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) 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 - 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. 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 > _________________________________________________ > time-nuts mailing list -- email@example.com > To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts> and follow the > instructions there. 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 _______________________________________________ time-nuts mailing list -- firstname.lastname@example.org To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.