We are not using a single reference file, we use a sequential technique that follows the SBAS approach. Each pair in the time series is made up of a reference and dependent image. The pair is co-registered together in order to extract an deformation map of the time period. To create the time series trend, each new reference in a pair is the dependent image from the most recent pair. This ensures that all images are accurately aligned tighter and generates a time series deformation product. “