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Comment by shadowgovt | Hacker News Reader

Radio's got some crazy physics.

So baseline, a modern cell phone can connect to a tower about 45 miles away. Stsrlink satellites orbir about 10 times out from that. They are solving this problem by some very clever phased array technology to be deployed on the satellites themselves, because when it comes to radio, you can often solve the problem of signal reception by boosting either the transmitter or the receiver side of the story.

This is how we've kept in touch with the Voyagers all this time. The Voyager radio technology is getting no better, but the technology we can throw at it from a ground station has been out-stripping the loss due to distance and degradation (or at least approximately keeping pace).

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Also 10x may not be representative since the 45 miles to a cell tower is all through atmosphere but Starlink is 400 miles up, so there's only 5-10 miles of atmosphere at any noticeable density.

The atmosphere is transparent at typical cell phone frequencies (and the part that can be a problem are clouds and the ionosphere).

> So baseline, a modern cell phone can connect to a tower about 45 miles away.

Isn't that primarily because of the Earth's curvature and obstructions/hills?

Good question. I've had a bear of a time sourcing good info on this and would welcome a solid analysis.

Radio horizon is somewhat complex, but 45 miles is not entirely unreasonable from an elevated position.

https://en.wikipedia.org/wiki/Line-of-sight_propagation#Radi...

There could also be timing related constraints that limit max range: radio waves propagate at the speed of light which is fast, but not instant.

microwaves go about 10% further than the horizon, barring any tropospheric ducting - which cell towers shouldn't, in general.

The issue you have with long distances is multipath, which can throw off the complex timing required to have multiple users in realtime on a single radio.

I've heard that US spy satellites used to eavesdrop on soviet telephone calls transmitted between line-of-sight ground-based microwave relay towers. The relay towers used directional microwave antenna aimed at each other, but satellites in space could pick up those signals.

I can't find a direct reference to it on wikipedia, but I suspect this is what the Vortex satellites were doing: https://en.wikipedia.org/wiki/Vortex_(satellite)

This [1] may help but I'm wholely unfamiliar with the field. A guy at Defcon did a talk about how HAM radio in Florida allocates their bandwidth and goes into the nitty details of propegation.

[1] https://youtu.be/fH-yyTZffAk

Line of sight (1/r^2) often has lower attenuation than urban propagation (1/r^3 to 1/r^4), so it’s often easier to hit a LEO satellite than a terrestrial tower.

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