Equivalent power flux density

Lore & Background

The EPFD concept emerged from a regulatory dispute in the 1990s over how non-geostationary broadband constellations could share spectrum already used by geostationary systems. The 1995 Radio Regulations obliged non-GSO systems to 'cease or reduce to a negligible level' any emissions found to cause unacceptable interference to GSO networks, without specifying what threshold of interference that meant in quantitative terms. The issue came to a head around proposed non-GSO constellations such as Alcatel SkyBridge and Teledesic, which sought to reuse Ku- and Ka-band spectrum already allocated to GSO fixed-satellite systems. At the 1997 World Radiocommunication Conference, European administrations proposed resolving the ambiguity by defining quantitative interference limits, expressed as EPFD, that a non-GSO system could meet in order to be deemed compliant without negotiating individually with every GSO operator. These provisional limits were refined using a set of reference GSO links, rain fade statistics, and unavailability criteria, and were formally incorporated into Article 22 of the Radio Regulations at WRC-2000. Limits have subsequently been applied to portions of the Ku, Ka, and C bands.

Three related quantities are defined: EPFDdown (interference from NGSO satellite downlinks into a GSO earth station's downlink reception), EPFDup (interference from NGSO earth stations into a GSO satellite's uplink reception), and EPFDis (inter-satellite interference, in which an NGSO satellite's downlink emissions fall into bands where GSO satellites receive uplink signals). Because EPFD depends on constellation geometry that changes continuously, verifying compliance requires simulation rather than a single closed-form calculation. Recommendation ITU-R S.1503 sets out the standardized methodology used by the ITU Radiocommunication Bureau to check whether a filed non-GSO system meets the Article 22 limits, based on parameters an administration submits describing its system's technical envelope.

Reader's Guide

EPFD's fixed technical limits have drawn sustained criticism as broadband satellite constellations have grown far larger and more capable than the systems the framework was designed around in the 1990s. The International Telecommunication Union itself noted in 2023 that the existing Ku- and Ka-band EPFD provisions are 'spectrally inefficient' and unnecessarily constrain non-GSO systems without providing additional protection to GSO operators, calling for the limits to be revisited. When the ITU studied a comparable sharing problem for the higher-frequency Q band and V band ahead of a 2019 conference, it deliberately chose not to replicate the EPFD approach, instead developing a different framework subsequently addressed under Resolution 770. In the United States, the Federal Communications Commission moved unilaterally away from EPFD for domestic purposes in 2026, voting on April 30, 2026 to replace its EPFD-based satellite spectrum-sharing rules with a performance-based framework that instead relies on voluntary coordination between operators, backed by specific throughput-degradation and service-unavailability backstops, along with a minimum avoidance angle from the GSO arc. Proponents, including non-geostationary operators, argued the decades-old EPFD limits were based on outdated technical assumptions and constrained the throughput of modern low-Earth-orbit systems. Geostationary operators generally opposed the change, warning that relaxed limits could cause new interference to their in-service satellites. The FCC's action applies only within United States jurisdiction, and EPFD continues to apply internationally.

Did You Know?

• EPFD weighs each transmitter's contribution by the receiving antenna's gain in that transmitter's direction, relative to the antenna's peak gain.
• The formal definition of EPFD is set out in ITU Radio Regulations Article 22.5C.1 and mirrored in US regulation at 47 CFR § 25.103.
• A non-GSO filing that exceeds Article 22 EPFD thresholds must pursue individual coordination with affected GSO networks rather than proceeding without negotiation.
• The aggregate case for multiple co-frequency NGSO systems is addressed using an assumed effective number of 3.5 systems to apportion the Article 22 limits.

Regulatory Purpose: Enabling Constellation-Scale Spectrum Sharing

EPFD exists to solve a practical regulatory problem: how to let large constellations of non-geostationary satellites coexist with geostationary satellites on the same frequencies without requiring bilateral coordination deals between every pair of operators. The International Telecommunication Union codified this concept in Article 22 of its Radio Regulations, and national authorities such as the U.S. Federal Communications Commission adopted it into their own rulebooks, specifically at 47 CFR § 25.103. Under this framework, an NGSO fixed-satellite service system can demonstrate compliance by showing that its aggregate interference at a protected GSO receiving station stays within a defined EPFD limit. This approach replaces the older, more cumbersome method of individually negotiated coordination agreements for each satellite-to-satellite pair, effectively scaling the regulatory burden to match the operational reality of constellations containing dozens or more active transmitters simultaneously visible from a single protected station.

From Single-Transmitter PFD to a Constellation-Wide Metric

The starting point is the familiar power flux density of a single transmitter, which follows directly from the inverse-square law: the effective isotropic radiated power in a given direction divided by four pi times the distance squared. For a lone interferer this is straightforward. A geostationary receiving station in practice, however, sees an entirely different picture. A non-geostationary constellation can place dozens of satellites above the horizon at any instant, each at a different slant range, each radiating at a different off-axis angle relative to its own boresight, and each arriving at the GSO antenna from a different direction. EPFD generalizes the single-source PFD by treating every visible transmitter as a contributor, scaling each one's flux density by the receiving antenna's gain in that particular direction relative to the antenna's peak gain, summing all contributions in linear power, and then converting the total back to a logarithmic expression in decibels per square meter within a specified reference bandwidth.

The Formal Summation and Its Governing Variables

The formal expression, as set out in ITU Radio Regulations Article 22.5C.1 and mirrored in U.S. regulation, sums over every transmitting station in the NGSO system that is visible from the protected GSO station. For each contributor, the formula incorporates the transmitter's radio-frequency power in the reference bandwidth, the transmitting antenna's gain evaluated at the specific off-axis angle between that satellite's boresight and the direction toward the GSO station, the slant distance between the two, and the receiving antenna's gain at the off-axis angle toward that particular transmitter, normalized by the receiving antenna's maximum gain. The result of each per-satellite term is a linear-power quantity; all such terms are added together, and the final sum is converted to a logarithmic value expressed in dB(W/m²). This structure ensures that satellites closer to the boresight of either antenna contribute more heavily, while those far off-axis are naturally attenuated by the antenna-pattern terms.

Why Antenna Geometry and Distance Shape the Result

A critical feature of the EPFD formulation is that it is not simply a sum of raw radiated powers. Each satellite's contribution is modulated by two antenna-pattern factors: the transmitting antenna's gain in the direction of the GSO station, evaluated at the off-axis angle between the transmitter's boresight and the receiver, and the receiving antenna's gain toward that satellite, evaluated at the off-axis angle at the GSO station and normalized by the antenna's peak gain. This means a satellite radiating strongly but pointing well off the GSO station's direction contributes little, while one in a favorable geometric alignment contributes more. The inverse-square distance term further weights nearer satellites more heavily. Together, these geometric and pattern-dependent factors ensure that EPFD reflects the actual electromagnetic environment at the receiving aperture rather than an abstract total of transmitted power, making it a physically meaningful interference metric for spectrum-sharing decisions.