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docs: add "first flight" docs for airspeed scale handling (#25987)

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Co-authored-by: Hamish Willee <hamishwillee@gmail.com>
Co-authored-by: Silvan Fuhrer <silvan@auterion.com>
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Mahima Yoga
2025-11-27 10:51:54 +01:00
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co-authored by Hamish Willee Silvan Fuhrer
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docs/en/advanced_config/airspeed_validation.md
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@@ -10,36 +10,13 @@ By default, the [Missing Data](#missing-data-check), [Data Stuck](#data-stuck-ch
You can configure which checks are active using the [ASPD_DO_CHECKS](#aspd_do_checks_table) parameter.
:::
## Airspeed in PX4
PX4 handles multiple types of airspeed:
- **IAS (Indicated Airspeed):** The raw measurement from the airspeed sensor, directly influenced by sensor characteristics and installation effects (e.g., pitot-static errors).
- **CAS (Calibrated Airspeed):** IAS corrected for sensor-specific and installation-related errors.
- **EAS (Equivalent Airspeed):** _Not explicitly handled by PX4_ - Calibrated airspeed corrected for compressibility effects.
While PX4 does not currently model EAS separately, this correction is negligible at low speeds and altitudes, so EAS is treated as equivalent to CAS for simplicity.
- **TAS (True Airspeed):** CAS adjusted for atmospheric effects such as air pressure and temperature (i.e., altitude and atmospheric conditions).
The standard conversion chain used in PX4 is: `IAS → CAS (= EAS) → TAS`.
## CAS Scale Estimation
PX4 estimates the IAS to CAS scale (referred to as the CAS scale) during flight using GNSS ground speed and wind estimation.
To compute the final TAS, standard environment conversions are applied (CAS → TAS).
This CAS scaling plays an important role in keeping the [innovation check](#innovation-check) reliable, since a well-estimated CAS is key to spotting inconsistencies between measured and predicted airspeed.
Calibrated Airspeed (CAS) is the measured Indicated Airspeed (IAS) scaled to correct for sensor-specific and installation-related errors.
CAS scaling plays an important role in keeping the [innovation check](#innovation-check) reliable, since a well-estimated CAS is key to spotting inconsistencies between measured and predicted airspeed.
If the estimated CAS scale is inaccurate, it can mask real airspeed faults or trigger false positives.
If you observe that the CAS scale estimate is consistently off, or if it is converging too slowly, you can manually set it using [ASPD_SCALE_n](#aspd_scale_n_table) (where `n` is the sensor number).
[ASPD_SCALE_APPLY](#aspd_scale_apply_table) can be used to configure when/if the estimated scale is applied.
::: info
For a quick manual CAS scale estimate, compare groundspeed minus windspeed (from the [VehicleLocalPosition](../msg_docs/VehicleLocalPosition.md) and [Wind](../msg_docs/Wind.md) messages, respectively) to indicated airspeed values (in the [Airspeed](../msg_docs/Airspeed.md) message).
The ratio of indicated airspeed to groundspeed minus windspeed can provide a reasonable starting estimate for [ASPD_SCALE_n](#aspd_scale_n_table).
:::
If you observe that the CAS scale estimate is consistently off, or if it is converging too slowly, follow the steps outlined in [Airspeed Scale Handling](../config_fw/airspeed_scale_handling.md#recommended-first-flight-process).
## Validation Checks