Combining Ground- and LEO-RO-Based GNSS Observations in Real-time Operational Space Weather Products
Creators: name: Durgonics, Tibor
name: Fang, Tzu-Wei
name: Onsager, Terry
name: Centinello, Frank
name: Wang, Jun
name: Fuller-Rowell, Dominic
name: Codrescu, Mihail
Publisher: name: Zenodo
Year: 2023
Subjects: GNSS
Radio Occultation
Ionosphere
Scintillation
LEO

description: NOAA's Space Weather Prediction Center (SWPC) has responsibility for continuously monitoring, forecasting, and alerting on conditions in the Earth's space environment that impact our technological infrastructure and human health and safety. One of the main space weather products is the ionosphere specification. Ionospheric density changes impact our technologies utilizing global navigation satellite systems (GNSS) and signals propagating between satellites and the ground stations as it can cause signal delays and disruptions. SWPC focuses on improving existing and creating new real-time, operational products of the two main ionospheric descriptive quantities: total electron content (TEC) and ionospheric scintillations. SWPC is also one of the centers that are responsible for space weather services identified by the International Civil Aviation Organization (ICAO). The ionospheric models developed at SWPC target GNSS and HF communication advisories. A new real-time specification tool being developed will support the decision making at the forecast office on more accurate GNSS advisories in the near future. The current ionospheric TEC and scintillation products, such as 2D TEC maps and rate of TEC index (ROTI) maps, have been derived from ground-based GNSS measurements. The exclusively ground based approach has its advantages and disadvantages. One of the prominent disadvantages is that it cannot provide observations over, e.g., oceans and remote areas. Empirical ionospheric models and various smoothing and interpolating techniques have been used to fill these coverage gaps. The introduction of GNSS receivers to low-earth-orbit (LEO) satellites represented a breakthrough, since they allowed GNSS observations to be performed anywhere globally regardless of ground conditions, permitting to perform radio occultation (RO) type remote sensing of the ionosphere, in a so-called limb-sounding geometry (GNSS-to-LEO satellites). The computed excess phase can be used for deriving TEC, ROTI, and phase scintillation (σφ) calculations, while the rapid changes in the signal's carrier-to-receiver noise density ratio or signal-to-noise characteristics allow for the calculation of the amplitude scintillation index (S4 ). Ground-based (disc-sounding geometry) GNSS measurements can also be used to infer the ionospheric TEC and scintillation indices. Therefore, the combination of these limb- and disc-geometry observations should, in theory, provide a more complete global coverage than any of the single geometries alone. The existing ground GNSS networks in combination with the increasing number of RO-capable LEO satellites and constellations may provide better ionospheric products and finer resolutions both spatially and temporally. RO data covering low- and mid-latitudes are available from the COSMIC-2 constellation, while commercial data providers are able to deliver global data from polar orbiting satellites. However, the combination of these two measurement geometries is not trivial, and some fundamental physical questions need to be answered before it can be implemented into a practical application. SWPC's plan is to combine ground- and space-based measurements to build improved real-time operational products of ionospheric TEC and scintillations. These products will be used to monitor the ionosphere and provide alerts for potential disruptions to radio communication and navigation, thus providing society with actionable space weather information. The main challenges are currently being addressed in close collaboration with Boston College and Aerospace Corp.: 1) the validation of ground-based scintillation measurements, 2) geolocation of RO scintillation events and 3) the determination of the limb-to-disc conversion algorithm between ground- and RO-based scintillation magnitudes. There are established methods to geolocate scintillation events along a given signal-path for ground observations, but the magnitude can depend significantly on the direction of the signal propagation in relation to the plasma structures. On the other hand, the RO geolocation algorithm that could cover all latitudes is still being researched and developed. Furthermore, performing a physically sound limb-to-disc conversion of the measurement magnitudes is necessary due to the different geometries of the measurements, which cannot be interpreted together without such a conversion. This has not been completed yet and requires additional work, especially for high-latitudes. Further challenges arise from the fact that the ionosphere is a non-linear, dynamic, and complex system, which makes its modeling, forecasting and even nowcasting quite an intricate task. Both the RO and ground-based GNSS measurements are subject to errors which need to be taken into account and continuously estimated. Such a complex, real-time system and operational product must be updated frequently because data and processing latencies can significantly affect the quality of the service and the ability to base decisions on its output. Despite the challenges, the benefits of using a real-time operational ionospheric product outweigh the costs. This product can be used to improve the safety and reliability of radio communications and navigation globally. It can lead to improved situational awareness of the ionosphere, early warning capability and improved decision-making for mitigation of ionospheric disruptions.