A space-based augmentation system (SBAS) provides delay corrections to the Global Navigation Satellite System (GNSS) along with the residual error bounds falling within a high confidence interval. SBAS considerably improves the safety of an aircraft during flight even under all weather conditions. The dispersive nature of the ionosphere is the largest contributor of range error in GNSS, thus threatening its accuracy. Scrutiny of the ionospheric behavior over low latitudes is one of the most challenging tasks for any SBAS system. In this letter, an attempt has been made to detect the spatial gradients in the ionospheric vertical total electron content (VTEC), using statistical hypothesis tests, for the chosen probability of false alarm (    $P_{fa}$  ) and probability of detection (    $P_{d}$  ). Logarithm of likelihood ratio test (log LRT) and generalized likelihood ratio test (GLRT) were performed individually on GNSS data recorded by the multifrequency receiver at Koneru Lakshmaiah University, Guntur (GNT), Andhra Pradesh, India. The tests were performed on the data recorded on January 22, 2013, and March 17, 2013, which were geomagnetically quiet and disturbed days, respectively. The tests were validated by selectively introducing external noise in the VTEC and observing the outcomes. GLRT not only demonstrated a superior performance over log LRT, but it was also simpler to implement as it did not require any prior knowledge of the probability distribution of TEC values.
['Swapna Raghunath', 'D. Venkata Ratnam']
Ionospheric Spatial Gradient Detector Based on GLRT Using GNSS Observations