Introduction to GNSS


GNSS: Global Navigation Satellite System.  The US GPS was the first system, but an increasing number of systems. are coming online.


System Country  Satellite PRN Numbers seen in software Notes
 Global Positioning System GPS US   1-32  
 Global Navigation Satellite System GLONASS Russia  65-96  
Galileo positioning system European Union     
 BeiDou Navigation Satellite System China  205-210 (what I've seen; don't have specs)  
NAVIC India     
 Quasi-Zenith Satellite System. (QZSS) Japan  193-200 Supplement to other system only for countries near Japan, which insures a satellite will be overhead and minimize multipath effects in urban jungles.


Measure time of travel from satellite.  Make corrections; refaction in ionosphere, lengthening the path, is probably the largest

Trilaterion: intersection of 3 spheres around the satellites.  While there can be more than one intersection point, the correct one should be obvious.



Image from Wikipedia showing that satellite orbits occur in three broad bands.  The orbit height, period, and speed are all related by physics--an orbit requires that the centripetal and gravity forces be equal.

  • Highest are the geostationary, which must be at one specific height above the equator.  They thus rotate at the same speed as the earth, and remail above that point.  They are used for communications, satellite TV, and weather satellites.  They can see only a portion of the earth, and do not have a good view of the high latitudes.   At this height it is hard to get very high resolution imagery, even at nadir.
  • The navigaton satellites all cluster an an period between 10-15 hours.  A geostationary satellite would not work for this application, since all the satellites would be coplanar.  The lower set of orbits, with periods of 1-2 hours, and a time visible from a single point of only about 10 minutes, would not be in view long enough to provide good operations, and would require way too many satellites since the view from one satellite would not cover much area.
  • No satelliltes in the Van Allen Radiation Belt.
  • The remote sensing satellite like Landsat, SPOT, Aqua, Terra, and the polar orbitting weather satellites all have orbites very close to the earth at this scale.  The image shows only Hubble, Iridium, and the ISS, but the others all have orbits with similar characteristics.


GPS Signal Errors Sources (from Garmin)

 GPS signal accuracy can be affected by:




  • In Urban areas, buildings can block the direct signal.   The signal recieved will have travelled a longer path, with one or more reflections. The GPS unit will place the position too far away from the satellite.  Depending on the signals from the other satellites being used, the computed postion will be incorrect.

Improving GPS Positions

Mulitple Latitudes 


Good reference, for a course at Penn State:

 Nov 2016 article

Last revision 9/30/2018