  Due to their low cost and low power consumption, single-frequency GPS
receivers are considered suitable for low-cost space applications such as small
satellite missions. Recently, requirements have emerged for real-time accurate
orbit determination at sub-meter level in order to carry out onboard geocoding
of high-resolution imagery, open-loop operation of altimeters and radio
occultation. This study proposes an improved real-time kinematic positioning
method for LEO satellites using single-frequency receivers. The C/A code and L1
phase are combined to eliminate ionospheric effects. The epoch-differenced
carrier phase measurements are utilized to acquire receiver position changes
which are further used to smooth the absolute positions. A kinematic Kalman
filter is developed to implement kinematic orbit determination. Actual flight
data from China small satellite SJ-9A are used to test the navigation
performance. Results show that the proposed method outperforms traditional
kinematic positioning method in terms of accuracy. A 3D position accuracy of
0.72 m and 0.79 m has been achieved using the predicted portion of IGS
ultra-rapid products and broadcast ephemerides, respectively.
