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Tools for the job

In this article Dr Tim Jackson looks in more detail at the tool required for powering a satellite in space.
A satellite in orbit over a cloud-covered earth as viewed from space.
© University of Birmingham / UKESF

Electronic Engineering in space

We’ll look at a satellite as a closed volume in space – an electrical system. The diagram below shows some of the inputs and outputs.

A flow diagram illustrating the inputs, outputs and internal parts of an electrical system in a satellite, the input is sunlight, the output is radio communications and heat, and the internal parts consist of a power generation, power management, battery and signal transmission blocks. (Click to expand)

Nanosatellites

We’re going to consider a “CubeSat” [1, 2]. The CubeSat programme at NASA in the United States of America enables education and research institutions to conduct experiments in space from low-cost miniature platforms also known as “nanosatellites”. The satellites are built from cubes about 10 cm on each side, with a volume of 1 litre and weighing less than 1.5 Kg. Up to 6 cubes can be stacked to make a single payload for launch. A typical mission might last for 120 days, after which a satellite begins a fall into the atmosphere and is burned up by the heat of re-entry.

The “transmit signals” sub-system contains three components, a waveform generator, a power amplifier to create a signal of sufficient strength to be reliably detected at the base station, and an antenna. A power of 0.5 W was sufficient in [3]. A battery capacity of 30 Wh was shown by modelling of the solar power generation to assure the supply of sufficient electrical power. The antenna presents a particular challenge. Like the solar panels aboard Tiangong-2, the antenna must be folded away inside the satellite at launch and then deployed in orbit. The transmitter would send signals twice a day to allow two receiver locations to be operational, during three orbits of the Earth.

An undergraduate project out of this world!

The modelling of electrical power generation is a sophisticated task yet something that can be tackled in a research project by an undergraduate student. Such a project was undertaken by a University of Birmingham student, Ade Adepegba, in 2015–16, for a 5x5x5 cm satellite.

Don’t worry if you don’t follow all the detail in this project, it gets fairly complicated! We really just want to highlight what a university-based project can look like, and this is a good example of the type of work a research project includes.

The first stage of the project was to simulate the tumbling of the satellite in orbit to get data for the angle of the solar panels mounted on five sides of the satellite to the sun at any instant within the orbit. This required knowledge of the orbital trajectory, a model of the rotation of a satellite around its own axes in orbit, and models of disturbances to the satellite from the friction with atmosphere, radiation from the Earth and the buffeting of the satellite by the solar wind. It was found that the mechanical disturbances to the satellite increased the rate of tumbling of the satellite around its own axes.

It was found that if a polar orbit was chosen instead of an equatorial orbit, the satellite would spend less time in eclipse (in the Earth’s shadow), allowing power to be generated for a greater proportion of the orbit. The higher the orbit, the lower the atmospheric drag; this did not affect the generated power much but reduced the rate of tumbling. At higher altitude, there was again a reduction in eclipse time compared with a lower altitude.

The data from the second stage was used to simulate the power produced from the five solar panels. In what is called “hardware in the loop simulation”, the simulated orbit data was used to programme a power supply to deliver the predicted power to a dummy load and an energy storage device.

Each of the solar panels was expected to have a maximum power output of 25 mW. A maximum-power point tracking algorithm was implemented. In the simulations, an average output power of around 40 mW was predicted for the 50 minutes or so while the satellite was in sunlight, followed by no power generation while the satellite was in the Earth’s shadow. A complete orbit took around 100 minutes. Thus, the total energy generated was of the order of 0.016 W-hours, or around 60 J.

Here’s an example of some of the output from the simulations. The left-hand column shows the orbit and the total output power, which drops to zero when the satellite is in the Earth’s shadow.

On the top left an illustration of the position of the satellite throughout one orbital period of earth, on the bottom left a graph of the total output power generated by the satellites solar panels which is around 40 milliwatts except when the satellite is in the Earth's shadow and drops to zero, on the right a selection of graphs showing the results of the models of the satellites orientation and the electrical power generated by different sections of the solar panels.(Click to expand)

The first two graphs in the right-hand column show the results of the models of the satellite’s orientation. The tumbling while in orbit is clear (we won’t try to define the co-ordinate systems used here, so don’t worry about the titles of the graphs). The lower three graphs show the electrical power generated by the panels.

Our system diagram also shows the generation of heat. Losses in the electrical systems (remember Ohm’s law) means heat is generated in all electronic circuits. This could be radiated out into space (the space stations have radiators to do just that, using pressurised cryogenic liquids as the operating fluids). In the case of the CubeSat there is no room for this so instead the heat energy needs to be stored. You might have an idea how to use this, you might remember from Chemistry that some chemical reactions absorb heat (are endothermic) and that changes of phase of matter can also absorb heat. The greatest amounts of heat are generated in the transition from solid to liquid; so the heat can be absorbed in melting a solid substance. You might also have seen that phase change cooling is even available in clothing [4].

Electrical and Electronic Engineering in space is a complex business, with many challenges that aren’t present on Earth. Some of them are though. You may know that advanced solar electricity farms employ rotation of the panels to catch the sun, mechanisms to mitigate against heating of the panels in the sun and maximum power point tracking control systems. You will also see that like most Engineering challenges, success requires a multi-disciplinary approach. In modelling the power generation on a satellite, our student had to make use of knowledge generated by Physicists and Mechanical Engineers to make his project realistic. That’s why it is important to at least have an understanding of general scientific concepts as well as of mathematical methods in order to be successful in Electronic and Electrical Engineering. It is helpful to be studying alongside Engineers from other disciplines, so that you develop experience of making the most of others’ expertise.

References:

  1. CubeSat Launch Initiative
  2. CubeSat Deployment
  3. The wideband ionospheric sounder cubesat experiment
  4. Techkewl Cooling Vest
  5. M. Angling, SERENE Annual Report [University of Birmingham, 2016]
© University of Birmingham / UKESF
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