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Wayward Satellites Test Einstein's Theory of General Relativity (scientificamerican.com)
An anonymous reader quotes a report from Scientific American: In August 2014 a rocket launched the fifth and sixth satellites of the Galileo global navigation system, the European Union's $11-billion answer to the U.S.'s GPS. But celebration turned to disappointment when it became clear that the satellites had been dropped off at the wrong cosmic "bus stops." Instead of being placed in circular orbits at stable altitudes, they were stranded in elliptical orbits useless for navigation. The mishap, however, offered a rare opportunity for a fundamental physics experiment. Two independent research teams -- one led by Pacome Delva of the Paris Observatory in France, the other by Sven Herrmann of the University of Bremen in Germany -- monitored the wayward satellites to look for holes in Einstein's general theory of relativity.
Einstein's theory predicts time will pass more slowly close to a massive object, which means that a clock on Earth's surface should tick at a more sluggish rate relative to one on a satellite in orbit. This time dilation is known as gravitational redshift. Any subtle deviation from this pattern might give physicists clues for a new theory that unifies gravity and quantum physics. Even after the Galileo satellites were nudged closer to circular orbits, they were still climbing and falling about 8,500 kilometers twice a day. Over the course of three years Delva's and Herrmann's teams watched how the resulting shifts in gravity altered the frequency of the satellites' super-accurate atomic clocks. In a previous gravitational redshift test, conducted in 1976, when the Gravity Probe-A suborbital rocket was launched into space with an atomic clock onboard, researchers observed that general relativity predicted the clock's frequency shift with an uncertainty of 1.4 x 10-4. The new studies, published last December in Physical Review Letters, again verified Einstein's prediction -- and increased that precision by a factor of 5.6. So, for now, the century-old theory still reigns.
Einstein's theory predicts time will pass more slowly close to a massive object, which means that a clock on Earth's surface should tick at a more sluggish rate relative to one on a satellite in orbit. This time dilation is known as gravitational redshift. Any subtle deviation from this pattern might give physicists clues for a new theory that unifies gravity and quantum physics. Even after the Galileo satellites were nudged closer to circular orbits, they were still climbing and falling about 8,500 kilometers twice a day. Over the course of three years Delva's and Herrmann's teams watched how the resulting shifts in gravity altered the frequency of the satellites' super-accurate atomic clocks. In a previous gravitational redshift test, conducted in 1976, when the Gravity Probe-A suborbital rocket was launched into space with an atomic clock onboard, researchers observed that general relativity predicted the clock's frequency shift with an uncertainty of 1.4 x 10-4. The new studies, published last December in Physical Review Letters, again verified Einstein's prediction -- and increased that precision by a factor of 5.6. So, for now, the century-old theory still reigns.
There are a lot of reasons for having their own system, including control. But regardless of the reasons, right now there are pieces of five different positioning systems in operation right now. There's GPS (US), GLONAS (Russia), Galileo (Europe), Beidou (China), and QSZZ (Japan). New crops of GPS units, including the very impressive ZED-F9P chip from U-Blox, can see all of these satellites, allowing for more accurate and faster fixes. Also more satellites provides more redundant information for weeding out any bad satellite data, overcoming deliberate jamming, or when a country intentionally degrades the signal. Apparently recently near Georgia and South Carolina there was some GPS jamming going on as part of a naval exercise. Having more systems to work with mitigates this somewhat, although they all use similar frequencies to GPS's L1, L2, and L5 bands.
And recently the FCC has finally allowed American users of GPS receivers to be able to use these other satellites. Odds are your phone is now using GPS, Galileo, and Glonass for positioning. It's a really a win win for those that rely on this technology. I can't see a downside, either for end users or countries to have more of these systems up and running, other than cost.
It's also much higher precision - by an order of magnitude. The US system cannot be trivially upgraded, you have to replace all of the satellites.
It's a small world and it smells funny; I'd buy another if it wasn't for the money; Take back what I paid (SoM)