Frequency agility is applied to dual-frequency scattering from the ocean using an L-band microwave system in order to reduce the clutter background which has previously limited signal detectability in such scatterometers. Doppler spectra of the return show improvements of up to 13 dB in signal-to-clutter ratio using this technique compared with the standard non-frequency-agile technique. This improvement greatly enhances the ability of a dual-frequency scatterometer to measure ocean surface currents. Composite surface scattering theory is applied to explain the signal improvement. The clutter background is shown to be proportional to the ocean surface wave spectrum evaluated at a wavenumber corresponding to the wavenumber separation between the carrier frequencies of two transmitted pairs of frequencies. Thus, if the frequency separation between these carriers is equal to the frequency separation between the lines of each pair, the entire signal received by the system is proportional to the ocean surface wave spectrum evaluated at the common wavenumber difference. This three-frequency technique completely removes previous limitations on the measurement of ocean wave spectra by dual-frequency scatterometers by converting the clutter background into part of the desired signal. Signal detectability is then limited only by thermal noise.
展开▼
