altimeter!
>
I've not even got an altimeter off switch. I guess I'll just have to smash the front
of the instrument.

[Ron Natalie]

> wrote in message ...
>
>
> Ron Natalie wrote:
>
> > >
> > They can tell you to stop altitude squawk if they want. They have no business to
> > tell you to "go VFR"
>
> When was the last time you had a controller ask you to turn off your altimeter? ;-)
>
I didn't repeat that error. I did have them tell me to stop altitude squawk. After three
weeks on the ground at VKX due to 9/11 hysteria, they let us flush out of there. We
got checked over by the local PG County cops and then given special void time clearances
out of there. As I broke ground I realized something was seriously wrong (static system
plugged). Fortunately, it was severe clear. I called ATC and asked what the mode C
was reporting (just for jollies), they said 100 feet. Told them that was in error but I really
didn't know where I was but figured about 2000'. After they verified I was in no danger
of crashing, they just told me stop altitude squawk and procede on course.

The only fun was figuring out how to tell when I was below 87 knots so I could drop the
gear (did a quick calculation with the GPS groundspeed and the AWOS winds which were
fortunately 10 knots right down the runway).

[Tarver Engineering]

> wrote in message ...
>
>
> Bob Gardner wrote:
>
> > I think the whole WAAS thing is in flux. The AIM gets wordier and more
> > difficult to interpret with each change to its discussion of GPS. IMHO,
> > until there are a whole lot of WAAS capable boxes flying in the system
we
> > will be flailing about in the dark.
>
> You're right on. And, in an effort to sell LPV (WAAS) minimums, the
friendlies
> "tweaked" the VNAV missed approach criteria so that VNAV minimums will
increase,
> all to make WAAS look better.

Just like selective availability. :)

[Peter R.]

William W. Plummer ) wrote:

> Aren't controllers supposed to tell you to turn off your altimeter
> and go to VFR when an error that big is detected?

Mode C is not always mandatory when flying IFR, at least in the US's
Northeast.

Last spring, I was returning from the AOPA Fly-In when I discovered I had a
mode C problem with my altimeter. The weather leaving Frederick, Maryland
(US) that day was low IFR, a really crappy day for a fly-in.

Upon calling for my clearance, I mentioned to the CD controller that I
suspected my mode C would not be functional. He had me wait while he
contacted others (perhaps the center controllers?). A moment later he gave
me the clearance and stated that I would be responsible for calling out my
altitude throughout the flight.

I then departed and as I was told, the departure controller had me call in
with my altitude every few thousand feet while climbing. From that point
on, I would receive a periodic "Cessna XXX, verify level at 9,000 feet"
call. I was impressed with the service.



--
Peter












----== Posted via Newsfeed.Com - Unlimited-Uncensored-Secure Usenet News==----
http://www.newsfeed.com The #1 Newsgroup Service in the World! >100,000 Newsgroups
---= 19 East/West-Coast Specialized Servers - Total Privacy via Encryption =---

[J Haggerty]

It may well have been due to the cold temperatures. See the table at
this link;
http://www.faa.gov/ATpubs/AIM/Chap7/aim0702.html#TBL%207-2-3

Your GPS may have had a correct altitude, and your barometric altitude
or mode C may have been reading a different altitude due to the cold air
temperature.

JPH

Hankal wrote:
> Long CX IFR at 7000
> Controller comes on "say Altitude" My response "7000" He says you show 300
> feet lower and gives me the altimer reading. I confirm. He says "turn off you
> mod C"
> I comply.Look at my GPS which is at 7060.
> I go down 50 feet the gps stays at 7060. I go up 100 feet the gps stays at
> 7060.
> The controller comes back and says" I think this is due to a cold front in my
> flight area.
> 15 miles further my GPS altitude goes to 7000. I queery ATC to turn on my mod
> C. He concurs my altitude at 7000.
> Request lower for decend for landing. The gps works fine now, ATC is happy.
>
> HAS ANYONE ELSE EVER EXPERINCED THIS?
>
>
>

[Hankal]

>Your GPS may have had a correct altitude, and your barometric altitude

But why did it freeze up for the next 15 miles. I was not near the Bermuda
triangle

[Roger Halstead]

On 05 Apr 2004 00:29:34 GMT, (Hankal) wrote:

>Long CX IFR at 7000
>Controller comes on "say Altitude" My response "7000" He says you show 300
>feet lower and gives me the altimer reading. I confirm. He says "turn off you
>mod C"
>I comply.Look at my GPS which is at 7060.
>I go down 50 feet the gps stays at 7060. I go up 100 feet the gps stays at
>7060.
>The controller comes back and says" I think this is due to a cold front in my
>flight area.
>15 miles further my GPS altitude goes to 7000. I queery ATC to turn on my mod
>C. He concurs my altitude at 7000.
>Request lower for decend for landing. The gps works fine now, ATC is happy.
>
>HAS ANYONE ELSE EVER EXPERINCED THIS?

It can be quite common and many reasons, but with weather you can
often expect it.

Headed home North bound just West of Toledo. They had given me the
altimeter setting a few minutes earlier. I received pretty much the
same call. I was flying into a front.

Roger Halstead (K8RI & ARRL life member)
(N833R, S# CD-2 Worlds oldest Debonair)
www.rogerhalstead.com
>
>