Ground-Station Pass & S-Curve Doppler Predictor
Evaluate daily pass count, contact minutes, and real-time 437 MHz Doppler S-curves for any ground station latitude.
Show the data as a table
| Time from TCA (min) | Doppler shift (kHz) |
|---|---|
| −6.0 | — |
| −3.0 | 9.3 |
| 0.0 | −0.0 |
| 3.0 | −9.3 |
| 6.0 | — |
DETERMINISTIC PHYSICS & DATA METHODOLOGY
How This Calculator Works
Source modules: lib/physics/orbit.ts
Propagates a circular LEO orbit with Earth sidereal rotation and J2 secular nodal precession over 5 days (Δt = 10 s) to compute daily pass count, mean pass duration, and total contact minutes above a station elevation mask, alongside single-pass 437 MHz Doppler S-curves.
Governing Equations & Physical Model
Ω̇ = −(3/2)·J₂·(R_E/a)²·n·cos i, sin ε = (r_sat − r_gs)·r_gs / (|r_sat − r_gs|·R_E)Integrates visibility intervals sin ε ≥ sin ε_mask across 5 sidereal cycles to average out initial orbital phase.
Δf(t) = −f₀ · (dr/dt) / cSamples 91 points from horizon rise to set for the selected peak pass elevation ε_max.
Inputs, Parameters & Units
| Parameter | Unit | Physical definition |
|---|---|---|
| h, i | km, ° | Circular orbit altitude and inclination (97.5° for 525 km Sun-synchronous). |
| φ_gs, ε_mask | °N, ° | Ground-station latitude and minimum usable elevation angle. |
| ε_max | ° | Peak elevation of the plotted reference pass for Doppler S-curve inspection. |
Assumptions & Validity Limits
- Spherical Earth geometry is used for horizon masks; local terrain obstructions and antenna rotator keyhole delays are excluded.
- Single-pass S-curve holds Earth rotation fixed during the ~10 min pass window (<4% duration difference).
Worked Default Reference Case
Produces an average of 3.0 passes/day above 10° elevation with a 6.5 min mean pass duration (7.7 min longest overhead pass) and 19.4 min/day total contact.
- Passes per day (≥ 10°)
- 3.0 passes/day
- Mean / longest pass
- 6.5 min / 7.7 min
- Total daily contact
- 19.4 min/day
- 437 MHz horizon Doppler
- ±10.25 kHz