I'KKMVNKVP KCIIOKS 



135 



long-distance beam should receive the most atten- 

 tion although the other combinations should be 

 checked also. 



If the beam is highly directive and can be changed 

 in elevation, a low angle such as would be used for 

 distant search should be used for recording perma- 

 nent echoes. In some situations several elevations 

 should be used. On plan position indicator [PPI] 

 scopes it may be more convenient to photograph the 

 screen if proper equipment is available. Care should 

 be taken not to confuse storm and fog echoes with 

 permanent echoes on microwave sets. 



A more detailed procedure is required where A- 

 scope presentation is used. After the initial adjust- 

 ments have been made the next step is to decide on 

 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 connected 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. 



15.5.3 jj ge Q £ p ermanen t 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 

 echoes 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 



