Comment by cedws
4 days ago
In summer I was lying on a beach in Thailand and used an app on my phone to look at things in the sky. Pretty much every moving glistening object I could see was a Starlink satellite. I know nothing about how their constellation works but I wonder why so many are needed. Surely you only need one or two in line of sight for it to work? I was seeing many more than that.
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.
15 minutes from horizon to horizon? No, you've got a handful of minutes at most.
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.
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.
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Thanks.
> I wonder why so many are needed. Surely you only need one or two in line of sight for it to work?
Only if you're not bandwidth limited. Having more satellites per steradian of sky allows reusing the same frequency via (physically or electronically) aiming at a particular satellite.