Published17 Feb Abstract In recent years, an increasing number of countries have shown a growing interest in developing their indigenous space capacity building through national small satellite programs. These satellites, which were initially focused on educational and training missions, currently are more scientific and operational-oriented. Thus, small satellite missions are being considered not only as educational tools but also as technological demonstrators or, even, mature enough for commercial and scientific missions, which might generate a huge amount of data to be transmitted to the ground segment. Therefore, an increasing demand on channel capacity will be needed for downloading the generated housekeeping and scientific data for missions based on small satellites. This paper analyses the communication subsystem of a real Cubesat. The influence of geometrical parameters is rigorously calculated both in the signal-to-noise ratio and in the capacity to transmit information.
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Published17 Feb Abstract In recent years, an increasing number of countries have shown a growing interest in developing their indigenous space capacity building through national small satellite programs. These satellites, which were initially focused on educational and training missions, currently are more scientific and operational-oriented. Thus, small satellite missions are being considered not only as educational tools but also as technological demonstrators or, even, mature enough for commercial and scientific missions, which might generate a huge amount of data to be transmitted to the ground segment.
Therefore, an increasing demand on channel capacity will be needed for downloading the generated housekeeping and scientific data for missions based on small satellites. This paper analyses the communication subsystem of a real Cubesat. The influence of geometrical parameters is rigorously calculated both in the signal-to-noise ratio and in the capacity to transmit information.
Subsequently, which parameters of the radio link can be modified to increase the transmission capacity, including the pointing requirements and its practical implementation, is studied. Finally, and as a future line, the technical feasibility of using optical links on small satellites that might greatly increase the transmission capacity, including the satellite pointing problems that presents, is presented. In conclusion, this paper presents a rigorous calculation in different frequency bands of the signal-to-noise ratio and the pointing accuracy that is needed to achieve the maximum transmission speed from the satellite to the ground station, and therefore the requirements that the Attitude and Orbital Control Systems AOCS must have, as well as the limitations of current systems.
Introduction A small satellite is a spacecraft able to provide, within a low-cost framework, not only educational purposes but also space services and applications, reducing cost, facilitating the launch process as piggybacks or low-weight payloads, using cheaper designs easing the Assembly, Integration and Verification process , and allowing mission disaggregation.
They are built in quick timescales, at relatively low cost, and make maximum use of state-of-the-art commercial-of-the-shelf COTS technologies to achieve complex functionality, while at the same time minimizing dependence on complex mechanisms and deployable structures. Both the technology evolution and electronics miniaturisation have promoted a fast-growing path for the integration of this sort of spacecrafts in daily applications as well as in the university environment, where the design and development of small satellites provides a highly valuable experience to students and researchers opening up a low-cost space gateway.
Its main purpose is to approach space research opportunities to universities by defining a standard mechanical interface and deployment system for small satellites. The basic structure of a Cubesat 1U is a 10 cm wide cube with a mass of up to 1. It is a solid and skeleton-like structure made of aluminium whose weight can be reduced. Cubesats have undergone a large evolution in a few years. From the first Cubesats launched in - which were mostly test beds for technologies that could be applied to space systems and which were based on COTS components - to current Cubesat missions to provide direct services and applications, there is a really large leap in the development of their technology.
The first Cubesats built were mere student projects to provide some experience in space systems. These systems were mostly based on commercial components and had payloads which were purposefully built. Even though these satellites were complete and functional systems, they were not mature enough to sustain a complex mission or guarantee any performance.
Lately, Cubesats have been and will be used for missions planned for the future for mainly three kinds of missions: i Technology demonstrators ii Scientific missions iii Services Currently, the new space industry strongly believes that Cubesats will be used for low-cost space services and missions.
One of the main issues regarding the range of missions that Cubesats can perform is the payload accommodation. Cubesats have strict limitations in terms of mass and volume, which is not the common mindset in classical spacecraft design. A payload that carries a couple of Petri boards for a biology experiment may be larger than a Cubesat itself when designed to be integrated, for example, in the International Space Station. Therefore, the challenge is to adapt classical payloads so that they can be integrated in a Cubesat.
Two key factors must be taken into account when talking about payload adaptation for Cubesats: i Electronics Miniaturisation.
Electronic devices have undergone a miniaturisation trend in the last years. A single chip can provide a whole computer system with very low power consumption, weight, and volume, which makes them ideal for Cubesats. Building and launching multiple Cubesats is in most cases much cheaper than building a single payload with an incredible amount or redundancy. This mindset change allows miniaturisation to become a predominant aspect in the design of these spacecrafts.
All these orbital deployers can be employed for injecting Cubesats into appropriate orbits, which will lead to a single satellite or a constellation of satellites that will be able to achieve a certain level of performance. In Section 2 , a complete analysis of the link budget of the Xatcobeo satellite on a UHF radio amateur band is introduced, including how to compute the antenna noise at the spacecraft and the ground station antennas.
In Section 3 , a number of alternatives to increase the communication capacity are analysed, including the use of higher carrier frequencies, which allows increasing the antenna gain in the satellite and in the ground station, as well as optical links.
Finally, a feasibility analysis of AOCS for the different situations is presented. The readers of this paper, using the rigorous calculation of the signal-to-noise ratio and the pointing accuracy presented in it, can understand the requirements that AOCS must have, as well as the limitations of current systems in different frequency bands. Xatcobeo Satellite Xatcobeo [ 4 ] was the first satellite designed, manufactured, and operated by the University of Vigo.
It was launched on the Vega maiden flight, as part of the Educational Payload, and it can be considered as an example of a typical university Cubesat demanding a reduced transmission capacity. It was selected as an educational project focused on providing hands-on experience to undergraduate and PhD students.
This Cubesat included two payloads: a software configurable radio and a system for measuring the amount of ionizing radiation. There was also an experimental system for solar panel deployment validation. Xatcobeo was launched on February 13, , and re-entried on August 31, This section presents the Xatcobeo [ 5 ] communication subsystem, including a complete analysis of the link budget.
Xatcobeo Communication Subsystem The housekeeping and scientific data generated in a typical orbit of Xatcobeo was only bytes. Considering 29 data bytes per frame, only 54 frames were necessary to transmit the complete gathered data in each orbit.
Therefore, a bps communication link was enough for guaranteeing the necessary data exchange. Xatcobeo Link Budget The link budget is the comparison among the power given to the transmitter, the amount of power available in the receiver, and the noise at the same point of the receiver. When a communication satellite is used, there are two different link budgets, the uplink from the ground station to the satellite and the downlink from the satellite to the ground station.
The first step previous to calculating the link budgets is the determination of the parameters of the orbit of the satellite, which gives us the parameters needed to calculate the link budgets. Table 1 shows the parameters used in the analysis. STK input.
AOCS Requirements and Practical Limitations for High-Speed Communications on Small Satellites
Taukora The maximum rotation speed of the reaction wheels is rpm. All eventual failures must be detected. Based on telemetry data received from the satellite and sztellite data obtained from the tracking system, the control system is used to correct the position and attitude of the satellite. The TTCM subsystem present at earth station monitors the position of satellite. Altitude control subsystem takes care of the orientation of satellite in its respective orbit. You can also try this books 1. For redundancy XMM-Newton carries a second star-tracker telescope.
Satellite Communication - Subsystems