A wireless apparatus includes an electrically conductive casing housing a ground plane and GPS receiver circuitry. A spring 22, which urges the bypass valve closed, is mounted in a support member 24 attached to the housing. A device is provided for determining whether a clock edge of the delayed clock signal output by the delay device, or of a signal obtained therefrom, lies within a predetermined time window before a corresponding clock edge of the clock signal, or of the signal obtained therefrom.