Comment by Jtsummers

4 days ago

They're in LEO which means approximately 15 minutes of visibility (horizon-to-horizon). The specific time will vary based on the orbital elements but 15 minutes is a good rule of thumb. To maintain coverage you need there to be some overlap in their visibility for a location. There's also a limit to how many connections each satellite can support.

Not all the satellites that you can see will be "looking" in your direction for a signal. They support some number of cells (specific, small, geographic regions on the ground). No one satellite can cover the entire ground visible to it while overhead so more satellites are needed.

And to add to the above, Starlink is using laser crosslinks to connect their satellites to each other for routing. This crosslink network is improved with more satellites visible to each other.

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Jtsummers

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lxgr  3 days ago

That would still only require a couple dozens or few hundreds of satellites. For example, Iridium has 60-70, and Globalstar has less than 50 or so.

The actual reason for these new megaconstellations having so many is spatial frequency reuse through directional transmission/reception beams: More satellites means less users competing for each satellite's spectrum-limited bandwidth.

  • Jtsummers  3 days ago

    Iridium offers lower bandwidth and much larger cells than Starlink. But yes, the number of customers within a cell is also key to why there are so many Starlink satellites. Suburban (let alone urban) population density can easily consume the bandwidth available through one satellite.

    • lxgr  2 days ago

      Smaller spot beams are still technically possible for an Iridium-like constellation with fewer satellites. That's what e.g. ASTS is doing.

      In fact, more than one (or maybe two, for geometric reasons near the equator where polar orbits are sparse etc.) satellite concurrently visible is pointless if the ground station/mobile device isn't also heavily directional, which is not the case for small, quickly moving handheld devices at least.

      One other reason for wanting more satellites splitting footprint coverage between them would be if the satellite transmitters were transmit power limited.