Managing Gyro Noise with the Dynamic Harmonic Notch Filters

As discussed under the Vibration Damping topic, managing vibration in ArduPilot autopilot installations is extremely important in order to yield predictable control of an aircraft. Typically installations utilize mechanical vibration damping for the autopilot, internally or externally, in order to remove the worst of the vibration. However, mechanical damping can only go so far and software filtering must be used to remove further noise.

To the autopilot, vibration noise looks like any other disturbance (e.g. wind, turbulence, control link slop, etc.) that the autopilot must compensate for in order to control the aircraft. This prevents optimum tuning of the attitude control loops and decreased performance.

ArduPilot provides two filtering mechanisms for noise. Lowpass filters on the accelerometer signals, controlled by the INS_ACCEL_FILTER, and the gyro signals, controlled by INS_GYRO_FILTER, and Harmonic Notch Filters on the gyro signals.

As discussed in Measuring Vibration section, there are basically two classes of noise/vibrations: those generated within the bandwidth of the gyros/accelerometer sampling and noise above those frequencies which are “aliased” down to within that bandwidth and can cause the “leans”. The aliased noise must be eliminated at the source with improved mounting or frame rigidity, but the above filters can deal with the other sources, typically generated from the motor/propellers at their rotation frequency and its harmonics.

For multicopters and QuadPlanes, virtually all vibrations originate from the motor’s rotational frequency. For helicopters and planes, the vibrations are linked to the main rotor/prop speed.

ArduPilot has support for two notch filters whose filter frequency can be linked to the motor rotational frequency for motors, or the rotor speed for helicopters, and provides notches at a primary frequency and its harmonics.

While the lowpass filters can effectively diminish the impact of this noise, having low frequency set points creates a lot of phase lag and therefore reduces how aggressive the tune can be before oscillation occurs, which results in a poorer tune.

For the gyro based rate controllers, this reduces their ability to respond to fast disturbances. If the gyro lowpass filter can be set higher, the phase lag induced is lower and the tune can be more aggressive. But this allows more noise and vibration, effectively canceling that gain out. Enabling either one or both of the Harmonic Notch filters gives the ability to target the noise generated by the motors, allowing a higher frequency of the following low pass to be set and therefore allowing tighter tune.

Notch Filter Setup Overview

  1. Select how the notch center frequency will be controlled. See Notch Filter Control Types.

  2. If a static notch (not usually recommended), or throttle-based control is used, the dominant noise frequencies will need to be determined in order to setup the notch. See Determining Notch Filter Center Frequency.

  3. Enable the notch filter.

  4. Setup the selected center frequency control method using INS_HNTCH_MODE. Then setup its associated parameters by reading its associated page linked in this secion Notch Filter Control Types.

  5. After a test flight and log analysis with the notch enabled (See Checking Notch Filter Effectiveness), the number and placement of higher harmonic filters implemented can be adjusted, the use of the multi-notch options, or even a second harmonic notch set (INS_HNTC2_ENABLE) can be configured to improve noise reduction effectiveness. The web based Filter Review Tool can be used to experiment with parameter changes based on the test flight log to determine best configurations without having to make iterative test flights.

Enabling the Notch Filters

The harmonic notch is enabled overall by setting INS_HNTCH_ENABLE = 1 for the first notch, and, if needed, a second set of harmonic notches using INS_HNTC2_ENABLE = 1. After rebooting, all the relevant parameters will appear.

Various methods of dynamically adjusting the notch(s) center frequency to track motor speed under different thrust conditions is provided, ie Dynamic Harmonic Notch filtering.

Notch Filter Control Types

Key to the dynamic notch filter operation is control of its center frequency. There are five methods that can be used for doing this:

  1. INS_HNTCH_MODE = 0. Static center frequency. Dynamic notch frequency control is disabled. The center frequency is fixed and is static. Often used in Traditional Helicopters with external governors for rotor speed, either incorporated in the ESC or separate for ICE motors.

  2. INS_HNTCH_MODE = 1. (Default) Throttle position based, where the frequency at hover throttle is determined by analysis of logs, and then variation of throttle position above this is used to track the increase in noise frequency. Note that the throttle reference only applies to VTOL motors in QuadPlanes, not forward motors, and will not be effective in fixed wing only flight modes. See throttle-based for further setup details.

  3. INS_HNTCH_MODE = 2 (RPM Sensor 1) or 5(RPM Sensor2). RPM sensor based, where an external RPM sensor is used to determine the motor frequency and hence primary vibration source’s frequency for the notch. Often used in Traditional Helicopters (See Helicopters) using the ArduPilot Head Speed Governor feature. See RPM Sensor for further setup instructions.

  4. INS_HNTCH_MODE = 3. ESC Telemetry based, where the ESC provides motor RPM information which is used to set the center frequency. This can also be used for the forward motor in fixed wing flight, if the forward motor(s) ESCs report RPM. This requires that your ESCs are configured correctly to support BLHeli telemetry via a serial port. See ESC Telemetry for further setup instructions. If INS_HNTCH_OPTS, or INS_HNTC2_OPTS if the second set of notches is enabled, has bit 1 set, then a set of notches for each motor will be created, tracking its RPM telemetry, otherwise, the average frequency of all motors will set the center frequency.

  5. INS_HNTCH_MODE = 4. In-Flight FFT based, where a running FFT is done in flight to determine the primary noise frequency and adjust the notch’s center frequency to match. This probably the best mode if the autopilot is capable of running this feature. It requires that the autopilot firmware supports it (see Firmware Limitations on AutoPilot Hardware for GyroFFT feature) and has sufficient cpu power (F7/H7 autopilots). This mode also works on fixed wing only Planes. See In-Flight FFT for further setup instructions.

All of the above are repeated, independently, for the second notch and are prefaced with INS_HNTC2_ instead of INS_HNTCH_. The following will explain setup for the first set of notches.


only one filter can be mode 4(FFT).

Determining Notch Filter Center Frequency

Before actually setting up a dynamic notch filter, the frequencies that are desired to be rejected must first be determined. This is crucial if a static notch or Throttle Based Dynamic Notch Setup is used. While the other methods do not require this knowledge before setting up their parameters, it can still be worthwhile as a comparison point for the post filter activation analysis of the filter’s effectiveness.

Once the noise frequency is determined, the notch filter(s) can be further setup. Historically, Measuring Vibration with IMU Batch Sampler has been used for this (also for slow cpu’s like F4-based autopilots), logging short bursts of raw IMU data for spectral analysis.

As of firmware version 4.5 and later, a better method has been developed using continuous raw IMU data, if the autopilot is H7-based, and using a new web based tool. This method is now preferred.


if you see two noise peaks close together, it may be that your copter has significant yaw imbalance. See the Yaw Imbalance section.

Number of Harmonics Filtered

  • Set INS_HNTCH_HMNCS to enable multiple harmonic filters centered at up to 16x the base frequency (multiples of the center frequency) notches. For ESC INS_HNTCH_MODE tracking, each motor will get a set of these notches. If an octocopter sets up three harmonics, this results in 8 x 3 = 24 notch filters. Enabling triple notches (see below) would result in 72 filters! This would most certainly cause excessive cpu loading and performance issues. Other modes only provide a single set of harmonic notches.

Always enable only the number of harmonic notch filters actually required and be especially aware of what is being enabled if using ESC (INS_HNTCH_MODE = 3) tracking mode. Enabling too many will result in running out of CPU cycles with unpredictable results. Three harmonics is usually safe.

Checking Notch Filter Effectiveness

Once the notch filter(s) are setup, the effectiveness of them can be checked by again measuring the frequency spectrum of the output of the filters (which are the new inputs to the IMU sensors). Refer back to the Measuring Vibration with IMU Batch Sampler or Raw IMU Logging for FFT Analysis page for this.

Multi Notch

The software notch filters used are very “spikey” being relatively narrow but good at attenuation at their center. On larger copters the noise profile of the motors is quite dirty covering a broader range of frequencies than can be covered by a single notch filter. In order to address this situation it is possible to configure the harmonic notches as multiple notches that gives a wider spread of significant attenuation. The configuration is controlled by the INS_HNTCH_OPTS parameter. This is a bitmask parameter and multiple options are possible at the same time, but using bit 0, 1, and bit 4 at the same time should be avoided. Use only one of those in a given configuration.




Double overlapping Notches


MultiSource: if using FFT Mode, the three largest noise sources will have a notch assigned. If ESC Telemetry Mode, then each motor will have a notch assigned at its RPM.


Updates the filters at the loop rate. This is cpu intensive, but tracks noise variations faster. Only valid if frequency source updates at loop rate, ie Bi-Directional DShot telemetry.


Enables notches on every IMU instead of just the primary. This is cpu intensive, but allows better lane switching decisions in noisy situations and for debugging. Not recommended for F4 boards.


Triple overlapping Notches


double notch option is no longer recommended since the triple notch option has been added. With a double notch, the maximum attenuation is either side of the center frequency, so on smaller aircraft with a very pronounced peak their use is usually counter productive.


Each notch has some CPU cost so if you configure multiple notches you can end up with many notches on your aircraft. For example, triple single (no harmonics) notches, using ESC telemetry will result in 3 notches per motor or 12 total notches. For example, with F4 cpus this should be acceptable, but enabling a second group of triple notches with INS_HNTC2_ENABLE or multiple harmonic notches, could cause problems.