On Measuring the Ultra High Energy Cosmic Ray Spectrum above 3 $\times $ 10$^{18}$ eV by Pierre Auger Observatory

We report the Ultra High Energy Cosmic Ray (UHECR) spectrum above 3 $\times $ 10$^{18}$ eV measured by Pierre Auger Observatory (PAO). PAO uses both the Fluorescence detector (FD) and the water cerenkov tank surface detectors (SD). PAO offers an exciting opportunity to compare the systematics of and cross-calibrate FD and SD for the first time.