Introduction to GNSS (Global Navigation Satellite Systems)
A Calabia History • 1957: First satellite “Sputnik I” (Russian). Positioning based on Doppler effect. • 1967: TRANSIT (USA) -> Doppler. • 1974: Tsikada (Russian) -> Doppler. • 1978: First NAVSTAR GPS (NAVigation System with Time and Ranging Global Positioning System) (USA). -> triangulation. • 1994: NAVSTAR GPS reaches final configuration (24 sat.). • 1993: GLONASS (Global’naya Navigatsionnaya Sputnikovaya Sistema) (Russian) operational. -> triangulation. • 2012: BeiDou NS (China). Regional. -> triangulation. • 2016: Galileo (EU). -> triangulation. • 2020?: BeiDou (China). Global. -> triangulation. Segments of the system
Control Space Segment Segment • constellation of • the control and satellites monitor stations
User Segment • all GNSS receivers worldwide Navstar/GPS GLONASS Galileo BeiDou Ns
•24 sat. / 6 planes • 24 sat. / 3 planes • 27 sat. / 3 planes • 24 sat. / 3 planes • Td= 717.98 min • Td= 675.73 min • Td= 844.69 min • Td= 773.20 min •20,183km ; i=55.0◦ • 19,130km ; i=64.8◦ • 23,222km ; i=56.0◦ • 21,528km ; i=56.3◦ • WGS84 • PZ-90.02 • GTRF • CGS-2000 L1 = 1,575.42 MHz G1 = 1,609 MHz E1 = 1,575.420 MHz E2 = 1,561.07MHz L2 = 1,227.60 MHz G2 = 1,252 MHz E6 = 1,278.750 MHz E6 = 1,268.52MHz L5 = 1,176.45 MHz G3 = 1,205 MHz E5 = 1,191.795 MHz E5B=1,207.14 MHz
1 day 8 day 10 day 7 day Navstar/GPS GLONASS Galileo BeiDou Ns
• 1 master control • 1 master control • 2 master control • 1 central station. station (MCS). station (MCS) at station (MCS) in • 3 monitoring • 5 monitor stations Krasnoznamensk, Oberpfaffenhofen stations. monitoring 11 Moscow, Russia. (Bavaria) and Fucino satellites at the same • 9 monitor stations (Italy). time. • Several monitoring - Colorado Springs (MCS) stations. - Hawaii (HAW) - Ascension (ASC) - Diego Garcia (DIG) -Kwajalein (KWJ)
• Monitoring stations receive the GPS signals and transmit the data to the MCS, where parameters are calculated and sent back to the stations, where the data is sent back to the satellites. Satellite signals Modulation methods Composition example
Carrier L1 1575.42 MHz
Code C/A 1.023 MHz
Navigation message 50 bps Code P (Y) 10.23 MHz
Carrier L2 1227.60 MHz PseudoRandom Noise (PRN) codes
Period2n 1 24 1 15 bits
• Code P = 235469592765000 bits ~ 38 weeks. • Code C/A = 1023 bits ~ 1 ms Autocorrelation function Correlation Basic Principle Δρ = c Δt The satellites send an electromagnetic signal and the user has a receiver equipped with a clock.
• very accurate knowledge of satellite positions. • very accurate clocks aboard each satellite.
Electromagnetic signal in vacuum travels 0.3m in 1ns (speed of light c). Δρ
Navigation Message (25 pages) 1 page= 5 subframes = 5x10 word = 5x10x30 bits Bit binary system
Subframe 1 Subframe 2 Subframe 3 Ephemerides
Satellite coordinates X’
* γ = Earth rotation angle; Positioning Positioning Phase measurement Phase measurement Phase measurement Observation Equation Bibliography
• Hoffmann (2008), GNSS – Global Navigation Satellite System. Springer • Michel Capderou (2014), Handbook of Satellite Orbits From Kepler to GPS. Springer. • Hofmann-wellennhof, B., Lichtenegger, H., Collins J. (1992), Global Positioning System (GPS). Theory and practice. Springer. Verlag Wien New York. • Kaplan and Hegarty (2006), Understanding GPS principles and applications. Artech House. • Vallado D.A. (1997), Fundamentals of astrodynamics and applications, MGH.