Cascade Dual Frequency Smoothing for Local Area Augmentation System

Both temporal and spatial gradients of ionosphere errors have adverse effects upon local area augmentation system (LAAS). This article proposes a cascade dual frequency smoothing (CDFS) method to overcome this problem. One dual frequency smoothing filter is used to secure a precise estimate of ionosphere error, which is then used to modify ionosphere error in the code pseudorange measurement before it is put into another dual frequency smoothing (DFS) filter to attenuate noise. Thus, the ionosphere error is thoroughly removed from the smoothing process and excessive noise induced by GPS L2 code measurement is suppressed. Effectiveness of CDFS and accuracy of CDFS LAAS is analyzed based on data collected from LAAS test bed in Communication, Navigation, Surveilliance/Air Traffic Management (CNS/ATM) Labs, Civil Aviation Administration of China (CAAC). The analysis demonstrates that CDFS method can eliminate residual errors in the smoothed pseudorange raised by the ionosphere temporal gradient and differential correction errors caused by the ionosphere spatial gradient.