Graph-based Return-To-Land

[samuelsadok]

The Return-To-Land mode now makes the vehicle return along the previously flown path. This way, crashes due to tall buildings can be avoided. The new RTL is based on a flight path tracker, which records the flight path as a graph.

The implementation was tested in unit tests (on PX4-FMUv4) and SITL, but no flight tests have been made yet. Some test paths in src/modules/navigator/navigator_tests are quite verbose, maybe I should remove them?

Memory and CPU usage depends heavily on the flight pattern. A 2-4kB bound should be sufficient for most cases. During tests, the most critical algorithm blocked the navigator for 300ms. This should not be an issue if the system behaves as designed, but we still may want to run this on a separate low-priority task instead.

The new RTL mode is now called RTLA while the old mode was renamed to RTLB and is used as a fallback for some failure modes. Also, a new RCRecover mode enables the vehicle to just return a few meters to regain a good signal. Someone with more experience with the system could integrate this such that it is invoked when needed. Changes to QGroundControl may be required to understand the new flight modes.

Previous discussions on this pull request are here: #5735

[samuelsadok]

The failure of travis-ci and semaphore-ci is due to a faulty reference to std::lgamma in #include <algorithm>. I need this header for std::min and std::max. The same target (px4fmu-v1_default) builds fine on my machine. It looks like the test containers have a buggy C++ library or I'm using a part of it I'm not supposed to use. Should I just provide my own min/max functions or is there a clean fix?

[dagar]

FYI - travis-ci and semaphore use gcc 4.9.3 for the nuttx builds, and circle uses 5.4.

math::max(), min(), constrain() are here - https://github.com/PX4/Firmware/blob/master/src/lib/mathlib/math/Limits.hpp

[dagar]

Can you rebase and push to see if px4fmu-v2 has enough free flash now? The circleci failure is different. https://circleci.com/gh/PX4/Firmware/1223?utm_campaign=vcs-integration-link&utm_medium=referral&utm_source=github-build-link

[samuelsadok]

Arrgh I once again messed up the rebase, the pull request apparently gets closed and can't be reopened when I delete the branch.

[dagar]

Not a problem, pull requests are free.