
MIT Turns Wi-Fi into Indoor GPS - wallflower
http://spectrum.ieee.org/tech-talk/telecom/wireless/mit-turns-wifi-into-indoor-gps
======
fennecfoxen
> Although Chronos can run on existing Wi-Fi devices using just an app (or a
> firmware upgrade for an access point), each device has to undergo a one-time
> distance calibration.

I worked at at an indoor location-systems startup once. The real fun part of
the system was integrating wifi-based location with streaming video from
fixed-mount cameras to pinpoint location even better. But let me tell your
right off: wifi calibration of the sort they're talking about can be
incredibly obnoxious (think "carefully rolling a laptop in a cart at a
constant speed around a space, in a grid", which could easily be completed in
an hour or two if you are really good at it, but might take a day or two to
get right if you aren't.) Sure, this is all fine if you want to calibrate one
or two indoor spaces for your automated drones, especially if you've already
got APs deployed in a location-friendly mode (i.e. more around the perimeter
and less around the center, with antenna orientation to match), but don't
expect to see it in a corner store near you anytime soon.

Now, if you can talk about automating the calibration you'll do quite well.

~~~
anc84
Are Wifi signals even stable enough over time for this to make sense? I
thought their signal strength changes and adapts, sometimes even hopping
channels automatically.

~~~
Animats
They seem to be taking over control of the transmitter, not using ambient WiFi
data. So they're really a beacon system. There are lots of those; here's
MoteTrack, from Harvard.[1] There's a commercial one, Estimote.[2]

The WiFi industry is working on a standard, called the Fine Timing Measurement
protocol, which is now part of IEEE 802.11.[3][4] That provides 100ps
resolution timestamps in WiFi packets. That takes care of the RF layer, and
provides resolution to about 3cm. (Real world accuracy won't be that good.)

As with GPS, you need at least four fixed stations to get a fix, and
preferably line of sight to five, so you can compute the error. You might get
that in a big room such as a convention center or gym. If you need it in a
house, you'd probably need some additional nodes that are mostly for location,
not passing data traffic.

There's no technical problem doing this; it just hasn't been valuable enough
to be deployed widely yet. It will probably be a free feature in fixed WiFi
nodes soon.

[1]
[http://www.eecs.harvard.edu/~konrad/projects/motetrack/moteT...](http://www.eecs.harvard.edu/~konrad/projects/motetrack/moteTrack.pdf)
[2] [http://estimote.com/indoor/](http://estimote.com/indoor/) [3]
[http://www.wi-fi.org/beacon/rolf-de-vegt/never-lost-
indoors-...](http://www.wi-fi.org/beacon/rolf-de-vegt/never-lost-indoors-the-
promise-of-wi-fi-location) [4]
[http://www.ieee802.org/1/files/public/docs2014/asbt-
kbstanto...](http://www.ieee802.org/1/files/public/docs2014/asbt-kbstanton-
use-of-timing-measurement-0714-v01.pdf)

~~~
Unklejoe
100ps resolution timestamps? That's crazy impressive. I wonder how they
accomplished that. It seems unlikely that there is a 10GHz clock that they're
using to timestamp asynchronous events, but who knows.

My reference is the fact that 1588 systems usually get into the <25ns RMS
offset realm.

This might not even make sense, but maybe they're talking about using the
recovered clock from the incoming signal? Something like synchronous Ethernet?

The link posted doesn't go in to much detail, but I'd like to learn more.

~~~
Natanael_L
They transmit on multiple frequencies. The signals on every frequency will
have its own phase offset between the sender and receiver. Think of it as
constructive / destructive interference patterns which varies across space,
except they have a method where they measure across time multiple times.

At certain multiples of a distance, with synchronized clocks, you'll have
offset X between channel Y and Z (on channel Y you may be perfectly phase
synchronized, but Z is half the phase off).

Measuring the offsets for multiple sets of frequencies allows you to calculate
multiple sets of possible distances, to then figure out which exact distance
it is that repeats among all of the calculated distance multiples (like 1, 2,
3; 0.5, 2, 4; 1.5, 2, 3 - the number 2 occurs everywhere).

If for example you're perfectly in phase on 10 frequencies, the distance
between you must be a must be a specific shared multiple of the wavelengths of
all the frequencies. Least common denominator.

------
jdiez17
Very cool. They are using Time Difference of Arrival analysis[1], the fact
that most devices have two or more antennas, and clever software to get 10-80
cm accuracy at a good enough frequency to be used as a positional reference in
a quadcopter, see the paper[2] for more info

Existing indoor positioning systems such as Bluetooth LE beacons and WiFi
triangulation[3] only use the RSSI value and trigonometry to figure out the
position. This method is slow (5Hz or so) and the resolution of the RSSI is
bad unless you sample it directly, eg with SDR (which incidentally, is what my
dissertation is about!)

[1]
[https://en.wikipedia.org/wiki/Multilateration#Principle](https://en.wikipedia.org/wiki/Multilateration#Principle)

[2]
[https://www.usenix.org/system/files/conference/nsdi16/nsdi16...](https://www.usenix.org/system/files/conference/nsdi16/nsdi16-paper-
vasisht.pdf)

[3]
[https://www.qualcomm.com/products/izat](https://www.qualcomm.com/products/izat)
(unfortunately there are no public 'specs' I can find...)

~~~
fennecfoxen
Point of note: Multilateration (link [1] above) is the real name for the
process that ordinary people casually call "triangulation" when they're
talking about wifi location. It's not actually triangulation (which determines
location based on the angle) but it's based on circles (or spheres, I guess -
based on the distance).

------
qrv3w
For those interested, I'm working on an open-source project for doing this at
home: [https://github.com/schollz/find](https://github.com/schollz/find)

My solution basically uses the RSSI values for generating classifiers for each
room. It works well enough to replace motion sensors.

~~~
azinman2
Is this looking at the rssi of all networks seen, or playing with the device
you're on? Does this work for iPhones?

~~~
qrv3w
Yes, it looks at the RSSI of all networks seen, generates distributions of the
RSSI signals, and then leverages those distributions using some naive machine
learning to classify the location.

It does not work for iPhones. Unfortunately, Apple keeps the WiFi scanning in
a "private" api which would require jailbreaking because, as far as I know,
use of those apis won't get past the app-store review.

So, for now it only works on Android and on laptops/single-board computers
using a Python script (links to app and script in Github).

~~~
Snowdax
Any chance of getting it to work natively on android as a standalone library
without cordova? Being able to integrate a library over Gradle into an
existing app would be really really nice.

~~~
qrv3w
That's a good idea! I'll look into it.

------
dmritard96
Its amazing how everything MIT touches, they make sure its in the HN title.
Good branding I guess.

Anyone with inside knowledge know if this is something that will actually make
it to market. They are talking about commercializing supposedly, according to
the article, but I would be surprised if this isn't something that just gets
incorporated into ios and android if it actually is as good as they claim. Its
not like (i)beacons managed to truly become an apple owned thing.

~~~
shas3
Not taking anything away from this very novel work, but MIT definitely has a
well oiled PR apparatus to publicize their research. This is a good thing from
the perspective of science advocacy and outreach. But it is less good from the
perspective of other researchers working in similar areas whose work can get
eclipsed this way.

~~~
KKKKkkkk1
This is definitely a deceptive practice. Claiming this was developed by MIT is
kind of like claiming that the theory of relativity was developed by the Swiss
patent office. MIT is just the place where the researchers were working. To
add insult to injury, the names of the researchers are withheld until the 6th
(!) paragraph of the article.

~~~
ssalazar
Thats not an accurate analogy at all. These researchers presumably utilized
MIT resources extensively while being paid by MIT specifically to conduct
research in addition to other duties. Without MIT or a similar host
institution their ability to conduct research such as this would have been
severely constrained.

In fairness a better title would be "MIT researchers turn Wi-Fi...", and it
would definitely have been good to mention the researchers' names earlier.

------
aresant
If they can prove it works at scale this tech instantly eliminates the
usefulness of beacons.

Wifi has always been the obvious solution to indoor location (lower power than
GPS / works indoors / piggybacks on installed hardware / somewhat "open") but
fidelity was too low.

Hence the rise of beacons.

Beacons IMO are the least elegant solution to the problem of indoor location
because they require (a) a whole separate location framework (b) requires
vendors / stores to install & maintain hardware to drive coverage (c) are
owned by the OS.

I've been watching this game unfold for years - Apple bought WifiSLAM in 2013
to improve indoor location / wifi (1), and then bought "super accurate GPS"
firm coherent NAV last year. (2)

There are also several startups attempting to use sensor data + geomagnetic or
inertial navigation to solve this problem. (3)

If these guys actually got Wifi working, and figure out a way around the
callibration problem, they in one fell swoop solved one of the major barriers
to the hyper-localized advertising / consumer tracking that we've been
promised (threatened with?) for years.

(1) [http://www.theguardian.com/technology/2013/mar/25/apple-
buys...](http://www.theguardian.com/technology/2013/mar/25/apple-buys-indoor-
mapping-wifislam)

(2) [http://blogs.wsj.com/digits/2015/05/17/apple-buys-gps-
firm-c...](http://blogs.wsj.com/digits/2015/05/17/apple-buys-gps-firm-
coherent-navigation/)

(3) [http://www.navisens.com/](http://www.navisens.com/), GiPStech.com,
[http://www.PathSense.com](http://www.PathSense.com),
[https://www.indooratlas.com/](https://www.indooratlas.com/)

~~~
TeMPOraL
> _Hence the rise of beacons._

Good riddance. So far beacons are widely used for just one thing - spamming
peoples' phones with ads in stores. We've been promised indoor location
accurate to 5cm; I haven't seen anyone deliver it yet.

~~~
Natanael_L
Beacons with full AR will be marvelous (for as long as the end user is in
control of the experience).

------
chiph
We had this problem at a previous job - how to locate a piece of medical
equipment so that a nurse can quickly get it when needed (they get moved
around and their locations typically aren't communicated at shift change).
Ordinary WiFi wasn't good enough - it had roughly 1.2 meter accuracy, which
given that radio waves can pass thru a wall, couldn't tell you if the device
was in room 201 or the adjacent 203. Which is a problem when seconds counted.

The fact that they can do this with just a firmware update is excellent news.
Hopefully they'll license it to some of the big commercial access point
vendors (Cisco, etc).

------
deckar01
It seems that the advance here is not that WiFi can be used as an alternative
to GPS, but that sweeping multiple frequencies allows for high precision
timing measurements (hence the name "Chronos"). I suspect the same could be
done with cellular frequencies on a much larger scale with similar
improvements in accuracy. I am curious how the precision scales with more
channels and lower frequencies.

Edit: Apparently exploiting the phase change of a signal has been used to
improve the accuracy of GPS in a similar ways [0]. The dual radio angle
calculation seems to be a more novel advancement.

[0]
[https://en.m.wikipedia.org/wiki/Real_Time_Kinematic](https://en.m.wikipedia.org/wiki/Real_Time_Kinematic)

------
gipsies
Accurate Wi-Fi based indoor location positioning already exists, though with
different pros and cons. For example, the following system still works
reliably when walking around, but may be a bit less accurate. And it requires
no calibration
[https://www.youtube.com/watch?v=foVqKLE4Mpk](https://www.youtube.com/watch?v=foVqKLE4Mpk)

That demo is based on the new 802.11mc standard. Surprisingly, this new
standard isn't mentioned in the paper. So no comparison between this and their
system...

------
ch
This is the same tech Prof. Katabi demoed to President Obama. She demonstrated
using the tech to detect pulse rate if I recall correctly.

Here's a link: [http://news.mit.edu/2015/president-obama-meets-mit-
entrepren...](http://news.mit.edu/2015/president-obama-meets-mit-
entrepreneurs-white-house-demo-day-0806)

------
eridius
Very cool. But does this work if the device is actually trying to use its wifi
connection for data? I'd assume that rapidly hopping between all the channels
would interfere with the normal duties of the wifi card.

~~~
pmontra
I think that if a device hops in the 2.4 GHz band it loses the connection to
the AP and it goes offline. So this location system will be useful to locate
carts in a shop but not customer's phones connected to that shop free WiFi.
And even if the customer is not connected to the AP and the phone still has
the WiFi on (extremely common), I think you need an app to do that.
Furthermore I expect this to impact on battery quite a bit.

------
hartror
I was in a giant mall yesterday where I knew what I wanted, from which outlets
but with little idea where those outlets were. Indoor turn by turn navigation
would have saved me 30+ mins.

~~~
aaron695
Which the mall would not like, so they won't be helping you out here.

~~~
raihansaputra
depends on the mall. i just found out one mall in my city have an automated
direction on the directory machine itself. not true turn-by-turn, but helps a
lot rather then guessing the places of escalator on the maps and whatnot.

