Position vectors of dust source r_M and spacecraft r (in spherical coordinates (r_M, α_M, ω_M) and (r, α, ω), respectively), defined in the Suncentered system S (compare to Fig. 1). The particle velocity vector at the time of ejection (u, ψ, λ_M) is defined in the coordinate system $\tilde{S}$ with the z-axis aligned with the asteroid heliocentric radius-vector and the x-axis pointing toward the local north of the system S. The axes of the source-centered system A are co-directed with the axes of the system $\tilde{S}$. The inclined system $\tilde{A}$ is the system in which the distributions of the ejection direction can be described mathematically in a simple way.