Calculator Inputs
Example Data Table
Use these sample values to test the calculator quickly.
| UTC Date | Site Lat (°) | Site Lon (°E) | Target RAAN (°) | Inclination (°) | Half-width (min) | Direction |
|---|---|---|---|---|---|---|
| 2026-04-01 | 28.6139 | 77.2090 | 120.00 | 51.60 | 10 | Prograde |
| 2026-04-02 | 34.6328 | -120.6107 | 75.00 | 97.60 | 15 | Retrograde |
| 2026-04-03 | 13.7333 | 80.2333 | 210.00 | 45.00 | 12 | Prograde |
Formula Used
This tool estimates launch windows by matching the local sidereal time (LST) at the launch site to the target plane’s RAAN (and the opposite node, RAAN + 180°).
- LST (deg) = GMST (deg) + Longitude (deg East)
- GMST is computed from the Julian date using a standard polynomial + Earth rotation term.
- Window center: times when LST ≈ RAAN or LST ≈ RAAN + 180°
- Window bounds: Start = Center − tolerance, End = Center + tolerance
- Azimuth (ideal): cos(Az) = cos(i) / cos(φ) where φ is latitude and i is inclination.
How to Use This Calculator
- Enter the UTC date for which you need the day’s opportunities.
- Set launch site latitude and east-positive longitude.
- Provide the target orbit RAAN and inclination from mission requirements.
- Choose a tolerance (minutes) to widen or narrow the displayed window.
- Press Submit to see windows below the header.
- Use Download CSV or Download PDF to export.
Operational Notes
Sidereal Alignment and Window Center
Launch windows are driven by Earth’s rotation relative to the target orbital plane. When the local sidereal time at the site aligns with the plane’s right ascension, the launch azimuth can place the vehicle into the desired node with minimal plane change. This calculator identifies the daily center times and expands them into practical windows using a selectable tolerance for operations planning.
Daily Drift and Schedule Coordination
Local sidereal time advances about 360.9856 degrees per solar day, so the window center shifts earlier by roughly four minutes each day. That drift matters when coordinating range assets, ship positioning, and ground crew shifts. By exporting CSV and PDF, teams can share the next-day sequence quickly and compare changes across multiple dates without reformatting outputs, reducing coordination friction across teams; daily updates.
Latitude Limits and Inclination Band
Inclination feasibility is constrained by site latitude. A site at latitude φ can directly reach inclinations between |φ| and 180°−|φ|, ignoring dogleg and performance losses. The feasibility badge highlights this geometric boundary. If the requested inclination falls outside the band, planners should consider a different site, a dogleg, or a transfer strategy after insertion, and document the delta‑v impact clearly for reviews.
Azimuth Estimate for Range Planning
The azimuth estimate uses cos(Az)=cos(i)/cos(φ) as an idealized heading from north toward east. This provides a sanity check for range safety corridors and typical ascent headings. For retrograde missions, the azimuth flips across due south. Real guidance will adjust for winds, Earth rotation, and vehicle constraints, but the estimate supports early trade studies and briefing materials for leadership decisions and approvals.
Interpreting the Timeline Plot
The timeline plot visualizes each window as a span with a center marker. This makes it easy to see whether two windows are tightly clustered, whether a long tolerance overlaps nearby operational events, or whether only one meaningful opportunity appears on the chosen day. Use the plot to coordinate handovers and to communicate schedule risk clearly to stakeholders under time pressure.
When to Escalate to High-Fidelity Analysis
For higher-fidelity analysis, incorporate precise Earth orientation parameters, vehicle ascent modeling, and trajectory constraints such as maximum dynamic pressure, stage drop zones, and downrange safety rules. Treat the results as an engineering pre-screen: good enough to shortlist candidate dates and align stakeholders, but not a substitute for mission design tools, simulations, and flight readiness reviews with sign‑offs and traceability across requirements.
FAQs
1) What does RAAN represent in this calculator?
RAAN is the right ascension of the ascending node, describing the orbit plane’s orientation in inertial space. The tool finds when the site’s sidereal angle aligns with that plane for near‑zero plane‑change insertion.
2) Why are there sometimes two windows per day?
One alignment occurs when LST matches RAAN, and another when it matches RAAN + 180°. These correspond to opposite directions along the same node line, depending on mission geometry and constraints.
3) How should I choose the tolerance value?
Tolerance is an operational buffer around the center time. Start with 5–15 minutes for planning, then refine using vehicle performance, winds, and range rules. Narrower values help avoid overstating usable time.
4) What does “Not feasible” mean here?
It flags that the requested inclination is outside the direct geometric band for the chosen latitude. Achieving it may require a dogleg, significant plane change, or selecting a different site and trajectory plan.
5) Is the azimuth value mission-accurate?
No. It is an idealized heading based on geometry only. Real azimuth depends on Earth rotation, winds, guidance limits, and safety corridors. Use it for early checks and stakeholder discussions.
6) Why are results shown in UTC?
UTC avoids confusion across distributed teams and aligns with many range and spaceflight products. Convert locally for staffing, but keep UTC as the single source of truth in exported schedules.