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GNSS Module | GNSS, Satellite Navigation and Data Format (NMEA) - Relevant Information
Update #10129  |  14 Nov 2014
 

Before we continue with the positioning module development. It is necessary to introduce some basic concepts regarding to GNSS principles and systems, satellite navigation basics and different data format (navigation) related with the project implementation.

This summary is an excerpt from:

  • Compendium,  GPS Essentials of Satellite Navigation by Ublox.
  • European Space Agency - website.
  • Global Navigation Satellite System (GNSS) Manual by International Civil Aviation Organization

 

GNSS

Satellite Navigation is a method employing a Global Navigation Satellite System (GNSS) to accurately determine position and time anywhere on Earth. Satellite Navigation receivers are currently used by both private individuals and businesses for positioning, locating, navigating, surveying, and determining the exact time in an evergrowing list of personal, leisure and commercial applications. [1]

Using a GNSS system, the following values can accurately be determined anywhere on the globe [1]:

  • Exact position (longitude, latitude and altitude coordinates) accurate to within 20m to approx. 1mm.
  • Exact time (Universal Time Coordinated, UTC) accurate to within 60ns to approx. 5ns.
    Speed and direction of travel (course) can be derived from these values, which are obtained from satellites orbiting the Earth. Speed of travel may also be determined directly by means of Doppler shift measurements.

 

SATELLITE NAVIGATION

Satellite Navigation Systems all use the same basic principles to determine coordinates [1]:  

  • Satellites with a known position transmit a regular time signal.
  • Based on the measured travel time of the radio waves (electromagnetic signals travel through space at the speed of light c = 300’000km/s) the position of the receiver is calculated.  

Satellite Navigation Systems use satellites as time-signal transmitters. Contact to at least four satellites is necessary in order to determine the three desired coordinates (Longitude, Latitude, Altitude) as well as the exact time [1] .

Satellite Navigation Systems employ satellites orbiting high above the Earth and distributed in such a way that from any point on the ground there is line-of-sight contact to at least 4 satellites [1].


Each one of these satellites is equipped with onboard atomic clocks. Atomic clocks are the most precise time measurement instruments known, losing a maximum of one second every 30,000 to 1,000,000 years. In order to make them even more accurate, they are regularly adjusted or synchronized from various control points on Earth. GNSS satellites transmit their exact position and onboard clock time to Earth. These signals are transmitted at the speed of light (300,000km/s) and therefore require approx. 67.3ms to reach a position on the Earth’s surface directly below the satellite. The signals require a further 3.33ms for each additional kilometer of travel. To establish position, all that is required is a receiver and an accurate clock. By comparing the arrival time of the satellite signal with the onboard clock time the moment the signal was transmitted, it is possible to determine the signal travel time. [1]

 

The position is determined in three-dimensional space. In 3-dimensional space we will have four variables [1]:

  • Longitude (X)
  • Latitude (Y)
  • Height (Z)
  • Time error (Delta t)

 If the distance to the three satellites is known, all possible positions are located on the surface of three spheres whose radii correspond to the distance calculated. The position is the point where all three of the spheres intersect. [1]

 

Satellite Navigation systems are deliberately constructed in such a way that from any point on Earth, at least 4 satellites are “visible”. Thus, despite an inaccuracy on the part of the receiver clock and resulting time errors, a position can be calculated to within an accuracy of approx. 5 – 10m [1].

 

 

GPS

All GNSS systems function on the same basic principles. GPS is the pioneer and forerunner of GNSS technology and is the only fully functional GNSS system in operation. GPS and GNSS are often used interchangeably, although GPS specifically refers to NAVSTAR GPS, developed by the United States Department of Defense and managed by the United States Air Force 50th Space Wing. The GPS system has been fully operational since 1993 [1].

The GPS system is comprised of three functional segments, please see picture below:

  • The space segment (all operating satellites).
  • The control segment (all ground stations involved in the monitoring of the system: master control stations, monitor stations, and ground control stations).
  • The user segment (all civilian and military users)

 

 

All these three segments operate together to provide accurate three-dimensional position, navigation and timing (PNT) services to users.[2]

The satellite constellation is the set of satellites in orbit that provide the ranging signals and data messages to the user equipment. GLONASS, Beidou and Galileo use satellites in different orbits and orbit period. [2]

The control segment (CS) tracks and maintains the satellites in space. The CS monitors satellite health and signal integrity and maintains the orbital configuration of the satellites. Furthermore, the CS updates the satellite clock corrections and ephemeris as well as numerous other parameters essential to deliver PNT services to users. The GPS control segment consists of a master control station, five base stations and three data up-loading stations in locations around the world. The base stations track and monitor the satellites via their broadcast signals. These signals are passed to the master control station where orbital parameters and timing corrections are computed. The resulting corrections are transmitted back to the satellites via the data up-loading stations. [2]

Finally, the user receiver equipment performs the localisation, timing, or other related functions (e.g., surveying). User receiver equipments are capable of simultaneously processing the signals from a minimum of four satellites to obtain accurate location, velocity and timing measurements. However the accuracy and the reliability of the measurement is enhanced as the geometry of the satellites in good situation. [2]


Satellite distribution and movement

The space segment of the GPS system consists of up to 32 operational satellites orbiting the Earth on 6 different orbital planes (four to five satellites per plane). They orbit at a height of 20,180km above the Earth’s surface and are inclined at 55° to the equator. Any one satellite completes its orbit in around 12 hours. Due to the rotation of the Earth, a satellite will be at its initial starting position above the earth’s surface after approx. 24 hours (23 hours 56 minutes to be precise). [1]

GPS satellites orbit the Earth on 6 orbital planes

 

24 hour tracking of a GPS satellite with its effective range

The distribution of the satellites at a specific time can be seen in figure below. It is due to this ingenious pattern of distribution and to the high orbital altitudes that communication with at least 4 satellites is ensured at all times anywhere in the world. [2]

 

Frequencies [3]

 

 

All satellites broadcast at the same two frequencies, 1.57542 GHz (L1 signal) and 1.2276 GHz (L2 signal). [3]

 

GLONASS

GLONASS is an abbreviation for a GNSS system currently operated by the Russian Defense Ministry. The designation GLONASS stands for Global Navigation Satellite System. The program was first started by the former Soviet Union, and is today under the jurisdiction of the Commonwealth of Independent States (CIS). The first three test-satellites were launched into orbit on October 12, 1982. It is an alternative and complementary to other GNSS systems such as the United States' Global Positioning System (GPS), the Chinese BeiDou navigation system or the planned Galileo positioning system of the European Union (EU).

The most important facts of the GLONASS system are: 

  • 28 satellites in constellation with 24 satellites operational.
  • 3 orbital levels with an angle of 64.8° from the equator (this is the highest angle of all the
    GNSS systems and allows better reception in polar regions)