Propagation model Based on free space propagation (Re. Recommendation ITU-R P.525) Propagation model does not accommodate certain phenomena which are
{"Image DocumentName": "Handbook Volume II Frequency assignment planning", "Image ImageDescription": "Chapter 1 (2) General methodology. Propagation model. Based on free space propagation. (Re. Recommendation ITU-R P.525) Propagation model does not accommodate certain phenomena which are difficult to predict such as. Changes in the refractive index of the atmosphere. Ducting. ITU has developed propagation curves for aeronautical communication and navigation systems (Recommendation ITU-R P.528) Another essential element of the frequency assignment planning criteria is the radio-frequency propagation model that is to be applied. In aviation we normally use the free space propagation model as specified in Recommendation ITU-R P.525. Propagation of radio waves is affected by the refractive index of the atmosphere and allows VHF frequencies to be received beyond the optical horizon. Radio line of sight normally employs a 4/3 Earth radius to calculate the distance (path) over which the propagation of the (VHF) radio signals can be considered with the free space propagation formulas. This complies with the so-called standard atmosphere . In aeronautical frequency assignment planning, variations of the refractive index are not considered. Similarly, temporary propagation phenomena such as ducting is not accommodated in frequency assignment planning. The ITU has developed a set of aeronautical propagation curves for 125 MHz, 300 MHz, 1200 MHz and 5100 MHz. These may be used for a more precise estimation of the propagation of aeronautical radio frequencies. These curves are based on the IF-77 model and were developed in the late ninteenseventies. They basically apply to temperate regions, that is, between latitudes of about 25 \u2013 55 degrees, north or south.", "Image XResolution": "37", "Image YResolution": "37", "Image ResolutionUnit": "Pixels/Centimeter", "Image Software": "SlidePlayer", "Image YCbCrPositioning": "Centered"}