1. A detector for a user input to a mobile device, the detector comprising: an ultrasound demodulator having an input for receiving an ultrasound signal reflected from a user and an output for outputting a demodulated ultrasound signal, wherein the ultrasound demodulator comprises a switch arranged between the input of the ultrasound demodulator and the output of the ultrasound demodulator and wherein the ultrasound demodulator is operable to switchably couple the ultrasound signal to the output of the ultrasound demodulator; a gesture processor comprising: a time-frequency processing module configured to generate a time-varying ultrasound-image spectrum from the demodulated ultrasound signal; an image-feature extraction module configured to extract micro-doppler features from the time-varying ultrasound image spectrum; a feature selection module configured to select and compress the extracted micro-doppler features; and a gesture detection module configured to compare the selected micro-doppler features with a known feature set and to output a detected gesture based on the comparison, wherein the gesture processor comprises logic hardware or software running on a digital signal processor.