  Precise (sub-meter level) real-time navigation using a space-capable
single-frequency global positioning system (GPS) receiver and ultra-rapid
(real-time) ephemerides from the international global navigation satellite
systems service is proposed for low-Earth-orbiting (LEO) satellites. The C/A
code and L1 carrier phase measurements are combined and single-differenced to
eliminate first-order ionospheric effects and receiver clock offsets. A
random-walk process is employed to model the phase ambiguities in order to
absorb the time-varying and satellite-specific higher-order measurement errors
as well as the GPS clock correction errors. A sequential Kalman filter which
incorporates the known orbital dynamic model is developed to estimate orbital
states and phase ambiguities without matrix inversion. Real flight data from
the single-frequency GPS receiver onboard China's SJ-9A small satellite are
processed to evaluate the orbit determination accuracy. Statistics from
internal orbit assessments indicate that three-dimensional accuracies of better
than 0.50 m and 0.55 mm/s are achieved for position and velocity, respectively.
