• The look angles for the ground station antenna are the azimuth and elevation angles required at the antenna so that it points directly at the satellite. the look angles were determined in the gen-eral case of an elliptical orbit, and there the angles had to change in order to track the satellite. With the geostationary orbit, the situation is much simpler because the satellite is stationary with respect to the earth. . Although in general no tracking should be necessary, with the large earth stations used for commercial communications, the anten-na beamwidth is very narrow (see Chap. 6), and a tracking mechanism

• is required to compensate for the movement of the satellite about the nominal geostationary position. With the types of antennas used for home reception, the antenna beamwidth is quite broad, and no track-ing is necessary. This allows the antenna to be fixed in position, as evi-denced by the small antennas used for reception of satellite TV that can be seen fixed to the sides of homes.

• The three pieces of information that are needed to determine the look angles for the geostationary orbit are :

• The earth station latitude, denoted here by λE • The earth station longitude, denoted here by φE • The longitude of the subsatellite point, denoted here

by φSS

Sign convention

• Sign convention used for satellite positions• for lattitude, North will be positive and south will

be negative .• for longitude,east will be positive and west will

be negative .• For example, if a latitude of 40°S is specified, this

will be taken as -40°, and if a longitude of 35°W is specified, this will be taken as -35°.

•

• From figure we are having six different angles.• The three angles A, B, and C are the angles between

the planes.• Thus we get B =φE – φSS

• a =90°• c =90°-λE

• Using Napier’s rule we get b = cos-1 (cos B cos φE) A=sin-1 (sin|B|)

(sin b)

• TABLE 3.1 Azimuth Angles Az from Fig. 3.3

Figure 3 λe B AzΟ

a < 0 < 0 Ab < 0 > 0 360-Ac > 0 < 0 180-Ad > 0 > 0 180+A

Applying the cosine rule for plane triangles to the triangle of Fig. 3.2b allows the range d to be found to a close approximation d =

Applying the sine rule for plane triangles to the triangle of Fig. 3.2b allows the angle of elevation to be found:

E =COS-1

bRaaR gsogso cos222

bdaGSO sin