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China's version of GPS now has more satellites than US original (nikkei.com)
68 points by benryon 57 days ago | hide | past | web | favorite | 72 comments

Given that BeiDou-3 MEO has a different orbital altitude (21500km) than GPS (20500km) and thus different orbital characteristics, wouldn't a different number of satellites be needed?

I'm not sure that the number of satellites is that significant, and simply reflects design needs/constraints.

Ran across a good lecture series on GNSS a while ago going into more detail than most people care about ("3.3 - From Keplerian parameters to Earth centered Earth fied ECEF frame"). Module 6.7 - BeiDou:

* https://www.youtube.com/watch?v=Rpx1ySCKO3Q


* https://www.youtube.com/playlist?list=PLGvhNIiu1ubyEOJga50LJ...

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 seems that the situation virtually rules out the possibility of selectively denying some party GNSS access. So if you want to make your adversary location-blind, you'll have to accept losing location data yourself, right?

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.

Jammers are cheap and since a lot of expensive gear has GPS tracking these days, thieves are using simple jammers you plug into the cigarette outlet in cars. This is increasingly becoming a larger problem since they jam a relatively large area.

> 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.

> selectively denying some party GNSS access.

Military depends on an encrypted signal. If you have the encryption key, you can decode it even with a very high level of interference.

Disadvantage is the key is shared for all users, and if it leaks to the adversary, then they can jam the signal.

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.

>Everyone sharing four increasingly redundant systems starts to look like a waste of money

straight out of the "having two kidneys is a waste of energy" school of thought

You could buy yourself a dialysis machine "just in case" but you will have to give up something else to afford it. There is a trade off.

What level of redundancy is needed? Having three global systems seems reasonable, if it's an important service.

the problem with just one is that US will insist having a deadswitch on it, just like it did with Galileo.

so you must have your own if you don't want US to have a Damocles sword over you

Newer GPS satellites apparently don't support selective availability, so they would have no way of preventing access to only civilian devices. Of course, any of these systems can be jammed or shot down but that would affect anyone using it.

> In September 2007, the U.S. government announced its decision to procure the future generation of GPS satellites, known as GPS III, without the SA feature. Doing this will make the policy decision of 2000 permanent and eliminate a source of uncertainty in GPS performance that had been of concern to civil GPS users worldwide.


Why would a reformist country like China and Russia trust such a thing. We were also told that mass surveillance was a conspiracy/nutjob story, but the Snowden leaks has shown us a different world.

They wouldn't, and I can see why. I was just adding some information. As far as anyone in the public has been told, both GPS and Galileo will remain open for anyone to use, but depending on that requires trusting whoever runs it.

If I wanted my neighbours to trust my GPS system more, I'd tell them I don't have selective availability.

Sure. If you wanted GPS to be more widely used and depended on with no ulterior motives, you would also tell them you don't have selective availability. All of these systems require trusting the operators at the end of the day, and I'm not making the claim that one should depend on any single system.

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.

NavStar GPS can keep on running for quite a while even if the entire ground segment fails. The current generation of satellites even have an Autonav mode that lets them synchronize their clocks and compute corrections to their orbital parameters using cross-links between satellites without any ground assistance. Supposedly it can provide almost full accuracy for six months after a complete failure of the ground segment. Galileo just isn't very robust by comparison.

If everyone is thinking the same thing, then someone isn't thinking. (This time, I wasn't thinking, but I agree with the sentiment.)

In a perfect world yeah, but given the geopolitical and geoeconomical changes going on. The tradewar to seperate the Chimerica relationship, you probably want to own your own navigation systems.

> 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 different orbits.

Beidou-2 retains some backwards-compatibility with the two-way link, but if your cell phone's GPS chip supports Beidou, it'll be the one-way link. You'd notice the 20cm antenna and poor battery life if you had the two-way link!

Interesting, thanks! I was wondering how two-way communication would work at scale. I often talk to people who are worried about being tracked and a recurring topic is the one-way property of GPS.

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

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...

[1] https://space.stackexchange.com/questions/30839/is-two-way-c...

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?

I doubt this can be done with a standard "GPS" omnidirectional antenna. This would require a lot of power.

Might be referring to their proposed Search and Rescue features, SAR.

COSPAS SARSAT rides along on new GNSS (GPS-like) birds from several nations because it's a good place to put it. They move in the sky (which means a beacon without its own ability to determine location can be triangulated using the doppler effect, not possible from a Geostationary communications satellite), there are lots of them, and they cover the whole Earth (the weather sats used for COSPAS SARSAT today are focused on areas where people care most about the weather, rather than aiming for global coverage...)

The COSPAS SARSAT beacons run at a different frequency to GNSS, they emit bursts of digital data at 406 MHz when active, containing a unique ID for the beacon, optionally (if fitted) its own estimate of its location, and some other data. A global database maps unique IDs to owner/ operators and the nearest rescue agency will try to figure out if this is a genuine alert (e.g. phoning your emergency contact "Hi, is Gupie out on their yacht today? I see, and how many are with them?") and then deploy appropriate resources.

Beacons are entirely passive until you activate them, and then you really want absolutely anybody to know where you are because you want rescuing. "Privacy" feels like a distant problem if you are currently in a leaking raft a hundred miles out to sea or just broke your leg half way up a mountain.

This seems rather counter to what I know about GPS. It is a one way link.

At some point in mid-90s GLONASS had more satellites in orbit than GPS, so the sat count alone is not a terribly interesting metric.

Saw some comments on how countries should collaborate instead of building 4 different GNSS, well China definitely tried:

> The Chinese feel that they were badly used and partly humiliated by Europe. They were brought into the Galileo consortium as partners, and they paid the EU for the privilege, only to see themselves shut out of Galileo’s controlling bodies...China would have had less influence over Galileo than Japan and India have over GPS, and those two nations have not paid the US a single dime for their cooperation in the signal augmentation systems they are now building [0].

[0]: http://www.thespacereview.com/article/1307/1

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 Americas).

They use a lot of geosynchronous orbits over China to have a better coverage in this area.

Take a look at the orbits here : https://www.researchgate.net/figure/Ground-tracks-of-the-Bei...

It's the dynamics of the the satellites' movement that create this effect. At any point in time it is possible for there to be more satellites in one region of the sky than elsewhere. That does not mean that any one satellite stays in the same position. The satellites continue to move around the planet.

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...

It doesn't help that the GPS modernization project was delayed by a decade re-prioritization after 9/11.

Though this result is also because BeiDou has intentionally uneven coverage via satellites with inclined orbits that result in much better coverage (and better signal angles, high off the horizon) in asia.

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 US GPS, there are most likely countermeasures in place, for example if the US detects an NK ICBM launch, then it stops or perturbs the GPS signal.

New providers of GPS complicate this problem.

Is this just conjecture? It seems ridiculous that the US would rely on the fact they can control GPS satellites as form of missile defence... So shooting down their GPS satellites would also hamper there ability to launch ICBMs? Seems equally unbelievable.

It's a conjecture indeed, as many defense things are classified, but you can bet good money it is true.

> It seems ridiculous that the US would rely on the fact they can control GPS satellites as form of missile defense

The US most certainly would not entirely rely on that. It would only be one tool in the toolbox. Nuclear ICBM attacks are so dangerous, that the defense by necessity uses a multi-pronged approach. It starts from diplomatic pressure, and technology export controls (see for example the Wassenaar arrangement [1] that lists the controls for inertial and celestial navigation in chapter 7. The limits on the GPS receivers exist as well, unfortunately I can't point you to the exact source). The actual physical defense options against a moving missile are listed in [2], such as the Ground Based Interceptors, the Aegis system and the THAAD system. It's all but certain that in case of credible intelligence of an imminent hostile ICBM launch, the US has pre-launch options as well (the current NK ICBM used liquid fuel, and the launch facilities are known and monitored; you will notice that one or two days before any NK missile test the press already writes articles about the likely launch).

Considering all this, would the US neglect the possibility of altering the GPS signal in case it detects a hostile ICBM launch? For example, the reentry vehicle travels at many km/s. Early detonation by only half a second can reduce the impact of the bomb by a factor of 10 easily. If the missile has not GPS signal and only relies on less-than-state-of-the-art inertial and celestial navigation (which may be ok for horizontal coordinates, but not that great of altitude) that may make a huge difference. So my bet is not only that the US will promptly alter its GPS signal, but will have means to spoof the signal from the Russian and Chinese systems as well, and agreements in place with the European Galileo system too. However, as I said before, the problem is more complicated: the Russian and Chinese are not dumb, they most likely have their counter-counter-measures too.

[1] https://www.wassenaar.org/app/uploads/2018/12/WA-DOC-18-PUB-...

[2] https://en.wikipedia.org/wiki/United_States_national_missile...

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.

You cannot disable single receivers, they could only shut down or encrypt the whole system (at least for part of the world). Which could be standard procedure if they detect an incoming attack. With 3 other systems also available, that probably won't have much of an effect.

Is there any information about the accuracy of Beidu compared to Galileo?

In the article the say accuracy is coming down to 1m vs 3-5 for other GPS

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 don't transmit it.

Correct, if only L1 is used. However as GPS exists today receivers can track phase variations between L1 and L2 without the codes necessary to decode the signal on L2.

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.

Groundbased gps transmitters might help to increase accuracy.

> In the article the say accuracy is coming down to 1m vs 3-5 for other GPS

In which areas?

> 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

Isn't the triangulation being done on the phone/device GPS chip ? The same chip is doing it for all 3 services. The chip gets the data of the satellite positions and then shows its location in relation to them that how I thought gps/other systems worked.

Wouldn't it be nice instead of countries an open academic agency launch a series of open source satellites for locations. Hopefully likes of apple, Google, Microsoft, alibaba, benz, BMW, Toyota, Amazon, Qualcomm, European space agency can fund it.

But I doubt it will happen as it's considered more strategic investment with military implications. But I am still hopeful like internet(which was designed for military purpose but now controlled by civilian authorities), locations can be free off selfish national interests.

Something SpaceX could consider? Given their ambition with Satellite Internet backbone.

Yeah hopefully there can be a consortium of Automobile manufacturers, internet companies and academia which can launch such a constellation of open source satellites for location services. Hopefully that can be combined with terrestrial peer to peer network to make sure it works even if military or specific nation wants to block it.

IATA would also be a suitable member. All civilian aircraft still rely on the unencrypted part of these GNSS systems AFAIK.

I know nothing about satellites but couldn't the Starlink constellation be used for positioning in some way? It already has twice the amount of satellites and will have an order of magnitude more than any other current positioning system.

Every map in China is wrong. https://youtu.be/L9Di-UVC-_4

Pretty significant indicator that the intention is for military applications

The space race

Seems like that's the next goal of the big nations.

How can they have more control over the space

Oh, look. It's the bigger penis argument all over again.

Please don't do this here.

Welcome to 1978 China.

This reminds me of the arrogance of the British Empire... Does the US (govt/people/blob) really not see the writing on the wall with this?

Am I the only one not impressed with China copying 40 year old milestones? Surely...

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 these signals. Your device knows when exactly all the satellites sent their signal. It listens and counts how long it takes to receive each signal. With the length of time it takes to receive each signal from each satellite, it can pinpoint your location.

For basic operations you require three satellites for an approximate location.

For precise operations you need line of sight to four satellites, as the fourth enables you to calculate your altitude.

At no point do you need more than 4 in sight.

Why this is important is because the U.S. satellite constellation has 32 currently operating GPS satellites. In addition to those, we have 8 that are currently in space, ready to turn on, but they aren't because there's no reason to have them on.

There's no doubt in my mind these satellites have additional secret reasons for being up there. That's the only reason they'd need that many.

TL;DR:While they do have more active satellites, it's pointless for navigation purposes. Additionally, counting the backup satellites the U.S. Has operational but currently turned off in "standby" mode, we still have more.

EDIT: Not everything I stated is completely accurate. See comments below for some clarifications. Make sure to upvote them if you found them helpful.

> 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 network.

You are completely missing their point - you are in the _city_, a place with tall buildings that give you just a slice of the sky. So the more satellites you have above, the better you chances you will see _enough_ from where you are standing.

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

Both in cities and mountains you'll often not have 180° visibility. Having more satellites available increases the chances to always have 4 in sight.

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