We’ve all been there. You’re drivingdown the highway, just as your navigation app instructed, when Siri tells you to “proceed east for one-half mile, then merge onto the highway.” But you’re already on the highway. After a moment of confusion and perhaps some rude words about Siri and her extended AI family, you realize the problem: Your GPS isn’t accurate enough for your navigation app to tell if you’re on the highway or on the road beside it.
Those days are nearly at an end. At the Institute of Navigation GNSS+ conference in Portland, Ore., in September, Broadcom announced that it is providing customers samples of the first mass-market chip to take advantage of a new breed of global navigation satellite signals. This new chip will give the next generation of smartphones 30-centimeter accuracy as opposed to today’s 5 meters. Even better, it works in a city’s concrete canyons, and it consumes half the power of today’s generation of chips. The chip, the BCM47755, has been included in the design of some smartphones slated for release in 2018, but Broadcom would not reveal which.
GPS and other global navigation satellite systems (GNSSs), such as Europe’s Galileo, Japan’s QZSS, and Russia’s Glonass, allow a receiver to determine its position by calculating its distance from three or more satellites. All GNSS satellites—even the oldest generation still in use—broadcast a message called the L1 signal, which includes the satellite’s location, the time, and an identifying signature pattern. A newer generation broadcasts a more complex signal called L5 at a different frequency in addition to the legacy L1 signal. The receiver essentially uses these signals to fix its distance from each satellite based on how long it takes the signal to go from satellite to receiver.
Broadcom’s receiver first locks onto the satellite with the L1 signal and then refines its calculated position with the L5. The latter is superior, especially in cities, because it’s much less prone to distortions from multipath reflections than L1.
In a city, the satellite’s signals reach the receiver both directly and by bouncing off one or more buildings. The direct signal and any reflections arrive at slightly different times, and if they overlap, they add up to form a sort of signal blob. The receiver is looking for the peak of that blob to fix the time of arrival. But the messier the blob, the less accurate that fix, and the less accurate the final calculated position will be.
Skinny Signals: To be accurate, receivers need the signal that takes the shortest path from the satellite (green). Classic L1 satellite signals overlap with their reflections (blue and purple) to form signal “blobs,” which mask the shortest path. L5 signals don’t overlap with their reflections, so receivers can easily find the signal that arrives first.
However, L5 signals are so sharp that the reflections are unlikely to overlap with the most direct signal. The receiver chip can simply ignore any signal after the first one it receives, which is the shortest path. The Broadcom chip also uses information embedded in the phase of the carrier signal to improve accuracy.
Although there are advanced systems that use L5 on the market now, these are generally for industrial purposes, such as oil and gas exploration. Broadcom’s BCM47755 is the first mass-market chip that uses L1 and L5.
Why is this only happening now? “Up to now there haven’t been enough L5 satellites in orbit,” says Manuel del Castillo, associate director of GNSS product marketing at Broadcom. At this point, there are about 30 such satellites in orbit, counting a set that flies only over Japan and Australia. Even in a city’s “narrow window of sky you can see six or seven, which is pretty good,” del Castillo says. “So now is the right moment to launch.”
Broadcom had to get the improved accuracy to work within a smartphone’s limited power budget. Fundamentally, that came down to three things: moving to a more power-efficient 28-nanometer-chip manufacturing process, adopting a new radio architecture (which Broadcom would not disclose the details of), and designing a power-saving dual-core sensor hub. In total, they add up to a 50 percent power savings over the company’s previous, less accurate chip.
The BCM47755 is just the latest development in a global push for centimeter-level navigation accuracy. Bosch, Geo++, Mitsubishi Electric, and U-blox established a joint venture called Sapcorda Services in August to work toward that goal. Sapcorda seems to depend on using ground stations to measure errors in GPS and Galileo satellite signals due to atmospheric distortions. Those measurements would then be sent to receivers in handsets and other systems to improve accuracy.
Japan’s US $1.9 billion Quasi-Zenith Satellite System (QZSS) also relies on error correction, but it further improves on urban navigation by adding a set of satellites, guaranteeing that one of them is visible directly overhead, even in the densest part of Tokyo. The third of those four satellites launched in August. A fourth was planned for October, and the system is scheduled to come on line in 2018.
Competing GNSS chipmakers have not announced mass-market L1/L5 chips for smartphones, but some are working on similar products. U-blox says it is working on a dual radio chip but would not give details; Qualcomm says it will be delivering products “soon.” STMicroelectronics, as a member of a research consortium called the European Safety Critical Applications Positioning Engine, is working on a mass-market chip for the automotive sector. The chip will take advantage of the Galileo satellites’ new spoof-proof signals, which begin broadcasting in 2018.
A version of this article appears in our Tech Talk blog.