Elements of a cubesat
A cubesat is to perform some functions, like data gathering, navigation, communications, etc. and a number of subsystems are needed for that. Choosing the right set of elements of a cubesat will ease the project workflow, achieve an optimum result in terms of SWaP and increase the chances of success. In this article I present an overview of the parts that a cubesat can have. Below is the structure of ESTcube-1 cubesat.
Structure
The structure or frame of the cubesat is quite often overlooked but it serves a number of essential purposes:
- Keeps the mechanical integrity, especially during launch.
- Holds essential elements like antennas or solar panels that would terminate the mission if lost.
- Makes a Faraday cage that collects space plasma charge.
- Conducts, spreads and radiates excess heat.
- Shields from space radiation.
Electrical Power System (EPS)
The EPS functions are: collecting power, mostly with solar panels, storing it in batteries for eclipse periods or when extra power is needed and producing regulated voltages in the internal power busses. Its main subsystems are:
- Solar cells. Produce electric power from Sunlight.
- Solar Array Power Regulator (SAPR). Mainly adapt the widely varying SA output voltage to the battery voltage to optimize the charge.
- Battery Charge Regulator (BCR). Manages the charge of batteries from the Solar Arrays.
- Secondary regulators. Produce power for local loads, like 5V, 3.3V, etc.
Command and Data Handling (C&DH)
The C&DH system, sometimes called “avionics” is in charge of gathering data from the spececraft to be downlinked to ground and the reception and processing of commands from ground.
The OBC (On-Board Computer) is at the heart of C&DH and communicates with almost all the elements of a cubesat to control them and receive information. Some important functions of the OBC are:
- FDIR (Fault Detection, Isolation and Recovery), a combination of hardware and software that detects faults in systems (like shorts produced by radiation), isolates (typically turns off) and takes adequate actions to protect the spacecraft.
- Time distribution. Some subsystems need an accurate time. This is typically distributed with PPS (pulse per second) signals.
- Navigation. Controls the position and attitude of the satellite. Most cubesats cannot change their orbit but still need to locate themselves.
- Thermal control. As the conditions change during the orbit, depending on temperatures sensed in the spacecraft, some actions may be taken like turning heaters on or switching systems off.
- Telecommand processing. Commands uplinked from ground can be executed immediately or scheduled for later.
- Telemetry processing. Data to be downlinked to ground. It will contain housekeeping data, payload data (sometimes called “science” data) and requested data.
Attitude Determination and Control (ADCS)
Attitude (orientation) is another essential aspect to know and control. Most missions will need to point to a target (Earth for EO missions) or at least to point its antenna for communications.
As many cubesats have only body mounted solar panels, attitude control also sets the incoming power and serves to control temperature inside the satellite.
As with any control system, it has sensors and actuators. Sensors tell the OBC what the attitude is and actuators will modify it.
Common attitude sensors are:
- Star trackers. Look at the sky and recognize certain stars. The most accurate but also expensive. Not especially popular in cubesats.
- Sun sensors. Photocells that sense and measure Sunlight. Typically indicate the Sun direction with two angles. Low cost and simple. Many can be installed for redundancy.
- Earth sensors. Look at the Earth’s edge in IR and provide the nadir vector.
- Magnetometers. Measure the local magnetic field created by Earth. They only work as attitude sensors if location is known because magnetic field is not homogeneous. A world magnetic map is needed. Phenomena like Sun storms also affect the magnetic field.
Actuators to control a satellite attitude are:
- Reaction wheels. Very effective and precise but use power.
- Magnetorquers. Coils that interact with the magnetic field. Low cost but have a slow response.
Thermal Control System (TCS)
While large satellites have active TCS, with fluid heat pumps, most cubesats rely on passive thermal control, mostly insulation to reflect the Sun radiation and radiators to emit the excess heat generated inside.
Communications (COM)
Communications is another essential function in the elements of a cubesat. It keeps the satellite connected to ground so it can be commanded and data can be received. Without communications, even if the satellite is perfectly functional, it is useless therefore it must be a reliable unit.
Different radio bands and modulation schemes can be used for communications, some of them licensed, other (amateur bands) are free, but still there are regulations to comply with. Each band will have its own characteristics in terms of atmospheric and rain loss, bandwidth, etc.
Payload System (PLS)
All the elements of a cubesat above are just the ‘vehicle’ for the mission but not the mission itself. The Payload is custom for each mission but the general characteristic is that is commanded by the OBC and returns data (payload or “science” telemetry) to the OBC.
Like any other subsystem, it is susceptible of FDIR and it can be switched off if the OBC detects any problem in it or another subsystem and decides to put the satellite in safe mode.
In summary
Executing a cubesat project involves a number of different disciplines, like RF, electronics, software, materials engineering, optics, control engineering, etc. As the project advances, decisions are set and contracts or purchases signed, any potential change becomes more critical and difficult to implement without an impact to the rest of the spacecraft.
It is then very important for the overall project execution mainly in terms of schedule and cost to have a very clear and wide understanding of all the elements of a cubesat to start with the rightest choices.
At Blue Marble Space Systems, I am available as a consultant for satellite projects. While my focus is electronics and FPGA development I do have a general knowledge of satellite architecture.
