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In this letter, we present the design of a concentric annular ring slot antenna with right-hand circular polarization. Two concentric rings have been etched in the ground plane to generate a dual-band response. A broadband feeding network and a thicker substrate below the slots have been used to improve the axial-ratio bandwidth and gain. The proposed antenna covers the bands of the positioning systems GPS and Galileo. Experimental results show a 3-dB axial-ratio bandwidth for the lower and upper band of 19.2% and 18%, respectively. The measured gain is higher than 4 dB in both bands.