Customer Service Center

• SimCHAN

The accuracy of GPS height measurements depends on several factors but the most crucial one is the "imperfection" of the earth's shape.

• Height can be measured in two ways.
  • The GPS uses height (h) above the reference ellipsoid that approximates the earth's surface.
  • The traditional, orthometric height (H) is the height above an imaginary surface called the geoid, which is determined by the earth's gravity and approximated by MSL.

• The signed difference between the two heights , the difference between the ellipsoid and geoid  is the geoid height (N). The figure above shows the relationships between the different models and explains the reasons why the two hardly ever match spatially.
• Therefore  ellipsoid height = MSL  height + geoid height
• The GPS receiver uses a theoretical sea level estimated by a World Geodetic System (WGS84) ellipsoid, which does not perfectly follow the theoretical MSL. The MSL, approximated by an ellipsoid, is related to gravity or the center of mass of the earth. Discrepancies between a WGS84 ellipsoid, and the geoid vary with location.
• Spirent SimCHAN software sets the ellipsoidal height but NMEA reports the geiodal height. This difference is normal, and is OK as long as you understand the expected difference. SimGEN software shows the height  using a Geoid reference, If a 0m height (elipsoid) is entered as the reference height a – (negative) height is displayed  (geoid).