A sensor pallet was one of the module options available to Nebula-class starships, sitting above the vessel's primary hull. This module provided advanced or specialized sensor options to the science vessel.
The USS Newton was equipped with a sensor pallet module while assigned to Project Nexus to study the Nexus. (TNG eBook: The Stuff of Dreams)
See Also[]
Sensor pallet article at Memory Alpha, the wiki for canon Star Trek.