The principle of a navigation constellation composed of SIGSO communication satellites

The Chinese Area Positioning System （CAPS）, a navigation system based on geostafionary orbit （GEO） communication satellites, was developed in 2002 by astronomers at Chinese Academy of Sciences. Extensive positioning experiments of CAPS have been performed since 2005. On the basis of CAPS, this paper studies the principle of a navigation constellation composed of slightly inclined geostationary orbit （SIGSO） communication satellites. SIGSO satellites are derived from GEO satellites which are near the end of their operational life by inclined orbit operation. Considering the abundant frequency resources of SIGSO satellites, multi-frequency observations could be conducted to enhance the precision of pseudorange measurements and ameliorate the positioning performance. A constellation composed of two GEO satellites and four SIGSO satellites with an inclination of 5° can provide service to most of the territory of China with a maximum position dilution of precision （PDOP） over 24 h of less than 42. With synthetic utilization of the truncated precise code and a physical augmentation factor in four frequencies, the navigation system with this constellation is expected to obtain comparable positioning performance to that of the coarse acquisition code of the Global Positioning System （GPS）. When the new method of code-carrier phase combinations is adopted, the system has the potential to possess commensurate accuracy with the precise code in GPS. Additionally, the copious frequency resources can also be used to develop new anti-interference techniques and integrate navigation and communication.

Demand and key technology for a LEO constellation as augmentation of satellite navigation systems

A Low Earth Orbit(LEO)constellation augmenting satellite navigation is important in the future development of Global Navigation Satellite System(GNSS).GNSS augmented by LEO constellations can improve not only the accuracy of Positioning,Navigation,and Timing(PNT),but also the consistency and reliability of secure PNT system.This paper mainly analyzes the diverse demands of different PNT users for LEO augmented GNSS,including the precision demand in real-time,the availability demand in special areas,the navigation signal enhancement demand in complex electromagnetic environments,and the integrity demand with high security.Correspondingly,the possible contributions of LEO constellations to PNT performance are analyzed from multiple aspects.A particular attention is paid to the special PNT user requirements that cannot be fulfilled with existing GNSS,such as the PNT service demand in the polar regions and the onboard GNSS orbit determination demand of some LEO satellites.The key technologies to be considered in the constellation design,function realization,and payload development of the LEO-augmented navigation system are summarized.

Autonomous navigation method of satellite constellation based on adaptive forgetting factors

To address the problem that model uncertainty and unknown time-varying system noise hinder the filtering accuracy of the autonomous navigation system of satellite constellation,an autonomous navigation method of satellite constellation based on the Unscented Kalman Filter with Adaptive Forgetting Factors(UKF-AFF)is proposed.The process noise covariance matrix is estimated online with the strategy that combines covariance matching and adaptive adjustment of forgetting factors.The adaptive adjustment coefficient based on squared Mahalanobis distance of state residual is employed to achieve online regulation of forgetting factors,equipping this method with more adaptability.The intersatellite direction vector obtained from photographic observations is introduced to determine the constellation satellite orbit together with the distance measurement to avoid rank deficiency issues.Considering that the number of available measurements varies online with intersatellite visibility in practical applications such as time-varying constellation configurations,the smooth covariance matrix of state correction determined by innovation and gain is adopted and constructed recursively.Stability analysis of the proposed method is also conducted.The effectiveness of the proposed method is verified by the Monte Carlo simulation and comparison experiments.The estimation accuracy of constellation position and velocity of UKF-AFF is improved by 30%and 44%respectively compared to those of the extended Kalman filter,and the method proposed is also better than other several adaptive filtering methods in the presence of significant model uncertainty.

Long-term semi-autonomous orbit determination supported by a few ground stations for navigation constellation

In view of the present technology of autonomous orbit determination for navigation satellite constellation(NSC) and the geographical conditions of China,we propose a long-term semi-autonomous orbit determination scheme supported by a few ground stations for NSC in this paper.Since the effect of rotation and translation of the entire constellation relative to the inertial reference frame can bring large errors to the autonomous orbit determination using only cross-link range measurement,a few ground stations(such as 1-3) are supposed to construct the connection between the NSC and the ground.Supported by such a few ground stations,the NSC can realize long-term orbit determination called semi-autonomous orbit determination.The simulation results based on the IGS ephemeris indicate that,for a certain degree of measurement errors,the NSC can maintain its semi-autonomous orbit determination in a period of 240 days within 5 meters of URE.

Feasibility study of autonomous orbit determination using only the crosslink range measurement for a combined navigation constellation

In order to expand the coverage area of satellite navigation systems, a combined navigation constellation which is formed by a global navigation constellation and a Lagrangian navigation constellation was studied. Only the crosslink range measurement was used to achieve long-term precise autonomous orbit determination for the combined navigation constellation, and the measurement model was derived. Simulations of 180 days based on the international global navigation satellite system（GNSS） service（IGS） ephemeris showed that the mentioned autonomous orbit determination method worked well in the Earth–Moon system. Statistical results were used to analyze the accuracy of autonomous orbit determination under the influences of different Lagrangian satellite constellations.