Do Smartphones Have a Super Special Move Called Sensor Fusion!? | Explained in the Easiest Way in the Universe | Basics of GPS #18
Well, for a while now, I've been talking about various ways smartphones figure out their location. Moreover, since the last couple of times, the focus has been on indoor navigation, which has nothing to do with GPS.
That finally ends this time. (Or so I plan...)
This time, I'm going to talk about how the "sensors" inside your smartphone are working incredibly hard.
Smartphones are packed with all sorts of sensors!
I don't think it needs explaining anymore, but smartphones use various sensors to ensure a comfortable user experience.
"When I turned my smartphone sideways, the screen rotated automatically!"
"When I moved from a dark room to a bright garden, the screen brightness adjusted automatically, making it easier to see!"
"I can unlock the screen with fingerprint authentication, so it's convenient not to have to enter a password every time!"
There are many more, but all of these are thanks to sensors.
Various sensors, “like human senses,” detect all kinds of movements and changes to realize various functions.
As for what kind of sensors there are...
Accelerometer: Knows "I just moved!" or "This side is down."
Gyro sensor: Knows "I turned left" or "I turned right."
Geomagnetic sensor: Knows "North is this way."
Barometric pressure sensor: Knows "Am I climbing up?" or "Am I going down?"
By making full use of these sensors, it secretly observes every single move of the owner.
A stalker?? (Scary!)
And the theme for this time is: can't we navigate using these sensors even if GPS isn't available?
What is sensor fusion?
By the way, it's impossible to determine an accurate location using just one of these sensors. For example, even if an accelerometer knows "Oh, I moved!", it doesn't know which direction you moved in.
That's where
“sensor fusion”
comes in. Huh? What? Is it some kind of superhero's special move!?
Yes, it feels just like a superhero!
It's a technology that combines multiple sensors to use them like one super-powerful sensor.
Accelerometer: "I just started moving, I think I'm walking."
Geomagnetic sensor: "The smartphone is facing east, so I think it's heading east."
Gyro sensor: "Oh, I just turned right, maybe about 45 degrees."
Well, something like that.
Even if their individual power is weak, they can do this much when they work together!!
I think it's fair to call them a sentai hero team (lol).
But there's a weak point...
So, I've been talking about how "you can know your own movement with the sensors inside your smartphone," but actually, this method has a weak point...
That is, it gradually drifts off.
You might be wondering why, so let me use an analogy here.
When you're blindfolded and spinning around for a watermelon splitting game...
Think back to that classic summer activity, the "watermelon splitting" game.
What? You say nobody does that anymore?
Well, let's just overlook that for now...
It's that game where you hold a stick, get blindfolded, and spin around three times before you start.
Doesn't the outcome of the watermelon splitting game almost always depend on whether you can stop exactly facing the watermelon after those initial spins?
If it were just one spin, you could probably stop facing the right way.
But as you do two or three spins, you start to lose track, and by the time you stop, you might be facing the complete opposite direction...
From that position, no matter how straight you walk, you'll just be heading in a straight line toward absolutely nothing!
This kind of "drift" happens with smartphone sensors too.
Measuring the distance traveled is extremely difficult
And it's not just the direction that drifts.
Actually, it is incredibly difficult to accurately measure the "amount" of movement—that is, how much distance you have covered!
A smartphone's accelerometer can tell that you've "moved!" but...
to measure the distance traveled, you need to know "how fast" and "for how long" you were moving.
Let's use another analogy here.
Imagine you are standing on a train.
When the train starts moving, you feel a G-force in the opposite direction of travel, right?
I'm sure you've all had the experience of being caught off guard and bumping into the person next to you, saying, "Oh, sorry..."
However, while the train is moving, that doesn't happen unless it brakes suddenly, does it?
Actually, an accelerometer is what measures the G-force in these situations.
To be more precise, it is a sensor that detects the magnitude of change when speed changes.
So, in the train example, you have to measure how fast the train is going based on the magnitude of the G-force when it starts moving.
(To put it technically, you would "integrate" it, but I'll skip that as it might give you a headache.)
What do you think? Doesn't it sound complicated, even if you're not sure why?
It is quite difficult to determine your position using only sensors
For that reason, relying solely on sensors for navigation because GPS doesn't work indoors is quite difficult...
It might work for a short time, but if you keep using it for a long time, you might end up being guided in a completely wrong direction.
A more realistic approach is to use it in combination with methods like the "WiFi signal-based method" I introduced last time.
Other potential smartphone sensors
Regarding sensors used for indoor navigation, various other sensors are being researched; some have disappeared, while others are expected to be put to practical use in the future.
Here, I will introduce just a few of them.
IMES, which installs GPS signal transmitters indoors
A unique Japanese technology devised by JAXA about 15 years ago that allows you to use your smartphone's GPS as is.
While there are infrastructure development challenges, it is said that the reason it has hardly been put into practical use might have something to do with adult circumstances...
Geomagnetic fingerprinting using geomagnetic patterns
Since geomagnetic patterns differ slightly depending on the location, a magnetic map is created.
The smartphone's geomagnetic sensor detects the magnetism and...
determines, "If it's this magnetic pattern, it must be next to that Starbucks!"
Acoustic positioning that measures distance using ultrasonic waves
Speakers that emit special ultrasonic waves inaudible to humans are installed everywhere.
The smartphone's microphone picks up signals from those ultrasonic waves to measure the position.
Li-Fi, which uses LED light to determine position
At a glance, they look like ordinary LED lights, but they flicker at a speed humans cannot perceive.
The smartphone's camera reads the signals from that flickering to measure the position.
What do you think?
Does it feel like it doesn't quite click for you??
But it might become commonplace in a few years!?
So, this concludes the smartphone location information series that I have been explaining over five parts. It's finally over. That felt long...
Actually, there are still other latest features on smartphones that I couldn't introduce this time (such as UWB or new Bluetooth features...). That will be something to look forward to another time.