Research On Ultra-tight GNSS/INS Integration For Autonomous Spacecraft Navigation

Autonomous navigation systems (NavSys) are spaceborne apparatus todetermine the position, velocity and attitude of spacecraft in real time, withoutexternal aids. Technical requirements include, on the premise of keeping security andreliability of spacecraft, promoting the performance of autonomous NavSys forspacecraft’s different mission phases, but with decreased volume, weight, powerconsumption and cost. Preliminary research in this thesis was funded by the ShanghaiAcademy of Spaceflight Technology in2007(Contract NO. YYF08001). On the basisof the preliminary research of the project, the objective of this research is to introducemodern strapdown algorithms to spacecraft INS, and to design realizable space tightlyintegrated GNSS/INS NavSys and characterize its performance in orbit based oncurrent navigation-grade inertial sensors. And this research would also give attentionto the theoretical algorithms and advantages of ultra-tight coupling or deep integration,and make sure whether or the new integration technology is necessary and feasible forspaceflight applications. Furthermore, the thesis would like to explore a highlyintegrated space ultra-tight coupling NavSys with supreme reliability and integrity toimprove spacecraft performance and survivability.The thesis penetrates into space SINS technology, GPS measurements errormodels and simulator for spaceflight, the design of space integrated GPS/INS NavSys,ultra-tight coupling GNSS/INS integration and space integral ultra-tightly coupledreceiver, etc. The primary research contributions and technical approaches arepresented here:(1) Space-oriented SINS scheme based on modern multispeed strapdownalgorithms is realized, and the INS error dynamic equations in ECI are derived. Asimulation toolbox is built to generate inertial sensor measurements. By virtue ofMonte Carlo (MC) simulation, evaluations of performance of the new scheme andconventional SINS algorithms are carried out.(2) For the sake of R&D (research and development) of space NavSys, ahigh-fidelity GPS simulator is provided, which generates GPS L1C/A codepseudorange and carrier phase measurements. The simulated measurements are used as GPS measurement input to update space tightly integrated navigation filter.(3) Considering the technical requirements of autonomous spacecraft NavSys,the thesis proposes a tightly integrated GPS/INS NavSys with EKF for spaceflight. Ahigh-credibility simulation platform of space navigation is also constructed, forverifying the autonomous navigation accuracy in orbit.(4) A flexible low-cost time synchronizer with FPGA is developed, whichsynchronizes the measurements from IMU and other navigation sensors with GNSSreceiver time, mitigating unknown time errors among different apparatus.(5) As coherent and non-coherent algorithms of deep integration are studied, therelated vector tracking loop is discussed in depth firstly. An ultra-tight couplingNavSys with federated Kalman filter architecture in terms of the coherent algorithm istheoretically addressed. Consequently, the research tentatively proposes the schematicdesign of a space integral ultra-tightly coupled MEMS IMU/GNSS receiver. Andmore attentions are paid to its deep integration algorithms.The key technologies and innovations in the research focus on the followingpoints:(1) As the space-oriented SINS scheme based on modern multispeed algorithmsare proposed, the thesis draws up a series of instructive guidelines, depending onapplications, to promote practical realization of the multispeed scheme, withconsideration on various optimizations and tradeoffs where possible. Performanceimprovement of the multispeed scheme is verified using MC method withmeasurements from the IMU simulation toolbox.(2) In the GPS measurements simulator, an ionospheric model for LEO isadopted, and receiver clock model is also fixed for spaceflight usage. Range rate’scontributions to spaceborne GPS receiver measurements are considered. In this waythe accuracy and fidelity of the simulator is promoted. The tightly integrated GPS/INSEKF includes the URE states of all visible GPS satellites at any simulation epoch,which achieves optimal navigation results from INS and all-in-view GPS receiver.(3) With investigations on GNSS/INS ultra-tight integration, the new proposedtentative concept on space integral ultra-tightly coupled receiver is meaningfulexploration in the field of spacecraft navigation.(4) The flexible time synchronizer synchronizes measurements from IMU andother navigation sensors with GNSS receiver time, in which synchronized time of themeasurements reaches high accuracy, and further effectively improves performance ofintegrated NavSys. In accordance with the former research, explicit conclusions are summarized as:(1) One clear advantage of the space-oriented multispeed SINS scheme is of highaccuracy. Moreover, high computational efficiency inherited from modern multispeedstrapdown algorithms is a secondary benefit of the new scheme. Attitude errorsobtained from both the multispeed scheme and conventional algorithm stay at theequivalent level. Velocity and position errors of the multispeed scheme are onlyone-fourth of that of the latter.(2) Driven by the true reference trajectory of spacecraft, the space integratedGPS/INS NavSys is tested using the high-fidelity simulation platform. Simulationresults show that: the attitude error of pitch axis is about0.1deg; the attitude accuracyof roll and yaw axes is better than0.03deg; the RMS error of velocity is smaller than0.2m/s and the SEP error of position is smaller than3m. The results prove that thespace integrated GPS/INS NavSys can readily satisfy requirements of high-precisespacecraft navigation.(3) Onboard simulation experiments and a field van test of the low-cost timesynchronizer are delicately carried out. Experimental results demonstrate that: thesynchronizer can achieve an accuracy of dozens of microseconds between IMU andGNSS receiver. The time synchronization error is below dozens of nanoseconds.