Schematic illustration of the angular separation θ calculated in the plane of the two spacecraft (grey shaded area) from the Sun-source axis to the vector connecting the spacecraft (green arrow). The heliocentric angle θ_H is also shown for comparison. The red dot at R₁ represents the spacecraft observing the Langmuir waves and thus the radio source location, whereas the blue dot at R₂ represents the other spacecraft. The origin of the 3D Cartesian coordinate system is at the solar centre.