Ground-based Global Navigation Satellite System (GNSS) receivers have become an ubiquitous tool for monitoring the ionosphere. Total Electron Content (TEC) data from globally distributed networks of ground-based GNSS receivers are increasingly being used to characterize the ionosphere and its variability. The deployment of these GNSS receivers is currently limited to landmasses. This means that 7/10 of Earth’s surface, which is covered by the oceans, is left unexplored for persistent ionospheric measurements. In this paper, we describe a new low-power dual-frequency Global Positioning System (GPS) receiver, called Remote Ionospheric Observatory (RIO), which is capable of operating from locations in the air, space, and the oceans as well as on land. Two RIO receivers were deployed and operated from the Tropical Atmosphere Ocean buoys in the Pacific Ocean, and the results are described in this paper. This is the first time that GPS receivers have been operated in open waters for an extended period of time. Data collected between September 1, 2018 and December 31, 2019 are shown. The observed TEC exhibits a clear seasonal dependence characterized by equinoctial maxima in the data at both locations. Both RIO receivers, deployed near the geomagnetic equator, show an 18-35% increase in TEC during moderately disturbed geomagnetic periods. Comparisons with the International Reference Ionosphere model show good agreement. The new capability presented in this paper addresses a critical gap in our ability to monitor the ionosphere from the seventy percent of the Earth’s surface that is covered by water.

The ionosphere contains many small-scale electron density variations that are under represented in smooth physics-based or climatological models. This can negatively impact the results of Observation System Simulation Experiments, which use a truth model to simulate data. This paper addresses this problem by using ionosonde data to study ionospheric variability and build a new truth model with empirically-driven variations. The variations are studied for their amplitude, horizontal and vertical size, and temporal extent. Results are presented for different local times, seasons, and at two different points in the solar cycle. We find that these departures from a smooth background are often as large as 25\% and are most prevalent near 250 km in altitude. They have horizontal spatial extents that vary from a few hundred to a few thousand kilometers, and typically have the largest horizontal extent at high altitudes. Their vertical extents follow the same pattern of being larger at high altitudes, but they only vary from 10s of km up to 200 km in vertical size. Temporally, these variations can last for a few hours. The procedure for using these spatial and temporal distributions to add empirically-driven variance to a smooth truth model is outlined. This process is used to make a truth model with representative variations, which is compared to ionosonde data as well as GPS Total Electron Content (TEC) data that was not used to inform the model. The new model resembles the data much better than the smooth models traditionally used.