Global Positioning System

Artist's conception of GPS satellite in earth orbit.

The GPS (Global Positioning System) is a satellite based navigation system made up of a network of 24 satellites placed into orbit by U. S. Department of Defense (DOD).The system was originally developed by DOD for military navigation but in 1983, they made the system available for civilian use. Today anyone with a GPS device like Satnav, handheld GPS unit or a mobile phone with GPS can obtain the radio signals that the satellites broadcast and find their exact location.

How does it work?

GPS satellites circle the globe twice a day in its
orbit at an altitude of 20,200km with an orbital speed of about 14,000 km/hour and transmit signal information to earth. Each Satellite transmits information about the current time and its position at regular intervals. These signals, travelling at the speed of light, are intercepted by your GPS receiver, based on how long it acquired for the signal to arrive; it calculates how far away each satellite is. Once it has information about how far away at least three satellites are, using a trilateration process, your GPS receiver can show your location.

GPS receiver must receive the signal of 3 satellites to calculate latitude and longitude (2-D position), With four signal GPS device can calculate altitude also. Once the user’s position has been determined, based on previous information, the GPS device can calculate other information, such as speed, distance to destination, bearing, track, trip distance, sunrise and sunset time and more. The more satellites there are above the horizon, the moreprecisely your GPS unit can determine your position.

What’s the signal?

The GPS satellites mainly transmit two low power microwave carrier signals. The L1 frequency (1575.42 MHz) carries the navigation message and the SPS (Standard Positioning Service) code signals, and the L2 frequency (1227.60 MHz) is used to measure the ionospheric delay by PPS (Precise Positioning Service) equipped receivers. Civilian GPS just uses L1 frequency, but military GPS uses both L1 and L2 frequency to measure the ionospheric delay to give the precise location. A GPS signal contains three different bits of information • Pseudorandom code It is simply an I.D. code which represents data transmitting satellite. • Ephemeris data It represents the Status of the satellite (healthy or unhealthy), Current date and time. This part of the signal is essential for determining a position. • Almanac data It tells the GPS receiver where each GPS satellite should be at any time throughout the day. Every single satellite transmits almanac data showing the orbital information of that satellite& every other satellite in the constellation.

How accurate is GPS?

Regular GPS receivers are accurate to within 5-10 meters on average. Newer more sophisticated techniques like Differential GPS (DGPS) and Real-Time Kinematic (RTK) methods can provide centimeter-level precision within a few minutes of measurement.

GPS and Relativity

The performance of the clocks is fundamental to GPS. Each satellite has an atomic clock with it that “ticks” with an accuracy of 1 nanosecond (1 billionth of a second).To get 10-15 meter level of precision on location, time on GPS satellite must be known to an accuracy of 20-30 nanosecond. Special Relativity predicts that the satellites’ clocks are moving relative to the clock on Earth, they will appear to run slower in earth frame of reference. Due to this effect of time dilation the clock on GPS station fall behind 7microsecond per day. General Relativity predicts that curvature of spacetime leads to time dilation so time will appear to run slower under stronger gravitational pull. Hence the clock on GPS satellites gets ahead by 45 microseconds per day. Overall each satellites’ clock ticks faster than 38microseconds(45-7) per day.This leads to an error of 10 km each day! The whole system would be worthless for navigation in a very shortperiod. Some interesting facts about GPS • In average, a GPS satellite weighs around 1000kg. • In 1978 The first GPS satellite was launched. • Each Transmitter consumes power less than 50 watts. • Total 72 satellites have been launched, and 32 are in orbit.

Source: gps.gov,Garmin.com
Camels have been domesticated for 3,000 years. Camels can run at 25 mph (40 kph) for long periods. The 

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