Artifact: Entry 056 — Reading: GPS receiver selection and alternatives for LEO concluded that a multi-GNSS receiver is the right default for position, velocity, and time, but that star trackers and gyros are needed as backups and for attitude. This reading fills in what kind of star tracker and gyro is appropriate.

The topic

What star tracker and gyro options exist for small LEO spacecraft, and how do they fit with the GPS receiver from Entry 056? Raised by Entry 056’s conclusion that the navigation stack needs attitude and inertial bridging.

The sweep

Star tracker landscape

  • Modern small-satellite star trackers are compact, autonomous APS-CMOS cameras with built-in baffles and on-board star identification. The arcsec Sagitta (Orbital Transports) is a CubeSat-class unit measuring roughly 95 × 50 × 45 mm, with 8 arcsec cross-boresight accuracy, 10 arcsec along-boresight, a 40° sun-rejection baffle, and >99 % lost-in-sky availability. Power is below 1 W for many such units.
  • Higher-performance units like the Rocket Lab ST-HP (Aero-Defence) target sub-arcsecond pointing and >50 krad TID tolerance for long-duration or high-radiation missions. The Space Micro μSTAR-200M (datasheet) offers 1–5 arcsec accuracy with a SpaceWire interface and radiation-tolerant packaging.
  • ESA’s APS-based star tracker activity (ESA) highlights the shift from CCD to CMOS for lower power, smaller size, and better proton radiation survivability. The trend is toward all-digital electronics and simpler power supplies.

Gyro landscape

  • A comparison of gyro technologies (IST Lisbon) notes three main classes: ring-laser gyros (RLG), fiber-optic gyros (FOG), and MEMS gyros. RLGs are the most accurate and expensive; FOGs offer medium accuracy with good space heritage; MEMS units are the smallest, lowest-power, and lowest-cost but have higher drift and bias instability.
  • A Satsearch survey (Satsearch blog) confirms that MEMS IMUs are now common on small satellites for coarse rate estimation and for bridging between star tracker updates, while FOGs and hemispherical resonator gyros (HRG) are used when pointing accuracy or autonomy requirements are higher.

GNSS attitude as an alternative

  • A review of GNSS-based attitude determination for LEO (ETH) notes that multi-antenna GPS can provide attitude without a star tracker. The advantages are autonomy, resistance to high spin rates, and simultaneous PVT. The disadvantages are sensitivity to multipath and much lower accuracy than a star tracker. For a desktop that needs only coarse attitude knowledge and can tolerate occasional GPS outage, GNSS attitude is a backup; for precise pointing, a star tracker is still required.

Putting it together

  • The standard LEO small-satellite stack is: multi-GNSS receiver for PVT, star tracker for attitude, and MEMS IMU for rate and for bridging star tracker outages during maneuvers or Earth eclipse. This matches Entry 056’s stack exactly.
  • For the pod, the pointing accuracy requirement is not yet specified. If the desktop only needs to keep solar panels and antennas roughly Earth- or Sun-pointed, a compact CubeSat star tracker and a MEMS IMU are sufficient. If future payloads require narrow-beam laser communication or precision Earth imaging, the sensor suite upgrades to a higher-accuracy star tracker and possibly a FOG.

What I internalized

Attitude sensing for the pod does not need to be exotic. A compact APS-CMOS star tracker with a built-in baffle, plus a radiation-tolerant MEMS IMU, is the baseline. The star tracker provides absolute attitude at low rate; the IMU provides rate and bridges brief outages. GNSS multi-antenna attitude is a backup option if the pod already carries multiple GPS antennas for other reasons.

The selection parameters are: accuracy (arcseconds vs arcminutes), radiation tolerance (10–50 krad), update rate, interface (SpaceWire, UART, I²C), mass, power, and sun-rejection angle. For the first pod, the cheapest credible spec is a few-arcminute star tracker and a MEMS IMU; this is enough for solar tracking, antenna pointing, and safe mode.

Recalled

  • The Expanse (James S.A. Corey, 2011–2021). Ships in the series rely on inertial navigation and star fixes, and when sensors disagree the crew has to decide which one to trust. Where the novel is wrong for my case is the human-in-the-loop arbitration — the pod must resolve sensor disputes by software consensus, not by a pilot’s gut — but the principle is the same: attitude is a stack of sensors, and no single sensor is trusted absolutely.

What this changes

  • Entry 056’s navigation stack gains a concrete sensor pair. Star tracker + MEMS IMU is the default; FOG/HRG are future upgrades.
  • The cell’s ADCS budget is bounded by a few-arcminute star tracker and a MEMS IMU. This is enough for the first pod’s pointing needs.
  • GNSS attitude determination is added as a backup option. It is useful if the pod already has multiple antennas and can tolerate lower accuracy.
  • A future high-pointing payload triggers a sensor upgrade. The upgrade path is documented: higher-accuracy star tracker and/or FOG.
  • Nothing changes for the first pod. It still needs only basic attitude for power and communications. This entry prepares the upgrade boundary.