 The global navigation satellite system, GNSS, has been widely used to measure the electron density in the Earth's atmosphere. However, the GNSS pod, precision orbit determination, and ROPE, radio occultation, and TNE have been used increasingly in characterizing the global three-dimensional distribution and variability of ionospheric electron density, now. In this study, an optimal estimation, OE, method was developed to retrieve profiles from the slant total electron content, HTC, measurements acquired by the GNSS pod links at negative elevation angles, epsilon zero degrees. This method overcomes the large no-oscillations, sometimes negative values, seen in the onion peeling, OPE, retrievals in the lower ionosphere. The new OE results can also be validated against ground-based ionisans and radar observations, showing generally good agreement in no from all sites. The OE algorithm has been applied to all GNSS pod data acquired from the Cosmic-1, 2006 to 2019, Cosmic-2, 2019 present, and SP. This article was authored by Don Elwou, Nimoulin Swarnalingam, Cornelius Sarjoud H. Salinas, and others. We are article.tv, links in the description below.