the signal level due to rain, multipath fading, k-fading, etc. The clearance should be checked for various fade margins - k = 1.33, 0.6, 0.5, etc. The first Fresnel zone should be free of obstacles for k = 1.33. For hops respectively under or over 15 km, the clearance should be zero for k-factors of 0.3 and 0.5. Over-water hops should be avoided. If they are unavoidable, the antenna heights should be chosen such that the reflection point is not falling over water. Another method to avoid the reflection point falling over water is to place the antenna such that the reflected ray is blocked (e.g. place the antenna on the far side of the terrace SO that the reflected ray is blocked by the roof of the building itself). Choose higher antennas if the probability of k-fading is high in the region. In regions where the ducting phenomenon is high, choose higher antennas, as it is easier to move an antenna down rather than up on the tower (especially if the antenna is placed near the top of the tower). 3.3.9 Error Performance and Availability Once the link has been designed, its performance is measured against error performance and availability targets. The error performance objective defines the parameters to be measured against certain standard values, while the link availability is characterised on the basis of the error performance parameters. These objectives are defined in ITU recommendations ITU-R G.826. DETAILED TRANSMISSION NETWORK PLANNING Example 3: Error performance calculations for radio relay system Calculated hop from Site A Site B Radio frequency 8.1 GHz vertical Hop length 35.0 km Latitude Longitude Percentage pL 50,0 % Geoclimatic factor 1.41E-003 Rainrate (0.01%) 120.0 mm/h Station heights (ref. level) 200.0 m 200.0 m Antenna heights (above ref. level) 50.0 m 50.0 m Feeder lengths 60.0 m 60.0 m Feeder loss/100 m 6.1 dB 6.1 dB Feeder type EW 77 EW 77 Antenna diameters 3.7 m 3.7 m Transmitter output power (Pix) 27.0 dBm Free space loss (Lo) 141.6 dB Additional terrain loss (Lad) 0.0 dB Antenna branching lo