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Since the first spacecraft was put into space, scientists and engineers have been pondering ways in which they can remove satellites, after their end-of-life (EOL), from Earth orbits. Many people believe that the space surveillance is not really important, but in fact, the space situational awareness (SSA) in relation to space surveillance is an

important area of study, especially for the detection and monitoring of space objects. Others believe that the mitigation of orbital debris is a vital area of research that needs more attention for a better and sustainable use of Earth orbits in particular and space in general. The literature on space debris mitigation has described and discussed the difficulty and complexity of providing free access to space. Space is becoming increasingly congested with artificial space objects especially in low Earth orbit (LEO) and geostationary Earth orbit (GEO). In fact, space becomes an economic centre, and the space environment becomes one of the most dynamic areas in the space industry. Perhaps it is time to think more seriously about the space environment, and therefore, the goal of this project is to study in detail one of the proposed techniques to mitigate orbital debris in LEO and to increase the space situational awareness.

Investigating the effects of objects in the Earth orbits on the operational space systems including collision avoidance and debris mitigation measures is crucial for most space agencies. Currently, the research community focuses on issues related to SSA. Many of these problems arise due to the high number of satellites launched in orbits around the Earth, which eventually increases the number of space debris and space junk in the Earth orbits. That is why a major consideration in designing any satellite constellation is to provide the specified coverage area with the fewest number of satellites.

In order to perform a space objects observation, some obvious obstacles need to overcome such as the orbiting object must be in sunlight and the minimum elevation angle needs to be met. Therefore, more research needs to be conducted into looking at designing and describing concepts and ideas of an observation and tracking system to be used for the detection of faint objects and small space debris.

Orbital debris detection and tracking have been extensively studied over the past few decades as satellites operators principally demand these studies. There are a couple of software tools that have been used in industry to build trajectory prediction models of objects in space and provide a comprehensive analysis and simulation for early warning of potential collisions. Good examples of such software tools include systems tool kit (STK) and orbit determination tool kit (ODTK), which are both developed by Analytical Graphics, Inc. (AGI). MATLAB & Simulink software platforms, which are developed by The MathWorks, Inc. can also be used for orbital modelling. These computer software programmes are useful tools to create, design and display the

trajectory of space objects easily from user input parameters through a graphical user interface (GUI). These software tools are very useful in modelling the interaction of satellites with the space environment and small debris. Space debris impact risk assessment tool (SDIRAT) is a software code that is specifically developed for orbital debris density, relative velocity and directional flux estimation on a target satellite. This software has been used by Pardini [ 6 ] to obtain snapshots of the evolving object distribution during the considered time span, together with an estimation of the changing collision probability with a satellite of the operational navigation systems in medium Earth orbit (MEO). Traditional satellite constellation design has focused on achieving global or zonal coverage while minimising the necessary number of satellites in the constellation, which is the main goal for most satellite constellation designers. The satellite visualization (SaVi) tool, which was developed by Worfolk at the Geometry Center at the University of Minnesota and maintained by Wood at the University of Surrey is an another straightforward and useful software tool for designing the trajectory of a satellite constellation [7].

The space surveillance network (SSN) can be implemented by using satellites with capable radars and a network of electro-optical sensors that help to build up a catalogue containing the space objects. The large angular velocity of objects in space is one of the biggest challenges in optical observations of objects in Earth orbits, especially in LEO. Thus, the exposure times must be very short, and the telescope aperture needs to be large enough to detect objects. In fact, large telescopes are required for observation and tracking since small objects also need to be observed [8]. The Haystack 37 m radar is a good example of the space surveillance mechanical tracker. It is capable of imaging near-Earth and deep space objects [9]. It conducts measurements of orbital debris to sizes of 1 cm. Another example is the Millstone Hill Radar (MHR), which is a high power sensitive radar that routinely tracks debris, rocket bodies and satellites in the geostationary belt. It produces highly accurate orbital data, due to its high precision. Chilbolton Observatory has a high-power fully steerable radar with 25 m diameter dish antenna, see Figure 2, that has been used to carry out radar observations on intact satellites under the ESA’s SSA Preparatory Programme [ 10 ]. Future satellite constellation could be used to observe and monitor space junk in LEO to deliver space surveillance capability and protect operational space missions. We believe that a cohesive methodology for developing a capacity for detection and tracking of space

objects in the upper part of the LEOs region and the near-GEO regime is needed if we are looking for a sustainable free access to space. This is demanding especially with an ability to detect a substantial number of small debris as small as 1 cm up to 100 cm, to build-up and maintain satellite catalogues.

Figure 2: Chilbolton 25 m radar

The next generation of radars and telescopes will allow us to track moving targets in uncrowded areas from space-based radar satellites. This can be achieved by using a constellation of satellites in LEO. One of the major considerations in such a system is the number and the size of individual satellites in the constellation because it determines the overall constellation capacity. In general, coordinating smaller satellites has some benefits over a single satellite, including simpler designs, cheaper to launch, faster build times, cheaper replacement in addition to the constellation ability to search targets at multiple times or from multiple angles. Kayal [11] proposed a satellite constellation for the detection of orbital objects in space. He has briefly described his proposal and focused on using lightweight and low-cost nanosatellites for the detection of space objects in orbit. However, large satellites have other benefits such as generating more onboard power from its solar panels, higher bandwidth, higher bit rate, longer lifetime (15 years in average), carry large telescopes with high viewing resolution. Having said that, the bigger the satellite dry mass, the more the onboard fuel mass is needed to perform manoeuvres.