GPS(Global Positioning System) is a satellite-based navigation system. It provides time and location-based information to a GPS receiver, located anywhere on or near the earth surface. GPS works in all weather conditions, provided there is an unobstructed line of sight communication with 4 or more GPS satellites. GPS is managed by the US Air Force.
A GPS operates independently of the user’s internet connection or telephone signal. However, their presence increases the effectiveness of GPS positioning. GPS was initially developed by the US government for military purpose, but currently, anyone with a GPS receiver can receive radio signals from GPS satellites.
- Initially when GPS was developed for military use, there were 24 GPS satellites orbiting the earth every 12 hours at a height of 20, 180 km.
- 4 GPS satellites were located in each of the 6 orbits with 60 degree orientation between each other. These orbital planes do not rotate with respect to any star.
- Later, the number of satellites were increased to 32, to improve location accuracy.
- Localization of any GPS receiver is done through time of flight measurement.
- The greater the number of satellite in line of sight to a GPS receiver, the greater is the accuracy in determining the position of the receiver.
How GPS works?
Any instant of time, there are at least 4 GPS satellites in line of sight to a receiver on the earth. Each of these GPS satellites sends information about its position and the current time to the GPS receiver at fixed regular instants of time. This information is transmitted to the receiver in the form of signal which is then intercepted by the receiver devices. These signals are radio signals that travel with the speed of light. The distance between a GPS receiver and the satellite is calculated by finding the difference between the time the signal was sent from GPS satellite and the time the GPS receiver received the signal.
Once the receiver receives the signal from at least three satellites, the receiver then points its location using trilateration process. A GPS requires at least 3 satellites to calculate 2-D position(latitude and longitude on a map). In this case, the GPS receiver assumes that it is located at mean sea level. However, it requires at least 4 satellites to find receivers 3-D position(latitude, longitude, and altitude).
What is trilateration?
Trilateration is the process of determining your position based on the intersection of spheres. When a receiver receives a signal from one of the satellite, it calculates its distance from the satellite considering a 3-D sphere with the satellite located at the center of the sphere. Once the receiver does the same with 3 other GPS satellites, the receiver then proceeds to find the intersection point of the 3 spheres to calculate it’s location.
Once the position of a receiver is calculated, the GPS device can then easily calculate:
- Time of sunrise and sunset
- distance to destination
of the GPS receiver.
Technical challenges face by GPS:
- Time synchronization between individual satellites and the GPS receiver
- Real time update of the exact location of the GPS satellite
- Precise measurement of time of flight
- Interference with other signals
- Each of the GPS satellites are equipped with an atomic clock to keep the time updated and accurate. In order to update and provide precise timing on the receiver, the receiver uses the fourth GPS satellite to keeps it’s timing accurate. With the timing with the receiver and GPS satellites maintained by atomic clocks, the exact time difference can be calculated by the receiver.
The GPS satellites are constantly managed from ground-based stations to resolve time synchronization.
Significance of ultra precision of time synchronization:
- Electromagnetic radiation propagate with the speed of light. The accuracy in position is directly proportional to the time measurement, as speed * time = distance, therefore, even a slight deviation in time calculation can create huge difference in distance due to large speed of light.
Real time update of exact location of the satellite.
- This is done by monitoring the satellites from a number of widely distributed ground based system.
- Master station analyses all the measurements and transmits the actual position of each satellite.
DGPS is an improvement over GPS that provides enhanced location accuracy
- The GPS receiver also called as the base station must be set up at a precisely known location
- The base station receiver calculates it’s position based on the satellite signals and compares it’s location to the known location
- The difference between the two locations is applied to the data recorded by the GPS receiver
- Provides position accuracy in sub-meter to cm range
DGPS uses a network of fixed ground-based stations to broadcast the difference between the positions indicated by the GPS satellite systems and the known fixed positions. These stations broadcast the difference between the measured satellite pseudo-ranges and actual pseudo-ranges and the receiver stations may correct their pseudo-ranges by the same amount.
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