question: How does a signal from a cellphone know where to go?
comment: The signal from your cell phone initially goes everywhere. That is, it radiates out in a sphere. It should pretty quickly find a cell tower. From there it's up to your cell phone provider to look up your buddy's phone number and hand off the message to your buddy's cell phone provider. Then your buddy's cell phone provider sends that message to the closest cell phone tower to your buddy. The tower broadcasts the message in all directions and your buddy's phone picks it up out of the air.
passages: Mobile phones are commonly used to collect location data. While the phone is turned on, the geographical location of a mobile phone can be determined easily (whether it is being used or not) using a technique known as multilateration to calculate the differences in time for a signal to travel from the mobile phone to each of several cell towers near the owner of the phone.
  
  The movements of a mobile phone user can be tracked by their service provider and if desired, by law enforcement agencies and their governments. Both the SIM card and the handset can be tracked.
   BULLET::::- the radio link used by the phones is a digital link with extensive error correction and detection in the digital protocol
  
  BULLET::::- sufficient operation of mobile phone in tunnels when supported by split cable antennas
  
  BULLET::::- local repeaters inside complex vehicles or buildings
   BULLET::::- Tomographic motion detector: These systems sense disturbances to radio waves as they pass from node to node of a mesh network. They have the ability to detect over large areas completely because they can sense through walls and other obstructions.
   Section::::Calculating the time difference in a TDOA system.
  
  The basic measurements are the TOAs of multiple signals (formula_7) at the vehicle (navigation) or at the stations (surveillance). The distance formula_8 in equation is the wave speed (formula_9) times transit time, which is unknown, as the time of transmission is not known. A TDOA multilateration system calculates the time differences (formula_10) of a wavefront touching each receiver. The TDOA equation for receivers "m" and 0 is
   As noted above, the phone is free to roam anywhere in the operator's network or on the networks of roaming partners, including in other countries. So the first job of the Gateway MSC is to determine the current location of the mobile phone in order to connect the call. It does this by consulting the Home Location Register (HLR), which, as described above, knows which Visitor Location Register (VLR) the phone is associated with, if any.
  
  Section::::Voice all calls.:Incoming calls.:Routing the call.
   A mobile terminal, while connected using a RAT, performs neighbour cell measurements and sends measurement report to the network. Based on this measurement report provided by the mobile terminal, the network can initiate handover from one RAT to another, e.g. from WCDMA to GSM or vice versa. Once the handover with the new RAT is completed, the channels used by the previous RAT are released.
   With control plane locating, sometimes referred to as positioning, the mobile phone service provider gets the location based on the radio signal delay of the closest cell-phone towers (for phones without GPS features) which can be quite slow as it uses the 'voice control' channel. In the UK, networks do not use trilateration; LBS services use a single base station, with a "radius" of inaccuracy, to determine a phone's location. This technique was the basis of the E-911 mandate and is still used to locate cellphones as a safety measure. Newer phones and PDAs typically have an integrated A-GPS chip.
   The base station controller (a central computer that specializes in making phone connections) and the intelligence of the cellphone keeps track of and allows the phone to switch from one mast to the next during conversation. As the user moves towards a mast it picks the strongest signal and releases the mast from which the signal has become weaker; that channel on that mast becomes available to another user.
  
  Section::::Operation.:Geolocation.
   Section::::Means of achieving non-line-of-sight transmission.:Other methods.
  
  Earth–Moon–Earth communication, Meteor burst communications, and Sporadic E propagation are also other methods of achieving communications past the radio horizon.
  
  Section::::How is positioning accuracy affected by NLOS conditions?
   Cellular geolocation is less precise than by GPS, but it is available to devices that do not have GPS receivers and where the GPS is not available. The precision of this system varies and is highest where advanced forward link methods are possible and is lowest where only a single cell site can be reached, in which case the location is only known to be within the coverage of that site.
  
  An advanced forward link is where a device is within range of at least three cell sites and where the carrier has implemented timing system use.
   BULLET::::- Mobile satellite communications – Satellite communications systems use a directional antenna (usually a "dish") pointed at a satellite. The antenna on a moving ship or train, for example, must be pointed based on its current location. Modern antenna controllers usually incorporate a GNSS receiver to provide this information.
   Some mobile transmitter hunters use equipment based on exploiting the principle of Doppler shift. At least four antennas are mounted in a precise geometric pattern, often on the roof of a vehicle. Specialty electronics computes the amount of Doppler shift present in the received signals and determines a probable direction from which the signal originates. The direction is commonly displayed using LEDs oriented in a circle or a straight line. Advanced units can use a compass or GPS receiver to compute a direction relative to the instant motion of the vehicle.
   To each location area, a unique number called a "location area code" (LAC) is assigned. The location area code is broadcast by each base station at regular intervals. Within a location area, each base station is assigned a distinct "cell identifier" (CI) number, see also Cell Global Identity.
   BULLET::::- Mozilla Location Service - an open service which lets devices determine their location based on network infrastructure like WiFi access points and cell towers
  
  BULLET::::- CellMapper - cellular coverage and tower map
  
  BULLET::::- OpenCellID - an open source project, aiming to create a complete database of Cell IDs worldwide, with their locations
  
  BULLET::::- cellidfinder - Find the coordinates of any known CellID.
  
  BULLET::::- Navizon - cloud service with API to locate wireless devices using a global database of WiFi access points and Cell-ID locations
   Wireless tracking uses a set of anchors that are placed around the perimeter of the tracking space and one or more tags that are tracked. This system is similar in concept to GPS, but works both indoors and outdoors. Sometimes referred to as indoor GPS. The tags triangulate their 3D position using the anchors placed around the perimeter. A wireless technology called Ultra Wideband has enabled the position tracking to reach a precision of under 100 mm. By using sensor fusion and high speed algorithms, the tracking precision can reach 5 mm level with update speeds of 200 Hz or 5 ms latency.
   The location of a mobile phone can be determined using client software installed on the handset. This technique determines the location of the handset by putting its location by cell identification, signal strengths of the home and neighboring cells, which is continuously sent to the carrier. In addition, if the handset is also equipped with GPS then significantly more precise location information can be then sent from the handset to the carrier.
   BULLET::::- Cell phone – a portable wireless telephone that is connected to the telephone network by radio signals exchanged with a local antenna at a cellular base station (cell tower). The service area covered by the provider is divided into small geographical areas called "cells", each served by a separate base station antenna and multichannel transceiver. All the cell phones in a cell communicate with this antenna on separate frequency channels, assigned from a common pool of frequencies.
   Section::::Technology.
  
  Cell Broadcast allows a text or binary message to be defined and distributed to all mobile terminals connected to a set of cells.
   A network of base stations communicate with the Internet, or a private network, in order to pass information from devices to a computer system; and to pass information back to the devices. The downlink to devices uses time slots (TDMA) and the uplink to the base station is divided into sub-channels so that several devices can communicate to the base station.
  
  Section::::Implementation.:Communication and connection.
   There are also hybrid positioning systems which combine several different location approaches to position mobile devices by Wi-Fi, WiMAX, GSM, LTE, IP addresses, and network environment data.
  
  Section::::Operational purpose.
   When the MS is switched on, it searches for a mobile network to connect to. Once the MS identifies its desired network, it sends a message to the network to indicate that it has entered into an idle state. The Visitor Location Register (VLR) checks its database to determine whether there is an existing record of the particular subscriber. 
   Radiolocation is employed in a wide variety of industrial and military activities. Radar systems often use a combination of TOA and AOA to determine a backscattering object's position using a single receiver. In Doppler radar, the Doppler shift is also taken into account, determining velocity rather than location (though it helps determine future location). Real Time Location Systems RTLS using calibrated RTLS, and TDOA, are commercially available. The widely used Global Positioning System (GPS) is based on TOA of signals from satellites at known positions.
  
  Section::::Mobile phones.
   Section::::Problem statement & basics.
  
  In a multipath system, the wireless channel is frequency dependent, time dependent, and position dependent. Therefore, the following parameters describe the channel:
  
  BULLET::::- Direction of departure (DOD)
  
  BULLET::::- Direction of arrival (DOA)
  
  BULLET::::- Time delay
  
  BULLET::::- Doppler shift
  
  BULLET::::- Complex polarimetric path weight matrix
   Defining the position and orientation of a real object in space is determined with the help of special sensors or markers. Sensors record the signal from the real object when it moves or is moved and transmit the received information to the computer.
  
  Section::::Wireless tracking.
   Section::::Mobile phone network.:Small cells.
  
  Small cells, which have a smaller coverage area than base stations, are categorised as follows:
  
  BULLET::::- Microcell, less than 2 kilometres
  
  BULLET::::- Picocell, less than 200 metres
  
  BULLET::::- Femtocell, around 10 metres
  
  Section::::Mobile phone network.:Cellular handover in mobile phone networks.
  
presuppositions: Signals from cell phones can determine where they need to go.
corrections: Signals from cell phones radiate in every direction until they find a cell phone tower.
raw_presuppositions: Signals from cell phones can determine where they need to go. Signal from phone "knows" where to go.
raw_corrections: Signals from cell phones radiate in every direction until they find a cell phone tower. Signal goes every and finds a tower which then routes it to the correct location.