Attitude sensors play a very important role in space missions. As you know, the attitude of a space system is it’s orientation relative to a fixed (internal or external) reference. Appropriate references may be frames, such as an orbital plane, fixed points, such as the Sun or other stars, or internal, such as an onboard fixed mass or spinning wheel.
You need to be able to accurately, efficiently, and consistently measure attitude (with respect to the reference frame) so that it can be controlled as required during a mission. This is vital for navigation and for aligning onboard systems such as:
- Cameras, telescopes, or spectrometers for valuable data capture
- RF antennas or optical systems for communication and data exchange
- Solar panels for power generation
- Propulsion and de-orbiting systems for controlled maneuvers
Attitude control systems are made up of sensors that can measure the relative orientations of the orbital and body frame, along with attitude actuators, that change the body frame’s orientation by applying torque where required.
On this page you can see information on the 6 main types of attitude sensor, the key performance criteria for each of them, and links to up-to-date information, directly from suppliers, on attitude sensor solutions available on the global market.
Star trackers
Star trackers are widely used in the industry. They utilize known stars as points of reference in order to determine attitude. Relative positions are calculated using an onboard star catalogue, typically including information on 50-60 celestial bodies.
Star tracker key performance criteria include:
- Field of view (FoV)
- Pointing accuracy (pitch and roll)
- Slew rate
- Interference mitigation (e.g. from satellite surfaces or propulsion exhaust plumes)
- Star catalogue size and accuracy
Sun sensors
Sun sensors use the position of our solar system’s Sun to determine a spacecraft’s orientation. Various system formats are available on the market, depending on the accuracy required and the setup of the rest of the Attitude Determination and Control (ADCS) system onboard.
Sun sensor key performance criteria include:
- Type of sensor: coarse or fine / analog or digital
- Field of view (FoV)
- Pointing accuracy and angular resolution
- Update rate (for units with integrated electronics)
Earth and horizon sensors
Earth sensors use our planet as a fixed reference point to determine a satellite’s orientation, at higher orbits or for basic information. At lower orbits a horizon sensor is utilized to identify the Earth’s radiation boundary with cosmic background. Note that the names Earth sensor and horizon sensor are often used interchangeably in the market, or combined in Earth horizon sensor.
Earth / horizon sensor key performance criteria include:
- Number and configuration of sensors in unit
- Accuracy and operating bands
- Pitch and roll measurement ranges
- Operation throughout orbit
- Processing requirements (systems can include internal processing)
- Update rate
Magnetometers
Magnetometers make use of the Earth’s magnetic field to determine attitude. By making precise field measurements, in known directions and orientations, the position and motion of a spacecraft may be calculated. Magnetometers are sometimes combined with inertial attitude sensors in hybrid units.
Magnetometer key performance criteria include:
- Measurement range
- Orthogonality and linearity
- Resolution
- Update rate
- Magnetic field noise density
GPS receivers
GPS receivers make use of precise measurements across onboard antennas to determine a spacecraft or satellite’s attitude. There are a wide variety of GPS and GNSS systems available on the global market, with access to one or more global navigation networks depending on the model.
GPS receiver key performance criteria include:
- Number of channels
- Number of antennas
- Operating frequencies
- Position accuracy
- Velocity accuracy
- Update rate
- Time-To-First-Fix (TTFF)
Inertial sensors
Inertial sensors determine orientation and motion with respect to an inertial reference frame. This category consists of gyroscopes and/or accelerometers, sometimes combined with magnetometers, and arranged in the format; single-axis, Inertial Reference Unit (IRU), or Inertial Measurement Unit (IMU).
Inertial sensor key performance criteria include:
General:
- Dynamic range
- Bias and bias stability
- Sample rate and output resolution
- Input acceleration
- Minimum acceleration (Dead Zone)
- Output acceleration
- Velocity random walk
- Type: Microelectromechanical System (MEMS) Gyro vs. Fiber Optic Gyro (FOG)
- Angle random walk
Inertial Measurement Unit (IMU):
- Degrees of Freedom (DoF)
- Sensor redundancy
- Individual sensor performance
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General procurement advice for attitude sensors
Here are some additional criteria to assess when weighing up any attitude sensor solution for your next mission:
- SWAP-C (size, weight, power and cost)
- Interoperability options
- Redundancy
- Customization options
- Operating (and storage) temperature ranges
- Radiation and vibration tolerance limits
- Flight heritage and reliability
- Assembly and integration requirements
- After-sales support
- Supplier reputation
- Availability and lead time
