Observation results. (a) Intensity fit result: The peak fluxes at each frequency, corresponding to the longitudes of the four probes, are denoted by points. Vertical lines show the uncertainty, which is given to be 50% of the peak fluxes. The curves represent the best-fitted fluxes obtained using Eq. (1). (b) Source positions: The source positions were determined using both the intensity fit (plus symbols) and timing method (triangle symbols) at six frequencies. The shaded dots indicate the positions determined from the timing method by sampling peak times varied by t^pk ± Δt, while the triangle symbols show average positions of those sampling dots. The Parker spiral, with a solar wind speed of 400 km s⁻¹, is connected back to the Sun at −60° and indicated as a red dashed line. In the lower right corner panel, the positions of PSP (P1), SolO (P2), STEREO-A (P3), and Wind (P4) are projected onto the plane of the Earth’s orbit in the Heliocentric Earth ecliptic (HEE) coordinate system. The dashed circles represent 100 and 200 times the solar radius.