Coordination Arc
How the Coordination Arc Bounds Satellite Interference
In a 2°-spaced geostationary environment, dozens of satellite networks share the same C, Ku, or Ka band frequencies within a narrow band of orbital longitudes. A new earth station, especially a small-aperture VSAT, transmits a main beam toward its own satellite but also radiates sidelobe energy toward neighboring orbital slots. The coordination arc defines the slice of orbit, centered on the wanted satellite, in which those neighbors are close enough that the sidelobe energy could rise above an agreed interference threshold. Networks inside the arc are flagged for coordination; networks outside it are presumed compatible without further study, which dramatically reduces the regulatory workload from a global all-to-all check to a handful of nearby slots.
The conventional half-width is plus or minus 5° of geocentric longitude, codified in ITU Radio Regulations Appendix 5 and adopted by the FCC for routine processing in the conventional bands. This value is not arbitrary: it follows directly from the standardized earth station antenna sidelobe envelope. With reference gain G(φ) = 29 − 25·log(φ) dBi, the off-axis gain toward a network 5° away in topocentric angle is roughly 11.5 dBi, low enough that interference contributions from more distant slots become negligible. Larger arcs of 6° to 8° appear in some bands and for very small antennas whose wider beamwidth keeps off-axis gain high over a broader angular range.
It is important to distinguish the orbital geometry from the surface geometry. The coordination arc is measured in degrees along the geostationary orbit and governs satellite-to-satellite and earth-station-to-space-station interference paths. The separate coordination distance and contour analysis handle interference between a transmitting earth station and terrestrial fixed service stations sharing the same band on the ground.
Geometry: Orbital Longitude Versus Topocentric Angle
The angular separation an earth station antenna actually sees between two satellites, the topocentric angle, is larger than the difference in their orbital longitudes because the station sits closer to the arc than Earth's center does. The two are related through a geocentric-to-topocentric magnification factor that depends on the slant range, and therefore on the elevation angle to the satellites. Magnification is greatest when the satellites are near the station's zenith, where the slant range is shortest at about 35,800 km: there a 2° longitude spacing subtends roughly 2.4° as seen from the ground (a factor near the geometric maximum of 1.18). As the elevation angle drops toward the horizon, as it does for high-latitude stations or satellites far off in longitude, the slant range lengthens toward 41,700 km and the magnification falls back toward 1.0. The larger topocentric angle from a near-overhead satellite slightly eases coordination, since it pushes the same orbital neighbor further into the antenna sidelobe roll-off.
Off-Axis EIRP and the Coordination Trigger
G(φ) ≈ 29 − 25·log10(φ) dBi
Off-axis EIRP density toward an adjacent slot:
EIRPoff(φ) = Pt + G(φ) − 10·log10(BW) dBW/Hz
Coordination triggered when:
EIRPoff(φ) > EIRPmask(φ) for any network with |Δlong| ≤ arc half-width
Where φ = topocentric off-axis angle (°), Pt = transmit power (dBW), BW = emission bandwidth (Hz), and Δlong = orbital longitude separation. Example: a 1.2 m Ku-band VSAT at φ = 2° sees G ≈ 21.5 dBi; at φ = 5°, G ≈ 11.5 dBi, a 10 dB drop that defines the ±5° arc edge.
Coordination Arc Conventions by Regime
| Framework / Band | Arc half-width | Trigger basis | Orbital spacing | Typical antennas |
|---|---|---|---|---|
| ITU RR App. 5 (C/Ku) | ±5° longitude | ΔT/T & off-axis EIRP | 2° nominal | 1.2 to 9 m |
| FCC Part 25 (2° spacing) | ±5° longitude | Routine EIRP mask | 2° | VSAT, gateway |
| Ka-band (high-gain) | ±4° to ±5° | Narrow beam, PFD | 2° to 3° | 0.75 to 2.4 m |
| Small-aperture / wide beam | ±6° to ±8° | Higher off-axis gain | 2° | < 1.2 m |
| Planned bands (App. 30/30A) | Allotment-based | EPM / overall C/I | Pre-assigned | Home reception |
Frequently Asked Questions
How wide is the coordination arc and where does the plus or minus 5 degree value come from?
The standard is ±5° of geocentric longitude on each side of the target slot, set in ITU RR Appendix 5 and used in FCC Part 25 two-degree-spacing coordination for conventional C and Ku bands. The 5° figure reflects the standardized earth station sidelobe envelope: with G(φ) = 29 − 25·log(φ), interference from networks beyond about 5° falls below the trigger. Some bands and small-antenna cases use a 6° to 8° arc, so the exact value is band and service specific.
What is the difference between a coordination arc and a coordination contour?
The arc is an angular region along the geostationary orbit, measured in degrees of longitude, that identifies which satellite networks could interfere through earth station sidelobes pointed near the GSO. The contour is a closed geographic curve on the ground around a transmitting earth station, inside which terrestrial fixed service stations sharing the band must be coordinated. The arc governs paths along the orbit; the contour governs earth-station-to-terrestrial sharing on the surface.
How does earth station antenna off-axis gain set the coordination arc trigger?
Coordination fires when off-axis EIRP toward an adjacent network exceeds a regulatory mask. The reference envelope G(φ) = 29 − 25·log(φ) dBi applies for 1° ≤ φ ≤ 7°, giving about 21.5 dBi at 2° and 11.5 dBi at 5°. Because sidelobe gain drops roughly 25·log per decade of angle, networks beyond about 5° orbital separation usually clear the threshold, which is exactly why 5° became the default arc half-width.