A single-feed composite cavity-backed four-arm curl antenna is presented for use in global positioning systems (GPS). Its primary radiating element is fed by a vacant-quarter printed ring, which helps the antenna directly match to a $50-{Omega}$ coaxial line and produce a good circular polarization (CP). The cavity-backed reflector is employed to improve the CP radiation characteristics in terms of 3-dB axial ratio (AR) beamwidth and minimum AR value. The optimized design with an overall size of $90{ imes}90{ imes}25mm^3$ ($0.4725{lambda}_o{ imes}0.4725{lambda}_o{ imes}0.13{lambda}_o$ at 1,575 MHz) results in a ${mid}S_{11}{mid}<-10dB$ bandwidth of 8.66% (1,514-1,651 MHz) and a 3-dB AR bandwidth of 2.23% (1,555-1,590 MHz). The antenna radiates a widebeam right-hand circular polarization and operates with a measured radiation efficiency greater than 90% within its impedance matching bandwidth.