Cascade dual frequency smoothing performance evaluation study

Both temporal and spatial gradient of ionosphere error have adverse effects to Local Area Augmentation System (LAAS). Current LAAS cannot solve such problems due to its single frequency nature. Two dual frequency techniques, called DFree and IFree respectively, are explored and analyzed. However, previous studies indicate that both DFree and IFree cannot fully satisfy CAT IIIb requirements. A cascade dual frequency smoothing (CDFS) method is imposed in this paper to cope with these problems. CDFS uses one dual frequency smoothing filter to estimate ionosphere error precisely. Then the estimation is used to compensate ionosphere error in code pseudorange before it is put into another dual frequency smoothing filter to attenuate noise. Because ionosphere error is thoroughly removed from the smoothing process, CDFS is immune to ionosphere temporal gradient. Noise in L2 code measurement is filtered twice and excessive noise in smoothed pseudorange is avoided. Differential error relative to ionosphere spatial gradient is corrected by using ionosphere-free smoothed pseudorange of CDFS. Therefore, CDFS LAAS, which uses CDFS both on ground and airborne, will not be affected by ionosphere error and can increase accuracy of corrected pseudorange by using a longer smoothing time constant. Effectiveness of CDFS and availability of CDFS LAAS is analyzed. The analysis demonstrates that CDFS can eliminate error related to ionosphere and provide adequate availability for CAT IIIb.