GPS (Global Positioning System) | Satellite Navigation

Global  Positioning  System (GPS) 

The Global Positioning System (GPS), Commonly known as Navstar GPS, is a Satellite based Radio Navigation System owned and administered by the United States. The Global Navigation Satellite System (GNSS) gives highly accurate geolocation and time information to a GPS receiver anywhere on the World where four or more GPS satellites can be seen without obstructions. Mountains and buildings can obstruct the GPS signals, which are quite weak.

The GPS independently works  and doesn't require the users to send any data. GPS works on telephonic or Internet reception, however both technologies can improve the accuracy of GPS positioning data. Military, Maine, Civil Aviation, Ground Transport and commercial users all across the world rely on the GPS for accurate positioning. The US government designed, maintains, and administers the system, which is publicly accessible to anybody with a GPS device.

The  Global  Positioning  System  (GPS) is the primary source of navigation these days.  Almost everyone relying on GPS. 

GPS Data

GPS provides most accurate navigational  information. The Global Positioning System is based on 24 satellites in six orbits with four satellites per orbit. 

The GPS provides following basic information worldwide:

  • Latitude  
  • Longitude 
  • Altitude  
  • Time   
  • Speed. 

GPS History & Development

The GPS project was initiated in 1973 by the United States Department of Defense. In 1978, the first experimental spacecraft was launched, and in 1993, the full constellation of 24 navigational satellites became operational. After the Korean Air Lines Flight 007 disaster, President Ronald Reagan issued an executive order allowing civilian use beginning in the 1980s.

Previously, GPS was a spin-off from two experimental satellite navigation programmes run by the US Navy and Air Force, with the goal of facilitating precision-aimed weaponry and precise troop deployment. Despite the fact that the technology is now offered to the public sector, the US Department of Defense still controls and administers it. Although the system is highly accurate, the US military can reduce its accuracy to meet their needs if necessary (up to 1,000 m).

Advantage of GPS

The following are the key benefits of GPS:

  • Accuracy   
  • Global  Application   
  • Signal  fidelity

GPS Segment

Satellites in orbit that are very precisely positioned broadcast very accurate, coded satellite location and time data. This data is decoded by the receiver, which then calculates its position in relation to the satellite. For proper results, these features must be provided if the receiver is moving. Accuracy is improved by combining data from multiple satellites. In the end, each satellite's accuracy is determined by extremely accurate atomic clocks. 

The Space Segment, the User Segment, and the Control Segment are the three segments that make up GPS.

Space  segment   

The Space Segment part of GPS consists of a minimum of 24 operational Navigation Satellites in six circular orbits 20,200 km above the earth (Space), with a period of 12 hours and an inclination angle of each Satellites is 55 degrees. The satellites are spaced in orbit so that a minimum of six satellites are visible to users anywhere in the world at any given time. These Satellites continuously broadcast position and time data to users all over the world who are using GPS. Atomic clocks with a precision of 0.1 second per 10,000 years are used to preserve transmission accuracy. The accuracy of the system is determined by the exact synchronization of the receiver and satellite clocks, as well as the receiver clock's quality. It's worth noting that combining data from multiple satellites can help lessen these mistakes.

User  segment   

The receivers, computers, and antennas that allow land, marine, and airborne operators to receive GPS satellite broadcasts and compute their precise position, velocity, and time make up the User Segment.

Control  segment   

The Control Segment is made up of a master control station in Colorado Springs, as well as five monitor stations and three ground antennas spread across the globe. The GPS monitor stations keep watch of all GPS satellites in their area and collect all the data from the satellite broadcasts. The monitor stations communicate the data they acquire from each satellite to the master control station, which uses it to calculate exceedingly exact satellite orbits. After then, the data is formatted into new navigation messages for each satellite.Ground antennas, which also broadcast and receive satellite control and monitoring signals, transmit and receive the updated information to each satellite.

GPS  frequencies & Operation

L1 at 1,575.42 MHz and L2 at 1,227.60 MHz are the two carriers transmitted by each satellite. A 1.023 MHz coarse/acquisition code (C/A) modulates the L1 signal every 1 ms and repeats every 1 ms. Despite the fact that the coded signal is repeating, it appears random and is referred to as Pseudo Random Noise (PRN). 

The L2 carrier is modulated at 10.23 MHz by another PRN signal that appears to be random coded. The Precision (P) code is the name given to this signal, which occurs every 267 days. A unique seven-day portion of this code is assigned to each satellite. 

For error correction, a third signal carrying navigation data is superimposed on each already complex signal containing satellite status, ephemeris data, clock error, and tropospheric and ionospheric data. Interestingly, the US control section has the ability to 'dither' these frequencies at any time to invalidate any known codes, rendering the GPS useless to people who are unaware of the dither coding. When this is done, the code is known as the Y code, and it can only be used for military or political purposes. With the exception of Y coding, every GPS receiver knows the modulation codes for each satellite, therefore identifying the satellite is as simple as decoding the received signals and then eliminating the carrier to retrieve the navigation data.


The GPS consists of a minimum of 24 operational Navigation Satellites in six circular orbits 20,200 km above the earth (Space), with a period of 12 hours and an inclination angle of each Satellites is 55 degrees. The main advantage of GPS are: Accuracy, Global  Application, Signal  fidelity. 

The use of GPS change our lifestyle, we all are totally dependents on GPS. 

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