Measuring RF Power at 2,450 MHz with <0.1dB Error Over 20dB Range, 20–60°C

I need an ultra precise technique to measure RF power (hopefully in Watts). I need the error to be less than about 0.1dB over a 20dB range and from 20 Celsius to 60 Celsius. The frequency is fixed at 2,450 MHz.

Any Ideas?