In order to improve the performance of the navigation service and support Differential GPS precision approach and landing at airports, the US Federal Aviation Administration (FAA) developed the Local Area Augmentation System (LAAS). Differential processing means a variety of algorithms, including carrier smoothing pseudo range and the calculation of differential corrections etc. This paper gives a detailed discussion of the processes and algorithms of LAAS ground facility (LGF), calculate the B value and a Message Field Range Test (MFRT) calculation against the simulated data. It confirms that the differential processing can provide effective data for integrity monitor algorithms, and the results of the MFRT processing are available.