Simulating GPS outage due to tree foliage

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Howdy folks,

I'd like to simulate GPS outage/degradation due to tree foliage in Gazebo. This is conceivably "relatively easy" to do if I knew where all the tree model objects are located in Gazebo. I am totally new to Gazebo and I have just started studying the code for ROS Gazebo plugins (like the gazebo ros imu, gps, etc.).

Can someone point me to a tutorial or some demonstration code for querying specific Gazebo models (i.e. the models that represent the trees) within a certain vicinity to the robot model?

In the mean time I'll keep on keepin' on with going through the published Gazebo tutorials and searching for answers, but any advice, pointers or clues that would speed up this process would be greatly appreciated :)

Thanks in advance

Galto