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The development and the application of a cost-effective differential global positioningsystem for remotely piloted vehicles are presented. With the aid of an aerial-photographysystem, i.e., digital camera and charge-coupled device camera, the low-cost global positioning system receivers (Garmin SvyII and XL-25) can perform valuableand highaccuracy positioning and navigation measurements, which are competitive with the precise (but more expensive) Ashtech Z-12 receiver. Flight-test results reveal that the low-cost payload (XL-25 system) developed can reach 10 m in spatial accuracy of the static ight trajectory and 0.17 m/s and 1 deg in accuracy for ight velocity and heading angle. The heading angle and ight-path angle can also achieve 1-3 deg rms accuracy level. Relative deviations of the rms accuracy values between XL-25 and Z-12, such as the kinematic, three-dimensional position and the ight velocity, are found to be below 2.6 m and 0.5 m/s, respectively. Nomenclature a = semimajor axis of the ellipsoid C = speed of light d ion ; d trop = ionospheric and tropospheric delay effect dT = offset of the receiver clock dt = offset of the satellite clock d½ = global positioning system (GPS) ephemeris error and selective availability effect error e = geometric attening ratio P = pseudorange between the GPS satellite and the receiver O P u = rover station's corrected pseudorange R = rotation parameter for coordinate transformation S = scale parameter for coordinate transformation V n ; V e ; V d = local north, east, and down velocity of the navigation coordinate system V t = navigation velocity of the ight vehicle X; Y; Z = world geodetic system 84 (WGS-84) coordinate system X m ; Y m ; Z m = gravity center of the Taiwan datum 67 (TWD-67) coordinate system X u ; Y u ; Z u = position of the GPS receiver antenna 1 P = GPS pseudorange correction item for differential GPS processing 1X; 1Y; 1Z = translation parameter for coordinate transformation "= residual error (such as multipath effect) ½ = true geometric range from GPS satellite to receiver's antenna ½ m ; ½ P = meridian and prime radius of curvature Á;¸; h = latitude, longitude, and altitude of the geodetic ellipsoidal coordinate system P Á; P = geodetic latitude and longitude rate

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Applications using location-based services have been explored due to safety and emergency requirements on wireless communications services. The interworking among heterogeneous systems induces the hybrid positioning concept with multiple positioning technologies. This study focuses on the improvement of GPS orbit accuracy to enhance hybrid position computation. The proposed approach by converting precise orbit into broadcast ephemeris format is proved to be an effective method. Both twohour and 3.5-hour orbit conversion results show that an orbit accuracy of less than 0.4 meters can be achieved. The results suggest that the proposed approach has a potential in real time application.