Satellites with dual-frequency Global Navigation Satellite Systems (GNSS) receivers can measure integrated electron density, known as slant Total Electron Content (sTEC), between the receiver and transmitter. Precise relative variations of sTEC are achievable using phase measurements on L1 and L2 frequencies, yielding around 0.1 TECU or better.
However, CubeSats like Spire LEMUR, with simpler setups and code noise in the order of several meters, face limitations in absolute accuracy. Their relative accuracy, determined by phase observations, remains in the range of 0.1-0.3 TECU.
With a substantial number of observations and comprehensive coverage of lines of sight between Low Earth Orbit (LEO) and GNSS satellites, global electron density can be reconstructed from sTEC measurements. Utilizing 27 satellites from various missions, including Swarm, GRACE-FO, Jason-3, Sentinel 1/2/3, COSMIC-2, and Spire CubeSats, a cubic B-spline expansion in magnetic latitude, magnetic local time, and altitude is employed to model the logarithmic electron density. Hourly snapshots of the three-dimensional electron density are generated, adjusting the model parameters through non-linear least-squares based on sTEC observations.
Results demonstrate that Spire significantly enhances estimates, showcasing exceptional agreement with in situ observations from Swarm and Defense Meteorological Satellite Program (DMSP). The model outperforms contemporary climatological models, such as International Reference Ionosphere (IRI)-2020 and the neural network-based NET model. Validation efforts include comparisons with ground-based slant TEC measurements, space-based vertical TEC from Jason-3 altimetry, and global TEC maps from the Center for Orbit Determination in Europe (CODE) and the German Research Center for Geosciences (GFZ).