Angular Resolution of the Auger Surface Detector

We studied the angular resolution of the Surface Detector events using data collected during 2004 and 2005. The angular resolution depends directly on the measurement accuracy of the arrival time of the particles in the tanks. In this note, we describe this measurement accuracy using a model based on the detection technique and on some of the shower parameters. This model has zero, one or two free parameters which can be adjusted using the information from the doublet data. It is remarkable to note that the predicted values for those parameters (prediction based on the physics motivation of the model) and the fitted ones are identical. Our model is further validated by looking at the flatness of the chi2 probability distribution, and at the hybrid events. Given this modeling of our measurement errors, the angular resolution can be determined with high precision on an event by event basis.