A source of laser light at a second wavelength is injected into the cavity to reverse the direction of propagation and to produce emission at the second wavelength.

For example, Doppler filters might be rejecting moderate ground echoes in a particular area and thus weak meteorological echoes would not be detectible.

The ultrasound sensors (1 - 12) transmit ultrasound signals (110, 120) at a frequency f0, wherein the ultrasound sensors (1 - 12) receive bottom echoes (112, 122) reflected from the ground while the vehicle is travelling.

The E-3A/D’s multi-mode radar is able to separate moving targets from ground clutter by use of the so called ‘Doppler’ principle.

Embodiments of the present invention provide a Gaussian adaptive filter for ground clutter filtering and signal parameter estimation for weather radars in the time domain.