What Are EPFD Limits, and Why Do They Matter for NGSO Constellations?

If you work anywhere near NGSO constellation filings, EPFD limits come up constantly — and they're one of the more misunderstood parts of the coordination framework, because they don't work like ordinary frequency coordination at all.

What EPFD actually measures

Equivalent Power Flux Density, defined under Article 22 of the ITU Radio Regulations, measures the aggregate signal that an entire NGSO constellation — not just one satellite — directs toward a geostationary satellite or earth station, accounting for the receiving antenna's directional gain pattern. That "aggregate" part matters: a constellation of thousands of satellites is evaluated on its combined effect on the GSO arc, not satellite by satellite.

Why aggregation, not a simple limit per satellite

A single NGSO satellite passing briefly through a GSO antenna's field of view might contribute a negligible amount of interference. Thousands of satellites doing that continuously, at different times, add up — which is exactly the scenario Article 22 was built to bound. The limits are the product of years of technical study across multiple ITU conferences, developed using a large set of reference GSO links contributed by administrations, precisely because getting the aggregate math right required broad, careful modeling rather than a single simple threshold.

What compliance actually looks like

Unlike a frequency-overlap check, EPFD compliance isn't something you assess with a spreadsheet comparison. The ITU's own validation approach is described in Recommendation ITU-R S.1503, and NGSO operators typically run dedicated software to demonstrate their constellation stays within the Article 22 limits before their filing can be validated. This is a materially heavier technical exercise than checking whether two frequency ranges overlap.

An actively contested part of the rulebook

EPFD limits are not settled policy — they're currently a live regulatory fight. Some GSO operators argue the limits have worked well and shouldn't be relaxed; some NGSO operators, including SpaceX, have sought relaxation of the limits, first through national regulators and eventually at the ITU itself. In 2025 the FCC opened a review of the decades-old GSO/NGSO sharing regime in several bands, and the ITU has an agenda item on the topic heading into WRC-27. Whatever your position, it's worth tracking: a relaxation or tightening of Article 22 would directly change how much room NGSO operators have to maneuver in bands they share with GSO incumbents.

Sources

• "Understanding EPFD Limits: Why They Foster Innovation and Harmonious Operations in Space," SES
• "EPFD limits in the spotlight as ITU selects Shanghai for WRC-27," Runway Girl Network