Jizhang Sang

A Novel Method for Precise Onboard Real-Time Orbit Determination with a Standalone GPS Receiver.

Satellite remote sensing systems require accurate, autonomous and real-time orbit determinations (RTOD) for geo-referencing. Onboard Global Positioning System (GPS) has widely been used to undertake such tasks. In this paper, a novel RTOD method achieving decimeter precision using GPS carrier phases, required by China’s HY2A and ZY3 missions, is presented. A key to the algorithm success is the introduction of a new parameter, termed pseudo-ambiguity. This parameter combines the phase ambiguity, the orbit, and clock offset errors of the GPS broadcast ephemeris together to absorb a large part of the combined error. Based on the analysis of the characteristics of the orbit and clock offset errors, the pseudo-ambiguity can be modeled as a random walk, and estimated in an extended Kalman filter. Experiments of processing real data from HY2A and ZY3, simulating onboard operational scenarios of these two missions, are performed using the developed software SATODS. Results have demonstrated that the position and velocity accuracy (3D RMS) of 0.2–0.4 m and 0.2–0.4 mm/s, respectively, are achieved using dual-frequency carrier phases for HY2A, and slightly worse results for ZY3. These results show it is feasible to obtain orbit accuracy at decimeter level of 3–5 dm for position and 0.3–0.5 mm/s for velocity with this RTOD method.

A Real-Time Orbit Determination Method for Smooth Transition from Optical Tracking to Laser Ranging of Debris

A critical requirement to achieve high efficiency of debris laser tracking is to have sufficiently accurate orbit predictions (OP) in both the pointing direction (better than 20 arc seconds) and distance from the tracking station to the debris objects, with the former more important than the latter because of the narrow laser beam. When the two line element (TLE) is used to provide the orbit predictions, the resultant pointing errors are usually on the order of tens to hundreds of arc seconds. In practice, therefore, angular observations of debris objects are first collected using an optical tracking sensor, and then used to guide the laser beam pointing to the objects. The manual guidance may cause interrupts to the laser tracking, and consequently loss of valuable laser tracking data. This paper presents a real-time orbit determination (OD) and prediction method to realize smooth and efficient debris laser tracking. The method uses TLE-computed positions and angles over a short-arc of less than 2 min as observations in an OD process where simplified force models are considered. After the OD convergence, the OP is performed from the last observation epoch to the end of the tracking pass. Simulation and real tracking data processing results show that the pointing prediction errors are usually less than 10″, and the distance errors less than 100 m, therefore, the prediction accuracy is sufficient for the blind laser tracking.

Improved orbit prediction of LEO objects with calibrated atmospheric mass density model

Abstract The accuracy of atmospheric mass density models is critical for OP (orbit prediction) of LEO (low earth orbit) space objects. This paper studies the effects of three methods on the improvement of OP accuracy for LEO objects, where only TLE (two-line element) data are used. Fifty objects at altitudes between 400 and 600 km are chosen for the study, with 20 or 40 being used as calibration objects, and the others as verification objects. The results show that about a 30 percent reduction in OP errors can be achieved for a 10-day prediction time.

A Method of Estimating Atmospheric Mass Density from Precision Orbit Solution

This paper presents a method of estimating atmospheric mass densities from precise orbit data of low earth orbit (LEO) space objects. The method is based on the drag perturbation equation of the semi-major axis of the orbit of LEO space objects which relates the change rate of the semi-major axis to the atmospheric mass density. The feasibility of applying the method is studied with simulations. The method is experimented using the GFZ-ISDC GPS rapid science orbit (RSO) products of the CHAMP satellite over a time period of three months. The densities from this method and accelerometer are compared and good agreements are achieved. This method has a great potential to generate high quality atmospheric mass densities with a high temporal resolution for LEO satellite missions.

A multiscaling-based semi-analytic orbit propagation method for the catalogue maintenance of space debris

ABSTRACT The choice of orbit propagation method is essential for orbit prediction (OP) and determination (OD) of space debris, requiring both high accuracy and computational efficiency. This paper presents a semi-analytic method using the multiscaling technique. The 7-day OP errors are less than 200 m for orbits above 800 km. The 5-year semi-analytic solutions are well fitted to the numerically propagated orbit. OD performance of the semi-analytic method is examined using real data, and the determined position accuracy is at dozens of metres. The computational efficiency of the semi-analytic method against the numerical method is improved by about 95 percent.

Space-borne pseudo-range reconstruction and its performance analysis in dynamic orbit determination

The Global Positioning System (GPS) is nowadays widely used for navigation of spacecraft in low Earth orbit (LEO). Both real-time positioning information (navigation solutions) and raw GPS measurements (pseudo-range, carrier-phase, Doppler shift and signal-to-noise ratio) are usually transmitted to ground control centres for post-processing. However, for certain LEO missions, only navigation solutions are available due to the limited capability of space-borne GPS receivers. In this case, navigation solutions are commonly used as pseudo-observations in dynamic orbit determination (referred to as dynamic filtering) to obtain a smooth and continuous orbit; however, the achievable orbit accuracy is limited since broadcast ephemeris data from the GPS navigation messages are used and the more accurate GPS orbit and clock information are only available in post-processing. In this study, a method of reconstructing space-borne pseudo-range measurements from real-time positioning information is developed, thus, the more accurate GPS orbit and clock products from the International GNSS Service (IGS) are able to be used in dynamic orbit determination. Real GPS data from the TerraSAR-X mission are used to validate this approach and its performance in dynamic orbit determination is assessed as well. Results show that the orbit accuracy of navigation solutions can be significantly improved from 15 m to 5 m after a dynamic filtering process is performed. A remarkably better orbit accuracy of about 0.5 m is achieved in dynamic orbit determination using the reconstructed pseudo-range measurements and IGS rapid orbit and clock products; both the requirements of 24-h timeframe and an orbit accuracy of 2 m for generating proper SAR images are well satisfied.