# Step 4-A: Field Strength of Isotropic Radiator (Reference Value)

The field strength of the isotropic radiator at the point P is used as a reference. The isotropic radiator is located at Mgeo and has the coordinates (xgeo, ygeo, zgeo).

To get the field strength, the following steps are required:
• Distance d between P and Mgeo (using Pythagoras)
(1) $d=\sqrt{{\left({x}_{1}-{x}_{geo}\right)}^{2}+{\left({y}_{1}-{y}_{geo}\right)}^{2}+{\left({z}_{1}-{z}_{geo}\right)}^{2}}$
• Computation of power density Siso at point P
(2) ${S}_{iso}\text{\hspace{0.17em}}=\frac{{P}_{{t}_{0}}}{4\pi {d}^{2}}\text{\hspace{0.17em}}$
with Pt0 describing the sum of all transmitted antenna powers.
• Computation of the effective electric field strength Eiso at the pixel P based on the power density Siso and the impedance of the free space ZF0 = 120 πΩ
(3) ${|{\underset{_}{E}}_{iso}|}^{2}\text{\hspace{0.17em}}={S}_{iso}×{Z}_{F0}$