Most of us use Global Positioning System (GPS) every single day. It has become an inseparable part of our life. Let it be Google Map or any other GPS based navigation system, we are heavily dependent on it. This post will discuss what is Global Positioning System, its architecture, working principle, future of GPS, applications, advantages and disadvantages.
What is Global Positioning System (GPS)
GPS is a positioning system that is based on a network of satellites that continuously transmit coded information through radio signals. The receivers interpret the information transmitted from the satellites to identify the locations on earth accurately.
This system is made up of around 29 satellites which are located nearly 20,000 km above the Earths surface and operates in all weather conditions. This system is largely available in all parts of the world, 24 hours a day. The U.S. Department of Defence (USDoD) had originally put the satellites into orbit for military use but made them available for civilian use in the 1980s.
Global Positioning System (GPS) is also used for navigation in planes and ships. It helps military and civilian users have critical advantage with continuous real time and 3-dimensional positioning data.
Fig. 1 – Introduction to Global Positioning System (GPS)
Global Positioning System (GPS) Architecture
The Architecture of Global Positioning System consists of three segments or units namely:
- GPS Space Segment
- GPS Control Segment
- GPS Receiver (User) Segment
Global Positioning System (GPS) Space Segment
The Space Unit consists of 24 active satellites which are assembled with huge solar panels with rechargeable batteries that act as a power source. The function of the satellites in space is to route or navigate the radio signals received from the control unit to store and re-transmit the message to the respective Receiver Unit.
Global Positioning System (GPS) Control Segment
The Control Unit consists of several monitoring and control stations. The monitor stations monitor the GPS satellite signals. These signals are then sent to the master control station where operational specifications are checked and revised before transmitting the control signals back to the GPS satellites. They are sent back through ground antennas.
Global Positioning System (GPS) Receiver (User) Segment
The User Unit is the term given to all GPS receivers like mobile phones, laptops, PC or any other device. The devices receives the signals from the GPS satellites and determines how far away it is from each satellite.
Fig. 2 – Global Positioning System (GPS) Architecture
How does Global Positioning System (GPS) Work
The three elements of Global Positioning System work in unison resulting in accurate and reliable operation of the positioning system. GPS positioning is based on “Trilateration Principle” which determines the position by measuring distances to points at known coordinates. Trilateration requires three ranges to three known points at a minimum but Global Positioning System requires four “Pseudoranges” to four satellites.
Hence the positioning system uses two main factors in determining the position:
- Position of the Receiver (User) using Trilateration Principle
- Pseudorange Calculation
Position of the Receiver (User) using Trilateration Principle
To calculate the 2-D position (latitude and longitude) of a point of interest or track movement, a GPS receiver must be locked on to the signal of at least three satellites. A single satellite tracks a general location of the point of interest on the Earth’s surface. This location information is spread over a large area. Data from a second satellite, when added to this information, allows the Global Positioning System to narrow the location. This will be the point where the two areas of satellite data overlap. Adding data from a third satellite provides more accurate position of the point.
The distance is measured using the equation:
Distance = Travel Time X Speed of Light
Where Travel Time is the time taken by the signals to reach the receiver, which travels at the speed of light and the distance measured helps to locate the point of interest (User) on the face of Earth.
The fourth satellite is used to re-confirm and enhance the position of the point of interest(Receiver). The receiver determines the 3-D position (latitude, longitude and altitude) of the point of interest with the information from the fourth satellite. Precision increases with increase in the number of satellites in the vicinity.
Fig. 3 – Trilateration Principle
The GPS satellites which orbit the Earth twice a day transmits or broadcast signals on the same two frequencies called L1 and L2. The primary signal is broadcast on L1 and is modulated with two information signals called C/A (Coarse and Acquisition code) and P (for Precise code). This signal undergoes spread spectrum modulation technique that basically consists of a carrier signal that is repeatedly inverted having 180ᶿ phase shift. Replica of the Precise code is broadcast on the second frequency called L2.
In addition to transmitting a specific phase-change pattern, additional data is also added to the signal which comprises of the navigation message. This message has the information needed to calculate the current position of the satellite at the time of transmission.
The signal generator consisting of 10-bit shift registers generates the C/A code pattern which acts as the input to Exclusive–OR Gate. The resulting digital output is referred to as Pseudo-Random Number or PRN sequence.
In order to receive the spread-spectrum signal, the Receiver must know the desired PRN sequence. The receiver searches for specific satellites by generating its own copy of the sequence and shifting the pattern that matches with the transmitted pattern. This is achieved using correlator, which generates an output that corresponds to the degree of match between the two signals.
Once the signals are matched, the Receiver computes the distance to each satellite using time frames, called “Pseudorange”. The time that the signal is transmitted from a satellite is recorded by the atomic clocks on the satellite and the time of signal reception is recorded by the receivers clock. The Receiver measures the difference of the two time frames called the “Pseudorange”. It is measured in units of time.
Fig. 4 – Schematic Representation of Pseudorange Calculation
What is the future of Global Positioning System (GPS)
The United States of America has a monopoly on satellite-based navigation currently. Europe, Russia, China and even India are in the process of launching new systems to bring their own Global Positioning System.
Ground technology is also becoming more advanced such that the existing Global Positioning System network can pinpoint location to within an accuracy of 30 centimeters.
Either of these technologies may not be widely available any time soon but interesting changes are on the way for new technology.
Applications of Global Positioning System (GPS)
Though initially designed for the military, Global Positioning System today is used by military as well as civilians extensively. The applications of GPS include:
- Location – Determining a position
- Navigation –Moving from one location to another
- Tracking – Monitoring object or personal movement
- Mapping – Creating maps of the world
- Timing – Informing the world about precise timing
Advantages of Global Positioning System (GPS)
The advantages of Global Positioning System are:
- It provides reliable information as it is updated constantly.
- Allows the determination of exact location, distance and speed (also altitude, if required).
- Permits tracking and measuring weather conditions.
- Technology is available Free of Cost (mostly).
Disadvantages of Global Positioning System (GPS)
The disadvantages of Global Positioning System are:
- Global Positioning System satellite signals are too weak when compared to phone signals.
- Global Positioning System works less efficiently when indoors, underwater, under trees, etc.