Communication satellites used in the satcom industry are typically in geostationary orbit, appearing to be in a fixed position in the sky directly above the equator, relative to an observation point on earth. The entire field of geostationary satellites can therefore be found in an arc across the sky. In order to communicate with the satellite you wish to downlink from or uplink to, the antenna must be able to "see" the location in the sky above the equator in which the satellite is located. The situation of an unobstructed view between the satellite and the antenna is known as "having a line-of-sight" to the satellite.

You can also view the Antenna Installation Chapter of the VSAT Installation Manual below which shows the procedure for the LOS Calculation.