# Airspeed Sensors
Airspeed sensors are highly recommended for fixed-wing and VTOL frames. They are so important because the autopilot does not have other means to detect stall. For fixed-wing flight it is the airspeed that guarantees lift not ground speed!
# Hardware Options
Recommended digital airspeed sensors include:
- Based on Pitot tube (opens new window)
- MEAS Spec series (e.g. MS4525DO (opens new window), MS5525 (opens new window))
- EagleTree Airspeed MicroSensor V3 (opens new window) (eagletreesystems)
- Sensirion SDP3X Differential Pressure Sensor
- Holybro Digital Air Speed Sensor (opens new window)
- Based on Venturi effect (opens new window)
- TFSLOT Venturi effect airspeed sensor.
All the above sensors are connected via the I2C bus/port.
Additionally, the Avionics Anonymous Air Data Computer (opens new window) can be connected to the CAN bus to determine not only high-accuracy airspeed, but also true static pressure and air temperature via onboard barometer and an OAT probe.
# Enable Airspeed Sensors
Airspeed sensor drivers are not started automatically. Enable each type using its corresponding parameter:
- Sensirion SDP3X: SENS_EN_SDP3X
- TE MS4525: SENS_EN_MS4525DO
- TE MS5525: SENS_EN_MS5525DS
- Eagle Tree airspeed sensor: SENS_EN_ETSASPD
You should also check ASPD_PRIMARY is
1 (see next section - this is the default).
# Multiple Airspeed Sensors
If you have multiple airspeed sensors then you can select which sensor is preferred as the primary source using ASPD_PRIMARY, where
3 reflect the order in which the airspeed sensors were started:
-1: Disabled (no airspeed information used).
0: Synthetic airspeed estimation (groundspeed minus windspeed)
1: First airspeed sensor started (default)
2: Second airspeed sensor started
3: Third airspeed sensor started
The airspeed selector validates the indicated sensor first and only falls back to other sensors if the indicated sensor fails airspeed checks (ASPD_DO_CHECKS is used to configure the checks).
The selected sensor is then used to supply data to the estimator (EKF2). The EKF fuses the airspeed data if it's above EKF2_ARSP_THR and has a low innovation compared to groundspeed minus windspeed.
# Sensor-specific Configuration
Other than enabling the sensor, sensor-specific configuration is often not required. If it is needed, it should be covered in the appropriate sensor page (for example TFSLOT > Configuration).
The specific configuration for sensors that do not have a separate page is listed below:
- Sensirion SDP3X: CAL_AIR_CMODEL (provides overview of required settings), CAL_AIR_TUBED_MM, CAL_AIR_TUBELEN.
Airspeed sensors should be calibrated by following the instructions: Basic Configuration > Airspeed.
# Developer Information
- Airspeed drivers (opens new window) (source code)