[time-nuts] Terrestrial GPS

Jeremy Nichols jn6wfo at gmail.com
Wed Mar 14 18:47:19 UTC 2018


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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