Space Science With An Attitude

12:54:00 PM

Vishnuu Mallik, 2nd year M.S. student in Aerospace Engineering Sciences

Source:  University of New South Wales, Australia
Chances are you’ve never personally been to space, but have you thought about how many things you use every day are actually dependent on space systems technology? Take for example the Global Navigation Satellite System (GNSS; more popularly known as the Global Positioning System, or GPS). The system consists of a constellation of satellites in orbit around the Earth. Apart from being used in every cell phone, the entire global economy is linked to the GPS network, and ATM and large bank transactions are synchronized with the GPS atomic clock. Satellites are also used for monitoring weather and climate change, oceanography, television and radio broadcasting, as well as defense and cybersecurity. 

Because they are used for so many things, a major goal for the aerospace industry has been to lower the costs of manufacturing, launching, and operating space vehicles. Lowering these costs will make it easier to access space, and allow this realm to be better utilized for everyone’s benefit. This need for cost-effective technology has led to the creation of miniaturized satellites called CubeSats.

CubeSats weigh approximately 4.4 pounds and can be constructed and launched for about $150,000, hence allowing low cost access to space. In comparison, an average satellite weighs 4166 pounds and costs about $51 million to launch. 

The Von Karman Institute and European Space Agency have started the QB50 (CubeSat 50) project. This international initiative challenges institutions worldwide to design, build, and operate a CubeSat equipped with specialized sensors for atmospheric research. All of these CubeSats are a part of one mission. This mission will serve as a benchmark for demonstrating affordable access to space, and for studying the Earth’s atmosphere through small satellite platforms. 

The University of Colorado Boulder is one of the 50 institutions participating in the QB50 project. Led by Principal Investigator Dr. Scott Palo, a group of graduate students from the Aerospace and Electrical Engineering department and a few select undergraduates from Mechanical Engineering are building a CubeSat called Challenger. 

Challenger will be carrying an Ion-Neutral Mass Spectrometer (INMS), an instrument which will be used to study the Earth’s thermosphere. This region of the atmosphere, 100 miles above the surface of the Earth, is where the ionosphere and thermosphere coexist. This coexistence gives rise to positively and neutrally charged particles (ions) along with neutral atoms, which govern the characteristics of the region. The INMS will measure and determine the composition of these atoms and ions. Then, the data collected from this mission will be used to help us learn more about this part of the Earth’s atmosphere.

One of the key features on the CubeSat Challenger is the Attitude Determination and Control System (ADCS). As the name suggests, the purpose of the ADCS is to determine and control the attitude, or orientation, of the spacecraft. The ADCS is critical for the success of the mission: it controls the solar panels, which need to point toward the sun for maximum power when the CubeSat’s batteries are low. 

The ADCS is also important for collecting useful scientific data from the INMS. In order to take measurements, the INMS must point in the forward direction of the spacecraft’s motion. This direction is known as the RAM direction, and is tangential to the surface of the Earth at any point of the spacecraft orbit. 

The ACDS stabilizes the CubeSat so that it can be
correctly oriented. Image Courtesy: Jacob Cook and
Nicholas Rainville, University of Colorado Boulder
The CubeSat gets launched into orbit in a tumbling motion, which could be as high as 60 degrees per minute. It is the ADCS’s job to de-tumble and stabilize the craft so that it can deploy its solar panels and recharge its batteries. Once the batteries are recharged, the CubeSat is ready to do science, and must orient itself in the RAM direction so that the INMS can collect data. 

By now you may be wondering, how does this ADCS system work on board a 10x20 cm metallic box flying through space? Let’s start with the hardware aspect. The ADCS subsystem consists of three different types of sensors – magnetometers, gyroscopes, and sun sensors. The magnetometers measure the Earth’s magnetic field, while the gyroscopes measure the rate at which the spacecraft rotates along each direction. Finally, the sun sensors measure the intensity of light at various locations on the spacecraft. 

The information collected through these sensors is fed into the main flight computer, which is then processed to determine how the spacecraft is currently pointed. Based on this input, the computer energizes three coils also known as solenoids or torque rods. Driving a coil with a current creates a magnetic field which generates a rotational force or torque when operated in an external magnetic field. The Earth’s magnetic field is the external field and the generated torque is used to re-orient the spacecraft. 

The key elements of this system are the sensors, the torque rod and a set of two algorithms. The determination algorithms comprise the first set. These algorithms ingest data from different sensors, including light intensity from the sun sensors, magnetic field readings from the magnetometers, and rotation rates from the gyroscopes. The algorithms then combine these data in an intelligent manner to calculate the direction in which the spacecraft is pointing. 

Once the CubeSat’s flight computer knows its attitude, which is output from the determination algorithm, the control algorithm computes how to adjust the spacecraft’s current attitude to its desired attitude. These algorithms calculate when to turn the torque rods on and off such that sufficient force is generated to properly orient the CubeSat. The control system works in close partnership with the determination system; at every point in time, the control system is receiving information regarding the CubeSat’s orientation and varying the current provided to the torque rods. This is known as a feedback control system. 

Extensive development and testing are needed to ensure that the ADCS works as designed once the CubeSat is deployed into orbit. Currently, the Challenger team is working towards finishing final testing and integration for all CubeSat systems. Once testing is complete, the Cubesat will be shipped to Europe for integration of the INMS, and then prepared for launch in late 2016! 

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