[Bug] PurePursuit does not minimize *crosstrack_error*

[slgrobotics]

Describe the bug

While running make px4_sitl gz_rover_differential_lawn I noticed that crosstrack_error stays on the negative side in the 0.1...0.2 meters range.

To Reproduce

• use latest PX4 "main" branch
• run make px4_sitl gz_rover_differential_lawn
• use plan here (or make your own)
• plot pure_pursuit_status.crosstrack_error

Screenshot / Media

Expected behavior

The vehicle should stay close to the A-B line, possibly oscillating around it.

Flight Log

https://review.px4.io/plot_app?log=eb4949b3-e648-4e7e-8c60-6fac8e86ba4e

Software Version

latest PX4 "main" branch

Flight controller

SITL - GZ

Vehicle type

rover_differential

[slgrobotics]

Here is the ULG file:

log_Bug_25262.zip

@chfriedrich98 - please take a look. This has been a problem when using "survey" missions since PurePursuit was introduced.

[slgrobotics]

Decreasing param set PP_LOOKAHD_GAIN 0.1 makes line following better, but the negative bias remains.