Two point sources P1 and P2, located respectively at (x01, y01, z01) and (x02, y02, z02), in 3D space. In formulating the distance estimation problem, the 3D geometry of the pair is equivalently described by the six parameters d, φ, ω, sx, sy, sz. The parameter d is the distance that separates P1 and P2. The coordinates sx, sy, and sz specify the location of the midpoint between P1 and P2. The parameter ω is the angle between the line segment P1P2 and the positive z-axis. The parameter φ is the angle between the xy-plane projection of P1P2 and the positive x-axis.