The small squares are 59 accelerometers that measure the movement of the array.
GPM Core Observatory
The solar arrays provide all the electrical energy for the satellite, and must survive flying in Earth's thin upper atmosphere at an altitude of 407 km (253 miles), where they will be subjected to small drag forces and exposure to corrosive atomic oxygen.
Each time a new component is added to the spacecraft, rigorous testing is conducted to ensure the new components operate correctly and work effectively with existing systems.
NASA technicians spun the Global Precipitation Measurement (GPM) satellite up to just over 10 RPM in Goddard Space Flight Center’s High-Capacity Centrifuge facility March 31. At that speed, the spin exerted a lateral pressure of 2.4 G’s, or 2.4 times the force of gravity on the satellite.
Spin tests such as these are used to determine whether the forces of launch could adversely affect hardware we put into space, and to test spacecraft chassis design.
A computer visualization of the GPM Core Observatory passing over the Earth and out of darkness.