Contact windows
A schedule is a set of windows with a start, an end and a usable arc. Everything in Mission Control is built around computing those windows honestly.
Computation
Upload a set of orbital elements — two-line elements, an CCSDS OEM file, or an ephemeris in the formats your propagator already produces. Helion propagates the orbit, computes the antenna look angles against each station's latitude, longitude and elevation, and reports the interval where the elevation mask is satisfied.
Three numbers come out of that computation for every pass: acquisition of signal, loss of signal, and the usable duration between them. The margin above the mask at the closest approach is reported alongside, because a pass that clears the mask by four degrees is a different pass from one that clears it by thirty.
- Station coordinates from a built-in list or from your own file
- Per-station elevation and azimuth masks
- Propagation from SGP4, including the SGP4.2 corrections
- Recomputation whenever the elements change
Overlaps and contention
When two spacecraft are visible from the same station at the same time, the station cannot serve both at full rate. Mission Control surfaces the overlap explicitly, ranks the candidates by downlink priority and by how long the other option would have to wait, and leaves the decision with the operator. Silent priority resolution is not offered, because the trade-off usually depends on which spacecraft is more tolerant of a delayed pass.
Missed passes
A pass that passed without downlink is recorded as a missed pass with the reason: the station did not connect, the acquisition failed, the spacecraft was not commanded, or the pass was intentionally skipped. The reason drives the alert rule, so a configuration mistake alerts differently from a hardware fault.