Fixed-wing Auto Return without GPS (GPS Position Estimation)

I. Introduction

When GPS signal is lost, the INAV system allows fixed-wing aircraft to continue flying by estimating their position, with the primary purpose of achieving "GPS-less Return to Home (RTH)". This function is only applicable to fixed-wing.
To use this feature, the aircraft should be equipped with the following sensors:

• Accelerometer, gyroscope
• Barometer
• GPS
• Magnetometer (optional but highly recommended)
• Pitot tube (optional)
  By default, when GPS positioning is lost, all navigation modes are disabled. If RC (remote control) signal is also lost at this time, the aircraft will not be able to enter RTH mode, but will instead switch to LANDING mode. If the aircraft is in an area where it cannot land safely, it may be lost.
  After enabling GPS position estimation, the aircraft can continue navigation relying only on the magnetometer and barometer, thereby restoring the return path. Even if the GPS sensor experiences a hardware failure (such as a timeout), the system will attempt to estimate GPS positioning.
  Please note that GPS estimation is not intended as a complete navigation solution to replace GPS. Due to the lack of GPS corrections, position error accumulates rapidly. However, it is still acceptable in RTH scenarios. In the INAV system, GPS is considered the most trusted sensor, and its output is not additionally verified.
  Under normal conditions, the aircraft obtains its position through the GPS sensor, enabling it to maintain heading, perform RTH, or navigate waypoints.
  When GPS positioning is lost, the aircraft can only rely on the magnetometer for heading and the barometer for altitude.
  To achieve GPS-less navigation, we make the following assumptions:
• The aircraft flies along the direction its nose is pointing;
• If no pitot tube is installed, the aircraft is assumed to fly at a constant airspeed as set in the configuration.
  Based on these assumptions, the aircraft's position can be roughly estimated. To improve estimation accuracy, the aircraft will use wind direction and speed information measured before GPS loss for correction; if equipped with a pitot tube, it will also use this data to further improve groundspeed estimation accuracy.
  Based on the estimated flight direction and speed, the aircraft can roughly calculate its current position.
  The system assumes: the aircraft will fly toward the "Home point" in the estimated direction until GPS or RC signal is reacquired.
  Note: The aircraft must successfully acquire GPS positioning and record the Home point before takeoff. Position estimation is not possible without any GPS positioning.

II. Setup Instructions

To enable the GPS position estimation feature, use the following command in CLI:

Important

set inav_allow_gps_fix_estimation = ON

Additionally, you need to set the aircraft's cruise airspeed.
How to obtain cruise airspeed:

1. Perform a test flight;
2. Enable ground speed display on OSD;
3. Fly in opposite directions in CRUISE mode;
4. Take the average of the speeds in both directions.
   The unit for cruise airspeed is cm/s.

• To convert from km/h to cm/s: km/h × 27.77 = cm/s
  Example: 100 km/h = 100 × 27.77 = 2777 cm/s
  CLI setting example:

Important

set fw_reference_airspeed = 2777

The aircraft should maintain the set speed in CRUISE mode. If "Increase cruise speed with throttle" is enabled, do not use this function when GPS Fix is lost.
If equipped with a pitot tube, the system will prioritize pitot data as the airspeed source, in which case fw_reference_airspeed does not need to be set, but it is recommended to still set a reference value in case of pitot tube failure.

• To continue mission execution when RC signal is lost, set:

Important

set failsafe_mission_delay = -1

• After entering the command, be sure to save the settings:

Important

save

For testing purposes, you can disable the GPS sensor fix from the RC controller in the programming options tab:

III. Expected Error

The actual expected error is up to 1 meter per 200 km of flight path. In testing, 500m drift occurred per 5 km of path.
Reducing drift:

• Fly a large circle with GPS to get a good wind speed estimate
• Use an airspeed sensor. If no airspeed sensor is installed, fly in cruise mode without throttle override.
• Make smooth, large turns
• Ensure the compass accurately points to the nose direction
• Correctly calibrate the compass
  This video shows a real test with occasional GPS disablement. Southwest wind speed 10 km/h:
  
  The purple line shows the estimated position. The black line shows the actual position. The "EST ERR" sensor displays the estimation error in meters. When the satellite icon shows "ES", estimation is running. When GPS positioning is needed, the estimated position will snap to the actual position.

IV. Navigation Estimation Without Magnetometer

Without a magnetometer, navigation accuracy will be very poor, with the main issue being heading drift.

• The longer the aircraft flies without magnetometer or GPS support, the greater the heading estimation error.
• After flying for several minutes and making several turns, the flight controller's estimated direction for "north" may be completely wrong.
• Generally, the accuracy is only sufficient to perform an RTH turn when both RC and GPS are lost, and to roughly maintain the return direction in scenarios with occasional GPS signal interruptions.
  

Important

Illustration: Purple line: Flight controller's estimated position trajectory Black line: Actual flight trajectory

Usage Recommendations
Without a magnetometer, GPS Fix mode is only recommended as a last resort in extreme situations.
For example: If the aircraft is flying over a lake and a forced landing would result in losing the aircraft, GPS Fix estimation can be used for approximate RTH to avoid a direct crash.
————This article references the official INAV website: https://github.com/iNavFlight/inav/blob/master/docs/GPS_fix_estimation.md