Altimeter sensor model may not be correct

[daewok]

Currently, the altimeter is hacked together using a DVL. Need to either confirm the properties are similar enough to the glider's actual altimeter or fix it.

[woensug-choi]

The altimeter sensor sounds pretty basic but it does not exist on uuv_simulator. How is the altimeter sensor differ from DVL? Both dvl and altimeter output distance from the ground right? @greg-burgess

[daewok]

I think the primary difference is that the altimeter is one beam and the DVL is
multiple. The DVL computes "altitude" by averaging the distance to seafloor
from all of its beams, where as the altimeter measures it directly.

That means that they can return different values depending on the exact
topology of the seafloor. My guess is that getting the altimeter model better
is a relatively low priority thing, but I'll defer to @greg-burgess and Rich on
that.

[daewok]

From Rich: more differences are that the frequencies are different and the angles are different. Each DVL beam is ~2.5 degrees and the altimeter cone is ~10 degrees.

But what we have right now is definitely good enough.

[woensug-choi]

So, stick to the one we have for now. I am sure we can easily set beam degrees and widths.