2.2.1. GNSS-R Surface Reflection Global Distribution

The global distribution of GNSS-R surface observations is the result of an asynchronous process between the orbits of the transmitting GNSS satellites (GPS, Galileo, SBAS, etc.) in Medium Earth Orbit (MEO, ~20,200 km altitude) and occasionally Geostationary orbit (~35,786 km altitude) and a constellation of Low Earth Orbit (less than 2000 km altitude) receivers below the transmitters. The GNSS-R observation point is the Earth surface location between the transmitter and receiver that results in a forward specular reflection in the direction of the receiver. An Illustration of three example GNSS-R reflection geometries is shown in Figure 1, with the transmitters above and outside the figure and a single LEO receiver capturing the parallel surface reflections. The specular reflection location point between transmitter and receiver at a single epoch can be calculated using an iterative process as described in [3]. The resulting total of GNSS-R surface reflection points is therefore driven by the pseudo-random alignment of the multiple GNSS satellites and the multiple LEO receivers that result in fortuitous surface reflection locations in the receiver antenna footprint of the GNSS-R instruments. The number of GNSS transmitters can well exceed 100 if multiple constellations are utilized (GPS, Galileo, Beidou, GLONASS) for example, while the optimal number of receivers can range into several dozen in varied orbits resulting in a dense web of overlapping surface observations. Given the large numbers of transmitters and receivers a simulation is used to generate these large data sets of surface observation points over time (nominally one day). The achieved GNSS-R observation coverage was then analyzed with respect to optimizing the spatial and temporal coverage metrics described below.

**Figure 1.** General illustration of three GNSS-R reflection specular points captures by a single LEO receiver. Reprinted with permission from ref. [16], Copyright: 2014, IEEE.
