NanoSat Subsystems

So now we know what a NanoSat is and why we love them. Let’s take a closer look at what makes them tick shall we?

Chassis

Most Nanosatellites have a chassis to provide structural rigidity. You may ask why we classify the chassis as a subsystem because it doesn’t seem to do much – but it really does!

To save on weight, the Nanosatellite structures are usually made of aluminium. The CALPOLY spec designates 6061 or 7075 grades as recommended grades of aluminium and this is pretty much the standard across the board. 6061 aluminium is a very standard grade while 7075 aluminium is often used in industrial aerospace applications.

In principle the job of the chassis is very simple, it has to:

  • Provide mounting points for other components
  • Provide structural rigidity to the satellite
  • Interface with the nano-satellite deployer
  • Have minimum mass

However, this can be quite complicated and requires a decent amount of design effort, which is why we have called it a subsystem in its own right.
For example, because most of the mechanical loading is from vibration, the chassis has to be designed to avoid going into resonance during launch. At the same time, a good chassis is simple, easy to put together and has mounting points at appropriate locations based on the internal component layout.

The aluminium chassis is required to be anodised (a chemical process that coats the surface) to prevent cold-welding with the internal faces of the deployer. This is why most commercially available chassis are black in colour.

Image of a 1U Endurosat Cubesat chassis (www.endurosat.com)

Attitude Control System

Image of an PCB assembly with magnetorquer rods offered by Innovative Solutions in Space (www.isispace.nl)

The attitude of a satellite refers to its orientation in space. Therefore the attitude control system (ACS) is designed to allow the satellite owner to move and rotate the satellite to point exactly where they want.

Most of the smaller nanosatellites (eg. 1U cubesat) do not have an ACS as they usually use low-gain antennas that work no matter how they are orientated, however for larger cubesats that have a high gain antenna, being able to point at the ground station receiver is crucial. Often, a nano-satellite only has rotational capabilities, not translational.

There are multiple ways that a satellite can rotate itself, however the two used most commonly are magnetorquers and reaction wheels. A brief description of both is below – warning, there is some physics talk included!

Reaction wheels are spinning flywheels inside the satellite. They work because in space, the total angular momentum of the satellite must remain constant as there are no outside moments/torques being applied. So if you spin the flywheel up using a motor, the satellite will – by the laws of physics –  start to rotate the opposite way. Similarly if you engage the reaction wheel and slow it down, the satellite will spin in the direction the wheel was originally spinning. Having a reaction wheel oriented in each axis should give full rotational control over the satellite.

The effect of reaction wheels can also be achieved by using magnetism. A magnetorquer is usually a rod of ferrous metal wrapped in a wire to make an electromagnet. As you pass electrical current through the wire, an electric field is created that interacts with the Earth’s magnetic field to produce a torque on the wire-wrapped rod. Since the rod is fastened to the satellite, this torque is transferred to the satellite. As with the reaction wheels, you need 1 magnetorquer per axis for full rotational control.

The algorithms needed to run these systems are quite complex.

Communications

The communications system on the satellite will normally consist of two main components, the radio and the antenna. The flight computer will tell the radio all the data that it wants to send down to Earth, and then the radio will convert that into a radio signal. The signal is boosted in power and sent to the antenna where it is then sent out into space.

Generally there are two types of antennas that can be used. Low Gain antennas (eg. dipole) will send their signal in a very wide arc around the satellite. As a consequence the area of effect for these antennas is big, however the strength of their signals is usually quite low as the energy is spread out over the large area.

In contrast, High Gain antennas are much more directional and have a narrower signal spread. The benefit is that for the same transmission power, the signal strength is much higher.

Most nanosatellites will use low gain antennas as they are cheaper and do not require the satellite to have good attitude control. However larger CubeSats doing scientific missions or trasnmitting images will use a high gain antenna as the data transmission rates are much higher and it’s not as much of an issue to incorporate a good attitude control system.

A ground station is also needed to receive transmission but is covered below.

Image of the Endurosat UHF Antenna which is a low gain, dipole antenna. (www.endurosat.com)

Power

Pictured is a electronic power system made by Cubic Aerospace (https://www.cubicaerospace.com)

Ah the Power System. Such a crucial aspect of the satellite because if there is no power it essentially becomes an orbiting brick! Generally the power systems on nanosatellites need to do three things:

  1. Generate power
  2. Store energy
  3. Distribute the energy around the satellite

It sounds simple but in practice this is a complex task.

Power generation comes from using solar cells on the outside of the nanosatellite to turn the radiant energy from the sun into electricity. The solar panels will be connected to the batteries through charging circuitry.

The batteries are extremely important in the nanosatellite and are often a Lithium variant. They need to be able to go through many charging cycles and have a good energy density, but most importantly, good batteries will have a reasonably wide operating temperature range.

The real magic of the power system comes when it is time to distribute the energy stored in the batteries. Often there will be many components in the satellite drawing power at the same time and so the distribution system needs to provide multiple voltage rails using regulator circuitry, encompass short-circuit protection features, and should be designed so that the total power draw for the satellite is not more than the satellite can supply over a period of time. A key concept is the power budget which is roughly the average power draw the satellite can maintain through solar panel generation and battery storage.

 

Flight Computer

The flight computer is the brain of the nanosatellite and is usually just a micro-controller on one of the PCBs in the satellite. However this computer is responsible for the operation of the whole unit. A brief list of the basic functions often controlled by the flight computer include:

  • Scheduling of radio transmission
  • Composition of the transmitted beacon
  • Calculation of algorithms for attitude control*
  • Monitoring of health data
  • Control of the satellite’s power state
*Some commercial control units do this on their own.
Often the flight computer is simply a PCB assembly with micro-controllers on it. Pictured is the 'Cube Computer' made by CubeSpace. (www.cubespace.co.za)

Ground Station

Ground stations come in all shapes and sizes and can range from large dishes as shown here to single small antennas in someone's backyard.

Something a lot of people forget about when thinking of a satellite is that there is still a fairly sizeable ground based system involved as well. When most nanosatellites transmit, they just send out a signal all around, and it is up to the ground station to pick up that signal and interpret it. Also, the ground station sends signals and commands to the satellite in orbit if we need it to do something different.

At it’s most basic level, a ground station has the following components:

  • Antenna
  • Radio
  • Decoder
  • Computer

The antenna is the physical part that picks up radio signals, while the radio filters out the specific frequency from all the other radio signals out there.
Quite often nanosatellites will have their transmissions encrypted. After all, antennas and radios are quite common and anyone around the world could pick up the signal of a particular nanosatellite if they knew the transmission frequency. For satellites that do transmit encrypted signals, the ground station needs a decoder to make sense of it.

Finally the ground station will pretty much always have a computer hooked up to take the decoded signal (sometimes the computer does the decoding) and turn it into a format that we humans can understand and display the data.