Coordination Distance
How ITU-R Appendix 7 Bounds the Coordination Region
When a new earth station shares a frequency band with terrestrial fixed or mobile links, regulators do not require every distant station on Earth to be checked for interference. Instead, ITU-R Appendix 7 defines a coordination distance that encloses only the geographic region where a real interference risk exists. Any terrestrial station outside that distance, in a given azimuth direction, is assumed to be safely decoupled by propagation loss; any station inside it must be brought into a detailed coordination process. The locus of these distances around all azimuths forms the coordination contour, a closed curve plotted on a map and submitted to the relevant administration.
Two physically distinct propagation modes are computed independently. Mode 1 captures great-circle mechanisms along the direct bearing between the two stations: tropospheric scatter, elevated-layer reflection, ducting, and diffraction. Mode 1 distance is strongly azimuth-dependent because it follows the earth-station antenna gain pattern and the great-circle path. Mode 2 captures hydrometeor scatter, where rain in a common volume re-radiates energy from one station toward another even when neither antenna points at the other. Because rain scatter is nearly isotropic in azimuth, mode 2 produces a roughly circular contribution. The coordination distance reported for each bearing is the maximum of the mode 1 and mode 2 results, so the final contour is the outer envelope of an azimuth-varying lobe and a near-circle.
The method iterates on path loss. For a chosen required loss between interferer and victim, the propagation model is solved for the distance at which that loss is reached for a small percentage of time (commonly 0.01 to 20 percent, depending on the service). Because anomalous propagation grows weaker with frequency while rain scatter grows stronger, mode 1 distances contract and mode 2 distances expand as the band moves from C-band toward Ka-band, which is why the dominant mode changes across the spectrum.
Required Loss and the Distance Solution
Lmin = Pt + Gt + Gr − Pr(permissible) dB
Great-circle (mode 1) trial loss vs. distance:
L(d) ≈ Lbfs(d) + Agas(d) + Aanom(d, p%)
Lbfs = 92.45 + 20·log10(fGHz) + 20·log10(dkm)
Coordination distance per azimuth φ:
dc(φ) = max{ dmode1(φ), dmode2 }, with dmin ≤ dc ≤ dmax
Where Pt = interferer transmit power, Gt = interferer antenna gain toward the path (so Pt + Gt is its EIRP toward the victim), Gr = victim antenna gain toward the path, Pr(permissible) = permissible interference power, Agas = gaseous absorption, Aanom = anomalous-propagation loss exceeded for p% of time, and d is increased until L(d) ≥ Lmin. Appendix 7 sets dmin ≈ 100 km and dmax ≈ 1200 km for mode 1.
Coordination Distance by Band and Mode
| Band | Frequency | Typical Mode 1 | Typical Mode 2 | Dominant Mode | Notes |
|---|---|---|---|---|---|
| C-band uplink | 5.925 to 6.425 GHz | 250 to 400 km | 300 to 360 km | Either | Heavy FS sharing; both modes near max |
| X-band | 7 to 8 GHz | 180 to 320 km | 280 to 340 km | Mode 2 | Government and FSS links |
| Ku-band uplink | 14 to 14.5 GHz | 100 to 200 km | 250 to 320 km | Mode 2 | Rain scatter dominates |
| Ka-band uplink | 27.5 to 30 GHz | 60 to 130 km | 180 to 260 km | Mode 2 | High gaseous absorption shrinks mode 1 |
| Predetermined (fallback) | Any shared band | 1200 km | Band-specific max | Worst case | Used when no detailed calculation is run |
Frequently Asked Questions
What is the difference between mode 1 and mode 2 coordination distance?
Mode 1 is great-circle propagation (ducting, layer reflection, tropospheric scatter) along the direct bearing, so it is azimuth-dependent and runs 100 to 500 km in the 4 to 6 GHz bands. Mode 2 is hydrometeor (rain) scatter from a common rain cell, nearly independent of antenna pointing, so it is computed in all directions and often dominates above about 10 GHz. The coordination distance at each azimuth is the larger of the two, and the contour is their outer envelope.
How is the predetermined coordination distance used when a full calculation is not performed?
Appendix 7 lists conservative fixed values, such as 1200 km for mode 1, that bound the worst-case contour. An administration may draw a simple circle of that radius around the earth station instead of computing an azimuth-dependent contour. This guarantees no station outside is missed, but it sweeps in many stations a detailed analysis would exclude, so most operators run the full calculation to shrink the contour and reduce affected assignments.
Why does coordination distance shrink at higher frequencies?
Free-space loss rises with 20·log10(f), gaseous absorption from oxygen and water vapor adds long-path attenuation, and ducting becomes less efficient. The mode 1 distance therefore contracts from several hundred km near 4 to 6 GHz to under 100 km in many Ka-band cases. Rain scatter efficiency, by contrast, increases with frequency, so above roughly 10 GHz the mode 2 distance exceeds mode 1 and sets the contour.