The nuances of the 53-degree Starlink shells

Starlink satellites orbiting with ~53 degrees of inclination are of prime importance to SpaceX for several reasons:

• It is where it all started (first shell uses 550 km of altitude and 53 degrees of inclination)
• Currently, around 60% of all Starlink satellites orbit with an inclination of 53 to 53.25 degrees.
• Together with the 43-degree shell, the 53-degree orbits maximize the number of satellites “in view” for the US market (Alaska excluded), which accounts for over 50% of all Starlink subscribers worldwide.
• The 53-deg orbits are also key for aero and maritime mobility routes, as well as for the upcoming sat-to-cell /D2D service.

Due to the extensive use of 53-degree orbits in the Starlink constellation, it is tricky to categorize the (filed) shell that operating satellites belong to.?The NCAT4 server updates orbital readings daily for all Starlink shells and simultaneously inspects both the orbital radius and inclination to place each orbiting satellite into the appropriate (most likely) nominal shell. Those satellites that have not yet reached their destination altitude (within certain margin) are not considered “in orbit” and thus not used for the visibility and bandwidth calculations.

Orbital distribution

Of the three shells, the one that currently presents more variability in satellite altitude and inclination is -naturally- the youngest shell, specifically the Gen-2 shell filed with a nominal altitude of 525 kilometers. This shell is the one that SpaceX has been actively populating recently and it is also the one assumed to be used for the sat-to-cell (D2D) T-Mobile services in the U.S. (announced to start in the fall).

Due to its variability, within the latest NCAT4.2 release we implemented a modification in the interpretation of the daily orbital reading (dotted lines in the chart) to account for such variability. As NCAT4 users may have noticed, this modification in the parameters of orbital readings has increased the total count of in-orbit Starlink satellites, particularly for the 525 km shell.

SpaceX does not disclose which satellites are operational, so there is a chance, that NCAT may be slightly overcounting active satellites because satellites that are not yet in their destination orbits, can be in ascending (or decay) mode. However, it appears that towards lower altitudes SpaceX is exercising some flexibility in how satellites are placed into orbit.

Notes:

• NCAT4 plots how satellites are instantaneously distributed in location (subpoint), altitude, inclination and other metrics for the user-selected constellation shells.
• Since Earth is modeled as an oblate spheroid, the altitude of each satellite varies based on its instantaneous subpoint location (Earth's radius is ~ 21 kilometers greater at the equator than at the poles).
• Also notice that circular orbits for operational systems are not perfectly circular, the eccentricity also influences satellites’ instantaneous altitude relative to their nominal (filed) parameters.

We will continue to monitor developments and, as the 525 km shell gets populated and densified, further tuning to the NCAT4 server filters may be needed.

The Non-GEO Constellations Analysis Toolkit (NCAT) is an assembly of easy-to-use analytics models to assess and benchmark LEO and MEO satellite constellations.

To learn more:

• Watch brief videos on YouTube
• Read use cases here on Linkedin
• Download the NCAT white paper
• Visit the NCAT info page on the Analysys Mason website
• Contact Analysys Mason