8.4 Typical Requirements and Design Considerations

Mission-Derived Requirements

For the spaceflight mission, the ADCS system must be designed to know and/or control the spacecraft’s orientation per the payload and spacecraft bus requirements. The most fundamental ADCS problem is pointing a vector to track a subject that holds still or moves. Like with any subsystem, the size, weight, and powers are obvious requirements or design drivers. Other high-level mission design considerations could be the orbit, level of autonomy, and necessary onboard data and computation. Subsystem requirements for spaceflight include:

• Pointing accuracy – “proximity of measurement results to the true value” [Wikipedia]
• Pointing precision – “the degree to which repeated (or reproducible) measurements under unchanged conditions” [Wikipedia]

• Agility – “rapid retargeting, fast transient settling and low jitter pointing control” [NASA]
• Drift – system behavior where the control system slowly integrates error and “drifts” away from the desired orientation
• Motion constraints – maximum thresholds for attitude and derivatives (velocity, acceleration, and jerk) bound by sensitive spacecraft components, like flexible appendages and vibration-sensitive components [Kim]. These maneuvers are called rate-limited, acceleration-limited, or jerk-limited maneuvers.

• Sensor and actuator placement
  • Some sensors or actuators need unoccluded access to the space environment, like sun sensors, star trackers, and reaction control system thrusters.
  • Some sensors or actuators require or prefer a certain placement within the spacecraft system, like
    • a magnetometer at the end of a boom to mitigate electromagnetic interference,
    • an inertial measurement unit at the center of mass of the spacecraft, or
    • thrusters at the far corners of the spacecraft
  • Some sensors or actuators require a known or controlled orientation within the spacecraft system, like
    • momentum control systems that point orthogonally/perpendicular with respect to each other
    • thrusters that also point orthogonally/perpendicular and antiparallel with respect to each other
    • star tracker and sun sensor oriented on different faces with respect to each other to avoid pointing the star tracker toward the sun and harming the sensor.
• Reliability – the sensors and actuators must have an operable lifetime that is longer than the space mission lifetime
  • Sensors with sensitive electronics that are susceptible to failure after a total ionizing dose (TID) must have a high probability of surviving until the end of the mission.
  • Actuators with moving parts, like reaction wheels, control moment gyroscopes, and thrusters, can fail due to wear and tear.

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Design Drivers

Design drivers include limited technology capabilities for sensors and actuators, the other subsystem requirements, and configuration constraints. We will discuss the sensors and actuators commercially available in a later section. We’ll consider the other subsystem requirements and constraints:

• The payload system may need precision pointing or slewing to observe the desired science. For example, the Kepler observatory requires continuous pointing at a single starfield in the Cygnus-Lyra region with accuracy “< 0.009 arcsec 3σ single-axis pointing stability on ≥ 15 min time scales” to pick up enough light to distinguish exoplanets from their stars [NASA]. For other imaging payloads, the angular slew rate can relate to the imaging resolution of the target; a fantastic trade study is referenced here [Shell]. The payload system will most likely offer the most stringent and critical pointing or slewing requirements.

• The communications system may need the spacecraft to track a ground station while the spacecraft passes over a ground station or if the spacecraft is in deep space, the spacecraft may need to precisely point toward Earth. For establishing beaconless optical communication from Mars to Earth, the iROC design requires an optical beam pointing accuracy on the order of 2-5 µradians [NASA]! For most other spacecraft in Low Earth Orbit, spacecraft pointing can range from a passive tumble to coarse nadir-pointing (toward the Earth) during downlink-uplink operations. This number can be encapsulated in antenna pointing accuracy.

• The power system may need the spacecraft to point solar panels at the sun to charge the battery. Some missions have articulated arrays so the solar arrays may track the sun independently of the main spacecraft bus but the ADCS system is responsible for calculating how to articulate the solar arrays. Some missions have designed the ADCS system out of consideration by assuming the spacecraft has enough solar cells so that there is more than enough power generated regardless of the spacecraft’s orientation.

• The command and data handling system enables or limits the ADCS computation. For systems with active ADCS control, ADCS sensors generate a lot of data, and control/determination algorithms need a lot of computational processing power. If the CDH system cannot support the ADCS data and processing needs, the sensors will need to be downsampled, and/or the algorithms must be updated less frequently.

• The thermal system may need the spacecraft to preferentially orient itself to expose or shield certain components away from heat sources. In the case of the James Webb Space Telescope, the intricate sun shield protects the science instruments and mirrors from the heat of the sun, made possible by the ADCS pointing system.

• The structures and mechanism system may impact the spacecraft design if there are any structural components that are sensitive to motion or contribute to motion, such as long arms that act like flexible appendages. For example, SMAP’s boom deployment will contribute angular momentum to the overall spacecraft as the whole system must conserve angular momentum. The long boom also acts as a flexible link that vibrates at frequencies that the ADCS system must reject or dissipate.

^{SMAP boom deployment. Video courtesy of NASA.} 

• The propulsion system may need the ADCS to orient the spacecraft such that the thruster aligns with the desired direction of thrust or to orient the gimbal on the thruster. The ADCS needs to compensate for any tumbling that could occur during propulsive maneuvers.

Design Configuration

These requirements, design drivers, and configuration constraints should lead to trade studies in which you decide:

• The necessity of an active control system:
  • You could get by with a tumbling spacecraft or design your spacecraft such that it passively stabilizes itself [Rawashdeh]

• If you do need active control, does the control need to span 1 axis of stability or all 3 axes of stability? [Wikipedia]

^{Three-axis Stabilization of Cubesat. A MATLAB simulation by Timothy Dawson built from scratch to simulate the active orientation of a CubeSat with three magnetometers.}

• Computational Architecture
  • Do we need to have the algorithm computation run in real-time on the onboard computer or can the computation happen on the ground then have the results transmitted back to the spacecraft?
  • How computationally intensive should the attitude algorithms be limited to in the data budget?
• Sensor Selection – in-depth discussion in the Sensing section
• Actuator Selection – in-depth discussion in Control Actuators section

Artemis Kit Specific

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