Large discrepancy between VPS and baro altitude

[With The Birds]

Loosely speaking yes, the IMU contains the Baro.
In reality it is not co-located physically.

So the P4P has dual barometers as well as the advertised dual IMU and compass? If IMU and barometer are physically seperate I would think it more likely they supply readings to the flight controller independently.

 

[SkyHog]

Yes, they are independent measurements, even if loosely associated within the overall navigation system. Strictly speaking, the IMU is only the Inertial Measurement Unit, which is essentially a set of three orthogonal solid state gyros that fundamentally just measure acceleration in each of the three axes. Those acceleration measurements can be integrated in software to obtain velocities in three dimensions, and those velocities can be further integrated to obtain position in three dimensions (which is also corrected with the slower GPS data, to control gyro drift). You'll notice in your IMU calibration data that it always ends up with zero acceleration in the x-y plane, and -1 acceleration in the z plane (for the acceleration of gravity) which are all correct for the rest state used in calibration. If you get two zeros and a -1, for x, y, and z respectively, you have completed a good IMU calibration.

The barometer is simply a pressure sensor only, not oriented with any spatial dimension. It's just an altimeter, except it isn't calibrated for local barometric pressure as would be needed for absolute altitude; it just measures relative altitudes.

Likewise, the compass is an independent sensor that measures magnetic field strength (in two dimensions) for heading reference and spatial orientation. The flight computer puts all this disparate sensor information together with software to support the overall system navigation, and for information display to the user.

 

[With The Birds]

Yes, they are independent measurements, even if loosely associated within the overall navigation system. Strictly speaking, the IMU is only the Inertial Measurement Unit, which is essentially a set of three orthogonal solid state gyros that fundamentally just measure acceleration in each of the three axes. Those acceleration measurements can be integrated in software to obtain velocities in three dimensions, and those velocities can be further integrated to obtain position in three dimensions (which is also corrected with the slower GPS data, to control gyro drift). You'll notice in your IMU calibration data that it always ends up with zero acceleration in the x-y plane, and -1 acceleration in the z plane (for the acceleration of gravity) which are all correct for the rest state used in calibration. If you get two zeros and a -1, for x, y, and z respectively, you have completed a good IMU calibration.

The barometer is simply a pressure sensor only, not oriented with any spatial dimension. It's just an altimeter, except it isn't calibrated for local barometric pressure as would be needed for absolute altitude; it just measures relative altitudes.

Likewise, the compass is an independent sensor that measures magnetic field strength (in two dimensions) for heading reference and spatial orientation. The flight computer puts all this disparate sensor information together with software to support the overall system navigation, and for information display to the user.

Thank you sky hog, that is my understanding. My question was if the barometer is part of the IMU as it was suggested by another poster earlier altitude data comes from the IMU

 

[SkyHog]

TLDR.....The IMU doesn't measure altitude. It measures accelerations using solid state gyros. The barometer measures pressure with a separate chip. Two different things.

 

[N017RW]

I said loosely speaking and never meant to imply there are two Baros.

If you search DJI's Wiki type pages you'll see THEY include the Baro in their description of the IMU.

It's not meant to indicate physical location.

 

[SkyHog]

I said loosely speaking and never meant to imply there are two Baros.
If you search DJI's Wiki type pages you'll see THEY include the Baro in their description of the IMU.
It's not meant to indicate physical location.

Yep, I agree. It's just semantics, which is why I also used the term "strictly speaking" in my own description. In a very broad sense, the term IMU is often loosely associated with the broader nav system, which is all fine so long as everyone understands the context. But in some cases, such as understanding how the barometer actually works, we need more precision. I'm not trying to be a stickler on language, just clarifying how it works.

 

[Sky King]

I had the same problem between real alt and indicated alt. It was off sometimes by 150'. I tried everything, called DJI and almost had it sent back......until I did a VPS calibration and that stopped the discrepancies between the two. Before it would go up and down like a yo-yo. After, it was rock steady.