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China's version of GPS now has more satellites than US original

asia.nikkei.com

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Re: China's version of GPS now has more satellites than US original

#51

Post by gmueckl »

gandalfian wrote: post #26
Maakuth wrote: post #19

Isn't the proliferation of multiple GNSS constellations actually quite positive thing? The smartphone geolocation chips seem to have no problem receiving signals from all constellations. So effectively this competition between the systems allows for denser coverage of location satellites. This in turn should enable more accuracy and faster location acquisition even with limited sky visibility. In security sense, it s…

Everyone sharing one system is excellent. Everyone sharing four increasingly redundant systems starts to look like a waste of money. We need a carpooling equivalent gps system. Still better than more bombs I guess.

It stops looking that redundant when one system goes down because both of its ground based refence clocks go down at the same time. This recently happened to Galileo. Nobody noticed because NavStar and GNSS covered for it.

Re: China's version of GPS now has more satellites than US original

#52

Post by Ambroos »

raihansaputra wrote: post #48
credit_guy wrote: post #38

So, if North Korea wants to develop an ICBM, the main challenges are 4: take off (which they appear to have mastered), miniaturization of the bomb, navigation and reentry vehicle. All the remaining 3 are quite difficult, and navigation in particular was traditionally done by a mix of inertial (i.e. based on gyroscopes) and celestial (i.e. based on the observation of stars). GPS makes all this unnecessary. If NK uses…

The unencrypted GPS automatically disables themselves when they detect speeds of a missile to prevent exactly this.

That is a limit placed on the receiver, I can't imagine that being hard to get around with a custom receiver or maybe even just customized receiver firmware.

Re: China's version of GPS now has more satellites than US original

#53

Post by mshook »

huhtenberg wrote: post #14
GaurVimen wrote: post #11

TL;DR at bottom. While impressive, "how many satellites you can see over a major city" is not an accurate way to judge a GPS satellite network. To understand why, you have to understand how GPS works. Basically, each GPS has a super accurate clock on board. These are all synced up. These satellites transmit a signal regularly on predetermined schedules. Your phone or car or whatever else is using GPS listens for thes…

> At no point do you need more than 4 in sight. Unless you are measuring something with high precision over long periods of time. Like dam flexing or ground shifts. The more sats you have, the better you'll be at eliminating the signal noise (due to the ionosphere transition and what not). Also, regarding this - > "how many satellites you can see over a major city" is not an accurate way to judge a GPS satellite netw…

The city problem even has a name: Street Canyon or Urban Canyon https://en.wikipedia.org/wiki/Street_canyon#GPS_signal_recep...

Re: China's version of GPS now has more satellites than US original

#54

Post by PaulAJ »

londons_explore wrote: post #33
willis936 wrote: post #9

Earlier quoted context omitted.

With military receivers (or codeless phase tracking) the current constellation is good down to centimeters.

This isn't possible without third party data. Things like the refractive index of the atmosphere (which depends on weather patterns), and the fact orbits are slightly warped due to varying gravitational fields and radiation pressure from the sun, all add up to location innacuracies unless you can correct for these effects. That correction requires data, which needs to come from a third party because the satellites do…

Actually its a bit more sophisticated. Galileo ground stations measure their reported position against the actual, and then transmit correction data to the satellites which then retransmit it with the rest of the position data.

Re: China's version of GPS now has more satellites than US original

#55

Post by robocat »

PaulAJ wrote: post #18

This doesn't make sense. How can there be more satellites in the Eastern hemisphere than in the Western? Navigation satellites are in low earth orbit: they go around the earth every couple of hours. I could understand if one constellation had better polar coverage while another has better equatorial, but an east-west divide isn't physically possible (unless the Chinese are turning their satellites off when over the A…

Easily if using QZSS or synchronous orbits. For example the Japanese use QZSS orbits to for 3 of their satellites: https://en.m.wikipedia.org/wiki/Quasi-Zenith_Satellite_Syste...

Re: China's version of GPS now has more satellites than US original

#56

Post by 867-5309 »

xbmcuser wrote: post #20
stunt wrote: post #6

Earlier quoted context omitted.

> As of June 28, Chinese satellites were observed more frequently than GPS satellites in 130 of 195 countries (U.N. member states plus the Vatican and Palestine). More than 20 BeiDou satellites were observed over mainland China.

I don't get something you have satellites of glonass, gps and beidou what's stopping anyone from getting better accuracy using all 3 at the same time.

because they are independent systems; they cannot communicate triangulation data between themselves

Re: China's version of GPS now has more satellites than US original

#57

Post by throw0101a »

raihansaputra wrote: post #31
Maakuth wrote: post #19

Isn't the proliferation of multiple GNSS constellations actually quite positive thing? The smartphone geolocation chips seem to have no problem receiving signals from all constellations. So effectively this competition between the systems allows for denser coverage of location satellites. This in turn should enable more accuracy and faster location acquisition even with limited sky visibility. In security sense, it s…

The US is able to block GPS signal to a specific area (I think it was done on the Gulf War) or disable the unencrypted GPS signal (only the US military can decode them). I assume each system has the same capability too. The worst case scenario would be all parties decided to block the unencrypted portion and leaving all civilians to be location-blind, including civilian aircrafts.

> The US is able to block GPS signal to a specific area ...

Anyone can block the signal given how weak it is, and there are news reports of people doing so (and being charged for it). You can supposedly buy jammers online for not a lot of money.

> ... (I think it was done on the Gulf War) or disable the unencrypted GPS signal (only the US military can decode them).

What you are probably referring to is "Selective Availability" and is the purposeful degrading of the signal to make it less accuracy. The DoD said it will not do so, and has given assurances to the FAA (because planes are now highly dependent on it). The newest GPS III satellites no longer have the S/A hardware:

* https://en.wikipedia.org/wiki/Error_analysis_for_the_Global_...

By 2000 people had figured out to get quite accurate results even with the S/A enabled, so the whole thing not as useful:

* https://www.aviationtoday.com/2000/07/01/the-real-reason-sel...

If there's a war on it's probably cheaper to just jam in the area of operations.

Re: China's version of GPS now has more satellites than US original

#58

Post by throw0101a »

michaelt wrote: post #22
lolc wrote: post #7

> When using BeiDou for car navigation, the receiver could theoretically transmit the car's location to a satellite in orbit, said Dean Cheng, a senior research fellow at the Heritage Foundation in Washington. Details on this? What does "theoretically" mean? Are there known protocols for this? Implementations? A lot of things could "theoretically" be done but aren't in practice.

I suspect this is a misunderstanding on the part of Dean Cheng. Beidou-1 used a two-way link, because it used two satellites in the same orbit. This means the satellite can track all users. But Beidou-2 supports a one-way link - the same operating principle as GPS, GLONASS and Galileo - where the satellites can't track users. The downside to this design is it needs a lot more satellites, and they have to be in differ…

It should be noted that BeiDou-1 and -2 are/were regional systems. The BeiDou-3 currently being deployed is global.

Re: China's version of GPS now has more satellites than US original

#59

Post by jhayward »

raihansaputra wrote: post #31
Maakuth wrote: post #19

Isn't the proliferation of multiple GNSS constellations actually quite positive thing? The smartphone geolocation chips seem to have no problem receiving signals from all constellations. So effectively this competition between the systems allows for denser coverage of location satellites. This in turn should enable more accuracy and faster location acquisition even with limited sky visibility. In security sense, it s…

The US is able to block GPS signal to a specific area (I think it was done on the Gulf War) or disable the unencrypted GPS signal (only the US military can decode them). I assume each system has the same capability too. The worst case scenario would be all parties decided to block the unencrypted portion and leaving all civilians to be location-blind, including civilian aircrafts.

> The US is able to block GPS signal to a specific area

The GPS system does not have any area denial capability. If the US wants to block it in a certain area they have to deploy terrestrial jammers just like anyone else.

Allies get decryption keys for the military high-accuracy signal.

Re: China's version of GPS now has more satellites than US original

#60

Post by lolc »

mytailorisrich wrote: post #13
lolc wrote: post #7

> When using BeiDou for car navigation, the receiver could theoretically transmit the car's location to a satellite in orbit, said Dean Cheng, a senior research fellow at the Heritage Foundation in Washington. Details on this? What does "theoretically" mean? Are there known protocols for this? Implementations? A lot of things could "theoretically" be done but aren't in practice.

Seems that the experimental Beidou-1 worked that way, and that Galileo is also able to work that way [1]. However, this does not seem to be how the deployed version of Beidou works, and this does not seem required with Galileo either. I also note that Qualcomm produces Beidou receiver chips so these are under American control. Overall I have the feeling that the article is trying hard to whip up fears over Beidou...…

I do wonder where this "scare" comes from. I remember a Bloomberg article that also posited Beidou as a danger.

Sure these systems have military applications, but why not talk about those dangers directly instead of trying to portray civilian use as problematic?

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