If you want to feel like Tom Cruise in Minority Report, Google squeezed a feature into the Pixel 4 that’s just for you. The Pixel 4’s Motion Sense, and the Soli sensor that powers it, is probably the most interesting new thing inside the Pixel 4—and it’s powered by decades-old radar tech.
Google Pixel 4 XL Teardown
We tear down Google's latest and greatest smartphone, the Pixel 4 XL. Inside we find the new Project Soli radar chip, along with some surprises!
Motion Sense detects when you’re reaching for your phone and turns the screen on for you. It can also dismiss calls, change songs, and snooze your morning alarm with a well-placed wave of the hand. It seems futuristic, but hand-gesture sensing isn’t new—it’s just never caught on in the mainstream, and we rarely see it in a mass-market product. The Xbox Kinect is a rare exception, but even then, it remained mostly a gimmick, and Microsoft discontinued it in 2017.
A more direct comparison to Soli might be Leap Motion, a promising startup that introduced hand gesture controls to consumer PCs. Unfortunately, they fell on hard times and eventually were acquired by rival company UltraHaptics—we have yet to see anything big from them in the consumer market.
But Google is using a new approach. In the past, manufacturers have used infrared cameras to detect motion. This time around, Google (with help from German semiconductor manufacturer Infineon) is using radar, which uses pulsing radio waves to sense the movement of your face and hands. It’s much more accurate and saves battery life.
Many Pixel owners like it; some reviewers consider it an utter waste. The sensor can be finicky until you figure out how to manipulate it, but the reliability and responsiveness is a highlight. And, as a bonus, Soli’s sensing tech makes Face Unlock that much quicker.
This tech comes from the Pixel 4’s new Soli chip, which Google calls a “motion-sensing radar.” It’s been around for a few years, at least publicly—Google showed off the tech for the first time back in 2015, initially as an improvement for smartwatches. You’d think the technology behind Soli was revolutionary, but the concept is simple, relying on technology invented by the British in 1935 to detect German warplanes.
How Radar Works and What It’s Used It For
Radar technology isn’t just used by air traffic controllers and the military—it’s more central to your everyday life than you might realize.
“Radar is any radio frequency that’s transmitted out, hits something, and is reflected back,” explains Will Sanitate, radio frequency expert and Vice President of O & S Services. “From that reflection, there’s a time calculation that figures out something is there.”
Radar experiments began in the late 1800s when German physicist Heinrich Hertz (yes, that Hertz) proved the existence of radio waves, based on predictions by Scottish physicist James Clerk Maxwell. After years of experiments, Hertz eventually discovered that radio waves could reflect off of solid objects—eventually leading militaries to develop Radio Detection and Ranging (or RADAR) for detecting ships and planes. Hertz’s name would eventually become the official unit of frequency (megahertz, gigahertz, etc.).
Radar is everywhere you look, even if you can’t see it: weather forecasts, police speed traps, and airport body scanners, to name a few. It’s also the same tech that powers the newest features in cars, like blind-spot alerts, adaptive cruise control, and even autonomous driving.
Now, we’re seeing radar in smartphones. The iPhone 11 Pro’s U1 ultra wideband chip uses radar to accurately pinpoint locations within a few centimeters. The Pixel 4 has a radar chip for hand gestures and motion detection, but it’s a bit different.
A Closer Look at Google’s Soli Radar Chip
The big difference between Soli and traditional radar is the power and sampling rate.
Back in the early days of military radar, the transmitters could wield up to 100 megawatts of power. “If you stand in front of a transmitter that’s doing 100 megawatts, it will literally cut you in half,” Sanitate says. In order to prolong battery life (and not cut anyone in half), the Soli chip turns the power down quite a bit. We don’t know its exact wattage, but the Wi-Fi chip on your phone emits around 50-100 milliwatts—about one billion times less power!
Google’s Soli chip is also doing the same thing as traditional radar, just a lot more of it. “The only difference is that, in classic radar, they would send out one long powerful beam and were happy to get a single reflection back from a big, fast-moving airplane,” Sanitate explains. Google is using a newer, improved version of that, called phased array radar. “It takes a whole bunch of reflections—lots of reflections. If you get enough reflections, you get an outline of what exactly is going on, rather than just seeing one blip, and thinking ‘Oh yup, there’s something.’”
The Soli chip doesn’t have particularly fine spatial resolution—it’s more like taking a picture with a flip phone from the 90s. It pulls out the nuances, like how fast things change, and how intense those changes are. Soli then pushes that data through a ton of equations and interprets the results into hand movements.
Our Pixel 4 teardown found the Soli sensor that makes the black magic happen. It’s located at the top of the phone, hidden under the earpiece speaker, ambient light sensor, and microphone. It’s stunningly tiny—just 5mm x 6.5mm and 0.6mm thick, or roughly third of the size of a micro SD card.
The Soli radar operates at 60 GHz, which until recently has primarily been used for military purposes. That’s a much higher frequency than Apple’s U1 ultra wideband chip, which sits in the 3.1-10.6 GHz range. The higher frequency can transmit and receive a ton of data, which helps detect and parse quick hand movements and their subtle differences. But since the signal is only emitting a few inches out from the phone, it uses way less power than other radar systems operating in the 60 GHz frequency spectrum.
All of this only explains one type of radar, inside one phone, so we’re just scraping the surface of radar’s functionality and capabilities, and it can get pretty complicated in the higher realms. “The [radio frequency] science gets really, really wacky, and no one really understands it,” Sanitate jokes. “The higher you go, you realize, no one has a freaking clue what’s going on.”
A Minority Report future is far off, but Google’s new riff on old tech might be the jumpstart the industry needs to make hand gesture sensing mainstream.