Posts Tagged ‘gps signal’

How GPS Bends Time

Saturday, July 16th, 2011

Equation section, WIRED magazine:


Einstein knew what he was talking about with that relativity stuff. For proof, just look at your GPS. The global positioning system relies on 24 satellites that transmit time-stamped information on where they are. Your GPS unit registers the exact time at which it receives that information from each satellite and then calculates how long it took for the individual signals to arrive. By multiplying the elapsed time by the speed of light, it can figure out how far it is from each satellite, compare those distances, and calculate its own position.

For accuracy to within a few meters, the satellites’ atomic clocks have to be extremely precise—plus or minus 10 nanoseconds. Here’s where things get weird: Those amazingly accurate clocks never seem to run quite right. One second as measured on the satellite never matches a second as measured on Earth—just as Einstein predicted.

According to Einstein’s special theory of relativity, a clock that’s traveling fast will appear to run slowly from the perspective of someone standing still. Satellites move at about 9,000 mph—enough to make their onboard clocks slow down by 8 microseconds per day from the perspective of a GPS gadget and totally screw up the location data. To counter this effect, the GPS system adjusts the time it gets from the satellites by using the equation here. (Don’t even get us started on the impact of general relativity.)



TeenyChron: A Linux-based GPS-synched NTP server

Sunday, September 26th, 2010


The genesis of this clock stems from one of my other hobbies, Ham Radio. I wanted a reasonably accurate clock that would display both local and UTC time on a large LED display. Everything I could find missed the mark by at least one feature. So I set out to design a clock with the above features, and also with the additional feature of being a stratum one NTP time Server, that is synchronized to a GPS’s pulse per second (PPS) signal.

At the heart of the system I am using a small single board computer based upon an ARM processor running Linux. I actually purchased the board in 2006 for another undertaking that is still in my long list of projects. The TS-7400 Computer-on-Module is built and sold by Technologic Systems. In the configuration I bought the SBC I paid $155 for a single unit. Mine has 64MB of RAM, 32MB of Flash, a battery backed up real time clock (RTC), and runs a 200Mhz ARM processor. I’ve configured the board to boot and mount a file system from a 2Gig SD card. I love this board! It runs a full version of Debian Linux. To date, every standard software package I’ve loaded complies and runs without any trouble.

courtesy of TeenyChron

More functional GPS in minutes

Saturday, April 3rd, 2010


Sparky with his hack to allow interaction with the core of an Aldi GO Cruise 4300 GPS Windows CE OS. All that’s required is a few programs and registry edits to the GPS, which anyone can accomplish within a few minutes. But, suggested you go slow and double-check your work, anyway; nobody wants a bricked system. After you’re done you can run such great programs like the one Sparky suggest for 4WD enthusiasts,Ozi Explorer.

via Hack a Day

Researchers Demonstrate How to Spoof GPS Devices

Tuesday, September 30th, 2008


With millions of GPS-based navigation devices on the road today, it is time someone considered the question: What if there’s an attack on the GPS network itself?

Researchers at Virginia Tech and Cornell University spent more than a year building equipment that can transmit fake GPS signals capable of fooling receivers.

“GPS is woven into our technology infrastructure, just like the power grid or the water system,” said Paul Kintner, electrical and computer engineering professor and director of the Cornell GPS Laboratory in a statement. “If it were attacked, there would be a serious impact.”

GPS is a U.S. government-built navigation system of more than 30 satellites circling earth twice a day in specific orbits. The satellites transmit signals to receivers on land, sea and in air. Based on the signals received from the satellites, devices are able to triangulate their exact positions on the globe. But if those satellite signals were wrong — or were spoofed — a GPS device might come up with the wrong location based on the signals it was receiving.

The researchers started by programming a briefcase-size GPS receiver used in the research of the uppermost part of the Earth’s atmosphere, known as ionospheric research, to send out fake signals. The phony receiver was placed in the proximity of a navigation device, where it anticipated the signal being transmitted from the GPS satellite. Almost instantly, the reprogrammed receiver sent out a false signal that the GPS-based navigation device took for the real thing.

The experiments to show the vulnerability of GPS receivers to spoofing could help devise methods to guard against such attacks, says Brent Ledvina, an assistant professor of electrical and computer engineering at Virginia Tech, and will be detailed in a research paper to be released Thursday.

“It’s almost like someone nearby is spoofing your favorite radio station by transmitting at the same frequency but higher power fooling your receiver into believing it is getting the right station,” says Ledvina.

The idea of GPS receiver spoofing has already been considered by federal authorities. In a December 2003 report, the Department of Homeland Security detailed seven countermeasures including monitoring the absolute and relative GPS signal strength, monitoring the satellite identification codes and the number of signals received and checking the time intervals between the received signals to guard against spoofs.

Still those fall short and would not have successfully fended off the signals produced by a reprogrammed receiver, said the researchers.

Instead they have suggested a few countermeasures that involve both hardware and software changes. “We have two patent applications which include a software algorithm to help make changes to how receivers react to signals,” says Ledvina.

The other patent is around the spoofer tool used, he says. “The idea is to help government and other companies use it to potentially make better receivers,” says Ledvina.

Photo: NASA

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