Global Navigation Satellite System & Components of GNSS (Especially for GATE-Geospatial 2022)

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Global Navigation Satellite System

  • GNSS are a constellation of satellites designed to determine positioning and timing information for users on the Earth or in space. In 1970, International Civil Aviation Organization defined GNSS as “a worldwide position and time determination system that includes one or more satellite constellations, aircraft receivers and system integrity monitoring, augmented as necessary to support the required navigation performance for the intended operation” .
  • At present, there are mainly two core constellations for GNSS namely United States of America (USA) Global Positioning System commonly known as GPS and Russian Federation՚s Global Navigation Satellite System abbreviated as GLONASS.
  • In addition, Galileo - an European satellite navigation system, GAGAN - an Indian GPS aided geo-augmented navigation and Compass System - a China navigation satellite system are the emerging satellite constellations which will evolve into global constellations in the coming years. The operational GNSS such as GPS and
  • GLONASS are combined into a single category known as GNSS-1. The under developing second generation GNSS system e. g. Galileo, Chinese compass, Japanese quasi-zenith satellite system and Indian regional navigational satellite system are grouped as GNSS-2.
  • When GPS is used with corrections from ground or space based augmentation systems then it is called as a Differential GPS (DGPS) .
  • Currently, the complete navigation satellite technology is known GPS because it is the only fully functional GNSS with constellation of 32 satellites and most of existing global applications are related to it.
Illustration 2 for Global_Navigation_Satellite_System

Components of GNSS

  • GPS, GLONASS and Galileo are the three main GNSS systems and all these technologies consist of three main segments/components.
  • These segments are space, control and user.
    • Space Segment: It consists of GNSS satellites like space vehicles that send radio signals from space.
    • Control Segment: It consists of a system of tracking stations located around the world like master control and monitor network.
    • User Segment: It consists of the GNSS receivers and user community. GNSS receivers convert space vehicle (s) signals into position, velocity and time estimates which are used by GNSS user for navigation, positioning, time dissemination and other research.

How Do GPS and GLONASS Work?

  • The current generation of navigation systems such as GPS and GLONASS and those under development like Galileo determine the user terminal position through the time of arrival and mode of ranging. In general, this kind of ranging technique is based on the measurement of time interval employed by a signal transmitted by an emitter (e. g. , satellite, radio signal) at a known location to arrive at the user receiver.
  • A simple formula that gives distance of the receiver from each satellite. In fact, time is the most important parameter required to know the distance between the satellites and the receiver. The distance from a known position of a satellite to the receiver end is equal to the velocity of transmitted signal multiplied by travel time of radio waves transmitted from the satellites to reach the receiver. In this case, distance is calculated as:
  • Let us discuss this with a common example of a thunderstorm. If you work out the phenomenon of thunderstorm such as how far you are from a bolt of lightning, you would make the following calculations. This distance can be established quite easily . Distance is the time of the lightning flash perceived (start time) until the thunder is heard (stop time) , multiplied by the speed sound of at approx. 330m/s. The difference between the start and stop time is referred as the Signal Travel Time. In this case, signal is the sound wave travelling through air. Hence,
  • Now you can see that GPS works on the same principle, called ‘Time of Arrival’ (ToA) , sometimes known as Time of Flight (ToF) . Time of arrival is the travel time of a radio signal from a single transmitter to a remote signal receiver. Time of arrival of the signal measured by the user receiver is as given below:
  • You have read about thunderstorm, where speed of sound is considered but in the case of satellite navigation, electromagnetic signals are used which propagates at the speed of light
  • If the user receiver knows the speed of signal, then he/she is only able to determine the distance from the emitter, simply by multiplying the time of arrival with signal speed value. By considering the relationship between the speed of light in vacuum and carrier frequency of a signal, the time represents a measure for the distance between transmitter and receiver.
  • You can obtain geometric distance between the satellite and receiver by calculating the true time of arrival. This implies that the receiver has a precise knowledge of time instant of arrival and time instant of transmission of the satellite signal.
  • The time instant of arrival can be achieved through direct reading of the receiver clock whereas time instant of transmission is embedded in the signal, which is scheduled navigation signal.
  • To achieve a true difference between these time instants, discussed above, the satellite and receiver clocks have to be synchronized to the same time scale. Once the user receiver has a sufficient number of distance values from multiple satellites with known locations, it can specify its position, according to theoretical considerations.
  • In satellite-based navigation, transmitters are not having fixed points, as in the case of terrestrial.
  • We have discussed that the receiver is able to determine satellite position for each distance measurement.
  • For this purpose, each navigation signal modulates a message that includes satellite orbital parameters. Satellite orbital parameters are generally known as satellite ephemeris data.
  • This enables the receiver to propagate satellite orbit and then to evaluate transmitter position at each time instant. Orbital parameters are updated by the master control station and transmitted to the satellites once or twice a day.

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