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Sensor Calibration (Biases)

Sensor calibration establishes the zero-point (bias) for each pressure sensor and the IMU. This must be done on the ground, with the aircraft level and in zero-wind conditions.

When to Calibrate

  • Initial installation — always calibrate during first-time setup
  • After moving the controller — if you change the mounting location or orientation
  • After a firmware update — if the update changed sensor processing
  • If readings seem off — drifting altitude on the ground, non-zero pitch/roll when level

Prerequisites

  • Aircraft on the ground, level (check with a level on the longerons or a known-level surface)
  • Zero wind — cover the pitot tube and AOA probe if possible, or wait for calm conditions
  • Engine off — prop wash will contaminate pressure readings
  • Aircraft stationary — no movement during calibration

Calibration Procedure

Via Web Interface

  1. Connect to OnSpeed WiFi (OnSpeed / angleofattack)
  2. Navigate to the Sensor Config page: http://192.168.0.1/sensorconfig
  3. Verify the aircraft is level and still
  4. Click the Calibrate button
  5. Wait for the calibration to complete (takes a few seconds — collects 1000 samples)
  6. The page will display the new bias values

Via Console Command

  1. Connect to the OnSpeed USB serial port (921600 baud)
  2. Type: BIAS
  3. The system collects 1000 samples each of Pfwd (pitot) and P45 (AOA) pressure
  4. It also zeros the accelerometer and gyro biases
  5. New bias values are displayed and saved

What Gets Calibrated

Bias What It Is Stored As
Pfwd Pitot pressure sensor zero reading 14-bit ADC count (nominal ~8192)
P45 AOA pressure sensor zero reading 14-bit ADC count (nominal ~8192)
PStatic Static pressure offset PSI offset
GX Forward acceleration bias G offset
GY Lateral acceleration bias G offset
GZ Vertical acceleration bias G offset
Pitch Pitch angle bias Degrees offset
Roll Roll angle bias Degrees offset

Verifying Calibration

After calibration, check the SENSORS console command output:

  • Pfwd pressure: Should read approximately 0 PSI with no airflow
  • P45 pressure: Should read approximately 0 PSI with no airflow
  • IAS: Should read 0 or very close to 0 knots
  • Pitch: Should read approximately 0° (or your aircraft's known ground pitch angle)
  • Roll: Should read approximately 0°
  • VerticalG: Should read approximately 1.0G
  • LateralG: Should read approximately 0.0G
  • ForwardG: Should read approximately 0.0G

If any of these are significantly off, verify the aircraft is truly level and there's no wind, then recalibrate.

AHRS re-initialization

As of v4.15, the firmware re-initializes the AHRS algorithm after calibration to ensure the new bias values take effect immediately. You don't need to reboot after calibration.