Coordination Threshold
How the ΔT/T Trigger Governs Satellite Coordination
The coordination threshold exists because the geostationary arc and the shared satellite frequency bands are finite resources used by hundreds of networks operated by different administrations. Rather than evaluate the full, computationally heavy interference picture for every new filing, the ITU framework uses a simple screening metric to decide whether two networks are close enough, in orbit and in frequency, to interfere meaningfully. The metric of choice for most geostationary fixed-satellite-service bands is the apparent increase in equivalent satellite-link noise temperature, ΔT, that an interfering network produces in a wanted network, divided by the wanted network's own equivalent noise temperature, T. When ΔT/T exceeds 6%, the two administrations are deemed affected and coordination under Article 9 becomes mandatory.
The 6% value is not arbitrary. A 6% rise in total noise temperature degrades the wanted carrier's available carrier-to-noise ratio by 10·log10(1.06), or about 0.25 dB. That degradation is small enough to be an acceptable single-entry interference allotment yet large enough to be worth catching. The calculation depends on the off-axis discrimination of both the interfering earth-station transmit antenna and the wanted satellite receive antenna along the geometry between the two orbital slots, so antenna pattern compliance with the 29 − 25·log(θ) dBi reference envelope directly determines whether a network trips the threshold.
Crossing the threshold does not mean interference is unacceptable. It is a screening trigger, not a hard limit. Networks that exceed it routinely coexist after the administrations exchange technical data and agree on measures such as orbital separation, polarization isolation, or EIRP-density caps. Hard limits, such as the equivalent power flux-density (epfd) masks that protect geostationary systems from non-geostationary constellations under Article 22, are enforced separately and must be met regardless of any coordination agreement.
The ΔT/T and PFD Equations
ΔT / T > 6% → coordination required
Equivalent noise-temperature rise:
ΔT = (pi × ge(θ) × gs(φ)) / (k × L)
Carrier degradation at the trigger:
Δ(C/N) = 10·log10(1 + ΔT/T) ≈ 10·log10(1.06) ≈ 0.25 dB
PFD protection limit (Article 21 example):
pfd ≤ −152 dB(W·m−2) in any 4 kHz for δ ≤ 5°
Where pi = interfering carrier power density, ge(θ) = interfering earth-station off-axis gain, gs(φ) = wanted satellite receive off-axis gain, k = Boltzmann's constant (1.38 × 10−23 J/K), L = path loss, T = wanted-link equivalent noise temperature, δ = elevation angle.
Coordination Trigger Methods Compared
| Trigger method | Typical value | Service / band | What it screens | ITU reference |
|---|---|---|---|---|
| ΔT/T noise rise | 6% (≈ 0.25 dB) | GSO FSS, 4 / 6 / 11 / 14 GHz | Aggregate noise-temp increase | Appendix 8 |
| Coordination arc | ±7° to ±16° (band-specific) | GSO FSS / BSS feeder links | Adjacent-satellite geometry | No. 9.7, Appendix 5 |
| PFD mask (downlink) | −152 to −142 dB(W·m−2)/4 kHz | Space-to-Earth shared bands (e.g. 3.4–7.75 GHz) | Terrestrial-service protection | Article 21, Table 21-4 |
| epfd hard limit | Band / percentage specific | NGSO into GSO | Constellation aggregate | Article 22 |
| C/I ratio target | ≥ 20 to 30 dB | Bilateral negotiated | Post-coordination carrier quality | Coordination agreement |
Frequently Asked Questions
How is the delta-T/T coordination threshold of 6% calculated?
ΔT/T expresses the rise in a wanted network's equivalent satellite-link noise temperature caused by an interfering network, divided by the wanted link's own noise temperature, as a percentage. The interfering carrier power is referred to the wanted satellite receiver input through the off-axis gain of both the interfering earth-station antenna and the wanted satellite antenna, then divided by k and the receive bandwidth to get ΔT. If ΔT/T exceeds 0.06, coordination is required. The 6% value maps to about 0.25 dB of wanted carrier-to-noise degradation, an acceptable single-entry interference allotment.
What happens when two satellite networks exceed the coordination threshold?
The two administrations are formally identified as affected and must coordinate under ITU Radio Regulations Article 9. They exchange parameters such as EIRP density, antenna patterns, orbital position, and frequency plans, then negotiate measures to keep mutual interference acceptable. Common remedies include 2 to 3 degrees of orbital separation, 20 to 30 dB of polarization isolation, frequency offset or band segmentation, EIRP-density reduction, and antenna off-axis discrimination meeting the 29 − 25·log(θ) dBi envelope. Coordination must finish before either network gains protected status.
What is the difference between the coordination threshold and a hard interference limit?
The coordination threshold is a screening trigger, not a maximum permissible level. Exceeding it means the administrations are obligated to talk and reach agreement, not that interference is unacceptable. A hard limit, such as an aggregate PFD mask or an Article 22 epfd limit for non-geostationary systems, defines a level that must never be exceeded regardless of coordination. The threshold is deliberately conservative so potential interference is caught early, while the negotiated agreement and any regulatory masks set the actual operating constraints in service.