Comparison of a Bottom-Up GNSS Radio Occultation Method to Measure D- and E-Region Electron Densities with Ionosondes and FIRI  
[PERSON]  
12d Weather Squadron, Offutt Air Force Base, Omaha, NE 68113, USA; [EMAIL_ADDRESS]  
[PERSON]  
22 NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA; [EMAIL_ADDRESS] (D.L.W. ); [EMAIL_ADDRESS] (N.S)2  
[PERSON]  
23 Department of Physics, The Catholic University of America, Washington, DC 20064, USA 23 Department of Engineering Physics, Air Force Institute of Technology, Wright-Patterson Air Force Base, Dayton, OH 45433, USA; [EMAIL_ADDRESS]  
[PERSON]  
45 Space Vehicles Directorate, Air Force Research Laboratory, Kirtland Air Force Base, Bernaildo, NM 87117, USA; [EMAIL_ADDRESS]  
[PERSON]  
56 Space Vehicles Directorate, Air Force Research Laboratory, Kirtland Air Force Base, Bernaildo, NM 87117, USA; [EMAIL_ADDRESS]  
[PERSON]  
45 Space Vehicles Directorate, Air Force Research Laboratory, Kirtland Air Force Base, Bernaildo, NM 87117, USA; [EMAIL_ADDRESS]  
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(\(N_{e}\)) capable of absorbing and reflecting long-wave radiation such as AM radio [2]. At night, the D-region disappears and the E- and F-region shifts to a higher altitude, thus radio waves reflect at a higher altitude and travel further after reflection. Therefore, long-range high-frequency (HF) operations such as over-the-horizon radar and HF communications can be heavily impacted by D- and E-region dynamics [3], requiring accurate global measurements to optimize performance. A standard method of measuring the ionosphere is through the use of an ionospheric sounder, or ionosonde. Ionosondes generate ionograms, which are virtual height profiles of the ionosphere obtained by transmitting a sweep of HF pulses and measuring the time of flight for the signals to reflect off of the ionosphere and return to the sounder [4]. Subsequently, the measured virtual height profiles can be inverted to calculate \(N_{e}\) profiles providing a measurement of the local ionosphere [5]. Ionosondes are limited, however, in several ways. First, they can only measure the bottomside ionosphere up to the F\({}_{2}\)-region peak and are unable to provide any information about the topside [6]. Second, they are limited by their minimum transmitted frequency of \(\sim\)0.5 MHz [7], so they are unable to measure the E-region at night or the D-region even at peak daytime values. Finally, there are a limited number of global ionosondes, with only 44 current sites around the world in the Global Ionosphere Radio Observatory (GIRO) that generate ionograms in real time [8]. With the recent COSMIC-2 and Spire satellite constellations, there are currently over 20,000 GNSS-RO measurements per day that provide global coverage [9; 10].