Conjunction screening
The Conjunction tab screens two scenario objects for close approaches over a time window. It finds every range minimum, refines the time of closest approach (TCA), and displays a range-vs-time chart, three RIC relative-motion projections, and an events table with TCA, miss distance, and relative speed.
Quick start
- Add at least two scenario objects (e.g. load two catalog satellites).
- Press A to open the Analysis panel and click the Conjunction tab.
- Select the Reference object (the RIC-frame origin) and the Chase object (the RPO object whose relative motion you want to see).
- Set the Window (6, 24, or 48 hours) and the Threshold (km). Range minima below the threshold are highlighted red in the events table.
- Click Run.
▶ Open the GPS constellation to try conjunction screening
Range-vs-time chart
The upper canvas plots the center-to-center range (in km) between the two objects over the selected window. The chart includes:
- Blue curve — the range at each sample time (30-second sampling interval).
- Red dashed line — the threshold. Only visible when Threshold > 0.
- Dot markers — each refined TCA (parabolic refinement of the nearest range minimum).
- Crosshair — hover anywhere on the chart to read the exact range and UTC time at the cursor position. The time slider below the chart also scrubs to that moment.
The chart label confirms the span: range vs time (N h).
RIC relative-motion view
Below the range chart are three equal-scale projections of object B’s position in object A’s RIC (Radial, In-track, Cross-track) reference frame. The RIC frame is centered on the reference object, with axes defined from its position and velocity vectors:
- Radial (R) — aligned with the position vector from Earth’s center to the reference object (outward).
- In-track (I) — aligned with the orbital velocity, completing the right-hand system with R and C.
- Cross-track (C) — normal to the orbital plane.
The three panels show the R-I, R-C, and I-C projections. The relative trajectory for a circular co-altitude orbit traces an elongated oval in the R-I plane (the famous “hockey stick” shape of rendezvous trajectories); highly inclined crossing orbits produce very different patterns.
Use the time slider to animate the chase object’s position in the RIC views. Scroll to zoom, drag to pan, and double-click to reset any RIC panel.
Events table
Below the RIC view, each detected close approach (every range minimum in the scan) is listed with three columns:
| Column | Meaning |
|---|---|
| TCA (UTC) | Time of closest approach, refined to sub-second precision, in UTC. |
| Miss (km) | Center-to-center range at TCA, in km (to three decimal places). |
| Rel speed (km/s) | Magnitude of the relative velocity vector at TCA, in km/s (to three decimal places). |
Rows where the miss distance is below the threshold are highlighted. A summary line above the table reads: “ObjectA vs ObjectB — N minima”.
How the screening works
The tool samples both objects every 30 seconds on a shared J2000 timeline (using stateAtJ2000() for both — SGP4 states are converted from TEME, other propagators are already in J2000). Every sample where the range is a local minimum is bracketed, then parabolically refined until the TCA window is under 0.25 seconds. The relative velocity at the refined TCA is computed directly from the two propagated velocity vectors.
Range minima at the window boundaries (the run begins or ends at a minimum) are ignored; if you suspect an approach falls at a boundary, widen the window.
Mixed-propagator pairs
Both objects can use any propagator (SGP4, J2, two-body), and pairs with different propagators are handled consistently: both are evaluated in J2000, so the relative-position vector is frame-accurate regardless of the propagator combination. The per-propagator honesty disclaimer appears below the events table for every propagator in the run. For operationally meaningful TCA times, both objects should use SGP4 with fresh TLEs.
The app uses two-body geometry for orbital computations. For operationally relevant close-approach analysis (real satellites, real risk), use SGP4 with fresh TLEs on both objects.
Accuracy & model notes
Conjunction screening uses the same propagators as the 3D scene. The accuracy of the predicted TCA and miss distance is bounded by TLE age for SGP4 objects: fresh TLEs give kilometer-level position accuracy; stale TLEs degrade rapidly. For J2 or two-body objects the results are geometric exercises, not operational predictions. The per-propagator disclaimer — “Idealized two-body propagation — educational geometry only; real orbits diverge within hours” — is shown for any two-body object in the run. See Propagation models and the validation page.
Related
Why does the conjunction run show “0 minima” even though the objects come close?
The scanner finds local range minima within the window. If the objects are always getting closer (or always getting farther) over the whole window — or if the minimum falls exactly at the start or end of the window — no interior minimum is detected. Widen the window, or start the scenario at a time when the approach is more centered.
What is the RIC origin for the projections?
The RIC frame origin is the Reference object (the first dropdown). The chase object’s position relative to the reference is projected into R, I, C axes derived from the reference object’s state at each sample time. Swapping the two objects inverts the relative trajectory but gives the same miss distances and TCAs.