Global positioning system(GPS)

Mobile that we using can be used to locate all the people in the case of emergency.that's why GPS(global positioning receiver) located chip was inserted in new phone.Newer phones can be used to track people in 30ft range.cell phone essentially radio.When cell phone is on it sends radio signals to it's cell tower to know where the phone is,this will help a person to move one cell to another cell with out loosing a call.At the same time GPS system receives signal from outer space specifically at least 3 geosynchronous satellites.Using geometric method called tridimentional trilateration,the GPS chip using the satellite signal to calculate its exact location on earth.
If we put a sniffer on a node we can track someone.Node is something which takes all calls coming to a cell tower and release those signals to where they are going.but sniffer is a electronic device that detect specific calls then send that signal to a phone that we are using to tracking.

Global positioning system(GPS)

GPS is a satellite based navigation system.GPS works anywhere in the world under any weather conditions.GPS constellation consist of 24 satellites in 6 near circular orbits.An on board GPS receiver must locked in to at least 3 satellite to calculate 2D position or the movement,if 4 satellites or more receiver can determine the 3D position.Once the user position has been determined GPS unit will start to calculate other information such as speed,track,trip distance,time.These parameters are continually updated by a master control station,which transmits them up to the satellites where they are broadcast as part of a navigational message from each satellite.Civilian GPS uses L1 frequency of 1.5754 GHz in the UHF band(signal travels by line of sight,it can pass clouds,glass and plastic but fail to pass through solid objects).Defense GPS uses L2 frequency of 1.2276 GHz in the UHF band.In the GPS system the modulation used is direct sequence spread spectrum and it is one way transmission system(from satellite to user only,user does not require a transmitter).Modulation method makes GPS system jam resistant,secure and addressable.The in phase component is being C/A (coarse acquisition) code while quadrature component is P-code(precision code).
GPS signal contains 3 different bits of information:

1. Pseudorandom code,simply an ID code to identify which satellite is transmitting th signal.
2. Ephemeris data,which is constantly transmitted by each satellite,contains information sbout its status and this is essential to determine its position.
3. Almanac data,tells the GPS receiver about the position(orbital information) of each GPS satellite throughout the day.

Dilution of precision (DOP) is measure of the quality of the GPS data being received fro the satellite.The effect of DOP can be resolved in to horizontal DOP(HDOP),vertical DOP(VDOP),time DOP(TDOP) and position DOP(PDOP).