Starlink constellation

Starlink's first-generation shell 1 as filed with the FCC: 1,584 satellites at 550 km, in 72 orbital planes of 22, inclined 53°. The other Gen1 shells can be switched on in the side panel. Each shell is a Walker-delta pattern, with its planes spaced evenly around the equator and its satellites spaced evenly within each plane. Drag to rotate. Click the globe to select the satellite nearest overhead.

The phasing factor F sets how far each plane's satellites are shifted along the orbit relative to the previous plane. It isn't in the filings. The default is a modelled choice that keeps neighbours in adjacent planes close to abeam and at least about 100 km apart where the planes cross.

Each satellite links to four neighbours. Two are in its own plane, ahead and behind, and they stay put. The other two are in the adjacent planes on either side. Adjacent planes cross near the northern and southern ends of each orbit. There the side neighbour swings from abeam to straight ahead and on to the other side, so the side links drop and have to be pointed, acquired and tracked again.

Modelled terminal limits: a side link works while the neighbour stays within the field of regard around abeam and within range, the line of sight clears the Earth and 80 km of atmosphere, and the pointing slew rate stays under the limit. All of these limits are settings in the side panel. SpaceX hasn't published its terminals' limits or acquisition timings.

Pointing, acquisition and tracking, one link

A laser beam is only microradians wide, so a link has to be found before it can carry data. The terminal first slews to where the neighbour should be (coarse pointing). It then searches for the neighbour's beacon (acquisition) and locks its fine-steering mirror onto it (tracking) before the link comes up. Any limit that is broken drops the link back to idle.

User terminals talk to the satellites over Ku-band. Gateways connect the satellites to the internet over Ka-band. A terminal can only use a satellite above a minimum elevation, 25° in the Gen1 filings, so each satellite serves the area inside a ring about 940 km in radius. Click the globe to add a terminal, or click an existing one to remove it.

Gateway sites on this globe are illustrative example locations. They are not real Starlink gateway sites. SpaceX does not publish a complete, current list.

Reassignments and failed slots count the last 40 scheduling intervals. Select a row to make it the selected terminal.

Terminals are reassigned to satellites on a fixed interval, every 15 seconds according to independent measurements of Starlink. At each boundary the scheduler picks each terminal's satellite for the next interval. On the coverage globe, terminals that just changed satellite are drawn larger and labelled.

The scheduler's rule isn't public. This one takes the highest satellite that stays above the minimum elevation for the whole interval, checked at its start and end.

Pick two ground points. Traffic goes up from the terminal to its serving satellite and across laser links, then down to a gateway, which carries it the rest of the way over terrestrial fiber. The path shown is the one with the lowest one-way delay on the current link graph, compared with a fiber route between the same two points.

Starlink's routing isn't public. This uses a shortest path (Dijkstra) weighted by distance ÷ speed of light, plus a modelled processing delay at each satellite. The fiber comparison is the great-circle distance times a modelled route-inflation factor, at the speed of light in glass (c ÷ 1.47). Both values are settings in the side panel.

Every satellite's clock drifts. Each clock is disciplined in two ways. One is GNSS time from an onboard receiver, which is noisy. The other is the neighbours' time over the laser links (two-way time transfer), which is precise but only as good as the neighbours. If the selected satellite loses both, its clock runs free, and once the error passes the guard threshold every slot scheduled on it fails.

This is a model based on public information. SpaceX hasn't published its timing architecture, so the clock noise, loop gains, drift and guard threshold are modelled values. For how GNSS satellites themselves keep time, see the GPS satellite simulator and the GPS ground segment simulator.

Failed slots are counted for the terminals, and for a modelled grid of user cells (2° apart) around any satellite that is out of sync. Failing cells are marked red on the coverage globe.