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 *L _{b}(p)* as:

(1)L=_{b}(p)P+_{t}G+_{t}G-_{r}P_{r}(p)

where:

* L_{b}(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;

* P_{t}*: transmitting power level (in dBW) in the reference bandwidth at the input to the antenna;

* G_{t}*: gain (in dBi) of the transmitting antenna;

* G_{r}*: gain (in dBi) of the radio astronomy antenna in the direction of the transmitter;

* P_{r}(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), *G _{r}* = 0 dB, and equation (1) assumes the form:

(2)L=_{b}(p)P+_{t}G-_{t}P_{r}(p)

where *P _{r}* is to be taken from Recommendation ITU-R RA.769.

*L*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

_{b}(p)*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 *L _{b}* has a simple analytical form and equation (2) may be written as:

20 log (4 π(3)d) - 20 log (λ) =P+_{t}G-_{t}P_{r}(10)

where

* d*: distance in meters between transmitter and receiver;

λ : wavelength in meters.

In the above analysis *P _{t}* is the power transmitted within the bandwidth

*B*of the radio astronomy receiver. If the transmitter power

_{r}*P*is distributed over a bandwidth

_{T}*B*>

_{t}*B*(where

_{r}*B*is the bandwidth of the transmitter) then:

_{t}(4)P=_{t}P- 10 log (_{T}B/_{t}B)_{r}

assuming that the transmitter power has a uniform spectral density (* from:* “ITU-R

*Handbook on Radio Astronomy*“, 1995, section 5.2).