LOOK ANGLE DETERMINATION
Note.. • Earth stations that communicate with satellites are described in terms of their geographic latitude and longitude when developing the pointing coordinates that the earth station must use to track the apparent motion of the satellite. • The coordinates to which an earth station antenna must be pointed to communicate with a satellite are called look angles.
Subsatellite Point • The subsatellite point is the location on the surface of the earth that lies directly between the satellite and the center of the earth. • It is the nadir pointing direction from the satellite and, for a satellite in an equatorial orbit, it will always be located on the equator. Since geostationary satellites are in equatorial orbits and are designed to stay “stationary” over the earth, it is usual to give their orbital location in ter terms of the their sub subsatel tellite point.
Note.. • To an observer of a satellite standing at the subsatellite point, the satellite will appear to be directly overhead, in the
zenith
direction from the observing location. The zenith and nadir paths are therefore in opposite directions along the same path
Specifying directions • Antenna: Designers
of satellite antennas reference the pointing direction of the satellite’s antenna beams to the nadir direction. The communications coverage region on the earth from a satellite is defined by angles measured from nadir at the satellite to the edges of the coverage.
• Earth station antenna designers, however, however, do not reference their pointing direction to zenith. Instead..
Azimuth and Elevation Angles • They use the local horizontal plane at the earth station to define elevation angle and geographical compass points to define azimuth angle, thus giving the two look angles for the earth station antenna toward the satellite (Az, El).
Elevation Angle Calculation • Refer to diagram and corresponding eqn (Fig. 2.12, pp.33) and Eqns. 2.31 and 2.35 pp. 34.
Azimuth Angle Calculation • Because the earth station, the center of the earth, the satellite, and the subsatellite point all lie in the same plane, the azimuth angle Az from the earth station to the satellite is the same as the azimuth from the earth station to the subsatellite point. • Commercial software packages are available for predicting orbital dynamics and intercept solutions.
Azimuth Angle Calculation • Special case is of Geostationary Satellites • The subsatellite point is on the equator at longitude ls and the latitude is Ls = 0, we write Cos(γ Cos(γ) = cos(Ls).cos(ls –le) Distance d from the earth station to satellite and elevation angle El at the earth station can be found out. Refer to Eqn 2.36, 2.37, 2.38, 2.39, 2.40, 2.41a..d, pp35
Visibility Test • Refer Fig. 2.13, 36 • For a satellite to be visible from an earth station, its elevation angle El must be above some minimum value, which is at least 0o • For a nominal geostationary orbit, the last equation reduces to γ < or = 81.3o for the satellite to be visible.
Orbital Perturbations • In practice, the satellite and the earth respond to many other influences including asymmetry of the earth’s gravitational field, the gravitational fields of the sun and the moon, and solar radi radiat atio ion n press pressur ure. e. • For LEO satellite earth’s atmospheric drag also influences • If the effects are unchecked, the subsatellite point may change with time. Six orbital elements vary with time
Orbit Determination • Sufficient measurements are made to determine uniquely the six orbital elements needed to calculate the future orbit of the satellite, and hence calculate the required changes that need to be made to the orbit to keep it within nominal orbital location. • The calculations aim at determining azimuth and elevation of the satellite, as a function of several orbital elements.
Launches and Launch Vehicles • Two parameters that are uniquely coupled: velocity vector and orbital height
• Ex: CEO satellite • height of 35,786.03 km above the surface of earth (or 42,164.17 km radius from the center of the earth) • with an inclination of 0O • an ellipticity of 0o • Velocity of 3074.7 m/s tangential
Launch Vehicles • Multiple stages: As each stage is completed, that portion of the launcher is expended, until the final stage places the satellite into the desired trajectory. • Ex: ELVs, STS, RLV, SSTO, RLV
Orbital Effects in Communications • Doppler Shift: For GEO satellites, effect is negligible, for LEO the effect is pronounced necessitating use of frequency-tracking receivers. • Range Variations: For LEO, the effect is pronounced • Solar Eclipse: Occurs 23 days before and after equi equin noxes oxes,, batt batter erie ies s are are cons consum umed ed • Sun Transit Outage: The microwaves received from sun cause noise.
