Modern life now often depends on GPS (short for Global Positioning System), but it can err by several meters in cities. Now a new study from a team of Dutch researchers reveals a terrestrial positioning system based on existing telecommunications networks that can deliver geolocation info accurate to within 10 centimeters in metropolitan areas.
The scientists detailed their findings 16 November in the journal Nature.
GPS and other global navigation satellite systems (GNSSs) such as China’s Beidou are capable of reaching centimeter-level precision, notes study senior author Christian Tiberius, a navigation engineer at Delft University of Technology, in the Netherlands.
However, buildings and other obstacles in urban settings can block or reflect the satellite signals on which a GNSS depends, resulting in major errors, Tiberius says. In addition, GNSSs are vulnerable to attacks such as jamming, spoofing, or forging, which have increasingly led scientists to explore possible backup positioning systems.
The atomic clocks onboard GNSS satellites also help keep clocks synchronized worldwide. This results in timing accurate to several nanoseconds, says study lead author Jeroen Koelemeij, who is an optical timing specialist at Vrije Universiteit Amsterdam. The scientists note, however, that emerging technologies such as quantum communication require sub-nanosecond accuracies.
Although researchers have designed a number of alternatives to GNSSs, these often require their own new infrastructure, forming a major barrier to large-scale deployment. Many also rely on two-way communications from the mobile transceiver to a sensor infrastructure. This means anyone using them has to reveal their presence and position, which is less privacy-friendly than a GNSS receiver.
Now scientists have developed a terrestrial positioning system called SuperGPS based on existing telecommunications networks.
Like a GNSS, SuperGPS does not require two-way communications. That’s where they part ways. This prototype is not only independent of any GNSS but can offer superior performance.
The researchers also noted SuperGPS’s compatibility with existing 4G and 5G telecommunications networks.
“Today’s mobile networks may, technically speaking, not be too far from an upgrade with potentially dramatic consequences,” says Koelemeij.
“The mobile infrastructure that now provides only connectivity services may in the future be upgraded to provide an additional crucial service—namely, positioning that is independent from, and more precise than, satellite positioning systems.”
A GNSS depends on satellites that each possess atomic clocks that are tightly synchronized in time. These satellites transmit precisely timed signals broadcasting their positions. A GNSS receiver can then pinpoint its own location by analyzing how long it took signals from each visible satellite to arrive.
Similarly, SuperGPS makes use of precisely timed signals from a mobile telecommunication network’s constellation of radio transmitters. These are connected and synchronized through a fiber-optic Ethernet network linked to an atomic clock.
“There is a global fiber-optic infrastructure and a wireless mobile infrastructure out there that telecom companies worldwide have invested hundreds of billions into,” Koelemeij says. “During the development of the project, we chose signal formats and equipment that are compatible with [those] of existing network infrastructure so that hopefully one day it can be integrated at reasonable additional cost.”
The hybrid optical-wireless system employs a bandwidth of radio signals about 10 times as large as what’s commonly used by GNSS. This helps it detect and deal with reflected signals, enabling higher positioning and timing accuracy.
Bandwidth within the radio spectrum is typically scarce and therefore expensive. The scientists accounted for this by using a number of small-bandwidth radio signals over a large “virtual bandwidth.”
In experiments outdoors with six radio transmitters dispersed over 660 square meters, SuperGPS delivered a positioning accuracy pinpointing objects to within 7.4 to 10.2 cm, as well as sub-nanosecond timing. The researchers could improve the positioning accuracy to within 2.2 cm by taking advantage of information about the phase of the signals—that is, the fraction of a wave that the signal has completed at any given time.
Enabling highly accurate positioning in cities “could advance the general introduction of safe automated driving, in particular if the positioning system is independent of and complementary to GPS positioning,” says Koelemeij.
Future research will examine how to best use the virtual bandwidth on which SuperGPS depends “to implement virtual wideband signals such that available resources are optimally used,” says study coauthor Gerard Janssen, a radio communication engineer at Delft University of Technology.
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