(and hence to the square root of the effective aperture area), the density (in W/m2) of the EIRP within the radiometer field of view can be related to the maximum tolerable brightness perturbation:
where A is the radiometer footprint area (m2); Equation C.3 shows that it is the density of EIRP per area (computed over the radiometer footprint) that determines the interference to the radiometer. EIRP limits on individual transmitters must be combined with information on the expected number of transmitters within a specific area in order to predict or interpret observed interference levels δT.
As an example, a 6.9 GHz Advanced Microwave Scanning Radiometer-Earth (AMSR-E) observation (2,500 km2 footprint area) with a bandwidth of 350 MHz will experience a brightness increase of 1 K if even a single interferer having a 130 milliwatt EIRP (in the direction of the radiometer antenna) is included in the footprint area. That such low radiated interference powers can perturb observed brightness temperatures demonstrates the high sensitivity of Earth Exploration-Satellite Service observations to interference. The fact that multiple interference sources may reside within any radiometer footprint substantially exacerbates the problem. The impact of a specific interference level on a particular geophysical measurement depends on the sensitivity of the measurement to changes in brightness temperature, as discussed in §2.2 in Chapter 2 of this report. The accuracy achieved in current radiometer systems typically makes even small changes in brightness caused by radio frequency interference to have a significant impact on measured products.