Star Tracker vs IMU: Core Comparison of Spacecraft Attitude Determination Systems

In aerospace, attitude determination forms the foundation for satellites, spacecraft, and probes to operate normally. It determines how spacecraft precisely point at targets, maintain stability, or perform maneuvers. Two mainstream sensors—Star Tracker and Inertial Measurement Unit (IMU)—play key roles in this system.

Star trackers mount on the spacecraft exterior. They feature optical lenses and baffles. They capture images of stars in the night sky. Thus, they determine the spacecraft’s absolute attitude by photographing the star field.

Image Acquisition: Built-in CCD or CMOS cameras capture stars within the field of view.

Star Pattern Recognition: Internal algorithms match observed star patterns with a pre-stored star catalog that contains positions of thousands of bright stars.

Attitude Calculation: Algorithms use triangulation or quaternion estimation. They compute the spacecraft’s precise orientation relative to the inertial frame.

Star trackers provide absolute attitude. Moreover, time accumulation does not affect them. Therefore, engineers prefer them for precise spacecraft pointing.

IMU consists mainly of gyroscopes and accelerometers. It sometimes integrates magnetometers.

Angular Rate Measurement: Gyroscopes, such as fiber optic gyros (FOG) or MEMS gyros, directly measure rotation rates around three axes.

Acceleration Measurement: Accelerometers capture linear acceleration and gravity components.

Attitude Propagation: The system integrates angular rates. It calculates attitude changes and provides relative attitude. Additionally, Kalman filtering enables continuous high-frequency updates, typically hundreds of Hz.

IMUs respond quickly. They track attitude changes in real time during dynamic environments. However, integration causes errors. These errors accumulate over time as drift.

Comparison of Practical Application Scenarios

– Star Tracker-Dominant Scenarios: Engineers rely on star trackers for Earth observation satellites, such as high-resolution imaging ones. They also use them for astronomical telescopes, like Hubble’s successors, and deep space probes. When missions demand extremely high pointing accuracy, star trackers become essential.

– IMU-Dominant Scenarios: IMUs lead during launch phases, orbital maneuvers, and atmospheric re-entry. For example, space shuttles depended on IMUs. They tracked attitude during high-dynamic ascent. In contrast, star trackers struggle to function in these stages.