116 



SITING AND COVERAGE OF GROUND RADARS 



profile is usually necessary for estimation of the 

 effective antenna height and the reflection charac- 

 teristics of the terrain. Profiles should be prepared 

 of several representative azimuths in the operating 

 sector. The accuracy required decreases with the 

 distance from the transmitter. In most cases suffi- 

 cient detail is not available on maps so that a 

 personal inspection of the terrain should be made 

 to become familiar with the nature of the soil and 

 the degree of roughness. Special attention should be 

 given to ridges, flat areas, bodies of water, distance 

 to the shore, hills to the rear, obstacles in the operat- 

 ing area and at the boundaries. A knowledge of the 

 antenna pattern in both the vertical and horizontal 

 planes is necessary for judging what parts of the 

 terrain should be more closely examined. 



15.3.4 



Orientation 



Where long distances and directive beams are 

 involved fairly accurate orientation is required. This 

 is especially true of the narrow beam, precision type 

 radars. Of the many ways of determining the direc- 

 tion of north, one of the most convenient is observa- 

 tion of the azimuth of the sun. Care must be taken 

 when using compasses because of local attractions 

 or inadequate information of the declinations. Star 

 observations are capable of good accuracy, but where 

 Polaris is not visible they require the same procedure 

 as solar shots. Caution must be used in aligning on 

 permanent echoes because nonstandard refraction 

 may bring in confusing distant echoes, or side lobes 

 may give false echoes. In general several methods 

 should be used in order to obtain independent checks. 

 When an accurate orientation has been obtained 

 reference marks should be provided so that the 

 azimuth may be readily checked. 



Solar azimuths, correct to the nearest quarter of 

 a degree, may be determined from the date, time 

 to the nearest minute, and the latitude and longitude 

 to the nearest degree. Two methods will be given 

 for obtaining the azimuth of the sun: (1) by calcu- 

 lation, (2) from tables. A third method gives true 

 south only. 



The azimuth of the sun may be calculated from 

 the formula: 



tan (3 = — 



sin HA 



cos * tan 5 — sin * cos HA 



(1) 



j3 = bearing of the sun. 



The bearing is east or west of south when *-5 is 



positive. The bearing is east or west of north when 

 *-5 is negative. The bearing is east in the morning 

 (/3 will be negative), and west in the afternoon (/? 

 will be positive). 

 HA = hour angle of the sun. 



During the morning hours when the hour angle is 

 greater than 12 hours, its value should be subtracted 

 from 24 hours for use in the formula. 

 * = latitude of the place of observation. 

 5 = declination of the sun at the time of observation. 



The signs of * and 5 are important and each is 

 positive when north of the equator and negative 

 when south. 



The hour angle HA is the local apparent time 

 [LAT] minus 12 hours. To convert the observed time 

 into LAT the civil time at Greenwich [GCT] must 

 be found and combined with the equation of time 

 to correct for the apparent irregular motion of the 

 sun. This gives Greenwich Apparent Time [GAT] 

 which is converted to LAT by allowing for the lon- 

 gitude. The equation of time and the declination of 

 the sun are plotted in Figure 1 for 1945. The annual 

 change is small, and these curves may be used for 

 radar work without regard to the year. Standard 

 time meridians are every 15° east or west of 

 Greenwich, each zone corresponding to 1 hour. Care 

 should be used to take daylight saving, or other 

 changes from standard, into account correctly. 



Example 1. It is desired to compute the azimuth 

 of the sun. 



Given: Date March 16 



Time 1345 hours PWT 



Latitude 40° North 



Longitude 118° West 

 Solution : 



The hour angle will be determined first: 



Observed time PWT 13* 



Zone difference - 7* 



Greenwich civil time 



Equation of time (Figure 1) 



Greenwich apparent time 



Longitude difference for 118° W 



Local apparent time 



LAT - 12 = HA 



Hour angle of sun + 0* 44 



HA in arc. (4 m = 1°) + 11' 



Latitude * + 40 



Declination of sun 5 (Figure 1) — 2' 



Substituting in equation (1): 



45" 



tan (3 = 



sin ll c 



cos 40° tan (-2°) - sin 40° cos 11° 



0.19 



0.766 X (-0.0349) - 0.643 X 0.982 



