Spectrum / regulatory
EPFD Screening — NGSO vs. GSO
A first-order screen for the equivalent power flux-density (EPFD) a non-geostationary constellation throws at geostationary networks — the single-entry epfd↓ from one satellite versus the ITU Radio Regulations Article 22 mask, so you can sanity-check a design before a full study.
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.
How this screen works, and what it deliberately is not
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.
// screening only — not an S.1503-2 validation
This estimates 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 mask value. It is a first-order screen to run before a full study — not an aggregate ITU-R S.1503-2 EPFD compliance validation over the whole constellation and all percentages of time. A "compliant" result here does not establish Radio Regulations compliance. Verify every limit against the current RR Article 22 tables.
// pick a scenario, then dial the EIRP density and off-axis geometry.
NGSO transmitter
// downlink band, EIRP density, and altitude.
GSO victim earth station
// antenna size + off-axis geometry set the discrimination.
Applicable limit
// pick a representative preset or enter your own.
Marginal
// single-entry epfd vs. Ku-band · 1 m ES · 40 kHz (representative) — verify vs. current RR Article 22
-161.7
single-entry epfd dB(W/m²)
-160.0
applicable limit dB(W/m²)
+1.7 dB
margin (limit − epfd)
-24.2 dB
off-axis discrimination
132.6 dB
spreading 10log(4πd²)
39.6 dBi
ES boresight gain
~2
equal sats to breach (crude)
// epfd vs. off-axis angle (screening sensitivity), with the applicable limit
// aggregate hint: ~2 equal-strength NGSO satellites, seen at once, would sum (power-wise) to breach this limit. This is a crude single-entry → aggregate proxy, NOT the S.1503 aggregate epfd.
// screening only
Single-entry worst-case geometry (satellite overhead, victim antenna pointed off it by the given angle), an ITU-R S.1428 earth-station reference pattern, and a representative Article-22 mask value. For a compliance determination, run the full aggregate ITU-R S.1503-2 analysis over the propagated constellation and every percentage-of-time point, and read the exact limit from the current RR Article 22 tables.
// shareable URL encodes every input. no backend.
// ai-generated breakdown of what these numbers mean — with diagrams.
Common questions
Why do EPFD limits exist?
Because non-geostationary (NGSO) broadband constellations share Ku- and Ka-band spectrum with the geostationary (GSO) networks that were licensed first. Article 22 of the ITU Radio Regulations protects the incumbents by capping the equivalent power flux-density an NGSO system may produce toward GSO earth stations (epfd↓), toward GSO satellites (epfd↑), and between satellites — each as a mask of not-to-be-exceeded levels per percentage of time, victim-antenna size, and band. Every NGSO filing has to demonstrate it stays under them.
What is the difference between single-entry and aggregate EPFD?
Single-entry is one satellite at its worst-case geometry — what this tool computes. Aggregate is the sum over every simultaneously visible transmitting satellite, evaluated statistically across the whole propagated constellation (the ITU-R S.1503-2 reference-software run). The distinction bites hard: the compliant preset carries 17.7 dB of single-entry margin, yet the tool's aggregate hint estimates ~59 equal-strength satellites would erase it — a megaconstellation spends single-entry margin very quickly, which is why a passing screen here never substitutes for the full study.
Why does the off-axis angle dominate the result?
Because the victim antenna's side-lobe gain falls as 29 − 25·log₁₀(θ) (the ITU-R S.1428 envelope): in the presets' geometry the earth station sees an interferer 1.5° off boresight at ~24.6 dBi but one at 8° at only ~6.4 dBi — an 18 dB swing from geometry alone. Keeping transmissions angularly separated from the GSO arc is the entire geometric basis of NGSO/GSO sharing — that discrimination, together with a lower EIRP density, is how the presets swing from 12.5 dB over the limit to 17.7 dB clear.
What does "EIRP density in the reference bandwidth" mean?
Article 22 masks are written against a reference bandwidth, not a total carrier power — the Ku-band presets use dBW per 40 kHz, the Ka-band preset dBW per 1 MHz. So the input is the transmitter's EIRP spectral density in that same bandwidth: a wideband carrier's total EIRP spread across its occupied bandwidth, plus the antenna gain toward the victim. Getting the bandwidth normalisation wrong is the most common way to mis-screen by tens of decibels.
Can this tool tell me if my constellation is Article 22 compliant?
No — and it says so deliberately. It is a screening estimator: one satellite, a representative scalar limit standing in for a full mask, and a crude power-addition aggregate hint. Formal compliance is established only by the aggregate ITU-R S.1503-2 analysis at every percentage-of-time point, against the current Radio Regulations tables for your exact band and victim-antenna size. Use the screen to kill clearly-failing designs early and to see which lever (power density, altitude, off-axis discipline) moves the margin — then commission the full study for anything that survives.
References
- // ITU Radio Regulations, Article 22 — epfd↓ / epfd↑ / epfd_IS single-entry and aggregate limits (Tables 22-1A…22-4B). Verify current values here.
- // Resolution 76 (Rev.WRC-15) — additional operational epfd limits and the S.1503 validation obligation.
- // Recommendation ITU-R S.1503-2 — functional description of the software determining conformity of NGSO FSS systems with the Article-22 limits (the aggregate EPFD definition).
- // Recommendation ITU-R S.1428 — reference FSS earth-station antenna radiation pattern for interference assessment (1–70 GHz).
- // Recommendation ITU-R S.465 — the classic 32 − 25·log10(θ) side-lobe envelope (cross-check).
- // Maral, G. & Bousquet, M. Satellite Communications Systems (Wiley) — antenna gain, pfd, and interference geometry.
