Performance of GEM detectors for SBS in commissioning with cosmic ray data

The Super Bigbite Spectrometer (SBS) at Thomas Jefferson National Accelerator Facility (TJNAF) was proposed to perform a series of high precision nucleon form factor experiments at large momentum transfer. The SBS will be capable of operating at very high luminosity and provide a large solid angle acceptance. A set of large Gas Electron Multiplier (GEM) detectors, each with an active area of 60 x 200 $cm^2$, is being commissioned for a novel neutron polarimeter based on elastic and charge-exchange recoil proton detection. These GEM chambers are expected to provide a position resolution of $\sim$ 70 $\mu m$, while operating in at high rate conditions up to 1 $MHz/cm^2$. This talk will report on the commissioning activity and performance of the GEM detectors with cosmic ray data.