EPFD Screening - NGSO vs. GSO

This is a screening estimator, not a compliance tool. It computes the worst-case single-entry epfd↓ from one NGSO satellite toward one victim GSO earth station and compares it to a representative Article-22-style limit. Formal EPFD compliance is established only by running the full aggregate ITU-R S.1503-2 analysis over the propagated constellation, across every percentage-of-time point - weeks of CPU, not a browser calculation. A "compliant" verdict here does not establish Radio Regulations compliance.

Single-entry epfd = EIRP density − 10log10(4πd²) + (G(θ) − Gmax), using an ITU-R S.1428 earth-station reference pattern, vs. a representative Article-22 mask. A screen to run BEFORE an ITU-R S.1503-2 validation - verify every limit against the current RR tables.

This EPFD calculator evaluates three terms in dB: the interfering satellite's effective isotropic radiated power (EIRP) density in the reference bandwidth, the free-space spreading loss over the slant range (taken worst-case, satellite directly overhead), and the victim earth station's off-axis discrimination from the Recommendation ITU-R S.1428 reference antenna pattern. The sum is the single-entry epfd↓, compared against a representative Article 22 mask value with the margin and an epfd-vs-angle curve. The full aggregate method - every visible satellite, every percentage of time - is defined by Recommendation ITU-R S.1503-2 and is deliberately out of scope here.

Non-geostationary (NGSO) satellite systems - LEO/MEO broadband megaconstellations - share Ku- and Ka-band spectrum with the geostationary (GSO) networks that were there first. To protect the incumbents, the ITU Radio Regulations Article 22 caps the equivalent power flux-density (EPFD) an NGSO system may radiate toward GSO earth stations (epfd↓), toward GSO satellites (epfd↑), and inter-satellite (epfd_IS). Each limit is a mask: a set of not-to-be-exceeded pfd levels, one per percentage of time, per victim-antenna size, per band (Tables 22-1A…22-4B).

EPFD, per Recommendation ITU-R S.1503-2, is the aggregate downlink pfd summed over every transmitting NGSO station, each weighted by the victim earth-station antenna's gain toward that station relative to its peak gain: epfd = 10·log10( Σ 10^(Pi/10)·Gi(θ)/Gmax / (4π·di²) ). This tool evaluates the single entry - one satellite, worst-case overhead geometry - in dB: epfd = EIRP_density − 10·log10(4π·d²) + (G(θ) − Gmax). The three terms are the transmitter's EIRP density in the reference bandwidth, the free-space spreading over the slant range (taken as the altitude, i.e. the satellite directly overhead - the closest, hottest geometry), and the victim antenna's off-axis discrimination toward the interferer.

The discrimination term is where the physics bites. The GSO earth station is pointed at its wanted GSO satellite; the NGSO interferer sits some off-axis angle away, so it is seen through the earth station's side lobes. We model that with the ITU-R S.1428 reference earth-station pattern (the 29 − 25·log10(θ) side-lobe envelope for the D/λ ≥ 100 regime, flooring in the far lobes). This is exactly why the off-axis angle dominates the result - the epfd-vs-angle curve falls steeply as the interferer moves out of the main beam, which is the whole geometric basis of NGSO/GSO sharing. The peak gain uses the parabolic-aperture form G = η(πD/λ)².

What this tool deliberately is not. It is a single-entry screening estimator, not an aggregate ITU-R S.1503-2 validation. Real EPFD compliance is determined by the ITU's reference software (implementing S.1503): the entire constellation is propagated over time, the aggregate epfd from all simultaneously-visible transmitting satellites is computed at each instant, and the resulting statistical distribution is checked against the mask at every percentage-of-time point - a multi-CPU-hour Monte-Carlo run. A single satellite passing the screen does not mean the aggregate passes; a "compliant" verdict here is a design sanity-check, not a Radio Regulations determination. The aggregate hint (how many equal-strength satellites would sum to breach the limit) is a crude power-addition proxy, not the S.1503 aggregate. And the preset limits are representative values - a single scalar cannot capture a full mask - so verify every limit against the current RR Article 22 tables for your antenna size, band, and percentage, or enter your own. Over-claiming EPFD compliance is a regulatory liability; treat this only as a screen to run before committing to a full S.1503 study.