Newest phones (like iPhone 8) work with GPS, Galileo, Glonass, and QZSS. Beidou support will come in the future.
Sidenote: the ublox M8 chips are a pleasure to work with, and there are variants that have IMUs (gyros and accelerometers) on board for sensor fusion. All for <10$. What an exciting time we live in!
I wish someone would build a USB data logger using one of these. I still use one to geotag photos and record trips, but all the available models use old GPS-only chips and can be pretty jumpy when reception is poor. Having one that fused multiple GNSS and had an IMU to smooth out some of the random jumpiness would be ideal.
It doesn't make any sense to have the ground station transmit to figure out its location. To give a baseline, that's how the original analogue COSPAS SARSAT worked last century, using doppler measurements to get a location over time - and its accuracy was appalling, a human operator has to use equipment to narrow down where the signal is, it takes significant time to get a 1 nautical mile search area. Which is much better than "I dunno, in the sea?" for a lost ship or plane, but you're still going to need a serious search-and-rescue operation to find anybody.
A modern digital COSPAS SARSAT system you may have seen if you're a serious hiker or outback flyer just does GPS, it uses GPS to figure out where it is, then sends a digital signal that says "I need help, here are my GPS co-ordinates" hence the slogan "Taking the search out of search and rescue".
The current Breitling Emergency (a bulky men's watch, but it wouldn't look out of place on some big out-of-doors type guy's wrist) is a working PLB transmitter.
So that means signals from it can be received not just 21000km up, but 36000km up where Geostationary birds live.
If you activate it, that watch will transmit the digital signal with its unique ID in it, and assuming you filled all the paperwork out soon after somebody from the relevant local emergency response (maybe a coastguard, mountain rescue, that sort of thing) will be phoning your emergency contacts
"Hi, do you know nraynaud? They went flying today? OK, do you know where they were headed? We have an emergency transmission from their beacon so we're going to send somebody to take a look and check they're OK. We'll let you know, thanks".
A "real" beacon would be better, more robust, better battery life, easier and more obvious for random strangers to operate if you're unconscious, not to mention much cheaper. But the Emergency has the advantage that you'll actually have it when shit goes wrong, because it's your watch.
The idea was realisedly lunatical even back then, but party boffins really though that reality and laws of physics will yield to party's mandate.
The explanation that beidou 1 was a glorified, super expensive satellite pager, with navigation functionality as an afterthought does not hold water. This "explanation" came much later than the commencement of beidou 1 project.
The reason Beidou I needs two way transmission is they wanted add pager functionality to its navigation functionality. The party wants more control has nothing to do with Beidou I's two way transmission mechanism. When Beidou I was developed, it's mostly used by military. The party doesn't need "more control" there.
China already had military digital satellite coms working by late nineties. For them, it was of even lesser utility.
My point is that many things in China may be explained by the party wanting more control, but two way transmission of beidou I is not one of them.
Why do some people view it as a with-us-or-against-us problem? Combining beidou with GPS gives a better accuracy. Some competition and collaboration in the GPS field can benefit us as a whole.
Why just two? If our stranglehold on the world breaks, the EU and Russia are easily capable of carving out their sphere of influence.
People really underestimate how populous and wealthy and powerful the EU is.
It's strange how the narrative changes in such a short time period. Just 10 years ago, I remember people talking of "chimerica" as if the US and china were going to merge into a single world ruling entity.
Let's hope even more countries get into GPS "rivalry."
So if GPS were to be jammed, or disabled by a hack into the ground control systems -- PLA missiles would still work just fine. Hmm.
GPS is too easy to jam, no serious military application would use GPS exclusively.
Edit: I'm wrong.
GPS satellites are transmit only so they can’t impose such limits directly it’s device manufacturers that implement COCOM.
The new military M code transmitted from the latest satellites is encrypted with a frequently changing key, so in the event of a war the DOD can disable the civilian signal as the M code will work standalone, hence a third party nation cannot rely on GPS.
The previous P(Y) code was also encrypted, but is theorised to need the civilian C/A code as well to work.
They also had the option in older satellites of turning on selective availability, which would degrade the civilian location service from 5-meter to 100-meter accuracy without affecting the military service. Purportedly newer GPS satellites don't have the option anymore.
That does sound suspiciously like something the military would want you to believe
There are probably other, newer ways to prevent an adversary to use the GPS signal, but there is little reason to doubt that SA is gone.
There's a good reason and precedent for China (and others; see GLONASS, IRNSS, Galileo) to be afraid of this.
The US, as well as virtually all other technological powers, does have the capability to jam GPS signals using ground, sea, or air-based jamming platforms but those are not part of the GPS system.
Edit - link:
> Additionally, we have demonstrated the capability to selectively deny GPS signals on a regional basis when our national security is threatened.
Building your own GNSS system does nothing to protect against electronic warfare platforms.
As well as anyone that has access to basic electronic parts. It's very easy to swamp a 30 watt transmitter that's ten thousand miles away.
Navstar (owned by USDoD, what is commonly called "GPS"), GLONASS and soon Galileo all have global coverage.
Although one should note that name is the intention; it has not always had global coverage (it does now), and may not always have global coverage in the future (say, if they decide to increase the precision in a strategic area at the cost of loss of coverage in others).
Does this make anyone else nervous?
Maybe Trump has a point when he says we should stop letting China steal our technology.
Any country that can launch a satellite has a technology to destroy one. It's called a missile. You don't even need to go to orbit - just intersect the orbit of the target and dump a bucket of nails in its path.
It's easier to put something in orbit than to knock something out of orbit.