[time-nuts] Terrestrial GPS

Wed Mar 14 15:24:08 EDT 2018

Hi

There *are* multiple different things that get tagged as “WiFi location” services.
One is a passive approach. They look at what your device can “see” and guess
your location. Another is an active approach. You are logged on to a WiFi system
and they try to back track your location though a trace route. I believe the initial
comment was related to the passive system.

Why does this stuff matter? 

Well, as you stroll around the mall with big piles of cash bulging in your pockets, 
the store you just passed wants to send you a coupon to drag you into the store. 
They feel the need to do this *after* (but not long after) you stopped and looked 
in the window. If you are on the floor above or the floor below the store … no
coupon. 

Doing this with timing based navigation (like GPS) is a bit tricky. You need to get
things sync’d good enough for < 3 M accuracy in a space with a lot of multi-path. 
Setting up a bunch of basestations that are sync’d to a few ns might seem simple, 
but it’s not. Without that sort of timing accuracy, they need to guess where you 
are by other means. 

All of this is independent of the issues regarding “do I want this to happen”. Diving
into that part heads even further off subject for this list …..

Bob

> On Mar 14, 2018, at 2:47 PM, Jeremy Nichols <jn6wfo at gmail.com> wrote:
> 
> Apparently the database in my area is poor. Another location I am sometimes
> found in is *Fremont*, almost 100 miles southeast. Fremont most frequently
> shows up as the "closest store" when I am looking at (for example) hardware
> store web sites.
> 
> Jeremy
> N6WFO
> 
> 
> On Wed, Mar 14, 2018 at 10:15 AM Bob kb8tq <kb8tq at n1k.org> wrote:
> 
>> Hi
>> 
>> WiFi based location can be pretty good or it can be a real joke. It all
>> depends on
>> how good the database happens to be and how many stationary WiFi setups you
>> can “see”. I’ve had it pop up with locations that are off by many miles.
>> ISP’s
>> feed you IP addresses via DHCP and the “on net” numbers are rarely visible
>> to
>> the casual passerby. , They likely are depending on other information
>> (like SSID
>> surveys from vehicles).
>> 
>> Bob
>> 
>>> On Mar 13, 2018, at 11:15 PM, Bill Byrom <time at radio.sent.com> wrote:
>>> 
>>> Have you noticed that your mobile devices (smartphone, iPad, tablet PC,
>>> laptop PC) can often know your location when you inside a  building
>>> shielding you from GPS satellites (or producing multipath confusing the
>>> GPS receiver)? Here is a quick test you can do if you have a PC with no
>>> GPS receiver but with WiFi capability:
>>> Start up a browser and go to http://maps.google.com (which redirects to
>>> https://www.google.com/maps/...) with a WiFi connection. Near the lower
>>> right of the screen you should see the + - zoom buttons, and above these
>>> a target icon. Click that target icon. If asked, enable location
>>> finding. You may also need to enable your browser to release location
>>> information.  In my case, I am now sitting near the middle of my house
>>> and the laptop Windows 10 PC Google Maps locator places my location on
>>> the street adjacent to my house, about 25 meters or so from my actual
>>> location. My iPhone iOS map shows my location more closely (inside my
>>> house) and it very accurately shows the location of the minivan I parked
>>> in the driveway several hours ago (as "parked car").  My iPad also shows
>>> my location within my house.
>>> How do these devices know your location without GPS? Several methods are
>>> used to produce a hybrid positioning system[1]:(1) Your IP address from
>> your ISP. This gets me within a few km of my
>>>   location. See: https://www.iplocation.net/(2) WiFi positioning
>> system[2] - This makes use of databases which
>>>   contain the geographic location of WiFi access points. The data is
>>>   collected by methods such as comparing the GPS receiver location
>>>   reports of mobile devices with the signal strength of access
>> points.(3) Cellular radio location - Various techniques allow accurate
>>>   mobile phone tracking[3]. The signal strength and propagation
>>>   delay from cellular base stations allow moderately good
>>>   determination of location.
>>> If you are in an area without GPS receiver coverage, your mobile device
>>> or PC can determine the time using various techniques:(1) Crystal
>> oscillator for short-term time stability.
>>> (2) NTP: https://en.wikipedia.org/wiki/Network_Time_Protocol
>>> (3) Cellular timing - cellular phone networks require very accurate
>>>   timing of the RF signals.
>>> It would be hard to place terrestrial transmitters on the GPS satellite
>>> frequencies without dynamic range and other problems, and of course
>>> someone could use this technique to jam GPS reception in an area. But
>>> several terrestrial geolocation and timing dissemination systems have
>>> been proposed, and some limited deployment has been achieved.
>> https://en.wikipedia.org/wiki/NextNav
>>> http://www.nextnav.com/technology
>>> http://esatjournals.net/ijret/2013v02/i04/IJRET20130204031.pdf
>>> 
>> https://spectrum.ieee.org/tech-talk/telecom/wireless/us-master-clock-keepers-test-ground-alternative-to-gps--
>>> Bill Byrom N5BB
>>> 
>>> 
>>> 
>>> On Tue, Mar 13, 2018, at 5:17 PM, Stewart Cobb wrote:
>>>> Peter Reilley suggests a backup to GPS using terrestrial
>>>> transmitters. This> idea has been around since the early days of GPS.
>> The terrestrial
>>>> transmitters were called "pseudo-satellites", or "pseudolites"
>>>> for short.> The big problem with this idea is that the GPS signal
>> format has
>>>> a narrow> dynamic range. The signal strength from a terrestrial
>>>> transmitter varies> widely (inverse square law) from positions near the
>> transmitter to
>>>> positions far away. The variation in any practical system is
>>>> larger than> the GPS signal format can handle. This is called the
>> "near-far
>>>> problem".> For an extensive discussion of the pseudolite concept,
>> including the
>>>> near-far problem, see my dissertation. You can find it with a
>>>> web search> for my full name and the word "pseudolites".
>>>> 
>>>> Cheers!
>>>> --Stu
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>>> 
>>> 
>>> Links:
>>> 
>>> 1. https://en.wikipedia.org/wiki/Hybrid_positioning_system
>>> 2. https://en.wikipedia.org/wiki/Wi-Fi_positioning_system
>>> 3. https://en.wikipedia.org/wiki/Mobile_phone_tracking
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> -- 
> Sent from my iPad 4.
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