Geometry of the toy model. The ionosphere is a layer of thickness b at height a above a reference location x₀. In general, ΔTEC is the TEC along one geodesic minus the TEC along another parallel geodesic. Usually, these geodesics are originating at antennae i and j (locations x_i and x_j), and pointing in directions and , respectively. One common choice is to have a fixed reference antenna for all ΔTEC measurements. The corresponding zenith angles are ϕ₁ and ϕ₂.