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From: Paul H. Christiansen11/2/2017 10:23:00 AM
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Superaccurate GPS Coming to Smartphones in 2018

We’ve all been there. You’re driving down 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.

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.

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