Evaluation of Two Types of Dual-Frequency Differential GPS Techniques under Anomalous Ionosphere Conditions

Strong ionosphere storms are a potential threat for the Local Area Augmentation System (LAAS). During these storms, large spatial and temporal gradients of the ionosphere component on the GPS signals could cause significant errors in user position estimation. Mitigating these errors is demanding for LAAS, especially for Category III LAAS. Dual-frequency GPS techniques are known to be an effective means of reducing or removing ionosphereinduced errors and thus improving the robustness of LAAS to ionosphere anomalies. We selected two dual-frequency methods and examined their effectiveness against anomalous ionosphere situations. These two methods are divergence-free smoothing (denoted here as “DFree”) and ionosphere-free smoothing (denoted here as “IFree”). These methods have the same filter structure as the single-frequency carrier-smoothing methods used in conventional single-frequency LAAS. Accordingly, we can compare the results of these methods directly to single-frequency LAAS under consistent assumptions. In order to investigate the effectiveness of DFree and IFree, we evaluated the availability of these methods under various ionosphere conditions. Simulation results show that DFree would provide much better availability than IFree under nominal ionosphere conditions and under most anomalous conditions. However, IFree proved to be superior under extremely anomalous ionosphere conditions. Therefore, optimal availability would be obtained by implementing both DFree and IFree in real-time and switching between them based on an ionosphere monitor’s best estimate of the current ionosphere state. This paper begins by introducing the theory of DFree and IFree and then evaluates the availability of both methods under different ionosphere conditions. This evaluation is followed by a discussion of the concept of a dual-frequency LAAS architecture in which both DFree and IFree are utilized.