Sun-Synchronous Orbit Calculator
Find the inclination that makes an orbit's node precess 360° per year, holding the local solar time of the ground track constant. Driven by Earth's J₂ oblateness: Ω̇ = −³⁄₂ · n · J₂ · (R⊕/p)² · cos i.
Inputs
Orbit
Most SSO missions fly near-circular (e < 0.005). Altitude is above the 6,371 km mean radius.
Local time of descending node
Does not change the inclination — it sets which RAAN you launch into. 10:30 is the classic imaging choice; 06:00/18:00 is dawn-dusk for continuous power.
Result
Retrograde by construction — cos i must be negative for the node to precess eastward with the Sun.
Need frozen-orbit eccentricity, repeat-ground-track tuning, or drift budgets over a mission lifetime?
Sign up →Frequently asked questions
What makes an orbit sun-synchronous?
Earth's equatorial bulge (the J₂ term) torques the orbit plane, rotating the right ascension of the ascending node. Pick the inclination that makes that rotation exactly 360° per tropical year — about 0.9856°/day — and the orbit plane keeps a fixed angle to the Sun. Every pass then happens at the same local solar time.
Why is the inclination always greater than 90°?
The required node rate is eastward (positive), and the J₂ rate carries a minus sign, so cos i must be negative. That puts i above 90° — a retrograde orbit. Typical LEO values land between 96° and 101°.
What is LTDN and why 10:30?
Local Time of Descending Node is the solar time under the spacecraft as it crosses the equator southbound. Mid-morning times like 10:30 give strong illumination with shadows long enough to read terrain, and less cloud than early afternoon over many land masses. Dawn-dusk (06:00/18:00) keeps the array in near-continuous sunlight, which suits radar missions.
What does this tool NOT account for?
Higher-order geopotential terms (J₃, J₄), drag-driven altitude decay and the inclination maintenance it forces, third-body luni-solar perturbations, or frozen-orbit argument-of-perigee control. Those are in the Aerospace Pack's specialized-orbit and propagation tools.
Upgrade for full orbit design
- Repeat-ground-track and frozen-orbit designers
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