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A compact active integrated microstrip antenna with circular polarisation (CP) diversity for a global positioning system (GPS) receiver is presented. As a radiator, a diamond-shaped micro- strip ring patch with four embedded slots has been newly proposed to realise wide impedance bandwidth, compact size and dual CP. The active circuitry, consisting of both switching circuits and a small signal amplifier, is placed at the square opening inside the radiator. Thus, all electrical parts are mounted on the top layer of the circuit board. CP diversity is achieved by the switching circuit, and its output signal is amplified. The proposed antenna has been simulated and fabricated on the FR-4 PCB board. The size of the fabricated active antenna, including the radiator and active circuitry, is only about 57% of a conventional square-microstrip antenna with a side of half wavelength. From the measured radiation patterns and CP bandwidth for either polarisation selection, it is proven that this antenna has successfully performed the polarisation diversity. In addition, the measured minimum antenna gain of 12 dBi is estimated to be good enough to improve the GPS receiver performance.