Target-Centric Satellite Observation Platform

Target-Centric Satellite Observation Predictor

What is this?

Given a ground location and a satellite, this tool predicts when the satellite's sensor can actually observe that target — not merely pass overhead.

How it differs from standard overpass prediction

Conventional overpass predictors (e.g. N2YO, Heavens-Above) compute visibility windows: time ranges when a satellite is above a minimum elevation angle from the observer. This answers "when can I see the satellite?" — useful for visual observers and radio operators.

This tool answers a different question: "when does the satellite's sensor footprint cross my target?" It models the sensor's geometry (swath width, off-nadir pointing capability) and finds the precise moment the target falls within the sensor's observable area. This is what matters for Earth observation, remote sensing, and satellite imagery tasking.

The algorithm: bearing-convergence

Rather than sweeping a time grid for elevation crossings, the predictor uses an iterative bearing-convergence method:

1. Propagate — Advance the satellite position forward in time using SGP4/SDP4 orbital propagation from TLE data.

2. Compute bearing angle — Calculate the angle between the satellite's velocity vector and the direction from satellite to the ground target. When this angle is exactly 90°, the satellite is at its closest approach to the target.

3. Detect zero-crossing — Monitor the bearing difference (angle − 90°). When it changes sign, the satellite has passed the point of closest approach.

4. Bisect and converge — Reverse the time step direction and shrink by 20×. Repeat until the bearing converges below a precision threshold (0.0001°), pinpointing the exact overpass time.

5. Sensor filtering — At the converged time, compute the ground distance and off-nadir scan angle. Apply the sensor model constraints to determine if the target is actually observable.

Sensor models

Pushbroom — Models a wide-swath scanning sensor (e.g. VIIRS, MODIS). The target is observable if the ground distance is within half the swath width (default 3,000 km). This is the legacy mode and captures most polar-orbiting imager passes.

Off-Nadir — Adds a scan angle constraint on top of swath filtering. The target must be both within the swath and within the maximum off-nadir pointing angle (default 30°). This models agile sensors that can point but have a limited look angle (e.g. high-resolution commercial imagers).

Visualization

Orbit track (white line at altitude) — One full orbital period (~90 min) of the satellite's 3D trajectory, centered on the predicted pass time.

Ground track (cyan line on surface) — The sub-satellite point traced on the Earth's surface.

Swath footprint (cyan ribbon) — The sensor's ground coverage corridor along the track, sized by the configured swath width.

Day/night — Globe lighting and terminator line are set to the predicted pass time, showing whether the observation occurs in daylight or darkness.