## 1 Introduction  
Space-based technologies have become integral to everyday life and present unprecedented opportunities for research and application development. But their operation depends on the ability to monitor, evaluate, and mitigate space climate phenomena with long-term effects on the Earth's ionosphere. The ionosphere, as the ionized part of the Earth's atmosphere, extends between ~60 km to ~1000 km. Plasma concentrated around the ionospheric ionization peak (height range of about 250-450 km) impacts the space system operation most significantly, affecting sub- and trans-ionospheric radio wave propagation, satellite communication links, and navigation system precision. There are challenges of practical interest in the study of the Earth's ionosphere that demand detailed knowledge of ionospheric plasma density distribution in space and time. The ionospheric electron density profile (EDP) Ne(h), as a function of electron density Ne versus height, is one of the most important products for operational space weather monitoring and ionospheric research. Our knowledge of how plasma is distributed in the ionosphere at different altitudes is largely based on remote sensing observations: ionosondes [1; 2], incoherent scatter radars (ISR) [3], and radio occultation (RO) measurements from Low-Earth-Orbiting (LEO) satellites [4; 5]. Ionosondes, a type of high-frequency (HF) sounding radar, are used for remote sensing of the ionosphere for many decades [6; 7; 8; 9; 10; 11]. The ionosondes provide accurate unbiased measurements of the ionospheric plasma density due to direct relationships between the measured frequency of full reflection from the particular ionospheric layer and the electron density in that layer. The ionosondes sweep in frequency from about 1-20 MHz and record tracings of reflected high-frequency radio pulses in the form of an ionogram. Typically, ionograms are recorded every 5-15 min. Directly from ionogram image recordings, one can identify echoes in the ordinary and extraordinary modes and can scale critical frequencies of major ionospheric layers and their virtual heights [12]. The critical frequency of the F2 layer (foF2), which is the largest o-ray frequency of the F2 layer reflection, is related directly to electron density at the F2 peak as NmF2 [el/cm\({}^{3}\)] = 1.24 \(\times\) 10\({}^{4}\)\(\times\) (foF2 [MHz])\({}^{2}\). The height of the ionospheric layer can be obtained throughout the true height inversion procedure based on the reflection layer's virtual height when the time delay of the echo is measured and how the electron density from the bottomside altitudes effect the wave propagation in plasma. For more details on ionogram inversion principles and specific implementations, see [13; 14; 15]. An important limitation of the ionosondes is that they only measure the bottomside part of an EDP up to the F2 peak height; the topside part of an Ne(h) profile is derived by fitting a model to the measured peak electron density. Therefore, ionosondes remain the \"ground truth reference\" for electron density specification in the lower ionosphere up to the height of the main ionospheric F2 peak. The prevalent type of operational ionosondes now are digital ionosondes--DPS (Digisonde-Portable-Sounder) [16; 17]. RO observations from GNSS receivers onboard LEO satellites have served as an important source for specification of the lower neutral atmosphere, ionosphere, and plasmasphere for over 25 years since the seminal GPS/MET experiment [18; 19]. The GNSS RO technique allows reliable measurements of ionospheric electron density vertical distribution on a global scale, tested and validated through extensive comparisons with independent observations. COSMIC-1 (Constellation Observing System for Meteorology, Ionosphere, and Climate) [20; 21; 22], the first multi-satellite RO constellation, launched in April 2006, delivered an impressive amount of 1500-2500 RO soundings of the ionosphere and atmosphere per day on a global scale for more than ten years.