ATMOSPHERIC STRATIFICATION AND INFRACTION 



20') 



misleading operators as to the overall performance 

 of the equipment. Long-range echoes caused by 

 superrefraction have frequently been assumed to 

 indicate good condition of the equipment, when 

 precisely the opposite is actually the case. The 

 phenomenon of superrefraction does not, however, 

 in the same degree invalidate the measurement of 

 signal-to-noise ratio of nearby echoes, as a criterion 

 of relative overall set performance. Field strengths 

 from nearby objects well within the optical horizon 

 are far less subject to propagation variations. Echo 

 strengths (signal-to-noise ratio) from nearby objects 

 are still considered a good relative index of overall 

 performance, provided that easily recognized echoes 

 can be measured which are not sensitive to very 

 small changes in the radar frequency. There are 

 other sources of echo fluctuations such as the motion 

 of objects (trees, towers) caused by the wind (import- 

 ant at wind speeds above 15 miles per hour). Great 

 care is needed in the choice of fixed echo "standards" 

 so that they are kept free of the effects enumerated. 

 Sometimes artificial echoing objects are constructed 

 of flat mesh screens perpendicular to the beam in 

 order to secure suitable echoes which are not 

 frequency sensitive. The extreme variability of long- 

 range fixed echoes emphasizes the operational need 

 for reliable test equipment for making quantitative 

 tests on the components as well as on the overall 

 performance of the equipment aspects of radars, as 

 distinct from propagation effects. 



In addition to the direct electrical checks on set 

 performance there are a number of ways of making 

 sure indirectly whether any failures of detection by 

 radar may be due to a deformation of the coverage 

 pattern by superrefraction. In the first place, super- 

 refraction rarely affects detection at angles of eleva- 

 tion above about 1.5°. Any irregularity at higher 

 angles must be attributed to other causes. Even 

 between 0.5° and 1.5° failures of detection are excep- 

 tional and occur only where there are very strong 

 ducts. A clue to the probability of occurrence of such 

 conditions can be ascertained from a study of the 

 primary meteorological effects which cause them; 

 and even with only a moderate amount of meteoro- 

 logical information it is usually possible to make an 

 estimate of this probability. Such superrefractive 

 conditions almost invariably show up in intensified 

 and extended ground echoes (ground clutter on the 

 scopes) and, in case of an overwater path, in extended 

 ranges of ship detection. A record of meteorological 

 data will be very helpful in deciding, after the fact, 



whether any specific failure of aircraft detection 

 might have been ascribed to weather. Even if this 

 is probable, there are, of course, a number of other 

 operational causes that might be responsible rather 

 than the weather. 



Experience gained in England indicates that the 

 technique of forecasting whether or not superrefrac- 

 tion occurs is, on the whole, fairly successful, but 

 there are still many occasions when the predictions 

 are not fulfilled. It has been intimated that in 

 England this was due, at least partly, to variations 

 in the sensitivity of the 10-cm set used; when the 

 set is not at peak efficiency, maximum ranges of 

 surface targets appear shortened, and the coverage 

 in the duct may be reduced to a value corresponding 

 to standard conditions. 



A major problem in any early warning radar 

 system is that of heightfinding by means of maximum 

 ranges. On this it is difficult to make general state- 

 ments. The method of heightfinding usually employed 

 in long-range radar work consists in using the boun- 

 dary of the lowest lobe as a height indicator, assum- 

 ing that when the target is first sighted it has just 

 entered the lowest lobe. When superrefraction is 

 present, the height estimated in this way can be 

 seriously in error. It may be too high if the enemy 

 is flying in the duct, so that he is discovered earlier 

 than he would be normally; or it may be too low if 

 the enemy is flying in the region above the duct and 

 so he is discovered later than he would be under 

 standard atmospheric conditions. Here, again, it 

 should be possible to find out whether repeated 

 errors in height determination are the result of 

 superrefraction or whether they are due to faulty 

 calibration or to other features not related to the 

 weather. Other methods of heightfinding, such as 

 are used in fighter control and control of antiaircraft 

 fire, are usually carried out at angles of elevation too 

 large to be affected by nonstandard types of atmos- 

 phere. 



VHF Communications and Navigational Aids 



The extension of the maximum range of very high 

 frequency [VHF] navigational aids has already been 

 mentioned as an important consequence of super- 

 refraction. Similar extensions of communication 

 ranges of VHF radio sets also occur. Because VHF 

 air-to-ground communications are relied upon only 

 for comparatively short-range communications, thie 



