
China's version of GPS now has more satellites than US original - benryon
https://asia.nikkei.com/Business/China-tech/China-s-version-of-GPS-now-has-more-satellites-than-US-original
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throw0101a
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/watch?v=Rpx1ySCKO3Q)

Playlist:

* [https://www.youtube.com/playlist?list=PLGvhNIiu1ubyEOJga50LJ...](https://www.youtube.com/playlist?list=PLGvhNIiu1ubyEOJga50LJMzVXtbUq6CPo)

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Maakuth
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?

~~~
gandalfian
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.

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Barrin92
>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

~~~
gandalfian
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.

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lolc
> 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.

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michaelt
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!

~~~
lolc
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.

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huhtenberg
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.

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bgee
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](http://www.thespacereview.com/article/1307/1)

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PaulAJ
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).

~~~
ZeKZ
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...](https://www.researchgate.net/figure/Ground-tracks-of-
the-BeiDou-satellites-and-distribution-of-the-experimental-
tracking_fig1_236080831)

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nullc
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.

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credit_guy
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.

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

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Ambroos
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.

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

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

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willis936
With military receivers (or codeless phase tracking) the current constellation
is good down to centimeters.

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londons_explore
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.

~~~
willis936
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.

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dragonsh
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.

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

~~~
dragonsh
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.

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

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DaniloDias
Every map in China is wrong. [https://youtu.be/L9Di-
UVC-_4](https://youtu.be/L9Di-UVC-_4)

Pretty significant indicator that the intention is for military applications

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flywithdolp
The space race

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

How can they have more control over the space

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not_a_cop75
Oh, look. It's the bigger penis argument all over again.

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dang
Please don't do this here.

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jaimex2
Welcome to 1978 China.

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onethought
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?

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jaimex2
Am I the only one not impressed with China copying 40 year old milestones?
Surely...

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GaurVimen
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.

~~~
huhtenberg
> _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.

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mshook
The city problem even has a name: Street Canyon or Urban Canyon
[https://en.wikipedia.org/wiki/Street_canyon#GPS_signal_recep...](https://en.wikipedia.org/wiki/Street_canyon#GPS_signal_reception)

