Distinguish between water and air

[mabelzhang]

Trying to be better at documenting issues along the way, instead of putting them in a little drawer in my brain, especially issues that are not explicitly outlined in the SOW.

Expected behavior:

MBARI controller assumes the robot cannot fly (why would they assume that?), which means when the vehicle is positively buoyant (VBS volume greater than <neutral_volume>) and at water surface, it should stay at water surface.

Actual behavior:

Currently, we do not distinguish between water and air, so when the vehicle is at depth 0 (water "surface", which we do not have a concept of) and positively buoyant, it continues to float upwards into the sky, since the "sky" is just a continuation of the water column.

More information

Scenario encountered in #48 (comment)

Measure of success

Ignition isolated test:
Start robot at depth = 0 in Ignition, VBS volume greater than <neutral_volume> (say 945, which is the default in MBARI control parameters). At startup, robot should stay at the same depth (0) and not float up.

Mission integration test (not entirely needed if the above works, but need to check the default parameters match between MBARI and Ignition side at startup):
Run the VBS unit test mission:

$ bin/LRAUV
> run RegressionTests/IgnitionTests/testDepthVBS.xml

Robot should stay at the surface and not float up.

[arjo129]

I had started some work on this here: gazebosim/gz-sim#818 (due for rewrite). Its been on hold for some time. I shall pay more attention to it since it seems necessary.

[mabelzhang]

Ah I remember hearing about that. Guess it'll have to be forward-ported to Fortress or rebased.