GNSS System: NAVSTAR Global Positioning System & GLONASS (History and Development) (Especially for GATE-Geospatial 2022)

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The two main satellite-based navigation systems operational today are GPS and GLONASS. Both systems were originally military networks, with each one based on constellations of 24 operational satellites in order to give the user an accurate position. They are very important:

  • Despite of their military origins, GPS and GLONASS had innumerable Civilian applications that were far beyond the original contemplation. Satellite navigation has completely changed the lives of common man. The civil aviation sector has greatly benefitted from this technology.
  • Civilians like hikers, private pilots and boat owners are making extensive use of satellite positioning.
  • It is estimated that nine out of every ten new satellite navigation receivers are currently being sold for civilian or commercial use. Let us discuss about the different operational and under development navigational systems.

NAVSTAR Global Positioning System (GPS)

  • NAVSTAR is commonly known as NAVSTAR-GPS (Navigation System with Timing and Ranging Global Positioning System) .
  • It is planned for both civilian and military use. NAVSTAR GPS was developed in 3 phases. They are:
    • Phase 1: 1973 - 1979 - Concept Validation
    • Phase 2: 1979 - 1985- Full development and tests
    • Phase 3: 1985-till date - Production and Deployment
  • GPS is based on GNSS and maintained by US government. It is freely accessible by anyone with a GPS receiver. In 1978, the first experimental Block I, GPS satellite was launched. By 1985, eleven more experimental Block I satellites had been launched to validate the concept.
  • On February 14,1989, a new type of satellite was activated and the first modern Block II satellite was launched. By December 1993, GPS achieved initial operational capability, indicating a full constellation of 24 satellites was available and provided the Standard Positioning Service (SPS) .
  • In 1995, Full Operation Capacity (FOC) was achieved and five years later accuracy for civilian users increased from about 100 m to 20 m as the result of deactivation of Selective Availability. In 2005, the modernization of GPS system began by launching the first satellite of type IIR-M, which supported a new military signal on L1 and L2 and the addition of the second civil code (C/A) on L2 frequency.
  • As of 12 February 2007, the space segment was built-up by 30 operational satellites of Block IIA, 12 satellites of Block IIR and 3 satellites of Block IIR-M (Crews, 2007) . The addition of third civil signal L5 at 1176.45 MHz in Block II F satellite has given an entirely different and more efficient structure.
  • The fully operational capability of GPS was achieved by April 1995, signifying full availability of the military՚s secure Precise Positioning Service (PPS) . The oldest GPS satellite was launched on November 26,1990 and became operational on December 10,1990. The recent operational GPS satellite was launched on May 28,2010. Currently, we have 31 operational satellites providing signals.
24 GPS Satellites Ring

GLONASS

  • GLONASS is satellite-based radio navigation system which provides the positioning and timing information to users, like GPS (Fig. 1.3) .
  • It is operated by the Ministry of Defence of the Russian Federation. During the period of over three decades GLONASS has gone through numerous improvements which can be divided into three generations:
    • The original Glonass is out of service (since 1982)
    • GLONASS-M (since 2003)
    • GLONASS-K are operational now (since 2011)
  • The most important specifications of this system were:
    • 24 planned satellites (21 standard + 3 reserve satellites)
    • 3 orbital levels with an angle of 64.8° from the equator
    • Orbital altitude of 19,100 km
    • Orbital period of 11h 15.8 min
    • Every GLONASS satellite transmits two codes (C/A and P-Code) on two frequencies. Every satellite transmits the same codes PRN (Pseudorandom noise) , but at different frequencies in the vicinity of 1602 MHz and 1246 MHz
    • There are three segments in GNSS satellite systems.

Components of GLONASS

  • GLONASS also comprises three segments as discussed below:
    • Space segment
    • Control segment
    • User segment

Space Segment

  • Space Segment consists of satellites located in the middle of the circular orbit at 19,100 km altitude with a 64.8 inclination and a period of 11 hrs and 15 min. The constellation operates in three orbital planes with 8 evenly spaced satellites on each.
  • A fully operational constellation with global coverage consists of 24 satellites, while 18 satellites are necessary for covering the territory of Russia. Glonass is especially suited for usage in the northern latitudes.

Control Segment

  • Control Segment consists of the system control centre and a network of command tracking stations across Russia.
  • Its control segment is like GPS that monitors the status of satellites and determines the ephemerides and satellite clock offsets with respect to GLONASS time and Coordinated Universal Time (UTC) . It uploads the navigation data to the satellites twice a day.

User Segment

  • User Segment consists of equipment that tracks and receives the satellite signals. This equipment is capable of simultaneously processing the signals from a minimum of four satellites to obtain accurate position, velocity and timing measurements. Like GPS, GLONASS has a dual military and civilian usage.
Illustration 2 for Components_of_GLONASS

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