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Garmin 450 slow "reaction" to high altitude variations


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I have it for around 4 years.

Traveling by plane I use to check altitude variations: altitude and vertical speed

It was good compairing to airplane displays info.

Now I see slow data variations compairing to real values. If the plane is at 30000ft the gps displays 8000ft and ascending slowly. Same is while descending.

Vertical speed also is wrong. I remember 60ft/sec variations. Now I only see, in the same situation, 2ft/sec.

 

Real slow variations seem to be ok traveling by car in mountains where variations are not radical.

 

What can I do?

 

Thank you

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slightly less flippant answer!

 

It's because your GPS uses a barometric altimeter, i.e. it measures the change in air pressure. Because the plane cabin is pressurised the pressure inside the cabin will never be the same as outside (otherwise you'd be REALLY uncomfortable) so it will never show 30,000ft.

 

If you can disable the barometric altimeter in the GPS (have a look on the setup screens or in the manual), it will then calculate the height from the GPS system, under normal conditions this would be less accurate than the barometric method, but in this case it will be better - assuming you have good satellite reception inside the cabin.

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There isn't a way to shut down the barometric input to the device for the altimeter on a 450. It will always use that method, not GPS measurement, so it only records the cabin pressure in an aircraft.

The two options are for the device to act as an altimeter or barometer (the first assumes you are moving in altitude to explain the difference in pressure, the second assumes you are stationary and that the barometric pressure is changing due to weather).

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There isn't a way to shut down the barometric input to the device for the altimeter on a 450. It will always use that method, not GPS measurement, so it only records the cabin pressure in an aircraft.

The two options are for the device to act as an altimeter or barometer (the first assumes you are moving in altitude to explain the difference in pressure, the second assumes you are stationary and that the barometric pressure is changing due to weather).

 

The strange thing is I've travelled that route several times and I always have had real data like 60 feet/sec vertical speed descending instead of no more than 2 ft/sec like last two times.

Big planes fly near 30,000 ft altitude. My 450 reaches that height but much later than real live. Same as when the plane is landed, altitude takes a long while to display real data.

 

That's what Garmin's support answered:

 

The calibrated altitude maintains its accuracy as long as there are no significant changes in air pressure from certain outside forces. Changes in weather are the most significant cause for changes in air pressure, but change in location can also affect the accuracy of the sensor as air pressure can vary from location to location.

 

When experiencing a change in location or weather consider recalibrating the sensor with the known elevation or correct pressure for best accuracy; refer to owner's manual for instructions on calibrating the altimeter.

 

Calibrating the sensor periodically when experiencing changes in air pressure will ensure the highest level of accuracy.

 

The current software version of the Oregon 450 is 6.60. If your device is not up to date, you can update the software with the WebUpdater program:

 

http://www8.garmin.com/support/collection.jsp?product=999-99999-27

 

You might also try a master reset to address the problem:

 

To perform the master reset:

 

Power device off

Press and hold a finger on upper-left corner of display

Power device on while continuing to press corner of display

Release power button once Oregon powers on

Touch 'Yes' on Do you really want to erase all user data? message

 

The device should now be reset to default factory settings, all user data should be removed, and the device should function as expected.

 

Next flight I'll post what happpened after those instructions.

 

Thank you all

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The user cannot disable the barometric sensor (as ECA said) nor is there a need to.

 

Look on the satellite page. The elevation shown there is the GPS derived elevation. That is not affected by baro pressure. The elevation shown on the altimeter page is the Baro pressure derived elevation.

 

If you have auto calibration feature turned on, any differences between the two will (over time) gradually be eliminated.(in theory)

 

Inside a plane, the cabin pressure will affect the "perceived" Baro pressure or ambient pressure elevation, but will not affect the GPS derived elevation (on the sat page)

Edited by Grasscatcher
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I think the confusion is that the various available plots are only for the baro data, rather than the GPS data, so if he's looking at those plots, I don't know why it ever seemed to work for him before.

 

I guess it would help if the OP would tell us what screen he's looking at when assessing altitude.

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At least you're getting a signal. Last time I flew, I couldn't hold a single satellite, even at the window. I kinda wonder if Delta installed hardware in their planes to block communications in the cabin. Got great signal on the ground at the gate, lost it when the engines fired up. That's the first time this happened to me. Then again, a year ago I got reception sitting in an aisle seat on a US Airways flight.

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I think the confusion is that the various available plots are only for the baro data, rather than the GPS data, so if he's looking at those plots, I don't know why it ever seemed to work for him before.

 

I guess it would help if the OP would tell us what screen he's looking at when assessing altitude.

Here it is my last flight landing report from gps from BaseCamp. Look at speed and elevation (5th and 6th column)

 

Screen_Shot_2015_05_10_at_15_14_42.png

 

Here's an old report also landing on the same airport

 

Screen_Shot_2015_05_10_at_15_50_51.png

 

Same last flight half an hour before

 

Screen_Shot_2015_05_10_at_15_53_11.png

 

Here's the plot of the recent flight

 

Screen_Shot_2015_05_10_at_16_36_16.png

 

And here's the plot of the old one

Screen_Shot_2015_05_10_at_16_35_05.png

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The user cannot disable the barometric sensor (as ECA said) nor is there a need to.

 

Oh, that is sooooo wrong.

 

The Garmins always use barometric elevation in the trackfiles which is bad for a number of reasons.

 

Barometric elevation is fine if you are moving slowly (e.g. hiking) and not climbing/descending quickly. It has very poor time resolution, however, and usually lags your current elevation by about a minute.

 

And it doesn't work in a pressurized cabin, period. Thus you cannot make a GPX file of a flight profile.

 

This design decision is one of the more brain-dead things Garmin did.

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The user cannot disable the barometric sensor (as ECA said) nor is there a need to.

 

Oh, that is sooooo wrong.

 

The Garmins always use barometric elevation in the trackfiles which is bad for a number of reasons.

 

Barometric elevation is fine if you are moving slowly (e.g. hiking) and not climbing/descending quickly. It has very poor time resolution, however, and usually lags your current elevation by about a minute.

 

And it doesn't work in a pressurized cabin, period. .....

Firstly, I never disagree with fizzymagic.

 

However, as one who has designed cabin air pressurization systems for commercial airliners, allow me to augment, rather than contradict, the terminal statement in the quoted post:

While not be able to provide altitude data from inside the cabin, such a pressure instrument may be used to verify the functionality of a cabin pressurization system.

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Let's presume that the cabin air pressure set point is 8,000 feet. The pressure should be constant, essentially at that value, at altitudes greater than 8,000 feet. However, during descent below 8,000 feet, the cabin pressure will rise to the ambient pressure at the landing site. Consequently, altitudes barometrically derived inside the cabin will decrease during that descent below 8,000 ft.

 

DISCLAIMER: In previous technical discussions I have been described as not being knowledgeable and having practiced pseudo-science; you may want to consider my offerings appropriately before accepting their veracity. (Link to description provided upon request.)

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I believe if the barometer is set for "Fixed elevation," the elevation readings come from the GPS signal and not the barometric altimeter. My "flight" profile is set up that way and it always displays the "correct" altitude ( usually around 30,000 feet) in flight.

 

*I say "correct" because I know there is more error in the GPS elevation than in horizontal position.

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I believe if the barometer is set for "Fixed elevation," the elevation readings come from the GPS signal and not the barometric altimeter. My "flight" profile is set up that way and it always displays the "correct" altitude ( usually around 30,000 feet) in flight.

 

*I say "correct" because I know there is more error in the GPS elevation than in horizontal position.

No, no, no! Tried to explain this months ago, and again above.

 

The barometric sensor measures local pressure. The sensor doesn't know whether or not the pressure is changing due to an elevation change or 'weather'. YOU, the user, must provide the explanation to the unit.

 

It can be configured so as to assume that barometric changes are the result of changes in altitude, or it can be configured so as to assume that barometric changes are the result of local pressure changes ('weather'). It NEEDS for you to configure it to supply the context so it knows what to report.

 

If you set it for 'Fixed Elevation', then the device assumes that barometric changes are a function of actual pressure changes, and the device functions as a barometer. However, if you change altitude, you'll throw the numbers off.

If you set it for 'Variable Elevation', then the device assumes that barometric changes are a function of changes in altitude, and the device functions as an altimeter. However, there seems to be a large damping factor involved (mechanical or software) so it's slow, and if a weather front comes through on that mountain climb, that will throw the numbers off, too.

 

At least now we know what the OP was looking at (at least he's shown us what data he's looking at NOW), a recorded TRACK. See Fizzy's comments about that, above.

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