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Space Engineering
Sun-Synchronous Orbit (SSO) Design Simulator
Design a Sun-synchronous orbit — the standard low-Earth orbit for Earth-observation satellites — by matching the J2-driven nodal regression to the Sun's mean motion. Tune altitude, LTAN and repeat cycle to see the required inclination, orbits-per-day, equatorial ground-track spacing and eclipse fraction update in real time.
Parameters
Orbit altitude h
km
Mean altitude above Earth's surface. SSOs live in the 200 to 1500 km LEO band.
Eccentricity e
Nearly circular (e < 0.01) is the SSO standard.
LTAN — Local Time of Ascending Node
h
10:30 / 13:30 for optical EO; 6:00 / 18:00 (Dawn-Dusk) for SAR.
Repeat cycle
day
16 days for LANDSAT, 10 days for Sentinel-2.
Observation latitude
°
SSOs cover up to ±(180-i)° (about ±82° for i≈98°).
Results
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Orbital period (min)
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Required inclination i (°)
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Orbits per day
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Orbits per repeat cycle
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Equatorial track spacing (km)
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Eclipse fraction (%)
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Orbit-plane nodal regression animation
Yellow = Sun, blue = Earth, white dot = satellite. The retrograde orbit plane precesses eastward (counter-clockwise) due to J2, keeping a constant angle to the Sun. The blue arc on the surface is the ground track, the yellow dashed line marks the LTAN meridian.
J2-driven nodal regression rate. Setting it to 360°/year gives the SSO inclination i. Typical values are i≈97.4–99.5° (retrograde) for h=500–1000 km. μ: Earth's gravitational parameter, a: semi-major axis, R_E: Earth radius, J2≈1.08263e-3, e: eccentricity.
$$T = 2\pi\sqrt{\frac{a^{3}}{\mu}}, \qquad a = R_{E} + h$$
Keplerian orbital period T. At h = 700 km, T ≈ 98.8 min with about 14.6 orbits/day.
Equatorial longitude spacing Δλ and surface distance d_eq between adjacent tracks. At 700 km this is about 24.7° (2749 km), giving 14.6 ground tracks per day.
I've heard the term "sun-synchronous orbit". Does it mean the satellite is somehow going around the Sun together with Earth?
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Half right, half wrong. The satellite itself is in a normal low-Earth orbit — typically 600–800 km up, circling Earth in about 90 minutes. What's "synchronous" is the orbital plane. As Earth goes around the Sun by 360° per year, the satellite's orbital plane is tuned to rotate slowly eastward by exactly 360° per year too. The net result is that the angle between the Sun direction and the orbital plane stays almost constant year-round. That's the whole point of an SSO.
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But what makes the orbital plane rotate on its own? I thought orbits just sat still in space.
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Earth isn't a perfect sphere — the equator bulges out a little, like a tangerine. That oblateness is captured in the J2 term of Earth's gravity field. The extra mass at the equator gives any inclined orbit a continuous sideways tug, and the orbital plane (the ascending node Ω) precesses like a slow gyroscope. The rate is dΩ/dt = -(3/2)·n·J2·(R_E/a)²·cos(i)/(1-e²)², and the sign of cos(i) sets the direction. Sliding the altitude in the left panel automatically solves this for the inclination that makes the rate equal exactly 0.9856°/day, which is what Sun-synchronous means.
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It shows 98.19° at the default 700 km. That's more than 90°, isn't it? Is the satellite going backwards?
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Yes — that's a retrograde orbit. It goes against Earth's rotation. To get the required eastward precession we need cos(i) < 0, which means i > 90°. That's why every SSO satellite — LANDSAT, Sentinel-2, ALOS, you name it — flies "slightly northwest" as it crosses the equator northbound, instead of straight north. And if you push LTAN to 6:00 or 18:00 you get a Dawn-Dusk orbit whose plane lies along the day-night terminator, so the satellite is hardly ever eclipsed. That's the trick SAR missions like RADARSAT use to keep their power-hungry radars fed.
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Why is LTAN preset to 10:30? Is that a magic number?
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It's the sweet spot for optical imaging. With LTAN 10:30 the spacecraft crosses the equator at local 10:30 a.m., so the Sun is east of zenith — shadows are long enough to reveal terrain relief, but not so long that they hide everything in darkness. LANDSAT has always used roughly 10:00–10:30, Sentinel-2 uses 10:30, SPOT uses 10:30, and WorldView-3 uses 13:30. Different applications nudge it: agriculture wants early morning before clouds build up, ice-sheet monitoring may prefer longer shadows to see cracks. Try setting LTAN to 6:00 in this tool and you'll see the eclipse fraction drop almost to zero — that's the Dawn-Dusk regime.
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Changing the repeat cycle changes "orbits per cycle". What does that actually mean in practice?
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It's how many days you have to wait until the satellite flies over exactly the same point with exactly the same viewing geometry. A 5-day cycle means the satellite completes 73 orbits, and on day 6 it lines up over its day-1 track. LANDSAT uses 16 days/233 orbits, Sentinel-2 uses 10 days/143 orbits, ALOS-2 uses 14 days/207 orbits. Shorter cycle = better temporal resolution; longer cycle = denser ground-track spacing that fully covers the surface. Disaster monitoring wants short cycles, cartography wants long ones. The "Equatorial track spacing" output is your density gauge — if it climbs above 4000 km the tool flips to a warn verdict because the gaps between tracks are wider than your sensor swath can fill.
Frequently Asked Questions
A Sun-synchronous orbit (SSO) is a special orbit whose nodal regression rate, driven by Earth's J2 (oblateness) perturbation, is tuned to match Earth's mean motion around the Sun (about 0.9856°/day). In a plain polar orbit (i = 90°) the orbital plane is fixed in inertial space, so the local time over the same ground point drifts day by day. In an SSO (i ≈ 98°, retrograde) the orbital plane slowly rotates eastward by 360°/year, following the Sun, so any point on Earth is overflown at roughly the same local time year-round. This stabilises the Sun-illumination angle and dramatically simplifies the design of Earth-observation payloads and solar arrays.
LTAN is the mean local solar time at which the spacecraft crosses the equator going northbound, and it sets the illumination conditions for every image it takes. 10:30 and 13:30 (Morning/Afternoon orbits) are the standard for optical Earth-observation satellites such as LANDSAT and Sentinel-2 because the shadows are neither too long nor too short and terrain relief is clearly visible. 6:00 and 18:00 (Dawn-Dusk orbits) keep the orbital plane along the day-night terminator so the spacecraft is almost never eclipsed; that is the choice for SAR satellites (RADARSAT, SAR-Lupe) that need continuous solar power for an active sensor.
The J2 nodal regression rate is dΩ/dt = -(3/2)·n·J2·(R_E/a)²·cos(i)/(1-e²)², and the sign of cos(i) sets the sign of the regression. The Sun-synchronous condition requires an eastward (positive) regression in the same sense as Earth's orbital motion, but the leading sign is negative, so we need cos(i) < 0, i.e. i > 90°, which is a retrograde orbit. Typical SSO inclinations are 97.4° to 98.6° at 500–800 km altitude and about 99.5° at 1000 km. A prograde orbit (i < 90°) would precess westward and could never be Sun-synchronous.
The repeat cycle is set by the integer condition D·86400/T_orbit = N (with Earth-rotation correction), where the satellite completes N orbits in exactly D days and the ground track re-aligns. LANDSAT-8/9 uses 16 days/233 orbits, Sentinel-2A/B uses 10 days/143 orbits. A short cycle gives high temporal resolution but wide track spacing; a long cycle gives narrower spacing and denser surface coverage. This tool lets you enter the desired repeat days and immediately shows the orbit count and equatorial ground-track spacing produced by your altitude.
Real-world applications
Optical Earth-observation satellites: LANDSAT 8/9 (705 km, i = 98.2°, LTAN 10:00), Sentinel-2A/B (786 km, i = 98.5°, LTAN 10:30), WorldView-3 (617 km, i = 97.97°, LTAN 13:30) and ALOS-3 (669 km, i = 97.9°, LTAN 10:30) are all SSO satellites. Most commercial and governmental optical remote-sensing platforms — used for agriculture, forestry, urban planning, disaster response — rely on the SSO property that "every pass uses the same Sun angle", which is essential for reliable change detection in time series.
SAR and meteorological satellites: SAR satellites such as RADARSAT-2 (798 km, i = 98.6°, Dawn-Dusk LTAN 18:00) and TerraSAR-X (514 km, i = 97.4°, Dawn-Dusk LTAN 18:00) consume large power for radar transmission, so they fly in Dawn-Dusk orbits to keep their solar arrays continuously illuminated. Meteorological constellations like NOAA POES and Suomi NPP / JPSS use SSO to give daily global temperature and cloud-cover maps at consistent local times.
Orbit design and launch planning: Reaching an SSO requires the launcher to inject the satellite at i ≈ 97–100° with high accuracy, which is energetically penalising from low-latitude sites. Tanegashima, Vandenberg, Kourou and Vostochny are the dominant launch sites for SSO missions. The nominal orbit produced by this tool feeds directly into Δv budgeting and operations planning at the concept-design stage.
Station-keeping and maintenance: An SSO is self-maintaining in theory but drifts in practice due to residual atmospheric drag (significant below 600 km), third-body perturbations and solar radiation pressure. LANDSAT-class missions perform a handful of station-keeping burns per year to keep LTAN within ±10 minutes. This tool gives you the nominal orbit that those corrections aim at.
Common misconceptions and caveats
The first myth is, "an SSO satellite is always in sunlight." That is only true in Dawn-Dusk orbits (LTAN 6:00/18:00). For Morning/Afternoon SSOs (LTAN 10:30/13:30) the spacecraft is eclipsed for roughly 20% of every orbit — about 20 minutes per 98-minute revolution at 700 km. Power-budget design must include the solar-array-off / battery-discharge phase during eclipse. The "Eclipse fraction" in this tool is a simplified approximation that ignores seasonal variation, but it captures the LTAN trend correctly.
Second, "J2 alone keeps the orbit Sun-synchronous forever." J2 is by far the largest perturbation, but in reality J3 and J4 zonal harmonics, lunar and solar third-body gravity, atmospheric drag, solar radiation pressure and tidal effects all nudge the orbit. After a year, LTAN can drift by tens of minutes if no propellant is used to correct it. Leap-second style changes in Earth-rotation also matter on long timescales. This tool gives the ideal SSO condition; operational design needs full propagators (STK, GMAT, Orekit).
Third, "SSO gives complete global coverage." Because the inclination is 97–100°, the satellite never flies directly over latitudes higher than 80–83°. The polar caps within a few hundred kilometres of each pole are simply blank in SSO imagery. Missions that must image the poles use near-polar orbits like ICESat-2 (i = 92°) or dedicated polar platforms. Conversely, when your target is mid-latitude or tropical, SSO is the optimal choice — increase the "Observation latitude" slider above 80° and the tool will flag the constraint in the verdict.
How to Use
Enter altitude (400–2000 km) using altNum or altRange slider; this sets orbital height above Earth's surface.
Set Local Time of Ascending Node (LTAN, 06:00–18:00 hours) via ltanNum or ltanRange; SSO requires specific inclination to maintain constant sun angle relative to ground tracks.
Define repeat cycle (1–16 days) using repNum or repRange; this determines how many orbits complete before ground track repeats, affecting equatorial spacing and mission coverage.
Adjust eccentricity (0.0–0.002) via eccNum or eccRange; near-circular orbits minimize altitude variation for Earth-observation instruments.
Click simulate to compute required inclination (typically 97–99°), orbital period, eclipse fraction, and ground-track geometry.
Worked Example
Landsat-9 configuration: altitude 705 km, LTAN 10:00 (mid-morning), repeat cycle 16 days, eccentricity 0.0001. Simulator outputs orbital period 98.9 min, required inclination 98.21°, 14.57 orbits/day, 233 orbits/repeat cycle, equatorial track spacing 15.9 km, eclipse fraction 34.2%. These values match real sun-synchronous missions where the 0.985 precession rate equals Earth's mean solar motion (~1° per day).
Practical Notes
Lower LTAN (06:00–09:00) produces morning overpasses with minimal cloud shadows; higher LTAN (14:00–16:00) maximizes sun illumination angle for radar backscatter consistency in SAR imagery.
Tighter repeat cycles (1–3 days) require higher inclination and narrower equatorial spacing; 16-day cycles (Landsat standard) balance global coverage with manageable ground-station revisit frequency.
Eclipse fraction exceeds 35% only near equinox seasons; polar regions remain fully sunlit year-round, essential for solar-panel power budgets in Earth-observation constellations.