According to certain embodiments, a temperature measurement system includes a radiometer coupled to an antenna and a signal processor. The radiometer is operable to generate two or more signals representing electro-magnetic radiation received at a corresponding two or more differing frequencies. The signal processor receives the two or more signals from the radiometer while the antenna is placed adjacent to a surface of a substrate, and compares the two or more signals with one another to determine a brightness temperature of the substrate at a depth of the substrate from the antenna.