Link Engineering

Coordination Contour

Q: What is the difference between a coordination contour and a coordination distance?

A coordination distance is a single number: the maximum great-circle range from an earth station beyond which interference is negligible. A coordination contour is the full closed curve traced by the coordination distance evaluated at every azimuth from 0 to 360 degrees. Because terrain, antenna gain off the main beam, and propagation conditions vary with direction, the distance is rarely constant, so the contour is typically an irregular, lobed shape rather than a circle. The contour is the operational quantity overlaid on a map to identify which existing or planned stations fall inside it and therefore require detailed coordination.

Q: Which ITU-R propagation modes are combined to build a coordination contour?

ITU-R Appendix 7 and Recommendation SM.1448 combine two propagation modes. Mode 1 is great-circle propagation including diffraction, ducting, layer reflection, and tropospheric scatter, evaluated for low time percentages of 0.001 to 1 percent. Mode 2 is hydrometeor (rain) scatter, where a common volume of rain illuminated by both stations couples energy along a non-great-circle path. The contour for each azimuth uses the larger of the two resulting distances, so a final coordination contour often shows a smooth great-circle lobe plus a separate, roughly circular rain-scatter contour centered near the earth station.

Q: How does the required loss term determine the coordination distance at each azimuth?

The minimum required loss is the permissible transmission loss between the interfering transmitter and the victim receiver, computed as the transmitter EIRP toward the horizon plus the victim antenna gain toward the horizon minus the receiver interference threshold. That threshold is set by a fractional-degradation criterion, commonly an interference-to-noise ratio of minus 6 to minus 10 dB exceeded for no more than 0.005 to 0.01 percent of the worst month. The propagation model is then inverted: the coordination distance for that azimuth is the range at which predicted path loss equals the required loss. Higher required loss pushes the distance outward, enlarging the contour.

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Drawn around an earth station, this closed curve marks the boundary inside which a potential for harmful interference exists, so any terrestrial or other earth station falling within it triggers detailed frequency coordination. The contour is generated under ITU-R Appendix 7 by computing a coordination distance at every azimuth, taking the worse of great-circle (Mode 1) and rain-scatter (Mode 2) propagation, then comparing predicted path loss against a required loss set by the victim receiver interference threshold. Because antenna horizon gain, terrain, and ducting vary with direction, the result is an irregular lobed outline rather than a circle, and it forms the first screening step of any satellite or fixed-link interference analysis.

Category: Link Engineering

Governing rule: ITU-R Appendix 7 / SM.1448

Time criterion: 0.005 to 0.01% worst month

How the Coordination Contour Bounds Interference Risk

Frequency coordination begins with a simple question: which other stations are close enough to matter? The coordination contour answers it geometrically. Around the earth station of interest, an administration computes, for each compass azimuth, the distance at which an interfering signal would just fall to a tolerable level. Joining those distances produces a closed boundary. Stations and service areas outside the boundary are presumed to cause and receive negligible interference and need no further study; everything inside must be examined case by case. This screening role is why national regulators and the ITU treat the contour as the entry point of the coordination process rather than a final result.

The distance at any azimuth is not arbitrary. It is the range at which the predicted propagation loss equals the minimum required loss between the two stations. The required loss is the transmitter equivalent isotropically radiated power directed toward the horizon, plus the victim antenna gain toward the horizon, minus the receiver interference threshold. The threshold itself comes from a fractional-degradation criterion, usually expressed as an interference-to-noise ratio that may be exceeded for only a tiny percentage of the worst month. Tightening that percentage, or pointing the earth station antenna near the horizon at low elevation angles, raises the required loss and pushes the contour outward.

Two propagation mechanisms shape the outline. Mode 1 covers great-circle paths including diffraction, tropospheric ducting, layer reflection, and scatter, and it produces the directional lobes that follow terrain and antenna pointing. Mode 2 covers hydrometeor scatter, where a common volume of rain visible to both stations couples energy along a bent path, producing a roughly circular contour centered near the earth station regardless of azimuth. The final coordination contour is the envelope of both, so a typical map shows a great-circle lobe extending tens to hundreds of kilometers superimposed on a more compact rain-scatter ring.

Computing the Coordination Distance per Azimuth

Minimum required loss (worst case):
L_b(p) = P_t + G_t + G_r − P_r(p) dB

Receiver interference threshold:
P_r(p) = 10·log₁₀(k × T_e × B) + (I/N) dBW

Mode 1 great-circle distance (inverted path loss):
d₁(φ) such that L_predicted(d, p) ≈ L_b(p)

Final contour per azimuth:
d(φ) = max{ d₁(φ), d₂ }, bounded 100 to 1200 km

Where P_t = interfering transmitter power into the reference bandwidth (dBW), G_t, G_r = transmit and victim antenna gains toward the horizon (so P_t + G_t is the horizon EIRP), k = 1.38 × 10⁻²³ J/K, T_e = system noise temperature, B = reference bandwidth, I/N ≈ −6 to −10 dB, p = worst-month time percentage (0.005 to 0.01%), d₂ = rain-scatter (Mode 2) radius.

Coordination Contour Parameters by Band and Service

Scenario	Band	I/N criterion	Worst-month %	Typical Mode 1 distance	Mode 2 radius
FSS earth station (uplink)	6 GHz (C-band)	−10 dB	0.01%	300 to 600 km	~350 km
FSS earth station (uplink)	14 GHz (Ku-band)	−10 dB	0.01%	200 to 500 km	~150 km
Gateway, high availability	28 GHz (Ka-band)	−6 dB	0.005%	150 to 400 km	~50 km
Receive-only TT&C	2.2 GHz (S-band)	−6 dB	0.01%	400 to 900 km	n/a
Low-elevation tracking	8 GHz (X-band)	−10 dB	0.005%	up to 1200 km	~250 km

Common Questions

Frequently Asked Questions

What is the difference between a coordination contour and a coordination distance?

A coordination distance is a single value: the maximum range from an earth station beyond which interference is negligible. A coordination contour is the closed curve traced when that distance is evaluated at every azimuth from 0 to 360 degrees. Since terrain, off-horizon antenna gain, and propagation vary by direction, the distance is rarely constant, so the contour is an irregular lobed shape rather than a circle. The contour is what gets overlaid on a map to find which stations fall inside and need coordination.

Which ITU-R propagation modes are combined to build a coordination contour?

ITU-R Appendix 7 and Recommendation SM.1448 use two modes. Mode 1 is great-circle propagation (diffraction, ducting, layer reflection, troposcatter) evaluated for low time percentages of 0.001 to 1%. Mode 2 is rain (hydrometeor) scatter, where a common rain volume couples both stations along a non-great-circle path. Each azimuth uses the larger resulting distance, so the final contour often shows a great-circle lobe plus a separate, roughly circular rain-scatter ring near the station.

How does the required loss term determine the coordination distance at each azimuth?

The minimum required loss equals horizon EIRP plus victim antenna horizon gain minus the receiver interference threshold. That threshold follows a fractional-degradation criterion, commonly an I/N of −6 to −10 dB exceeded for no more than 0.005 to 0.01% of the worst month. The propagation model is inverted: the coordination distance for an azimuth is the range where predicted path loss equals the required loss. Higher required loss pushes the distance outward and enlarges the contour.

Related Terms

← Coordination Arc Coordination Distance →