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I have a Technical Amateur Radio license so I know the basics of wavelengths. What I'm researching is how GPS works.

From what I've read, there are frequencies such as L1, L2, and L5 uses in GPS between the receiver and the satellite. These operate at at a frequency of about 1500 MHz. https://gisresources.com/everything-you-need-to-know-about-gps-l1-l2-and-l5-frequencies

What I'm having trouble understanding is how a receiver can communicate with an object 12,000 miles away at about 1 GHz.

I know my knowledge on GPS is rudimentary but I'm trying to make sense if it, thanks!

https://www.gps.gov/systems/gps/space

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4 Answers 4

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The first thing to know is the communication is one-way. There's a satellite-to-receiver transmission, nothing going in the opposite direction. So your cell phone doesn't have to find the radio power to send a signal all the way to space!

(exceptions: The decommissioned Chinese BeiDou-1 system - and any products where the GPS receiver chip is used alongside a satellite transmitter, like a Cospas-Sarsat emergency locator beacon)

The signal from GPS satellites is very faint - each satellite has to broadcast a signal to about half the planet, powered only by some solar panels! So the broadcast signal is modulated using a 'Gold Code' (in the case of the oldest public GPS signal) where part of the signal transmitted by the satellite is already known by the receiver - the GPS receiver can pick out the signal despite how faint it is, by tracking the cross-correlation between the received and expected signals.

This also means multiple satellites can transmit their signals at the same frequency - so long as they use different gold codes, the receiver can track both signals independently.

Newer GNSS signals replace gold codes with newer techniques - like 'multiplexed binary offset carriers' - which perform better, but do basically the same thing.

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    $\begingroup$ Nitpick: The emergency beacons are not talking to the GPS satellites, but to birds in a much lower orbit. $\endgroup$
    – Loren Pechtel
    Commented Oct 10, 2022 at 4:41
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    $\begingroup$ @LorenPechtel, actually, the emergency beacons are talking to GPS satellites: starting with the Block IIR satellites, COPAS-SARSAT capabilities have been part of the payload. They're also talking to Galileo, Glonass, BeiDou, and nine geostationary-orbit satellites (and a bunch of low-orbit satellites, but those don't have anywhere near the coverage of the others). $\endgroup$
    – Mark
    Commented Oct 10, 2022 at 21:39
  • $\begingroup$ @LorenPechtel Ya but, birds aren't real... $\endgroup$
    – SnakeDoc
    Commented Oct 11, 2022 at 22:08
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    $\begingroup$ @SnakeDoc So are they integer or complex? $\endgroup$
    – Loren Pechtel
    Commented Oct 11, 2022 at 22:26
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The receiver does not communicate with the satellite; it just receives.

Fundamentally, the receiver listens on one or more well-known frequencies. The receiver determines which satellites it can "hear" based on the PRN identifier code and each satellites position based on the ephemeris data that it transmits.

The satellites all broadcast a synchronized time signal. Since the receiver knows the location of the satellites and it can measure the differences in the time signals from the various satellites, it can triangulate its position on the surface of the earth.

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  • $\begingroup$ GPS satellites are definitively not geostationary. But they transmit their ephemeris data from which the receiver can compute the s positions. $\endgroup$
    – Jean-Marie
    Commented Oct 9, 2022 at 16:05
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    $\begingroup$ GPS/GNSS satellites are not geostationary, nor geosynchronous. Nevertheless, GPS receivers only receive (they don't need to send) satellite signals sent by quite powerful solar-powered transmitters located onboard each satellite. By the time a GPS signal gets to Earth, it is well buried under the noise floor. The used (CDMA) signal modulation allows receivers to pull out the signal info from the noise around. Modern receivers can even extract GPS signals that go through glass (but not metal nor walls). $\endgroup$
    – Fabrizio
    Commented Oct 9, 2022 at 17:40
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    $\begingroup$ Thanks for those comments. It's been a long time since I read the docs on GPS and a re-read was well worth it. :) $\endgroup$
    – David Hoelzer ♦
    Commented Oct 10, 2022 at 4:36
  • $\begingroup$ A-GNSS downloads the ephemeris and almanac data over a terrestrial data connection, so phone-based GPS receivers (and dedicated receivers with a data link via a phone like many GPS bike computers) do store the data (whether strictly a database or not) without the need to download it over the GPS signal. (@uhoh) The Time to First Fix is far faster than my 20-year-old Etrex, unless I haven't used GPS location on my phone for several days and I have no data signal $\endgroup$
    – Chris H
    Commented Oct 10, 2022 at 9:52
  • $\begingroup$ @user253751 total almanac time is 12.5 minutes, but that's for one satellite to transmit the whole constellation's positions. In the days of limited search computing power, it would be easier to acquire one satellite, listen for the positions of the others, then use this to find them more easily. Now correlating power is cheap and you can acquire all satellites in a few seconds, completely blind. Then, each satellite transmits its own ephemeris every 30 seconds, so you should be able to cold start within a minute. But we are spoiled, Internet reduces it to just a few seconds as you say. $\endgroup$
    – tomnexus
    Commented Oct 17, 2022 at 5:37
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The other answers already provide very detailed explanations, so I decided instead to suggest a resource which answers your question, but should also be very helpful with your further research:

GPS - Bartosz Ciechanowski (wayback link)

It's an interactive blog post which starts with the basics, then gradually goes into great detail about the inner workings of GPS.

Your question is answered near the very bottom:

Finally, it’s worth reiterating that the receiver doesn’t need to send anything to the satellites, it just listens to the signals that the satellites restlessly emit without knowing if anyone receives them.

In summary, to answer your question, they don't!

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UHF frequencies - LX - are in 1200 MHZ range and such frequencies propagate

  1. in line of sight
  2. are not effected ( much) by ionosphere

hence the GPS receiver can process them as necessary.

With little ( or lots - depending on you point of view ) of technology NASA kept receiving messages from a satellite OUTSIDE of our solar system. Maybe they still do... Radio waves (outside our ionosphere ) travel indefinitely , at lest in theory. But that is no longer about GPS , sorry...OT.

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