Trouble adding camera to gazebo simulation

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Hi

I am trying to add a camera to the iris drone to my gazebo simulation. I tried using many models (iris_fpv_cam, iris_downward_depth_camera, iris_stereo_camera ...) by changing px4 launch files' vehicle argument. But i keep getting:

Error: Unknown model 'iris_fpv_cam'
ERROR [px4] Startup script returned with return value: 256

I also tried to modify iris.sdf and iris.xacro files as described here. And I get the following errors:

Error [parser.cc:550] Required attribute[name] in element[collision] is not specified in SDF.
Error [parser.cc:769] Error reading element <collision>
Error [parser.cc:769] Error reading element <link>
Error [parser.cc:769] Error reading element <model>
Error [parser.cc:394] Unable to read element <sdf>
Error:   Could not find the 'robot' element in the xml file
         at line 81 in /build/urdfdom-UJ3kd6/urdfdom-0.4.1/urdf_parser/src/model.cpp
Error [parser_urdf.cc:3408] Unable to call parseURDF on robot model
Error [parser.cc:340] parse as old deprecated model file failed.

Can someone please assist me on how to add a downward camera attached to my drones?

Thank you