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Space Fence

Quick answer

Space Fence is the US Space Force's S-band radar on Kwajalein Atoll that detects and tracks orbiting objects, including debris down to roughly 5 cm in low Earth orbit. Operational since 2020, it multiplied the trackable catalogue and feeds the conjunction warnings satellite operators rely on.

📘 Full definition✓ Reviewed 2026-09-07
Space Fence is a large ground-based S-band phased-array radar operated by the US Space Force on Kwajalein Atoll in the Marshall Islands, purpose-built for space surveillance. Rather than pointing at individual targets, it projects a continuous fan of radar energy — the "fence" — across the sky; anything in orbit that crosses the fan is detected, measured and handed to cataloguing systems. Its sensitivity is the headline: objects down to around 5 cm in low Earth orbit, far below the ~10 cm floor of the legacy surveillance network, with capacity to maintain custody of hundreds of thousands of objects. Declared operational in 2020, it became the sharpest instrument in the space surveillance architecture, driving growth in the public satellite catalogue and sharpening the conjunction assessments that trigger collision-avoidance manoeuvres. Detections feed orbit determination that ultimately surfaces as TLEs and conjunction warnings used by operators worldwide.
Location
Kwajalein Atoll
Band
S-band radar
Min Object Size
5 cm in LEO
Operational Since
2020

Understanding Space Fence

How a surveillance fence works

The system uses separate transmit and receive phased arrays of thousands of gallium-nitride elements. The transmit array paints a narrow east-west fan; digital beamforming on receive carves that fan into thousands of simultaneous pencil beams, so a single crossing yields angle, range and Doppler in one pass. Follow-up micro-fences can be steered above the main fan to re-observe interesting objects, refining orbits without any moving parts.

What it changed

Within its first years, the trackable population roughly doubled as previously invisible fragments entered the catalogue — many from the 2007 Fengyun-1C ASAT test and the 2009 Iridium-Cosmos collision. More objects means more warnings: operators saw conjunction alert volumes rise sharply, pushing automation in screening and manoeuvre planning across the industry.

See it live The catalogue Space Fence helps build is live on the globe — every tracked object, propagated in real time. Open Live Globe →
📖 Learn More
Space Situational Awareness Guide Space Debris Map Tracker

Frequently Asked Questions

The radar transmits a thin, wide sheet of energy — like an invisible wall rising from the Pacific. Objects crossing the wall announce themselves, letting one fixed installation surveil everything that passes overhead rather than chasing targets one by one. The name carries over from an earlier VHF "fence" system retired in 2013.
Because centimetre-class debris is abundant, lethal at orbital speed, and was previously invisible — satellites could be hit by objects nobody tracked. Every size-class the fence adds converts unknown risk into warnings an operator can actually act on.
Its primary mission is LEO and MEO, where the fan geometry and radar physics favour it. Geosynchronous surveillance leans on other assets — dedicated telescopes and radars — since GEO ranges make small-object radar detection far harder.

Sources & References

Definitions are reviewed against primary sources. Last reviewed: 2026-09-07.

Related Terms
NORAD ID Tracking & Space Surveillance Unique tracking number for every space object SSA Tracking & Space Surveillance Knowing what's in orbit and where it's going Conjunction Assessment Debris & Space Safety Screening for potential collisions in orbit Who Tracks Debris? Glossary Surveillance networks
Related in the Library
LEO
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MEO
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GEO
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Heo
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