Modelling And Assessment Of The Current And Future Space Surveillance Network

Abstract

Two-Line Elements (TLEs) resulting from the Space Surveillance Network (SSN) are often the only available source for many Space Situational Awareness (SSA) activities such as conjunction analyses and re-entry predictions. The network consists of many different radar and optical stations, contributing in either a dedicated, collateral and contributing fashion. For low-Earth orbit radar stations are primarily employed. Radar station can be further distinct into phased-array and mechanically steered radars. Uncertainties in TLEs are primarily a product of sensor capabilities, model deficiencies, network geometry and configuration, and orbit determination setup. The aim of the paper is to separate and analyse the individual contributions and identify potential areas of improvement. Specifically, the configuration and geometry of the network is investigated. Only the LEO satellites are considered. Further, although mechanically steered radars are generally much more accurate, only phased-array radar stations, capable of tracking many object, are considered. The SSN is simulated as a collection of present and hypothetical dedicated and collateral radar stations. An investigation into the current state of the SSN is performed. The location and coverage of each station is accurately estimated and modelled. Range and range-rate measurements are generated for several setups over a number of revolutions. Noise is introduced to individual observations to account for sensor capabilities (e.g. power and resolution) and secondary disturbances. Lastly, observations are edited out to account for sensor availability, viewing conditions, and other limitations. TLEs are estimated from the simulated measurements using Simplified General Perturbations (SGP4) model. GOCE during its re-entry phase is used as the reference satellite. During this final phase the thruster was switched off. GPS derived orbits are considered as truth for assessing TLE accuracy. The optimal number of measurement and passes are investigated to achieve the best state estimates. The resulting simulated TLEs are compared against actual historical TLE estimates. Several network configuration consisting of different combinations of stations are investigated, including historical, present, proposed and hypothetical scenarios. Individual sensors are shown to be the primary factor in the overall accuracy of the initial state. Sensor accuracy, however, is difficult to improve, due to the associated cost and trade-off between accuracy and capacity. The network itself is found to be underrepresented in large regions of the world. The new proposed space fence, with sites in the Marshall Islands and Western Australia, improves the geometry and overall accuracy significantly.