The Rayleigh limit in spatial resolution of an image forming device is given by:
l = 1.22 * λ * L / D
Where λ is wavelength, L is distance and D is optical element diameter.
For satellite at an altitude of 150 km with 2.4 m diameter mirror and 0.4 μm blue light, this gives a spatial resolution of 0.3 m. To read a number plate requires an order of magnitude better than this.