152 TECHNICAL SURVEY 
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 4tmos- 
phere. 
VHF ComMuNICcATIONS 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, this 
extension of the normal range by atmospheric 
conditions is important primarily from a security 
standpoint. It must always be borne in mind that 
transmissions on VHF may frequently be propagated 
‘hundreds of miles beyond the normal limiting range 
and are subject to enemy interception. Superrefrac- 
ition has also been observed to cause very objection- 
able mutual interference between two control towers 
attempting to use a common VHF channel, although 
the distance between the airports was great enough 
to prevent serious mutual interference under normal 
conditions. Point-to-point VHF radio links are also 
affected by refraction, over longer paths than optical. 
Rapio COUNTERMEASURES 
The laws of radio propagation enter into the 
problem of jamming the enemy communication and. 
radar equipment. Since it is rarely possible to locate 
the jamming transmitter coincident with the enemy 
transmitter whose signals it is desired to mask, the 
efficiency of propagation of the signals from the 
enemy transmitter relative to those of the friendly 
transmitter enters into the problem. This has been 
worked out in detail for the standard atmosphere. 
When conditions are not standard, however, the 
effectiveness of the enemy transmitter, as determined 
for standard conditions, no longer applies. A case of 
special interest occurs when an airborne jamming 
transmitter is used as a countermeasure against an 
enemy radio communication link operating between 
two points on the ground. If the meteorological 
situation is such as to be favorable to formation of 
a ground-based duct the enemy signals may be 
propagated with small attenuation, whereas the 
signals from the jamming transmitter may be unaf- 
fected or even weaker than would normally be 
expected. 
Plans for the employment of ground-based jammers 
against enemy radio and radar systems should take 
into consideration the ability of atmospheric refrac- 
tion to increase, or occasionally to decrease, the 
signal propagated to the enemy’s installation for 
jamming purposes. However, there has been only 
limited use of ground-based jamming so far. Unin- 
tentional mutual jamming has occurred between the 
spaced radar sets of a coastal system on the same 
frequency, where nonstandard propagation condi- 
tions caused strong signals to be propagated between 
normally noninterfering radars. 
RADIO METEOROLOGY 
Temperature and Moisture Gradients 
This section is devoted to a survey of the meteoro- 
logical conditions which produce the various types 
of propagation described in the preceding sections. 
This brief outline is not intended to replace the 
assistance of a professional meteorologist in analyzing 
short and microwave propagation problems; but by 
familiarizing radar or communications personnel with 
the fundamental physical processes of low-level 
weather it may open the way toward a more fruitful 
consultation with the meteorologist. 
Duct formation is the most important phenomenon 
for which a detailed knowledge of the physical state 
of the lower atmosphere is required. Whenever a 
duct is formed, M decreases with height within a 
certain height interval. Since, according to text on 
pages 139- 143 ,M = (nm — 1) - 10® 4+ 0.157h, the 
