Abstract-Local Area Augmentation System (LAAS) is expected to enable the pilots to guide the aircraft more precisely and safely into busy airports even in poor visibility conditions. The anomalous low and equatorial latitude Ionosphere is severe threat to the LAAS system. To characterize the anomalous ionospheric gradients, the performance of an ionospheric threat model is evaluated. In our investigation, in contrast to the reported work available in the open literature, smoothed code phase measurements are used in the threat model to obtain precise ionospheric time delay. The three key parameters of the threat model gradient slope (mm/km), width (km) and front speed (m/s) are used in the analysis. Further, geometry screening using Maximum Ionosphere Induced Error in Vertical (MIEV) as a key parameter is carried out to identify the stationary gradients and its impact on system performance for CAT-I operations. A maximum ionospheric gradient of 355.74 mm/km over a distance of approximately 75 km is reported at mid latitudes. Whereas, in our findings at low/equatorial latitudes even within a distance of approximately 4 km a maximum gradient of 460 mm/km is observed, which is comparatively very high. Our results show that, there is necessity to enhance upper bound for the ionospheric gradients threat space over low latitudes.
In this paper, effects on DOP (Dilution of Precision) due to augmentation of Global Positioning System (GPS) with pseudolites are investigated. For this purpose, a typical Local Area Augmentation System (LAAS) scenario is consi-dered by placing pseudolites in various positions. It is found that only properly located pseudolites can improve the DOP. DOP values with two pseudolites located on either side of the run way are found to be the best. Geometric DOP (max) was found to be nearly 4 due to only GPS and came down to approximately 2 due to augmentation with two pseudolites. Implementation aspects of Bayes and Kalman filters while estimating DOP values are also examined
The Indian Regional Navigation Satellite System (IRNSS) is an autonomous and independent navigational system being developed by India. IRNSS will provide position, navigation and timing services for national applications. To improve accuracy, it can be augmented using GPS and pseudolites (pseudo-satellites). In this paper, the effect on DOP (Dilution of Precision) due to augmentation of the proposed constellation of IRNSS with pseudolites is investigated. GDOP is reduced to 1·75 (max) from 3·63 (max) due to augmentation of IRNSS with two airport pseudolites (APLs). Due to augmentation of IRNSS with GPS, GDOP is reduced to 2·4 (max). When IRNSS is augmented with an APL as well as with GPS, GDOP is further decreased to 1·65 (max). The regional effect on DOP due to IRNSS is also investigated at different locations in the Indian region.
Precise estimation of ionospheric time delay is essential for achieving better positional accuracy. The transionospheric signals are affected due to the existence of anisotropic plasma in the ionosphere. For the first time, a novel model based on anisotropic Inverse Distance Weighting (IDW) with 'Jackknife' technique is proposed and is applied in the context of ionospheric time delay modelling for Global Positioning System (GPS). The performance of the proposed anisotropic IDW model is evaluated by comparing with the prominent grid based models (Kriging, Spline and Modified Planar Fit (MPF)).
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