Abstract
The Constellation Observing System for Meteorology, Ionosphere, and Climate 2 (COSMIC-2) mission was launched into a low-inclination (24°) orbit on June 25, 2019. Six satellites, each with an advanced Tri-GNSS Radio-Occultation Receiver System (TGRS), provide a global and uniform data coverage of the equatorial region with several thousand electron density profiles daily. The COSMIC-2 electron density profiles, and specifically the derived ionospheric F2 peak parameters, are properly validated in this study with reliable “truth” observations. For this purpose, we used manually scaled ionograms from 29 ground-based ionosondes located globally at low and middle latitudes. For this validation campaign, we considered only geomagnetically quiet conditions in order to establish benchmark level of the new mission’s ionospheric observation quality and to evaluate the operational capability of the COSMIC-2 Radio Occultation (RO) payload at the background of normal day-to-day variability of the ionosphere. For reliable colocations between two independent techniques, we selected only COSMIC-2 RO profiles whose F2 peak point coordinates were within 5° of the closest ionosonde. Our comparison of the ionospheric F2 peak height (hmF2) derived from COSMIC-2 RO and ground-based ionosonde measurements showed a very good agreement, with a mean of ~5 and ~2 km at low and middle latitudes, respectively, while RMS error was of ~23 and ~14 km, respectively. That range corresponds to a deviation of only 6–9% from the reference, ionosonde observations. Examination of representative collocation events with multiple (2–5) simultaneous RO tracks near the same ionosonde with different RO geometry, multi-satellite and multi-GNSS combination give us observational evidence that COSMIC-2 RO-based EDPs derived from GPS and GLONAS links show good self-consistency in terms of the ionospheric F2 peak values and electron density profile shape. We can conclude that COSMIC-2 provides high quality data for specification the ionospheric electron density at the F2 peak region.
Highlights
The Global Positioning System Meteorology (GPS/MET) Experiment, which placed a GPS receiver onboard a LowEarth-Orbiting (LEO) satellite in 1995, was a pioneer in demonstrating the applicability of the GPS-based Radio Occultation (RO) technique to retrieve a vertical distribution of electron density in the Earth’s ionosphere (Hajj & Romans, 1998; Schreiner et al, 1999)
These values are in a good agreement with results of the COSMIC-1 RO frequency of the F2 peak layer (foF2) assessment made by McNamara & Thompson (2015) – using foF2 data manually scaled from the Australian ionosondes network and 1871 collocation points during 9 months of 2007/2008, they report standard deviation (SD) and root mean square error (RMSE) of ~0.5 MHz at midlatitude stations of the Australian region and increasing up to ~1.0–1.6 MHz at equatorial/low latitudes
We avoid errors related to auto-scaling processing by manual checking and editing of the ionosonde data, including cases of doubtful ionograms scaling under F-spread conditions
Summary
The Global Positioning System Meteorology (GPS/MET) Experiment, which placed a GPS receiver onboard a LowEarth-Orbiting (LEO) satellite in 1995, was a pioneer in demonstrating the applicability of the GPS-based Radio Occultation (RO) technique to retrieve a vertical distribution of electron density in the Earth’s ionosphere (Hajj & Romans, 1998; Schreiner et al, 1999). The objective of the present paper is to evaluate the accuracy of the ionospheric F2 peak parameters retrieved from RO EDPs from the new COSMIC-2 multi-satellite mission by direct and statistical comparison with globally distributed ionosonde observations. For our study, we consider as negligible any impact of MSTIDs propagation on accuracy of the F2 peak parameters derived from vertical sounding ionograms and on the RO–ionosonde cross-comparison results as well From these selected (and available) ionosondes, a major portion is represented by DPS-4D Digisondes – ionograms are scaled in automatic mode in real time using the ARTIST (Automated Real Time Ionogram Scaler with True height) software and sent to the DIDBase (Digital Ionogram Data Base) repository. For many cases when ionograms or the sequence of nearest ionograms within ±15 min were affected by strong spread F or blanketing sporadic E, these colocation events were removed from the analysis
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