GPS satellites don't transmit strong enough to reach indoors, through the roofs and walls of buildings, like cell phones do. GPS signals that enter buildings through windows are unreliable since they often have bounced and thus give the wrong distance measures by even hundreds of meters. Since cell phones work fine indoors, and GPS uses the same frequency, I suppose there's no need to use another frequency.

Would it suffice for today's kind of GPS satellites to orbit in low Earth orbits, such as planned communication satellites constellations like Starlink? Would it be feasible to equip them with larger solar arrays, or would nuclear generators be required?

One can imagine many commercial applications for precision indoor navigation (like more efficient robotic vacuum cleaners ;-) Just reaching through the roofs of single storage factories and shopping centers should make for a great market. The alternative today is to install radio or ultrasound beacons (or perhaps a camera system) in every room where one wants to have a location service. A single ubiquitous system that's already standard would have economic advantages.

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    $\begingroup$ Not all cell phones work fine indoors and not in every building. There are metall roofs and steel construction buildings. Multi path distribution of satellite signals does harm the GPS position precision. The speed of electric waves is different in air and in solid materials ( non metallic solids ). But GPS even handles different speed of light in vacuum and air. But how to handle the much lower speed in solids? $\endgroup$ – Uwe Apr 2 '19 at 11:20
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    $\begingroup$ All you have to do is construct your building appropriately. GPS works perfectly well inside my (wood frame, single story, non-metal roof) house. $\endgroup$ – jamesqf Apr 2 '19 at 15:56
  • $\begingroup$ @jamesqf Really? It's been some years since I disappointingly tried out GPS inside a hut, and maybe with already then old equipment. The receivers have been getting alot better. But how far could that development go? $\endgroup$ – LocalFluff Apr 2 '19 at 17:21
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    $\begingroup$ There is no need to install radio beacons for a indoor positioning system. Wifi access points which are ubiquitous these days can do that job. Wifi positioning system (en.wikipedia.org/wiki/Wi-Fi_positioning_system) is already used extensively by mobile phones and laptops and have a better accuracy than GPS indoors. $\endgroup$ – Kevin Selva Prasanna Apr 3 '19 at 8:36
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    $\begingroup$ The comparison with phones is really unhelpful. You yourself state why: "since they often have bounced". While that is a problem for GPS, your phone doesn't care at all, hence it works $\endgroup$ – Hobbamok Apr 3 '19 at 10:05

There are some big differences between GPS and cell phone signals:

  1. GPS relies on the exact timing of a signal (1 nanosecond off = 30 cm of position inaccuracy). Cell phones are much more tolerant to variations in signal timing. Basically they don't care as long as the packets arrive quickly enough not to cause a gap in the audio.

  2. GPS transmitters are 18,000 km away, cell phone towers are less than 5 km away. So a cell phone signal is much stronger at the receiver.

item 2 could be solved with a stronger transmitter, but that would make #1 worse: with a stronger signal, you get more multipathing: more reflections that are still strong enough to be picked up by the receiver.

You can't really use indoors transmitters either. Many buildings have a steel structure which reflects radio waves, so you get lots of multipathing when the transmitter is inside.

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    $\begingroup$ "... with a stronger signal, you get more multipathing..." This sounds like something that's made up and not based in EE fact. Any digital radio receiver system will have automatic gain control (AGC) before the ADC. So as far as the digital signal is concerned, the multipathed reflections will have the same strength relative to the direct path signal no matter what the strength of the satellite's signal is. $\endgroup$ – uhoh Apr 2 '19 at 12:06
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    $\begingroup$ if your signal is weak to begin with, reflections can drop below the noise floor. Strong signal->more reflections make it to the receiver above the noise floor. $\endgroup$ – Hobbes Apr 2 '19 at 12:28
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    $\begingroup$ It doesn't work like that. It's not like there is a "floor" and signals below it disappear. Every signal (of a given type, e.g. L1) from every GPS satellite in the sky, plus every reflection of every signal from every satellite in the sky plus all sources of noise, all go through one single front end amplifier and then one ADC, where it is digitized into one digital stream of 1's and 0's. That stream is then copied to dozens of identical correlaters, each looking for a different Gold code from a different satellite. $\endgroup$ – uhoh Apr 2 '19 at 12:35
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    $\begingroup$ @uhoh, would need to find a reference, and is a analog artifact but was taught a potential solution for en.wikipedia.org/wiki/Ghosting_(television) where the cause was proximity to the transmitter allowing reflections to be above noise floor was adding an attenutor or just making the antenna less effective. Thinking through may have also been result of of doing AGC later in the signal path than GPS. $\endgroup$ – GremlinWranger Apr 2 '19 at 12:43
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    $\begingroup$ @uhoh, other place I've met power reduction to reduce multipathing is in Sonar, which also does not say anything useful at all about GPS and their somewhat unique receiver design so think I get to learn something today. $\endgroup$ – GremlinWranger Apr 2 '19 at 13:12

There is existing technology for this, termed "{Active,Passive} GPS Repeater". It is composed of both an antenna outside and inside, with optionally active or passive components to forward the signal. My introduction to it was within an ocean-going vessel, and it worked fine.




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    $\begingroup$ Note that a GPS repeater doesn't actually give you indoor navigation - everything receiving the repeated signal computes approximately the same position, that of the outside antenna. (Source: we use these repeaters to be able to do indoor testing of GPS-based equipment that we make.) $\endgroup$ – jasonharper Apr 2 '19 at 13:19
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    $\begingroup$ A cite from your first link: "It should be noted that all the repeaters in a single network will transmit the coordinates of the outdoor antenna and not the position of the repeater unit itself." $\endgroup$ – Uwe Apr 2 '19 at 13:22
  • $\begingroup$ @jasonharper If they have chosen to transmit the computed GPS position directly to the GPS receivers (so that crew and cruise passenger easily can see on their phones where they are at sea when under deck) instead of via e.g. WiFi, that speaks for the advantage of GPS, as a dominating standard, over indoor positioning systems requiring specific hardware and software. $\endgroup$ – LocalFluff Apr 2 '19 at 13:31

In general, if you need a local indoor navigation system, then you are better off implementing your own. Generally speaking these are called Indoor Positioning Systems. There isn't yet a standard, but there has been some work to making one. It would be MUCH cheaper, more accurate, and overall just better.

GPS satellites are expensive, they are all space rated atomic clocks, some of the most accurate clocks ever created, and the most accurate in orbit. Making thousands of them would be very expensive, and not really gain a lot.

All that being said, there are a few things that can be done. The way indoor GPS typically works is by estimating your location from other means, and looking for the very low signal strength signal coming at the right time.

  • $\begingroup$ Low altitude GPS satellites do have conceptual problems with time in sight and Earth's shadow. But I'm pretty sure that putting nuclear reactors on the GPS satellites to up their power (if that would be a solution) is cheaper than installing and maintaining unstandardized indoor systems in billions of buildings. Just changing the beacons' batteries or cabling them to the power grid is a pain. GPS isn't more expensive than EU, Russia, China, Japan and India all have gotten their own (redundant) positioning satellite constellations. $\endgroup$ – LocalFluff Apr 2 '19 at 13:21
  • $\begingroup$ @LocalFluff Nuclear reactors in LEO ?!! $\endgroup$ – Antzi Apr 2 '19 at 15:43
  • $\begingroup$ @Anzi Nuclear rather as the alternative to having GPS satellites in LEO. With enough power generation they could beam their radio the more powerful from 30 times further away, $\endgroup$ – LocalFluff Apr 2 '19 at 17:17
  • $\begingroup$ They may not be cheaper though. Suppose you can either include an extra stage in your mobile phone's chipset to (somehow) pick up GPS indoors, or install an indoor positioning system. The former is cheap (add to a handful of chip designs => solved for most devices within 2 years, similar to adding GALILEO or GLONASS, low cost). The latter needs every building to buy new hardware and install it, effectively billions of individual boxes to buy, install, and run on mains power for the rest of time. $\endgroup$ – Stilez Apr 2 '19 at 21:06
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    $\begingroup$ No satellite from far far away will possibly give the precision that a local system would require. Also, indoor will always suffer from multipath, which will degrade the accuracy further. One of the interesting ideas I've heard in this regard is to use the local wifi signals as a kind of GPS signal, which would be totally doable. $\endgroup$ – PearsonArtPhoto Apr 2 '19 at 21:16

1. Assuming that your application can (or does) use an external GPS antenna, keep in mind that they are not all the same. They are available in different gains.

I use them for two lightningmaps.org receivers. One is far more sensitive than the other, and I might be able to get away from using it indoors (though I have never tried to).

2. Many have had success by placing GPS antennas next to windows. Glass does not attenuate the signals like foil-backed building insulation, wiring, etc. does.

A GPS receiver connected to an external (outdoor) antenna would determine the position of that outdoor antenna.

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    $\begingroup$ Question did not ask for GPS indoor receiption only, it was about GPS indoor navigation too. A GPS receiver connected to an external (outdoor) antenna would determine the position of that outdoor antenna. $\endgroup$ – Uwe Apr 3 '19 at 20:17
  • $\begingroup$ @Uwe Thanks. Comment edited to include that. $\endgroup$ – Mike Waters Apr 3 '19 at 20:27

You have a couple of big problems here:

1) GPS simply doesn't have the accuracy you need. In most cases the best it could hope for is to figure out what room you're in and that wouldn't be 100% accurate.

2) The more stuff in the way the more reflections become an issue and the more inaccurate your fix is even if you manage to get one. There are special (expensive) high accuracy GPS antennas--but they are utterly intolerant of obstructions.

Note that there is an obvious answer that fails--you can't have an antenna to relay the signals. A GPS receiver does not actually figure out it's location, but rather the location of the antenna used. In a normal civilian use the difference is so small it doesn't matter but when you have a GPS built into something large (say, a ship) the antenna can be a substantial distance from the electronics.


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