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The Global Positioning System consists of 31 Earth-orbiting satellites, each carrying an atomic clock that sends a highly accurate timing signal to the ground. Anybody with an appropriate receiver can work out their position to within a few meters by comparing the arrival time of signals from three or more satellites.
And this system can easily be improved. The accuracy of GPS signals can be made much higher by combining the signals with ones produced on the ground. Geophysicists, for example, use this technique to determine the position of ground stations to within a few millimeters. In this way, they can measure the tiny movements of entire continents.
This is an impressive endeavor. Geophysicists routinely measure the difference between GPS signals and clocks on the ground with an accuracy of less than 0.1 nanoseconds. They also archive this data providing a detailed record of how GPS signals have changed over time. This archival storage opens the possibility of using the data for other exotic studies.
Today Benjamin Roberts at the University of Nevada and a few pals say they have used this data to find out whether GPS satellites may have been influenced by dark matter, the mysterious invisible stuff that astrophysicists think fills our galaxy. In effect, these guys have turned the Global Positioning System into an astrophysical observatory of truly planetary proportion.
The theory behind dark matter is based in observations of the way galaxies rotate. This spinning motion is so fast that it should send stars flying off into extra-galactic space.
But this doesn't happen. Instead, a mysterious force must somehow hold the stars in place. The theory is that this force is gravity generated by invisible stuff that doesn't show up in astronomical observations. In other words, dark matter.
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(Score: 2) by VLM on Monday May 08 2017, @05:40PM
Yeah the noise level is due to propagation and is white enough that averaged long enough it goes away.
"long enough" being on the order of a satellite orbit.
I suppose this is a big problem if its assumed the dark matter is moving faster than the satellites (insert frown emoji)
There is an interesting way around this where a truly enormous mesh of receivers can do the bistatic radar thing. You'd get a nice high res map of the ionosphere and whatever else is interfering (weather fronts or something) Then I guess the experimental limit would be telling the difference between swirlies in the ionosphere vs dark matter just passing thru. I suppose that would be self evident over time.
I'm too lazy to run the math but its interestingly easy and useful to bounce radio waves off ionized gas in the upper atmosphere, likewise now that the world has its first gravity wave detector presumably clouds of "dark matter" would F with those gravitational signals, if only we could install gravity wave detectors on all the planets and moons. Its a good excuse to go colonize the solar system.