If geographical sharing is to be successful, the interfering transmitter and the interfered-with receiver must be separated by a distance at which interference is not considered harmful. Using the protection criteria developed in the ITU-R Handbook on Radio Astronomy (chapter 4), the attenuation over this distance must be sufficient to reduce the received signal below the appropriate level as given in Recommendation ITU-R RA.769, for all but 10% of the time.
Appendix 28 of the ITU-R Radio Regulations defines a basic transmission loss Lb(p) as:
Lb(p) = Pt + Gt + Gr - Pr(p) (1)
Lb(p): minimum permissible basic transmission loss (in dB) for p% of the time; this value must be exceeded by the actual transmission loss for all but p% of the time;
Pt: transmitting power level (in dBW) in the reference bandwidth at the input to the antenna;
Gt: gain (in dBi) of the transmitting antenna;
Gr: gain (in dBi) of the radio astronomy antenna in the direction of the transmitter;
Pr(p): maximum permissible interference power (in dBW) in the reference bandwidth to be exceeded for no more than p% of the time at the receiver input.
Using the protection criteria of the ITU-R Handbook on Radio Astronomy (chapter 4), Gr = 0 dB, and equation (1) assumes the form:
Lb(p) = Pt + Gt - Pr(p) (2)
where Pr is to be taken from Recommendation ITU-R RA.769. Lb(p) should be calculated using an appropriate model, such as in the comprehensive set given in Recommendations ITU-R P.452, P.526 and P.617, using p = 10% in the case of time-variable propagation loss.
For line-of-sight transmission the free-space transmission loss is not generally variable and the percentage of time criterion is not pertinent. In this case Lb has a simple analytical form and equation (2) may be written as:
20 log (4 π d) - 20 log (λ) = Pt + Gt - Pr(10) (3)
d: distance in meters between transmitter and receiver;
λ : wavelength in meters.
In the above analysis Pt is the power transmitted within the bandwidth Br of the radio astronomy receiver. If the transmitter power PT is distributed over a bandwidth Bt > Br (where Bt is the bandwidth of the transmitter) then:
Pt = PT - 10 log (Bt/Br) (4)
assuming that the transmitter power has a uniform spectral density (from: “ITU-R Handbook on Radio Astronomy“, 1995, section 5.2).