Comment by fennecfoxen | Hacker News Reader

Comment by fennecfoxen | Hacker News Reader

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> 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.

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.

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... [2] http://estimote.com/indoor/ [3] http://www.wi-fi.org/beacon/rolf-de-vegt/never-lost-indoors-... [4] http://www.ieee802.org/1/files/public/docs2014/asbt-kbstanto...

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.

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.

APs in the 5 Ghz band can hop channels automatically (IEEE N and AC) but APs in the 2.4 Ghz band (IEEE abgn) are locked to the channel they start on. Modern routers automatically choose the "clearest" channel when they start up, but they can't change the channel without user intervention. Once the channel changes all the clients would need to rediscover the network and redo the handshake, i.e they would disconnect and maybe reconnect automatically but maybe not.

Also, there could be multiple paths (signals bouncing off walls in multiple ways).

Wonder if you could use a Roomba-like robot to do this?

I experimented with position-tracking on a Roomba 780 a couple of weeks ago. I found that its sensors were far too imprecise to get any sort of reliable position. I think the Roomba 980 might work, but it costs > 900 USD.

tl;dr: maybe, but only the very expensive models

I guess you could easily strap a smartphone or laptop to the Roomba to do the actual position tracking.

How much "drift" do you guys have to deal with when using consumer electronics? I don't imagine the calibration being too difficult or numerous these days. Wi-Fi these days is cheap and of high quality.

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