Star Tracker (Ball Aerospace CT-633)

ABOUT STAR TRACKER (BALL AEROSPACE CT-633)

The Ball Aerospace CT-633 is a star tracker: a small space camera that photographs the stars, recognizes them, and tells a spacecraft exactly which way it is pointing. It is one of the most flight-proven attitude sensors ever built, having flown on NASA missions like WISE and DSCOVR.

Quick facts

• Type: Autonomous CCD-based star tracker (an attitude sensor that determines a spacecraft’s orientation). A CCD is the same kind of light-sensing chip found in early digital cameras.
• Maker: Ball Aerospace & Technologies Corp., Boulder, Colorado.
• Field of view: about 20 degrees of sky in each frame (a wide-field design).
• Sensitivity: can see stars down to roughly visual magnitude 4.5 (faint stars, near the limit of the naked eye).
• Accuracy: about 3 arc-seconds for a single unit; two units mounted at right angles reach roughly 1 arc-second. An arc-second is 1/3600 of a degree.
• Update rate: a fresh orientation answer about every 0.2 seconds (around 5 times per second) once it is tracking stars.
• “Lost-in-space” start-up: can find its orientation from scratch in about a minute.
• Validation: tested under the real night sky in Boulder, Colorado in January 1996, gathering data at 37 sky positions across 23 constellations, all yielding successful star identifications.

What it is and how it works

A star tracker is essentially a digital camera paired with a star atlas and a fast pattern-matching computer. The CT-633’s optics image a roughly 20-degree patch of sky onto a CCD detector. Onboard software measures the precise position (the centroid, or exact center point) and brightness of each star in the frame, then compares that pattern against an internal star catalog using a star-identification algorithm.

Think of it like recognizing a familiar constellation: you do not need to measure anything if you can match the pattern of bright dots to one you have memorized. Once the CT-633 matches enough stars (usually four or more) to known catalog entries, it calculates the camera’s exact orientation in inertial (fixed, star-based) coordinates. It reports this as a quaternion, a compact set of four numbers that describes a 3D rotation.

From a cold start with no idea where it is pointing, this “lost-in-space” solution takes about a minute. After that, it tracks the slowly moving star field and refreshes the answer roughly every 0.2 seconds. Spacecraft usually carry two trackers aimed in perpendicular directions, because the weakest axis for any single tracker is “roll” (rotation around the direction the camera faces). The second unit covers that weakness, sharpening the combined answer to about an arc-second. The trackers also work alongside gyroscopes: the gyros sense smooth, fast motion between updates, while the star tracker supplies an absolute, drift-free reference that keeps the gyros honest.

Why it matters

Knowing which way a spacecraft points underpins almost everything it does: aiming telescopes and cameras at targets, pointing antennas back at Earth, keeping solar arrays facing the Sun, and turning smoothly between tasks. Older methods, such as sun sensors and magnetometers combined with gyroscopes, tend to drift over time. The CT-633 gave missions a self-contained, arc-second-class reference that needs no ground commands and no prior knowledge to recover its orientation. As a moderate-accuracy unit, it deliberately trades the finest precision for lower cost and complexity, which is exactly why it became a standard building block for more than a decade.

Notable missions and examples

• WISE (Wide-field Infrared Survey Explorer, 2009): two CT-633 trackers provided the primary attitude information for this all-sky infrared survey, supporting precise three-axis pointing with low jitter (under about 1.3 arc-seconds).
• DSCOVR (Deep Space Climate Observatory, 2015): after the spacecraft’s gyroscopes degraded, NASA wrote a software patch to determine orientation using the CT-633 star trackers alone, returning the observatory to full operations in March 2020 with pointing comparable to the gyro-aided mode.
• Explorer heritage: the CT-633 grew out of Ball’s earlier CCD star-tracker family (such as the CT-601, which flew on the 1999 Wide-Field Infrared Explorer), and its autonomous star-identification software was characterized under the night sky before going on to fly on missions like WISE.
• CT-2020 (modern successor): Ball’s newer tracker traces its software lineage to the CT-633, though it is a distinct, higher-performance unit and should not be confused with it.

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