First, consider the radiometer concept, and the definition of the antenna temperature.  
    1: T_B   is the emitted radiation from a target, observed by the radiometer. What type of signal is TB? And does TB express magnitude or power?  
    2: Why do we get (TA+TN) as power output (slide 3 in the presentation of Ch4). Typically we cannot add power signals directly?  
    3: Consider a radiometer for measurements of sea surface salinity in a region near land bodies. Assume an antenna with a side lobe accounting for 2% of the total received power, and assume this side lobe to point towards an island with a brightness temperature equal to 300 K. Compute the antenna temperature, when the sea surface brightness temperature is assumed to be 120 K, covering the remaining part of the antenna beam.

1. T_B is a stochastic signal. It expresses power, given by the Rayleigh-Jeans approximation to Planc’s Radiation Law, P = k•B•T, where k = 1.23•10-23 J/K is Boltzmanns constant, B is the bandwidth, and T is the temperature.

2. The resulting power cannot be found through simple addition in the general case, as (a+b)2 = a2 + b2 + 2ab, where the last term also has to be accounted for. However, for stochastic signals, which are statistically independent, the last term represents the cross correlation, which is 0. (The same applies to deterministic signals, which are orthogonal.)

3. ¿Antenna temperature?

$$ T_{B-island} = 300 [K] \\ T_{B-sea} = 120 [K] \\ T_A = 0.02 \cdot 300 K + 0.98 \cdot 120 K = 123.6 K $$