# Near-Real-Time Global Thermospheric Density Variations Unveiled by Starlink Ephemeris
## 2 Data and Methods  
In this study, Starlink ephemerides published by SpaceX on space-track.org ([[https://www.space-track.org/#publicFiles](https://www.space-track.org/#publicFiles)]([https://www.space-track.org/#publicFiles](https://www.space-track.org/#publicFiles)), accessed on 13 January 2025) were used to calculate the mechanical energy loss of each Starlink satellite. These data feature 72-h predictions and are updated three times a day (even during geomagnetic disturbed periods), with a time resolution of 1 min. Each ephemeris file consists of a descriptive header followed by thousands of ephemeris points spanning the following three days. As detailed in the official Starlink specifications ([[https://www.space-track.org/documents/SFS_Handbook_For_Operators_V1.7.pdf](https://www.space-track.org/documents/SFS_Handbook_For_Operators_V1.7.pdf)]([https://www.space-track.org/documents/SFS_Handbook_For_Operators_V1.7.pdf](https://www.space-track.org/documents/SFS_Handbook_For_Operators_V1.7.pdf)), accessed on 13 January 2025), these ephemerides conform to the Modified ITC ephemeris format. Table 1 presents the contents of the header and the first ephemeris point from STARLINK-32392 in the batch of 2024-10-01 UTC13_21_09_01 as an example. The four-line header specifies the creation time and start and stop times of the ephemeris, along with its source and the covariance reference frame. Specifically, in this example, the header in the table reveals a 60-s interval between consecutive ephemeris points and a total timespan of three days, resulting in 4321 ephemeris points per file. The covariance reference frame is UVW, also known as RTN (Radial, Transverse, and Normal), a spacecraft-centered reference frame used to describe relative motion in orbit. For individual ephemeris points, the first data line contains the epoch timestamp (formatted as \"yyyyDOYhhmmss.sss\", provided in Coordinated Universal Time, UTC) and the 3-dimensional position and velocity vectors (in units of km and km/s, respectively, provided in the EME2000 reference frame). The subsequent three lines systematically arrange the corresponding covariance matrix elements. The currently deployed first-generation Starlink constellation is organized into five orbital shells [35]. Based on orbital altitude and inclination, as shown in Table 2, we can categorize each Starlink satellite into its respective shell. After excluding satellites with significant orbital fluctuations and anomalous behavior, the number of operational Starlink satellites (during October 2024) in Shells 1-5 is 1115, 388, 232, 1473, and 1335, respectively. Taking Shell 1 as an example, Figure 1a illustrates the geographic distribution of satellites within this shell. The satellites are densely concentrated between 53\({}^{\circ}\)S and 53\({}^{\circ}\)N, consistent with the designed orbital inclination of 53\({}^{\circ}\). In orbital mechanics, the right ascension of the ascending node (RAAN) specifies the orientation of a Keplerian orbital plane in space, while the argument of latitude defines the position of a spacecraft along the orbit. This implies that, over an orbital period, the RAAN remains nearly constant, whereas the argument of latitude varies continuously from 0\({}^{\circ}\) to 360\({}^{\circ}\). This characteristic allows us to divide satellites of each shell into distinct orbital planes. Here, we select values of RAAN on 00:00 UT (universal time) 3 October 2024 as a reference to number the orbital planes. For Shell 1, we divide the shell into 72 orbital planes at 5\({}^{\circ}\) intervals, as depicted in Figure 1b, where each plane is color-coded to indicate its corresponding region. Note that Starlink satellites in lower-numbered orbital planes cross the equator earlier in local time (LT) than those in higher-numbered orbital planes. Figure 1c presents a histogram quantifying the number of satellites per orbital plane. The results show that all orbital planes are populated, with satellite counts ranging from 8 to 22 and an average of approximately 15 satellites per plane.