Researches On Space-Based Surveillance System For Cataloging Space Debris

The number of on-orbit spacecraft is increasing gradually with continuous launch activities.On the other hand,dis-functional satellites,upper stages of the rocket as well as their fragmentation debris form a crowded space environment where tens of thousands of space debris reside.In order to avoid collision between debris and active satellites,it is necessary to catalog and maintain the orbits of the space objects.The catalog would also be useful for active debris removal and space traffic management.Cataloging and orbit maintenance of space objects are relying on the data collected by a space surveillance network.The United States has a space surveillance network(SSN)consisting of space-and ground-based facilities,which is well distributed around the world and has the best operational performance.The SSN currently catalogs about 23,000 debris objects.The catalog will soon be expanded sharply with the deployments of the new space fence system,5 3.5-m space surveillance telescopes(SST)and the space-based surveillance system(SBSS),which allow the regular surveillance of cmsize debris.The orbital cataloging of space debris includes both putting newly-found debris into the catalog(initial cataloging)and maintaining orbits of cataloged debris(catalog maintenance).In many cases,a debris may have only one short observed arc each day or even every two days.Therefore,there is a significant difference between the orbit cataloging of debris under the condition of sparse data and the precise satellite orbit determination with dense and highly accurate observations.First,under the sparse data condition,i.e.,a debris may have only one very short observed arc(VSOA)every 24~48h over which the data may be collected with a rate of a few Hz,the orbit determination(OD)accuracy would fluctuate in a wide range,even in the case that the number and length of VSOAs,the observation accuracy,and the distance from the observer to the debris are all the same.This phenomenon indicates that there are unknown factors affecting the debris OD accuracy.In this thesis,a phase parameter is firstly introduced to specify the relative position of a VSOA on a nearcircular orbit.Set the phase of the earliest VSOA to zero,the phase of a later VSOA is defined as the 360 degree-moduled geocentric angle between the VSOA and the earliest VSOA.Then the relation between the distribution of the VSOA phases and the OD accuracy is studied.Experiments show that,given 3 VSOAs of the same arc length and observation accuracy,the distribution of the 3 phases has a deciding effect on the OD accuracy,as well as the condition number of the correlation matrix derived from the covariance matrix of the orbit solution.When the 3 phases are uniformly distributed,the OD error reaches its minimum value,and the condition number is relatively small.When the phase of the middle VSOA is close to either that of the first or last VSOA,the OD error is at its maximum,and the condition number may be increased by one order of magnitude.In the case of the low Earth orbit(LEO)debris OD with 3 VSOAs over 2~3 days,the ratio of maximum to minimum OD errors could be as large as 3,and the ratio of the maximum to minimum condition numbers may reach 10.To measure the VSOA phase distribution,we use the area of the inscribed polygon of phase points on the unit circle as the main metric,and the condition number as the secondary metric,all of them can be obtained by straightforward computation.This finding provides a theoretical basis to schedule observation tasks with the limited surveillance resources,and to analyze the OD and orbit prediction accuracy under the sparse data condition.It may be also used to formulate the objective function for optimizing sensor locations.Second,due to the restrictions on the locations of ground-based surveillance stations and the sun-lit and weather conditions for their operations,the cm-size debris surveillance is most likely relying on the space-based optical technique in the foreseeable time.Therefore,it is of importance to make study on space-based optical surveillance system(SBOSS).SBOSS uses spacecrafts as observing platforms with onboard telescopes taking sky images from which angular observations of debris are extracted,and the angular data is in turn input to cataloging process.Set cataloging 200,000 LEO debris and 10 cm GEO debris as objectives,this thesis makes simulation studies on LEO SBOSS constellations and zero-inclination SBOSS satellite.Simulation results show that a LEO Walker constellation of 8 satellites each with a large-aperture and large field of view(FoV)(e.g.,1.5-m aperture,10° × 10° FoV)telescope is able to maintain a catalog of 200,000 LEO debris,among which about 120,000 can be constantly updated.For high orbit debris,a 4,163km-altitude zero-inclination satellite with a telescope of 1.5-m aperture and 20° × 20° FoV is able to make at least one VSOA each day for about 94.7% of 10 cm objects in the GEO belt.In addition,the initial orbit determination(IOD)and association of uncorrelated tracks(UCTs)with 496 VSOAs from 74 10 cm high-orbit debris over 7 days are assessed.The IOD success rate using the range-search based algorithm is 97.4%,with 70% of the estimated semimajor axes having errors less than 25 km.The rate of correctly associating two tracks originated from the same object is 89.7% when the pure geometrical track association method is applied,resulting in tracks belonging to 66 of 74 objects being correctly associated,which indicates 89.2% of observed objects can be put into a catalog in 7 days.It can be inferred from this experiment,therefore,the zero-inclination surveillance satellite can catalog the vast majority of high-orbit debris.Last,there is an engineering need to scientifically assess the variation of performance of a surveillance network due to changes of sensor parameters such as the location and FoV.This thesis makes such a study.The performance assessment is based on the simulated surveillance capability of sensors with different types,locations,parameters and operation modes.With this base,the AHP hierarchy analysis method is introduced to determine assessment criteria,and the ensemble method and rule-based reasoning method are used to calculate the performance merit and task execution level,respectively.The contribution level of a sensor is estimated by comparing the performance merits or the task execution levels before and after the removal of the sensor from the whole network.The whole process is reliable,in general,since it is based on the mathematical simulations,with the exempt that the experts' knowledge is used in the weight determination.A case study on the performance of cataloging space debris by a simulated space surveillance network composed of SBOSS satellite,ground-based optical surveillance stations and ground-based radars is presented.It shows that the contribution by the 600km-high SBOSS satellite is the largest,about 50% and 54%,respectively,assessed using the ensemble method and the rule-based reasoning method.This demonstrates the superiority of SBOSS in space surveillance.