How do satellites communicate with a GPS system? (2018) | Hacker News Reader

How do satellites communicate with a GPS system? (2018)

allaboutcircuits.com

56 pointsreqo15 comments

The title is very confusing.

Satellites don't generally communicate 'with' a GPS system.

I think they meant, "How do GPS satellites communicate with the GPS system?"

I believe that’s no longer true. That is, there are now orbiting satellites that use GPS in a client capacity for whatever reason.

I’m pretty sure I recall a paper I saw on the subject discussing how the Earth centered inertial frame can’t just be used unmodified.

Those satellites have GPS receivers, but they don't "communicate with" a GPS system (which implies they are transmitting to the GPS system).

I'm sure there are some Russian or Chinese satellites that can disrupt GPS satellites, maybe that would be considered to be a system that can "communicate with".

For those interested in GPS, check out this article which discusses in detail how each step of the GPS process works, including acquisition, tracking, and decoding. I was thoroughly fascinated and flash-card-ified it so I will remember the facts long-term.

https://news.ycombinator.com/item?id=40041198

Makes you wonder if there is encryption on the uplinks for the first batch and how quickly they will be hacked.

Remember how they didn't have encryption for years on the UAVs during the Iraq War and Afghanistan?

https://www.wired.com/2012/10/hack-proof-drone/

Of course the command and control uplink is encrypted. Always have been. All military satellite tt&c uplinks are encrypted. (Commercial as well, almost certainly, I just don't have personal knowledge.)

Exactly – otherwise, somebody would have most likely already deorbited them or disrupted them in some other way.

Even amateur satellites are allowed to use encryption for TT&C – a rare exception in (at least US) regulations that otherwise ban all form of encryption.

The title actually means "How do GPS receivers receive signals from the GPS satellites?" It's not about other satellites communicating with GPS satellites.

TL;DR: They use special-purpose antennas.

that is completely the wrong answer

How so? The summary of the article at the bottom of TFA, helpfully labelled "Summary", says this:

GPS antennas, located on satellites, spread across the globe, integrated into application receivers, make possible the systems that keep us on time and on track.

Sounds like parent might not be completely right, but they're certainly not "completely wrong".

The antenna is not the special part. The special part is the signal processing magic that allows decoding of extremely weak signals. Also, the antennas on receivers are dramatically different then the antennas on the satellites.

Then I suggest GP go argue with TFA author, rather than calling out a comment as "completely wrong" when TFA summary says otherwise.

this is correct, thank you

'they use special-purpose antennas' is as misleading an answer as 'they use electricity' or 'they use radio waves'. even the summary of the article doesn't claim the antennas are special

GPS system = Global positioning system system

I mean the GPS satellites are in medium earth orbit so most LEO satellites receive the signal more or less the same way as people a few hundred km further down, no?

The comms between the GPS satellites and the ground stations is pretty interesting, see figure A4-1 in this for what happens to accuracy over time if the satellites stop getting corrections from the ground:

https://navcen.uscg.gov/sites/default/files/pdf/gps/geninfo/...

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