Geostationary orbit - Wikipedia, the free encyclopedia
I assume the satellite is over the equator because they claim equal coverage, North and South.
The satellites are over the equator because they are Geostationary. They are stationary relative to the surface of the earth.
EDIT: This is significent because it determines the arc(s). If there were multiple pings and if the satellites were moving relative to the surface of earth we might know exactly where the plane is.