*2.1. LEO Data*

The LEO data used in this study consist of a total of 1188 reflection events on 1 March 2021, sourced from Spire Global Inc. Currently, the Spire Lemur-2 constellation comprises more than 80 GNSS radio occultation CubeSats, out of which about 30 have been adapted to acquire GNSS reflectometry measurements at grazing angles [19]. The Lemur-2 satellites follow a Sun-synchronous orbit, with altitudes ranging from 400 to 600 km and varying orbit inclinations. This orbital configuration enables them to conduct GNSS-R measurements, encompassing all latitudes of the Earth.

The Spire grazing angle GNSS-R products are collected with a focus on specific regions, including the polar areas, the Gulf of Mexico, and southeast Asia. These regions are selected due to their favorable characteristics, such as sea ice surfaces and calm ocean surfaces, which enable the best performance of coherent reflectometry measurements [10]. Figure 2 displays the track positions of the specular points distributed across both polar regions, as well as in the mid-latitude and tropical regions at different local times. Given the geographical distribution of the events, the dataset has been categorized into three distinct regions: north, covering latitudes between 40◦N and 90◦N; tropics, spanning latitudes between 40◦N and 40◦S; and south, covering latitudes between 40◦S and 90◦S.

**Figure 2.** Specular point tracks provided by the Spire Lemur-2 CubeSats on 1 March 2021, color-coded according to coordinated universal time (UTC) in hours.

Each Lemur-2 satellite event lasts an average of 4 min, resulting in a total of about 80 h of recorded data. The recording durations vary, with a minimum of 1 min and a maximum of 6 min. Table 1 shows the number of events per region (north, tropics, and south) along with their corresponding durations in minutes.


A total of 21 CubeSats from the Spire constellation are evaluated. The metadata include the space vehicle number (SVN) of the Lemur-2 satellite, as well as information about the GNSS satellite and constellation from which the CubeSat receives the reflected signals. For the simulation in this study, the GNSS constellation employed is GPS (global positioning system). Upon analyzing the Spire data, it is found that each Lemur-2 satellite receives the reflected signal from 4 to 19 GPS satellites during different time windows, depending on the positions of the transmitters and the receiver. The Spire SVN and GPS pseudo-random noise code (PRN) are presented in Table 2.

**Table 2.** Space Vehicle Numbers of Lemur-2 CubeSats and GPS Satellite PRNs on 01/03/2021.

