Effectiveness of CCD ionospheric monitors for air navigation based on GBAS: Investigation and implementation under Brazilian conditions
Creators: name: Costa, Renan Toledo
name: de Oliveira Jr., Paulo Sergio
name: Klein, Ivandro
name: dos Reis, Thiago Tiedtke
name: Silva, Weverton da Costa
name: Monico, João Francisco
Publisher: name: Zenodo
Year: 2024
Subjects: GNSS
Code-Carrier-Divergence
Monitor
Navigation
Ionosphere

description: CCD (Code Carrier Divergence) monitors play a crucial role in Ground Based Augmentation Systems (GBAS) and Satellite Based Augmentation Systems (SBAS). GBAS systems were designed to support precision approaches and landings in civil aviation, ensuring safety and integrity. In the context of GBAS, CCD monitors allow us to analyze the impact of satellite faults and, especially, ionospheric fronts, which result in the divergence between the code and carrier phase of the transmitted signal, causing significant differences between pseudorange and phase measurements. By detecting the divergence between the code and carrier phase, CCD monitors can alert the GBAS system to the presence of a fault and provide appropriate corrections to ensure reliable positioning information. The CCD monitor plays a fundamental role in real-time monitoring and evaluation systems of GNSS (Global Navigation Satellite System) positioning quality, such as GBAS and SBAS. It is capable of assessing the integrity of the GNSS signal and detecting any anomalies that may affect the accuracy of the estimated position. In this way, it is possible to ensure the reliability and safety of these systems, as potential failures can be quickly identified and corrected. The ionospheric dynamics over low latitudes, particularly in Brazilian regions, are highly active, with various phenomena resulting from the complex interaction between space weather and atmospheric elements. These phenomena can disrupt aviation communications, navigation systems, and surveillance systems. Due to this condition, the present study aims to evaluate the potential use of CCD monitors for autonomous GNSS navigation, especially for air navigation in the Brazilian region. Tests simulating landings with RPAs (Remotely Piloted Aircraft) were conducted on May 25, 2023 (low ionospheric activity), and November 17, 2023 (high ionospheric activity) to compare the performance of the CCD and determine if it is truly capable of identifying when positioning is safe or not. The project's results would be directly applicable to any GNSS-based autonomous navigation positioning. For the calculation of CCD, initially assuming that the ambiguity remains constant, the Code Minus Carrier (CMC) is applied, where common errors in pseudorange and carrier phase measurements, such as tropospheric delay and satellite and receiver clock errors, are cancelled. After this, the change rate is calculated as the difference between the current and previous CMC divided by the time interval between the two instants. Finally, the errors are smoothed using a first-order low-pass filter in cascade. A CCD fault of the satellite on the aircraft is indicated whenever the CCD monitor exceeds 0.0415 m/s. The satellite is then excluded. Incorporating the CCD as an additional parameter in evaluating internal positioning reliability in air navigation makes it possible to better understand the quality and integrity of positioning information. This contributes to the safety and accuracy of air navigation, allowing for the detection and mitigation of ionospheric errors and satellite failures. Based on the conducted tests, the performance of the CCD monitor was evaluated to determine its capability to identify truly safe positioning or errors in the meter range.