SITING AND COVERAGE OF GROUND RADARS 91 
the intervals in azimuth at which readings are to 
be made. The definition of the echoes will depend 
in part upon the beam width so that the narrow 
beam radars should be checked at closer azimuth 
intervals. Readings may be taken at intervals of 10° 
or 5° or even less depending upon the detail desired; 
in general an interval of about a fourth of the beam 
angle is sufficient. Permanent echo readings should 
be taken through 360° regardless of the sweep sector 
used, so that back and side echoes may be investi- 
gated also. 
At each azimuth the range of all permanent echoes 
is recorded from zero out to the extreme range. The 
width of the main pulse and local ground echoes 
should be noted as well. Echoes one mile or less in 
width are recorded by a single reading at the center 
of the echo. Wider echoes are recorded by two read- 
ings, one at the left of the echo where the trace leaves 
the baseline and a second at the right where the 
trace returns. Adjacent echoes less than 1 mile apart 
are recorded as a single echo. Where the separation 
is greater, care should be taken not to lump echoes 
together. 
For most purposes variations in amplitude may 
be disregarded. Amplitude is, “however, sometimes 
recorded for a few azimuths of special interest such 
as those used for test flights or in tactically important 
regions. 
To plot the data an overlay of a regional aero- 
nautical map or other chart with a scale of 1 to 
1,000,000 may be made showing some of the signif- 
icant features as coastlines, islands, and cities. On 
this should be drawn radial azimuth lines every 10 
degrees and range circles every 10 miles. The data 
are then marked on the chart as short lines, and 
these lines are connectéd as indicated by inspection. 
The enclosed areas may then be shaded lightly. If 
it is desired to represent amplitudes, a few equal 
amplitude contours may be shown within an echo 
area. More detail may be shown by plotting ampli- 
tude versus range on a rectangular graph for each 
azimuth. 
The completed permanent echo diagram should 
be compared with a topographical map to check the 
degree of shielding obtained and the range and 
azimuth accuracy of the equipment and back and 
side lobe radiation effects. Care must be exercised 
‘in identifying the cause of an echo, as distant echoes 
may come in on the second or third sweep on the 
scope after the main pulse. 
In Figure 35 is shown a permanent echo diagram 
which was selected for purposes of illustration rather 
than as an example of a good site. A few miles from 
the coast is an extensive range of mountains which 
are poorly shielded to the north. The large echo at 
200° is due to a mountainous island 260 miles away. 
Use of Permanent Echoes in Testing 
Permanent echoes are useful for tuning the equip- 
ment, estimating the output and sensitivity, and 
checking the range and azimuth accuracy. While 
such observations may be used as an overall test 
of performance, care should be used in selecting the 
test echo and in interpreting the indications. 
Careful tests have shown that, even though 
equipment performance is closely controlled, the 
strength of permanent echoes varies over a consider- 
able range. It is noted further that indications from 
aircraft also vary, but there is little correlation with 
the changes in permanent echoes. Other tests show 
that, as the performance of the set is reduced, the 
maximum range for small targets is reduced at a 
much faster rate than for large targets. Thus a 
reduction of receiver sensitivity may cause weak 
achoes to disappear .entirely without a noticeable 
effect on strong permanent echoes. 
Permanent echoes vary for the following reasons: 
1. Atmospheric changes affect both the direct and 
reflected rays. This may be due to a change in the 
amount of refraction from standard or in the degree 
of trapping. Under some conditions marked absorp- 
tion may occur. The changes may occur slowly or 
fluctuate erratically, being most marked in connec- 
tion with microwaves. 
2. If the reflecting surface is the ocean, variation 
of the reflected ray may occur if the tide changes 
or the roughness of the surface becomes excessive. 
3. Frequency variations will affect the echo from 
complex reflectors such as rugged terrain. Peaks 
which are separated in distance such that the returns 
from a single pulse overlap are said to be frequency 
sensitive. In the overlap portion the echo strength 
will depend upon the relative phase of the two 
returns. Thus if the pulse width is 10 usec the wave 
train will be about 2 miles long. If there are peaks 
at 10 miles and 1014 miles their echoes will appear 
as follows on an A scope: 
10 to 1014 miles near peak echo only 
1014 to 12 miles combined echo of both peaks 
12 to 1214 miles far peak echo only 
The combined portion of the echo may have a height 
from zero to twice that of the individual echoes and 
usually fluctuates rapidly as the frequency drifts. A 
change of a half wavelength in the separation of the 
peaks will change the combined echo from maximum 
to minimum. This means that a frequency stability 
of the order of one part in a million is required for 
a steady combined echo. 
Permanent echoes used for testing should therefore 
be (1) nearby but distinct from ground clutter and 
other echoes, (2) separated from the transmitter by 
rough nonreflecting land, (3) a single distinct target 
such as a steel tower, (4) weak in response, that is, 
comparable to that of a distant aircraft. 
The range of echoes which come in on the second 
or third pulse may be estimated by adding one or 
