72 ; TECHNICAL SURVEY 
the operation of friendly fighters against enemy 
aircraft. It has a range of about 50 miles and is 
capable of handling a large volume of traffic. In 
addition to the grid position and identification of 
the target it also determines the height. Surrounding 
the defense area is a region whose width depends on 
the time required to make an interception on an 
incoming enemy plane. The siting objective of the 
GCI stations is the continuous and effective cover- 
age of the interception region. Close coordination is 
maintained between early warning and fighter sta- 
tions, and the coverage deficiencies of one station are 
counteracted by favorable characteristics of the 
other stations. 
The coast defense gunlaying radar is concerned 
primarily with accurate location of ships. It has a 
range up to 100,000 yd and must be sited fairly high 
and within a few miles of the coast defense guns 
which it directs. This radar supplies aceurate data 
on the azimuth and range of the target. 
The antiaircraft gunlaying radar is used primarily 
for directing the guns. Long-range search features 
are usually provided so that they may function also 
‘as early warning radars, at least to a limited extent. 
They are sited near the guns which are located to 
meet artillery requirements. These units provide a 
continuous flow of data to the gun director giving 
the azimuth, elevation, and range with great accuracy. 
The searchlight control radar is a short-range high 
angle set which is located near the light it directs. 
It furnishes the azimuth, angular elevation, and 
altitude of the target. 
Radar Siting—Technical Aspects 
In the past some elaborate air warning systems 
have been set up without a competent analysis of 
terrain effects. This resulted in a waste of time and 
money and in failure to adequately provide urgently 
needed radar screens. This failure was caused in 
many cases by the use of prepared coverage diagrams, 
furnished with the equipment, which were computed 
for idealized sites. In mountainous regions where 
only limited reflection areas occur and where the 
sites are very much higher than those used in labora- 
tory tests, such diagrams are likely to be very 
misleading. A result of this experience is an unfor- 
tunate tendency to explain variations from expected 
coverage by resort to various abstruse speculations, 
with weather not infrequently bearing the brunt 
of the odium. 
It is the purpose of this report to provide an 
engineering type of solution for the bulk of the 
problems that arise in siting and in field computa- 
tion of coverage. A more accurate analysis, with 
increased attention to detail, probably is not war- 
ranted at this time in view of the relatively rough 
measurements which now are made in the field of 
radar. 
The common early warning radar uses horizontal 
polarization and operates in the VHF band. It must 
be sited from several hundred to several thousand 
feet high in order to obtain sufficiently low angles 
for the range and low coverage desired. Suitable 
sites of the required height may be far inland so 
that an important part of the reflecting surface may 
be rough land or sloping flat areas. Such features 
and also cliff edges, ridges, hills or other obstacles, 
nearby towers and structures will, in general, produce 
a marked effect on the coverage pattern. 
The GCI radar uses horizontal polarization, 
operates in the VHF band and should be sited on 
a large, flat area. The determination of the height 
of an airplane is accomplished by comparing signals 
from two antennas of different heights. If reason- 
able accuracy is to be attained the lobe structure in 
the vertical plane must be known with considerable 
precision. Best results are obtained by using a site 
of the extent and flatness prescribed in the instruc- 
tion manual. In practice it may be necessary to 
operate on rough ground or limited areas. The ques- 
tion may then arise concerning the benefit that will 
be obtained by grading the surrounding areas, or 
how much forest: or vegetation should be removed 
for acceptable operation. 
Similar problems arise in siting DF stations. Large 
errors may be introduced by reflection from sloping 
land or other terrain features. 
The effects described above, involving reflection 
from limited areas or rough land or passage of waves 
past an edge, may all be treated as problems of 
diffraction, for which solutions are well known or 
may be readily computed. This subject is unfamiliar 
to most Service personnel; but a working knowledge 
of the methods of computation may be obtained by 
anyone who has the usual engineering education. 
Since it is not possible to anticipate all problems 
which may arise in the field, a fairly comprehensive 
discussion of diffraction has been included in this 
report so that even in the absence of other references 
the majority of problems may be treated. 
Other important considerations such as orienta- 
tion, visibility, permanent echoes, interference, and 
test methods are discussed. There have been many 
ingenious developments in these subjects in different 
theaters, and where available they have been 
included in this report. Only standard atmosphere 
propagation has been considered. Those who are 
interested in nonstandard propagation should refer 
to the articles on this subject published in this series. 
TOPOGRAPHY OF SITING 
Introduction 
The performance of equipment which utilizes radio 
propagation depends upon the character of the inter- 
vening land or sea and in particular upon the local 
