Can anyone help me to know about the azimuth angle?

hi to all...
Can anyone help me to know about the azimuth angle?
My question is related to the satellite communication. What is the reason for the azimuth angle to be 180- when the earth station is southern and western to the satellite substation point. Also let me know about the angle 180+ in the case of the earthstation being in eastern and northern with respect to the satellite substation.