Hi @ben2,
A sonar is an active sensor, so it’s reliant on the pulse that it transmits getting back to it in order to estimate distances.
You can think of it like trying to take a photo of a camera in a mirror. If the camera is at an angle to the mirror that’s larger than its field of view then it won’t be able to see itself at all (get a sonar reflection) unless the ‘mirror’ is very bumpy.
In this case, the Ping Sonar has a beam width of ~30 degrees, and you’re tilting it at 20 degrees, so the closest ‘edge’ of the beam is still 5 degrees off vertical away from the sonar.
Is there a particular reason you’re trying/wanting to use the sonar at an angle?