I was always fascinated by this idea. You are literally able to communicate or open a web browser from anywhere in the world. I am OK with the low power device able to receive data from satellites, but directly transmit the signal.. How is it doing it? It doesn't even have a parabolic antenna.

Take a look: h small module

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    \$\begingroup\$ As a terrestial being you are used to thinking of distance as a challenge due to the horizon. But this is line of sight, 483 miles up and some distance over - really not that far. Flip it around and ask why you would need a lot of power - obstructions? (Unclear if it works indoors). Intereference? Cheapskate engineering? Wide bandwidth? When those are out of the way, hams work the world with a few watts via ionospheric reflection, so if this is well designed and operating in quiet spectrum, why shouldn't it work? \$\endgroup\$ Feb 9, 2015 at 0:21
  • \$\begingroup\$ @ChrisStratton Very good point on your side. You just got me interested in radio communications field. :) My question was How does it work? though. \$\endgroup\$ Feb 9, 2015 at 0:23
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    \$\begingroup\$ It works by being the strongest source of RF energy with particular filterable/correlatable properties visible to the satellite's antenna. \$\endgroup\$ Feb 9, 2015 at 1:17
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    \$\begingroup\$ Transmit power of Iridium ground unit is 2W. That's not such a low power. The stubby antenna is probably a helix, which has a directionality pattern that's mostly directed upwards. \$\endgroup\$ Feb 9, 2015 at 5:30
  • \$\begingroup\$ I'm no expert - I didn't do as well as I would like in communications circuits. But I do know that what you are interested in requires an understanding of far-field EM waves, which, once established, travel indefinitely until they are absorbed or destroyed. Also, directional antennas don't suffer the power density spread problem that an omnidirectional antenna has. +1 to Chris and Nick. \$\endgroup\$
    – Nohbdy
    Feb 12, 2015 at 0:15

1 Answer 1


It is a matter of link budget.

The people who designed Iridium did their homework and saw that the tiny fraction of those 2 watts that reached the big parabolic antenna of the satellite was sufficient. And for most cellular systems, you make cheap and low power terminals, while the base station (or satellite) has very expensive receiver equipment and high transmit power to compensate for the cheap terminals.

I found a very detailed paper on the subject, first describing the theory, then benchmarking the theory with actual numbers from Iridium. Rocket science level of the text though.


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