Research On Key Techniques Of Autonomous Navigation For Satellite Based Navigation Constellation

The autonomous operation of navigation constellation with inter-satellite links is an important technique to meet the navigation warfare demand under complicated circumstances. On the one hand, the application of this technique can reduce the depends of the satellite on the ground operational control segment, and after losing the contact with the operational control segment, the satellite can achieve autonomous operation for a certain time period, so as to enhance the entire survivability of the system. On the other hand, some satellite navigation systems, such as the BeiDou Navigation Satellite System of China(BDS) and the Global Navigation Satellite System of Russia(GLONASS), can hardly establish ground observation stations in the world due to the international political influence. As a result, the visible arcs for the satellites will be limited, and the system will be unable to maintain high precision running in the global scope. An effective way to solve this problem is the combined use of inter-satellite links and the satellite-station links of the native ground observation station. So the study on the technique of constellation autonomous navigation(AutoNav) has an important engineering value and strategic significance. In this context, the paper carried out the following aspects of work:(1)The traditional on-board numerical integrator uses satellite’s position and velocity as its state variable, whose accuracy is limited due to the computing capability of the satellite platform. A new integrator is proposed for the AutoNav constellation, which uses the orbital element as its state variable instead. After one-day propagation with 4th-order Runge-Kutta method and 15-minute step size, the traditional integrator leads to a orbital error of 6km,while the proposed integrator’s error is only 5cm.To achieve the same integration accuracy, the proposed integrator needs less computational time than the traditional one. The reduction percentage improves as the order of the earth non-spherical perturbation grows, whose value is 20% for 8*8 model and 40% for 20*20 model.(2)Under the condition of inter-satellite measurements, traditional Least Squares method fails to solve the satellite orbit parameters.A weighted parameter adjustment algorithm is proposed for the orbit determination problem in AutoNav. The inter-satellite ranging and velocity measurement matrix are both rank deficient, which results in the uncorrected system error of the longitude of ascending node(?). The weighted parameter adjustment algorithm uses both the a priori information of the orbit and the inter-satellite ranging datas to autonomously determine the satellite orbit under the rank deficient condition.If the inter-satellite velocity measurements are added in,the performance can improve further, while the position accuracy improvement is relatively small and the velocity accuracy improvement is relatively large. Further study shows that if the inter-satellite link can achieve decimeter-level or higher ranging precision, the added velocity measurements have little improvement on the adjustment performance of the constellation.(3) The earth orientation parameters(EOP) have prediction errors in the autonomous navigation mode and the satellite will have a large position error in the Earth-Centered Earth-Fixed coordinator. A mitigation technique is proposed based on the anchor station. The EOP are predicted for 180 day using the classic Fourier series model, and the largest prediction error will lead to a constellation deviation of 168.4m.The major deviation lies on the horizontal component and the radial component is relatively small and both components cannot be eliminated via inter-satellite links. While inducing the anchor station, the influence of the prediction error is effectively mitigated under the conditions of maximum EOP’s prediction error. The constellation position error is reduced to 1.7m with the inter-satellite and satellite-station links both at the ranging accuracy of 2m and the error will be further reduced as the anchor station ranging accuracy increases and the latitude of the anchor decreases.(4) The system error of ? is unobservable and leads to the rigid rotation of the entire navigation constellation. For the system level and terminal user level respectively, two mitigation techniques are proposed, which are the combined inter-satellite links and the satellite-ground links Rotation Mitigation Technique(CRMT) and the Differential Rotation Correction Technique(DRCT).The validity of CRMT for system level is analyzed and verified with simulations. With the ? error of 20″, the CRMT reduces the user range error(URE) from 289 m to 3.8m, and URE reduces as the ground ranging accuracy improves. If the ground ranging has relatively high precision, the increase of the ground stations has little improvement on URE.The basis of the DRCT is that the system error of ? will lead to an equivalent deviation for terrestrial users’ geodetic longitude, which is strictly proved from the mathematical point of view. The validity of DRCT for terminal user level is analyzed and verified with simulations. If the system error of ? is less than 1 ’, the horizontal error of the ground user is less than 1.5m and the altitude error is less than 3mm after using the DRCT.The differential correction capability may increase as the latitude of the base station decreases.(5) According to the testing and verifying requirement of autonomous navigation system, a semi physical testing system is designed and the autonomous navigation performance is verified for 60 days. In the research of the inter-satellite signal simulation subsystem, the relativistic effect error is presented for inter-satellite measurement in hybrid constellation for the first time, and the ranging measurement exists a change of 8.2us/day between MEO and GEO satellites; A 3rd-order numerically controlled oscillator is used to generate the real-time ranging signal with the large Doppler frequency offset between satellites, and the influence of truncation errors induced by hardware implementation is analytically presented. In the research of the autonomous navigation solution subsystem, the two-way inter-satellite ranging mode is adopted to decouple the position and clock resolving of the satellite; The ephemeris filtering algorithm is proposed based on the sum of the two-way ranging measurements, and the clock filtering algorithm is proposed based on the difference of the two-way ranging measurements. Based on the simulation conditions set in the paper, the testing results of the autonomous navigation performance show that after 60 days the URE of the constellation will be 10 m with inter-satellite links only. While inducing the anchor station, the URE reduces to 1m.The research findings of this paper have been applied to the demonstration of the autonomous navigation and the development of ground testing system project in China’s BDS.