Researches On Key Technologies Of Fast And Accurate Orbit Determination And Prediction Of Space Debris

The orbital information of space objects,of which the vast majority is the space debris,is not only a critical piece of information for the space and defense security,but also the prerequisite of many space operations to be efficiently carried out,such as the space collision conjunction assessments,space surveillance and debris removal,and so on.China is on the highway of the space technology developments and applications.With the launch and operation of the manned spaceship,Tiangong Space Station,and more and more strategically important satellites,one key to guarantee their safety and efficient utilization of the precious orbit resources is to have a fully and more accurate understanding of the space debris environment.There are approximately 18,000 unclassified Earth orbiting objects in the NORAD catalogue,about 6%of which are the functional satellites.These objects are mostly larger than 10cm in diameter.According to the National Aeronautics and Space Administration(NASA)data,it is estimated there are more than 500,000 space debris larger than 1cm in diameter.They pose realistic threats to the current space applications.Space collisions are reportedly becoming more frequent,highlighted by the collision between Iridium 33 and Cosmos 2251 in February 2009.This motivated the space situation awareness(SSA)community to search for the fast and accurate orbit determination(OD)and prediction(OP)for debris objects,which are required for the timely and reliable space conjunction warning.The debris OD and OP are essentially the same as the satellite OD and OP,but there are a few different features in terms of the practical applications.The satellite OD accuracy can reach centimeter level,and short-term OP meter level,while the debris OD accuracy is usually at dozens of meters,and the OP accuracy at several hundred or thousands of meters,or even worse.Hundreds of thousands of space debris involves millions of OD and OP computations,and this would demand an orbit integrator with both the accuracy and computing efficiency.The huge amount of the orbit data should be delivered to users in a manner of concise,without loss of accuracy and easy to use.The need for the orbital information of space objects drives the continuous improvements in the abilities of data acquisition,processing and use.This thesis,applying orbital mechanics fundamentals,is therefore focused on several key problems in the debris data chain:the highly efficient laser tracking which requires precision telescope pointing,the fast and accurate debris OD and OP,and debris orbit ephemeris presentation in a large-scale catalogue.The relevant theories and algorithms have been experimented extensively,and they are part of the basis in the developed orbit computation and analysis software for Earth orbiting objects.It is expected that the software would play an important role in the SSA capability development.To summarize,the work and contributions of this thesis mainly cover:1.A new real-time OD/OP algorithm for a smooth transition from optical to laser tracking of debris has been developed.The method use the short-arc(less than 2 minutes)angular observations and NORAD TLE/SGP4-derived positions as pseudo-observations to determine the orbit of the debris with a simplified force model,and then to predict forward from the last observation epoch to the pass end.The OP accuracy can satisfy the debris-pointing requirement of the narrow laser beam of the debris laser ranging(DLR)system,thus the DLR system can make full use of the valuable operation time to collect the ranging data.The key to use the method is to determine the appropriate number of the pseudo-observations and their corresponding weights,in order to make the OD computation converged to angular observations.The proposed algorithm is tested by using the simulated and real data under the ground-based and space-based debris tracking scenarios.Results show that,for the LEO space objects,the OD using angular observations over a 30s orbit arc can generate OP results in an accuracy of 10" for angles and 100m for distance,respectively.All the OD and OP computations are completed within 2s.The algorithm makes the DLR operations possible in a continuous,smooth,and automatic manner,and significantly improves the success rate and data collection efficiency.2.Compared with the lengthy time consumption using numerical orbit integration methods and the low accuracy using analytic methods in the OD and OP computation,the semianalytic satellite theory(SST)possesses advantages in both the accuracy and computing time.The SST method developed in this thesis,based on the multiscaling principle,is able to deal with more complicated perturbation forces,a feature superior to the SST based on the averaging method.The developed WHU-SST orbit propagator contains two main modules:a numerical integration module for the equations of motion of mean orbital elements,and an analytic module for the short-periodic variations of the mean elements.Experiments with space objects of different altitude,inclination and area-to-mass ratio show that the 7-day position prediction errors are less than 500m for an orbit above 700km and 200m for an orbit above 2000km.The 5-year OP results for an orbit of a shallow resonance agree well with the numerically-propagated orbit.For its performance in OD computation,two cases are demonstrated,one using dense data and the other using sparse data.In each case,two satellites,in a scenario similar to debris data availablity,are tested.The results show that the WHU-SST propagator can generate OD results in an accuracy of 50m for positions,just about 20m worse than those obtained by using the Cowell numerical propagator,but save nearly 95%of the computation time.These features make the proposed semianalytic method more attractive in the catalogue maintenance for hundreds of thousands of space debris.3.On the basis of the comprehensive analyses of the existing orbit ephemeris formats,two analytic ephemeris compress models to represent the precise orbit predictions for Earth orbiting space objects are developed,one termed TLE-based method and the other EE(equinoctial element)-based method.These two methods have the advantages of high accuracy,small storage,easy implementation,and fast computation,and thus are well suited for the development,maintenance and update of the catalogue for hundreds of thousands of space debris.In term of the ephemeris compress accuracy,the TLE-based method can achieve a representation accuracy of 100m,and the EE-based method obtains an accuracy of 50m to represent 5-day precise orbit positions for the near-circular orbits.For the eccentric orbits,both methods also show a highly accurate representation results.On the data storage aspect,if the 5-day precise orbit positions are output at 3 minute interval,there will be 2400 epochs which need about 300KB hard space.However,using the TLE-based method or EE-based method to represent them,only the coefficients of the fitting functions are stored and about 2KB-5KB memory is needed,saving significantly in the hard space and benefit for the file transmission and storage.In term of the computation time,the two orbit representation algorithms are both analytic ones,and thus extremely fast to compute orbit positions.For 100,000 debris objects,only 5.5 hours are consumed to reconstruct the 5-day precise orbits,only 5%of the time using the numerical orbit integration methods.4.Based on the existing platform,an orbit computation and analysis software for Earth orbiting objects has been developed.The software has the following main functions:orbit and tracking data simulations for the ground-based and space-based observations,accurate OD and OP using the numerical and semianalytic orbit propagation method,precise ephemeris compression,and so on.This software has been tested by a large number of experiments using the real and simulated data,showing its excellent reliability and validation,which can be used in the SSA field.