Topcon America Corporation 090531 Manuale Utente

HiPerII Chapter 1 

 
Currently, the following two global navigation satellite systems ( GNSS ) offer line-of-site radio 
navigation and positioning, velocity, and time services on a global, all-weather scale to any user 
equipped with a GNSS tracking receiver on or near the Earth's surface: 

  GPS - the Global Positioning System maintained and operated by the United States 

Department of Defense.  For information on the status of this system, visit the US Naval 
Observatory website ( http://tycho.usno.navy.mil/ ) or the US Coast Guard website 
( http://www.navcen.uscg.gov/ ). 

  GLONASS - the Global Navigation Satellite System maintained and operated by the Russian 

Federation Ministry of Defense.  For information on the status of this system, visit the 
Coordinational Scientific In-formation Center website ( http://www.glonass-ianc.rsa.ru/ ). 

Despite numerous technical differences in the implementation of these systems, satellite 
positioning systems have three essential components: 

  Space - GPS and GLONASS satellites orbit approximately 12,000 nautical miles above Earth 

and are equipped with a clock and radio.  These satellites broadcast ranging signals and 
various digital information ( ephemerides, almanacs, time and frequency corrections, and 
so forth ). 

  Control - Ground stations located around the Earth that monitor the satellites and upload 

data, including clock corrections and new ephemerides ( satellite positions as a function of 
time ), to ensure the satellites transmit data properly. 

  User - The community and military that use GNSS receivers to calculate positions. 

 

 
When calculating an absolute position, a stationary or moving receiver determines its 
three-dimensional position with respect to the origin of an Earth-Center Earth-Fixed coordinate 
system.    To calculate this position, the receiver measures the distance ( called pseudo-ranges ) 
between it and at least four satellites. 
The measured pseudo- ranges are corrected for clock differences ( receiver and satellites ) and 
signal propagation delays due to atmospheric effects.  The positions of the satellites are 
computed from the ephemeris data transmitted to the receiver in navigation messages.    When 
using a single satellite system, the minimum number of satellites needed to compute a position 
is four.  In a mixed satellite scenario ( GPS, GLONASS ), the receiver must lock onto five or 
more satellites to account for the different time scales used in these systems and to obtain an 
absolute position.