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NHS contact tracing app

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Kawasaki Jimbo
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PostPosted: 16:30 - 24 May 2020    Post subject: Reply with quote
The "downwind" aspect occurred to me while at the back of a queue outside Tesco's with a strong breeze in my face. Goodness knows how many individual's exhalations I was taking in, despite 2 metre distancing.
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Riejufixing
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Joined: 24 Jun 2018
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PostPosted: 16:38 - 24 May 2020    Post subject: Reply with quote
Kawasaki Jimbo wrote:
The "downwind" aspect occurred to me while at the back of a queue outside Tesco's with a strong breeze in my face. Goodness knows how many individual's exhalations I was taking in, despite 2 metre distancing.

I was thinking about that queueing for the PO last week. About 8 people in a row about 30 yards long, with the wind blowing along the queue. Compare aircon in offices, even desk/office fans etc.
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Polarbear
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PostPosted: 17:07 - 24 May 2020    Post subject: Reply with quote
Did anyone ever come out with a definitive answer whether those cruise ships air conditioning was instrumental in spreading the virus or if it was only when people were actually in contact with someone else?

It's one hell of a difference considering many offices have central ac and heating systems like the cruise boats have and that will make a mockery of any social distancing implemented if it is the case of possible transmission by air circulation.
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Im-a-Ridah
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PostPosted: 23:53 - 24 May 2020    Post subject: Reply with quote
Kawasaki Jimbo wrote:
I'm undecided as to whether I'd use the app. The NHS doesn't have a great track record with new IT systems (who does?) but I'm aware that Bluetooth supposedly has an outdoor range of at least 100 metres, so will the app be able to determine proximity, maybe by signal strength? Otherwise it'll get a lot of "hits."

I also think the app's primary purpose is to assuage the media by showing that the government is doing something about the situation and following examples (possibly questionable ones) set by other countries.


Signal strength isn't a good indicator of distance. You can just time how long it takes for the packet to travel the distance.
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Riejufixing
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PostPosted: 23:58 - 24 May 2020    Post subject: Reply with quote
Im-a-Ridah wrote:
Signal strength isn't a good indicator of distance. You can just time how long it takes for the packet to travel the distance.

Coo! How, using a cuckoo clock, or what?
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Im-a-Ridah
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PostPosted: 02:02 - 25 May 2020    Post subject: Reply with quote
Riejufixing wrote:
Im-a-Ridah wrote:
Signal strength isn't a good indicator of distance. You can just time how long it takes for the packet to travel the distance.

Coo! How, using a cuckoo clock, or what?


Just time stamps and/or hardware timers. Same way GPS works at least in principle. So yeah send out your carrier pigeon to your destination e.g MPD, guy on the other side hits it with a baseball bat and says go home you immigrant, then the pigeon arrives back with you and the cuckoo clock time divided by two is the one way travel time. Obviously immigrants never go home, unlike carrier pigeons Wink

Also new version of bluetooth can give angle of arrival for the signal though its not relevant to the app as not everyone has the latest version of bluetooth Razz
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Riejufixing
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PostPosted: 10:46 - 25 May 2020    Post subject: Reply with quote
Im-a-Ridah wrote:
Riejufixing wrote:
(You can just time how long it takes for the packet to travel the distance)
Coo! How, using a cuckoo clock, or what?


Just time stamps and/or hardware timers. Same way GPS works at least in principle. So yeah send out your carrier pigeon to your destination e.g MPD, guy on the other side hits it with a baseball bat and says go home you immigrant, then the pigeon arrives back with you and the cuckoo clock time divided by two is the one way travel time. Obviously immigrants never go home, unlike carrier pigeons Wink

Also new version of bluetooth can give angle of arrival for the signal though its not relevant to the app as not everyone has the latest version of bluetooth Razz

There are things to consider here.

First, what you are trying to time? The time it takes for a signal to travel about 2 metres. That, for radio, which is what Bluetooth is, will be around 6 ns. for 2m, and you want to see if it's less than that (down to 0 seconds, instantaneous contact).

Doubling the distance, you're obviously looking at somewhere between 0 and 12 ns, easy peasy, BUT.

What about all the other gubbins? Taking it very simply:

The sender app has to communicate with whatever O/S is on the sending device to schedule and dispatch instructions to make the packet, tell the the transmitter to send it, it has to be sent using a set time slice (only then does transmission time at the speed of light come in), and it has to be received by the receiving device with all the processing entailed there, then in the "ping pong" case the receiving device's O/S has to tell the receiving app about the packet, which has to tell the receiver O/S to chedule and dispatch instructions to construct another packet and, the transmitter has to be told to send it, it has to be sent again using a set time slice, it has to be received by the oricinal sender with all the processing entailed there, and only then can the sent and returned time be calculated and the distance found.

While nanosecond precision "in the machine" is possible, nanosecond accuracy is not. The process of sending can only happen (I think, but don't quote me) on Android (BLE) every 7 1/2 ms, which is a massive figure compared to the ~6 ns we're trying to accurately measure.

"It's not going to happen" is the result. It would be nice if it could, though!
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Kawasaki Jimbo
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PostPosted: 11:02 - 25 May 2020    Post subject: Reply with quote
So if it can't determine whether you were within 2 metres or at maximum range (100 Thumbs Up Wink ) from the diseased individual, that effectively means social distancing for smart-phone-app users is 5-10 metres.
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Riejufixing
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Joined: 24 Jun 2018
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PostPosted: 11:45 - 25 May 2020    Post subject: Reply with quote
Kawasaki Jimbo wrote:
So if it can't determine whether you were within 2 metres or at maximum range (100 Thumbs Up Wink ) from the diseased individual, that effectively means social distancing for smart-phone-app users is 5-10 metres.

I'm told it will use signal strength to determine range, but how exactly and how accurate that will be is a bit hazy in my mind. Social distancing is the same for app users as for anyone else, the app's an add-on. It's hard to find a decent write-up of how it will look to the user, as articles are very often littered with other stuff. I can see a lot of facebook-type people reporting "Oh I've been near Xpeople who've got [symptoms|the virus]!!" Smile
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Im-a-Ridah
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PostPosted: 12:09 - 25 May 2020    Post subject: Reply with quote
Riejufixing wrote:


First, what you are trying to time? The time it takes for a signal to travel about 2 metres. That, for radio, which is what Bluetooth is, will be around 6 ns. for 2m, and you want to see if it's less than that (down to 0 seconds, instantaneous contact).

Doubling the distance, you're obviously looking at somewhere between 0 and 12 ns, easy peasy, BUT.

What about all the other gubbins? Taking it very simply:

The sender app has to communicate with whatever O/S is on the sending device to schedule and dispatch instructions to make the packet, tell the the transmitter to send it, it has to be sent using a set time slice (only then does transmission time at the speed of light come in), and it has to be received by the receiving device with all the processing entailed there, then in the "ping pong" case the receiving device's O/S has to tell the receiving app about the packet, which has to tell the receiver O/S to chedule and dispatch instructions to construct another packet and, the transmitter has to be told to send it, it has to be sent again using a set time slice, it has to be received by the oricinal sender with all the processing entailed there, and only then can the sent and returned time be calculated and the distance found.

While nanosecond precision "in the machine" is possible, nanosecond accuracy is not. The process of sending can only happen (I think, but don't quote me) on Android (BLE) every 7 1/2 ms, which is a massive figure compared to the ~6 ns we're trying to accurately measure.

"It's not going to happen" is the result. It would be nice if it could, though!


That is sufficient for contract tracing if they just want a vague idea which is all they seem to be going for. You aren't literally trying to measure the distance with a high level of precision, though that is possible. The description you present of the protocol is the most inefficient possible method and involves totally unnecessary steps. For a start this technique does not a require an OS, it can be implemented in hardware or firmware and does not need to go up the network layer except to give a result. Timing can be done with standard 32 bit timers with a period of 0.2ns. You seem to be under the impression that this is an idea I have come up with, it's literally an established method of measurement that is already used so you can't "debunk" it.
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Riejufixing
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PostPosted: 12:18 - 25 May 2020    Post subject: Reply with quote
Im-a-Ridah wrote:
You seem to be under the impression that this is an idea I have come up with, it's literally an established method of measurement that is already used so you can't "debunk" it.

I'm not trying to debunk anything, I'm saying it won't work.

So educate me, please. An in-a-nutshell tefchnical walkthrough of how the process could work on a general purpose mobile device, 'cos that is what we're talking about, (including specific reference to transmission slot timings). I'd really like to know.

Edit: In addition, established real-life examples would be good to see.
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Lord Percy
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PostPosted: 20:47 - 25 May 2020    Post subject: Reply with quote
The app could use a standardised clock time that is equal across all apps in the country. The time in the app could be updated every minute/hour/whatever to make sure it still really is the official GMT time. So that's the start point: Every app has an internal clock running at exactly the same time.

Then use bluetooth to broadcast that time.

Nearby devices can detect those broadcasts and compare them to the time they are currently at. There'll be a difference of a few nanoseconds, due to the distanced the signal travelled.

Ta-daaa, distance between users can be figured out.

Only works if nothing gets in the way though. But I'm pretty sure that issue exists for any solution.

Extra accuracy could be easily achieved if there are multiple devices all interacting at once, as that would allow for GPS style triangulation.
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stinkwheel
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Joined: 12 Jul 2004
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PostPosted: 21:06 - 25 May 2020    Post subject: Reply with quote
My mate reckons they should make everyone vape constantly, then you will be able to see their droplet cloud.
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Im-a-Ridah
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PostPosted: 22:37 - 25 May 2020    Post subject: Reply with quote
Lord Percy wrote:
The app could use a standardised clock time that is equal across all apps in the country. The time in the app could be updated every minute/hour/whatever to make sure it still really is the official GMT time. So that's the start point: Every app has an internal clock running at exactly the same time.

Then use bluetooth to broadcast that time.

Nearby devices can detect those broadcasts and compare them to the time they are currently at. There'll be a difference of a few nanoseconds, due to the distanced the signal travelled.

Ta-daaa, distance between users can be figured out.

Only works if nothing gets in the way though. But I'm pretty sure that issue exists for any solution.

Extra accuracy could be easily achieved if there are multiple devices all interacting at once, as that would allow for GPS style triangulation.


It depends what you are trying to achieve. If its just this app then you can just mark all detected devices as "contact" if they are detected for a sufficient period of time. If you are asking about measuring the distance using RF then you can just use a 32 bit or 64 bit timer with a low latency processor. I think the idea was to limit battery life and protect privacy by not using GPS. I have no intention of using the app.
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Lord Percy
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PostPosted: 23:21 - 25 May 2020    Post subject: Reply with quote
Im-a-Ridah wrote:

I have no intention of using the app.


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Mart_er6
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PostPosted: 08:47 - 28 May 2020    Post subject: Reply with quote
"Timing can be done with standard 32 bit timers with a period of 0.2ns"

Don't think you'll find any *standard* 32 bit timers that can clock at 5GHz (0.2ns period).

What can determine distances to ~2m would be a form of radar which is not something bluetooth or smartphones were ever envisaged to be used for (so they can't accurately).

RF (Bluetooth) signal strength is grossly affected by the environment (and performance of the antenna in each smartphone, how it is held and so on) - using this for range is a massive approximation. Is the signal low because a body is in the way or a wall?

The people that really need an App like this to work are the elederly - they are the ones least likely to have a smartphone and/or least likely to be able to install/operate the App.

Then you have the privacy of data concerns..

It all adds up to money spent on an App that perhaps could have been spent on masks for the elderly to wear whilst out and about.
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Old Thread Alert!

The last post was made 3 years, 326 days ago. Instead of replying here, would creating a new thread be more useful?
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