Space based internet systems, like the one recently proposed by SpaceX and Google, are said to use satellites to communicate directly with end users, without using cellular ground stations. But how much power would a consumer device like a smartphone need to send data 1,000+ km? Wouldn't a separate outdoor antenna and amplifier be required anyway, reducing the mobility?

  • $\begingroup$ That, or a giant antenna on the receiving end. $\endgroup$ – Aaron M Nov 27 '16 at 15:27
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    $\begingroup$ Are you sure it is said that SpaceX and Google systems will provide internet directly to smartphones? Bidirectionally? Directly between satellites and smartphones over paths of 1200 to 1600 km? Can you show a credible link to someone saying this? This sounds challenging. Was it said that it would work indoors, or only outdoors with a clear view of the sky? $\endgroup$ – uhoh Nov 27 '16 at 19:18
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    $\begingroup$ @uhoh I think you are agreeing with my point. You CANNOT use a narrower "reception beam" when you don't have a base station. Because giant antennas work because you can point them. But you only want to point a satellite at something which isn't moving. Additionally the LEO sats will make tracking ANYTHING difficult. My point was large antennas aren't magic. $\endgroup$ – Aron Nov 28 '16 at 10:19
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    $\begingroup$ @Aron OK that last sentence is fine - "large antennas aren't magic." But now and especially in the near future electronically phased arrays will be electronically steerable and not prohibitively expensive. One can already go to the store and buy a 5GHz WiFi personal router with electronic beam-steering. While wide band receivers may still need dishes for now, if you are using a fairly narrow band of frequency a flat array can steer electronically under computer control just fine. $\endgroup$ – uhoh Nov 28 '16 at 10:38
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    $\begingroup$ @uhoh Amen to synthetic apertures/MIMO! $\endgroup$ – Aron Nov 28 '16 at 10:40

While power requirements are higher than for regular GSM service, they are not as high as one might think. Current satellite telephones use handsets of the size of 2000-era mobile phones and are able to transmit 15 kBit/s to geosynchronous satellites (the Thuraya system). These satellites are more than 30 times farther from Earth than the planned SpaceX system. Therefore, the received power is about 1000 times weaker.

Compared to e.g. Iridium, the planned satellite network has the advantage of a much larger number of satellites (4000 compared to 66), which allows the use of highly directional antennas with way better gain, even in the phone itself.

For high speed data transfer a larger antenna will be required, but lower bandwidth services are well within reach for smart phones, albeit with a slightly changed design to house a larger antenna. I don't have precise numbers, but a bandwidth of well above 100 kBit/s seems well possible when communicating with a satellite in LEO.

  • $\begingroup$ Implicit here is the need to be standing outdoors with a clear line-of-sight to the moving satellite, isn't it? And that's not really a typical smartphone internal antenna either - it's quite substantially sized. See i.stack.imgur.com/HTI1R.jpg and i.stack.imgur.com/rTEdR.jpg for example. So you are really talking about a smartphone, standing outdoors in the rain, with a specialized external antenna attachment - right? $\endgroup$ – uhoh Nov 28 '16 at 2:15
  • $\begingroup$ As frequency goes up and wavelength goes down, a smaller antenna will have the same gain referenced to a half-wavelength dipole, but that does not mean it will collect the same power - the effective collection area of a reference dipole of course scales with wavelength squared. At some point you can't shrink the antenna size no matter how small the wavelength. Think about receiving optical signals from satellites - one doesn't use a sub-micron dipole, one uses a telescope of tens of centimeters or much more to collect enough power. $\endgroup$ – uhoh Nov 28 '16 at 2:23
  • $\begingroup$ Iridium uses 1.6 GHz, SpaceX wants to use 10-12 GHz. It might be possible to implement a phasing array of antennas for a highly directional signal in a 1cm high bar along the topside of a smartphone-like device. $\endgroup$ – asdfex Nov 28 '16 at 10:10
  • $\begingroup$ OK, now it's changed to a smartphone-like device which translates in English to "Expensive, Specialized Handset". While some gain comes with directionality, for antenna arrays larger than a wavelength a large enough total collection area is still important. In addition to having a continent-sized synthetic aperture, the Square Kilometer Array (SKA) still boasts a square kilometer of actual dish area for sensitivity. This kind of speculation should be backed up with real math. $\endgroup$ – uhoh Nov 28 '16 at 10:29

Presumably, yes, having a satellite smart phone is possible without fundamental technological innovation.

There's mobile satellite phones that aren't bigger then late 90s / early 2000s cell phones. Look for Iridium or Thuraya phones.

  • $\begingroup$ and look at the sizes of their antennas $\endgroup$ – uhoh Nov 28 '16 at 2:20

If it is a geosynchronous satellite it would not be a issue. The satellite itself would transmit with sufficient power to connect with the smart phone. The satellite would a high enough gain to receive the smart phone. Any phone within the footprint of the satellite would connect easily. This is because the path of the signal is straight upwards and as a result the signal would not have and terrestrial obstructions or signal propagation issues blocking the signal. The satellites transponder would down link the to the phone company.

A low earth orbiting (LEO) satellite would not be feasible for a any cell phone service due to elliptical nature of the orbit. It would be prone only being with usable for 6-10 minutes during each pass overhead. The satellite would then take 90 mins give or take to complete an orbit of the earth. The satellites orbit would not change. However since it is orbiting Planet Earth which is rotating east to west-90 mins later the satellite would 15 degrees west of you and would not be overhead for hours if not days. Furthermore because of the elliptical orbit the speed as it passes changes. This causes something referred to as Doppler Shift as a result both smart phones and satellite would constantly have to adjust their frequencies to remain in contact.

Doppler Shift is the effect one hears when stand next to a railway track as a passing train blows the whistle. As it approaches the pitch is higher, in front of you the actual pitch, and as it passes the frequency drops in pitch.

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    $\begingroup$ This is all backwards. The Iridium LEO constellation is already fully functional (if quite expensive) and uses a fixed-position antenna pack on the surface side. (Either it's omnidirectional or it steers itself internally and probably electronically.) But GEO satellite Internet absolutely requires an accurately pointed dish of considerable size. You're not going to get that in a smartphone, most obviously because the dish areas used are larger than phones by two orders of magnitude. $\endgroup$ – Nathan Tuggy Nov 28 '16 at 4:34
  • $\begingroup$ Compensating for the Doppler shift is trivial -- far easier than compensating for either the inverse-square attenuation of the extra distance to geosynchronous orbit or the fact that, for much of the high-tech part of the world, geosynchronous satellites are low on the horizon. $\endgroup$ – Mark Nov 28 '16 at 6:11
  • $\begingroup$ 1 LEO satellite would not work for cell phone. However the Iridium setup is different. There are 60+ satellites linked together which hands off the call to the next satellite coming into range. Each satellite orbits the earth at 780 km above the surface every 100.6 mins, $\endgroup$ – Old_Fossil Nov 30 '16 at 3:54

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