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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