North-south horizontal gradient contribution to the atmospheric time-delay for observations at meridian. Left panel: sketch representing a transmitting source (labeled 1) being ground tracked simultaneously by three stations on Earth (labeled 2, 2′, and 2″). The source is passing in the meridian of the three stations, i.e. ι = π∕2 and λ = Ω, thus the direction of the line of nodes is the intersection between the propagation plane of photons and the Earth’s equator. Right panel: graph of Eq. (62) (normalized by the constant factor Rn_NS ∕ηc) for the three different paths which are depicted in the left panel. True anomalies have been replaced by colatitudes by applying the transformation in Eqs. (63). The plain and dotted curves are computed for q = 2.4 and q = 2.0 respectively. The computation has been carried out assuming , , , and .