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An improved approach to satellite-based navigation (e.g., GPS) is provided. In one embodiment, a method includes determining a nominal orbital path of a navigation satellite. The method also includes transmitting ephemeris data corresponding to the nominal orbital path from the navigation satellite

An improved approach to satellite-based navigation (e.g., GPS) is provided. In one embodiment, a method includes determining a nominal orbital path of a navigation satellite. The method also includes transmitting ephemeris data corresponding to the nominal orbital path from the navigation satellite to a plurality of navigation devices. The method further includes determining an actual orbital path of the navigation satellite locally at the navigation satellite. In addition, the method includes determining a deviation between the actual orbital path and the nominal orbital path locally at the navigation satellite. The method also includes autonomously adjusting the actual orbital path locally at the navigation satellite to reduce the deviation between the actual orbital path and the nominal orbital path.

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1. A method of adjusting a navigation satellite orbit, the method comprising: determining a nominal orbital path of the navigation satellite;transmitting ephemeris data corresponding to the nominal orbital path from the navigation satellite to a plurality of navigation devices;determining an actual

1. A method of adjusting a navigation satellite orbit, the method comprising: determining a nominal orbital path of the navigation satellite;transmitting ephemeris data corresponding to the nominal orbital path from the navigation satellite to a plurality of navigation devices;determining an actual orbital path of the navigation satellite locally at the navigation satellite, wherein the determining an actual orbital path comprises: measuring non-gravitational disturbance force effects locally at the navigation satellite to determine a relative position of the navigation satellite, wherein the measuring is performed by a drag free sensor of the navigation satellite, andusing the relative position to determine the actual orbital path;maintaining the drag free sensor substantially at a center of gravity of the navigation satellite;determining a deviation between the actual orbital path and the nominal orbital path locally at the navigation satellite; andautonomously adjusting the actual orbital path locally at the navigation satellite to reduce the deviation between the actual orbital path and the nominal orbital path. 2. The method of claim 1, further comprising measuring an actual position of the navigation satellite using a local navigation device onboard the navigation satellite, wherein the determining an actual orbital path comprises using the measured actual position to determine the actual orbital path. 3. The method of claim 1, wherein the adjusting comprises commanding propulsion actuators of the navigation satellite to counteract the measured disturbance force effects. 4. The method of claim 1 wherein the maintaining comprises: measuring angular motion acceleration vectors associated with the drag free sensor; andadjusting a position of the drag free sensor until a sum of the angular motion acceleration vectors is approximately zero. 5. The method of claim 1, wherein the determining a nominal orbital path comprises: receiving the ephemeris data from a ground station; anddetermining the nominal orbital path locally at the navigation satellite using the ephemeris data. 6. The method of claim 1, wherein the determining a nominal orbital path comprises: receiving satellite state vector data, state vector covariance data, and force model data from a ground station; anddetermining the nominal orbital path locally at the navigation satellite using the satellite state vector data, state vector covariance data, and force model data. 7. The method of claim 1, wherein the ephemeris data is long duration ephemeris data. 8. A method of adjusting a navigation satellite orbit, the method comprising: determining a nominal orbital path of the navigation satellite;transmitting ephemeris data corresponding to the nominal orbital path from the navigation satellite to a plurality of navigation devices;determining an actual orbital path of the navigation satellite locally at the navigation satellite, wherein the determining an actual orbital path comprises: measuring non-gravitational disturbance force effects locally at the navigation satellite to determine a relative position of the navigation satellite, wherein the measuring is performed by a drag free sensor of the navigation satellite, wherein the drag free sensor comprises a proof mass enclosed by a vacuum caging system, andusing the relative position to determine the actual orbital path;determining a deviation between the actual orbital path and the nominal orbital path locally at the navigation satellite; andautonomously adjusting the actual orbital path locally at the navigation satellite to reduce the deviation between the actual orbital path and the nominal orbital path. 9. A navigation satellite comprising: a receive antenna adapted to receive ephemeris data from a ground station, wherein the ephemeris data corresponds to a nominal orbital path of the navigation satellite;a memory adapted to store the ephemeris data;a transmit antenna adapted to transmit the ephemeris data to a plurality of navigation devices;a plurality of propulsion actuators;a drag free sensor adapted to measure non-gravitational disturbance force effects locally at the navigation satellite to determine a relative position of the navigation satellite;an assembly adapted to maintain the drag free sensor substantially at a center of gravity of the navigation satellite; anda processor adapted to: determine an actual orbital path of the navigation satellite using the relative position,determine a deviation between the actual orbital path and the nominal orbital path, andcommand the propulsion actuators to adjust the actual orbital path to reduce the deviation between the actual orbital path and the nominal orbital path. 10. The navigation satellite of claim 9, further comprising a local navigation device adapted to determine an actual position of the navigation satellite, wherein the processor is adapted to determine the actual orbital path using the actual position. 11. A navigation satellite comprising: a receive antenna adapted to receive ephemeris data from a ground station wherein the ephemeris data corresponds to a nominal orbital path of the navigation satellite;a memory adapted to store the ephemeris data;a transmit antenna adapted to transmit the ephemeris data to a plurality of navigation devices;a plurality of propulsion actuators;a drag free sensor adapted to measure non-gravitational disturbance force effects locally at the navigation satellite to determine a relative position of the navigation satellite, wherein the drag free sensor comprises a proof mass enclosed by a vacuum caging system; anda processor adapted to: determine an actual orbital path of the navigation satellite using the relative position,determine a deviation between the actual orbital path and the nominal orbital path, andcommand the propulsion actuators to adjust the actual orbital path to reduce the deviation between the actual orbital path and the nominal orbital path. 12. The navigation satellite of claim 9, wherein the assembly comprises: a plurality of inertial angular motion sensors adapted to measure angular motion acceleration vectors associated with the drag free sensor;a plurality of rollers;a plurality of extendable members connected with the rollers and the drag free sensor; anda plurality of sensor actuators connected with the extendable members and adapted to adjust a position of the drag free sensor until a sum of the angular motion acceleration vectors is approximately zero.