Orbital characteristics and operational flexibility in non‑GSO space stations

Julie Zoller, Head, Global Regulatory Affairs, Amazon Project Kuiper

There are two types of Earth orbits: geostationary-satellite orbits (GSO), where satellites are at an altitude of almost 36,000 km above the equator and appear motionless to antennas on Earth, and non‑geostationary satellite orbits (non‑GSO), the category for all other satellites.

Non‑GSO systems generally fall into two types based on altitude: low Earth orbit (LEO), between 400 km and 2000 km; and medium Earth orbit (MEO), between LEO and GSO levels.

Every satellite filing with the International Telecommunication Union (ITU) should include essential orbital characteristics, such as planned altitude and inclination. In addition, GSO operators must provide certain orbital tolerance details, including limits — meaning the extent of possible deviations from the orbital information provided to ITU.

For non‑GSO systems, however, orbital tolerance is neither required in the filing nor limited in practice.

Expanding non‑GSO systems

ITU filings for non‑GSO systems continue to grow in volume and size, with plans now calling for constellations of tens, hundreds, or thousands of satellites in low Earth orbit.

This has heightened the need to consider orbital tolerance in the non‑GSO environment.

The issue is on the table for the upcoming World Radiocommunication Conference (WRC‑23).

Under agenda item 7, topic A considers setting tolerance limits on certain orbital characteristics of non‑GSO space stations. Agenda item 7, topic B, considers a post-milestone procedure to address the scenario of a sustained reduction in the number of satellites in orbit after milestones have been met.

Besides numbers of satellites, the most important orbital parameters for a non‑GSO system are the altitudes and inclinations of the orbital planes making up the constellation. Orbital tolerance information would establish the permissible variation in altitude and inclination from the filed parameters.

A simple example illustrates the point: If WRC‑23 decides to limit orbital tolerance to 10 per cent for altitude and 10 per cent for inclination, then a non‑GSO system in a circular orbit, if filed with an altitude of 500 kilometres (km) and an inclination of 30 degrees, would have an orbital tolerance of +/– 50 km altitude and +/– 3 degrees inclination.

Why does orbital tolerance matter?

Orbital tolerance affects how the Radio Regulations are applied.

Orbital tolerance has a bearing on whether a particular space station is considered to be in the right place at the right time to meet the first major milestone in the life of a non‑GSO system, “bringing into use.”

Using the above example, a satellite at 450 km and 27 degrees inclination, or 550 km and 33 degrees inclination, or anything in between, would be regarded as maintained on a notified orbital plane. Thus, after a continuous period of 90 days, it would satisfy the criteria of bringing into use.

Orbital tolerance also affects the accuracy of the ITU Master International Frequency Register (MIFR). By extension, it may alter how we understand the ITU Radiocommunication Bureau’s findings on matters such as Article 21 of the Radio Regulations regarding power flux-density limits, or the approach to coordination between non‑GSO systems.

To calculate power-flux densities on the ground, or to avoid harmful radio interference, we need to know exactly where satellites are.

Space safety and orbital tolerance

While ITU does not manage physical aspects of objects in space, there is a relationship between space safety and orbital tolerance. Temporary operational variances in orbital parameters — such as during the re-phasing of satellites in a constellation or due to the solar cycle — are expected occurrences that should be excluded from orbital tolerance requirements.

Satellite operators require this flexibility. Among the operator community, the generally accepted best practice is to have adequate radial separation between large non‑GSO constellations for space safety purposes.

The preparatory report (CPM Report) for WRC‑23 reflects the general agreement “that any tolerances should provide the necessary flexibility to accommodate normal operations of non‑GSO systems and allow operational coexistence between systems filed at the same or close orbital positions.”

Decisions needed at WRC‑23

The decisions ITU Member States make at WRC‑23 on non‑GSO orbital tolerance will affect the rational, efficient, economic, and equitable use of the radio-frequency spectrum and satellite orbits, as well as influence the space safety ecosystem.

Ideally, the decision on orbital tolerance will meet the needs of countries and operators worldwide. This means enabling the operational flexibility needed for satellite operators to provide good service to their customers, while complementing space safety objectives.

These overarching objectives suggest that WRC‑23 should limit non‑GSO orbital tolerance.

Download your copy of the ITU News Magazine: Satellite connectivity.

Header image credit: Adobe Stock