IMU and Barometer Vario

The intent of this page is to explain how a barometric vario works, what you can expect from smartphone variometers and sensors and how you might benefit from an instant vario incorporating inertial measurements as done by AI Sense.

Before we dig into technology, let's state the practical aspect for pilots. Technology aside, the impact of combining IMUs and Barometer in a vario is:

So what you hear is finally what you instantly feel and thus perfectly synchronized to your natural sense of flying. Two small bumps in the air won't be smeared and represented as possibly one thermal you accidently try to turn. Once in a thermal centering and finding the core is easier due to the instant feedback of the variometer. Within our vario apps AI Sense is used to provide and instant vario using the IMU.

What is an IMU?

An Inertial Measurement Unit (IMU) is a combination of sensors that provide information on orientation, velocity and gravitational forces. Nowadays IMUs are included in most phones and typically refer to a combination of accelerometer, gyroscope and magnetometer. Those inertial measurements refer to gravitational forces / gravity, they play a key role in providing our instant vario app.

a video comparing the performance of phone varios

AI Sense and Barometric Mode

Our vario app uses the combination of accelerometers, gyroscopes, magnetometers often referred to as IMU and the built in barometer as well as readings from the GNSS receiver. While most phones shipped with built in barometers also offer decent IMUs, this is not always the case. That is why, depending on the sensor setup and integration of a phone, two modes can be chosen in our vario apps to calculate the climb rate. AI Sense for barometer + IMU or Barometric Mode in case the IMU does not match the requirement for AI Sense.

AI Sense

AI Sense means that the vario engine is using the sensor readings from all sensors to compute a combined and cross checked climb rate. AI Sense is built around the Inertial Measurement Unit (IMU) of a phone. Using multiple sensors instead of just the pressure readings from a barometer the climb rate is on average more accurate, precise and robust. As opposed to the barometric mode, the vario tone is instant!

Barometric Mode

The Barometric Mode is a fallback vario engine that uses a classic Kalman Filter that can be tweaked to your liking with regards to responsiveness. It will depend on your hardware and in this case the barometer or the pressure sensor how well it performs. Start with a setting in the middle and increase responsiveness until you feel comfortable.

Now let's check how a barometer compares to an IMU. 

How Barometric Varios Work

A barometer is only capable of informing about pressure changes. Assuming a vertical translation (up/down) caused  these, those pressure changes will be translated to altitudes. And the changes in altitude will be derived to calculate a climb rate. For a vario that is based solely on a barometer or a so called barometric only vario, 

😯 changes in altitude / pressure need to happen before they can be detected 😯

Read that again please :-)!  And these pressure changes only then can be translated into altitude changes or ultimately a climb rate which is what the pilot is really after. Having said this it should be clear that

a barometric vario needs time to actually integrate pressure readings ⌛

Time to get enough readings to reliably separate noise from signal. No matter what algorithm, whether it is a plain moving average, an exponentially weighted moving average or even some Kalman Filter, there is an inherent delay and a trade off between the accuracy and reliability of the climb rate and the integration time needed to calculate that climb rate. With that kind of setup a vario becomes solely dependent on the readings of one sensor.

How Instant Varios Work

Remember, a basic vario for paragliding uses the variation in pressure to calculate the difference in altitudes and from that difference the climb rate. However, that difference in altitudes will need to happen first which is why a barometric vario, one that is based on pressure alone, will be delayed, always! It can only indicate lifts or changes in altitude that already happened ¯\_(ツ)_/¯ 

That said, there are other sensors that can be used to calculate the change of the climb rate. And again it is about relative change rather than absolute values.

Acceleration - the INSTANT in the vario

To understand how an instant variometer works, imagine you just got into an elevator. (Thermals can actually be seen as our elevators to cloud base.) You pushed the very last button to get to the top (cloud base), closed your eyes and now you wait for the ride to begin. What will you be able to tell from your senses...?

Now, you naturally feel once the elevator starts accelerating. In this case you are experiencing a 

🔝 Positive G-Force: your climb rate is expected to increase! 🆙

which results in a higher vertical speed. The start is actually like entering a thermal.

Of course, you also feel once the elevator starts braking or more technically, decelerates. In this case you are experiencing a

⬇️ Negative G-force  your climb rate is expected to decrease!  ⬇️

which results in lower vertical speeds and ultimately no speed.

So whenever you are accelerating (positive or negative) your climb rate will change. But what is more important, you are anticipating the change in climb rate before any barometric variometer could have made that calculation from altitudes and hence changes in pressure.


Gravity changing is INSTANT

But what can you tell about the climb rate from the elevator going up at constant speed, once the acceleration (and deceleration phase to be correct) is finished. Can you tell how fast you are going up, your exact climb rate? No, you can’t! Without any visual clues it is impossible. Which is why we need the barometric climb rate calculated from changes in pressure due to changes in altitude. You cannot have an instant vario without using the pressure sensor of your smartphone.


change of altitudes is NOT INSTANT

Now, taking the instant part from gravity and fusing it with the not so instant part from pressure is the basic idea of an instant vario. Best of both worlds, plain simple. 

Feedback on AI Sense Instant Vario

Since the start of the instant vario feature AI Sense, the feedback we got so far underlines an interesting but not unexpected fact that we also noticed when developing the instant vario algorithms. If you are used to flying with a classic vario and a delayed response, you might need a little retraining (learning curve) on getting what you feel working together with the instant varios output. There is no counting, no delay that needs to be compensated by the pilot.

As a beginner this (re)learning curve is not experienced, it feels natural from the beginning.