Foundation Knowledge; Gnss; Ins; Gnss/Ins - Advanced Navigation Spatial OEM Reference Manual

2

Foundation Knowledge

This chapter is a learning reference that briefly covers knowledge essential to understanding
Spatial and the following chapters. It explains the concepts in simple terms so that people
unfamiliar with the technology may understand it.
2.1

GNSS

GNSS stands for global navigation satellite system. A GNSS consists of a number of satellites in
space that broadcast navigation signals. These navigation signals can be picked up by a GNSS
receiver on the earth to determine that receiver's position and velocity. For a long time the only
operational GNSS was the United States GPS. However the Russian GLONASS is now fully
operational with similar performance to GPS. The Chinese COMPASS is in the process of
becoming operational and the European Union's GALILEO should be operational within ten years.
GNSS is excellent for navigational purposes and provides fairly accurate position (2.5 metres) and
velocity (0.03 metres/second). The main drawback of GNSS is that the receiver must have a clear
signal from at least 4 satellites to function. GNSS satellite signals are very weak and struggle to
penetrate through buildings and other objects obstructing view of the sky. GNSS can also
occasionally drop out due to disturbances in the upper atmosphere.
2.2

INS

INS stands for inertial navigation system. An inertial navigation system can provide position and
velocity similar to GNSS but with some big differences. The principle of inertial navigation is the
measurement of acceleration. This acceleration is then integrated into velocity. The velocity is then
integrated into position. Due to noise in the measurement and the compounding of that noise
through the integration, inertial navigation has an error that increases exponentially over time.
Inertial navigation systems have a very low relative error over short time periods but over long time
periods the error can increase dramatically.
2.3

GNSS/INS

By combining GNSS and INS together in a mathematical algorithm, it is possible to take advantage
of the benefits of GNSS long-term accuracy and INS short-term accuracy. This provides an overall
enhanced position and velocity solution that can withstand short GNSS drop outs.
2.4

AHRS

AHRS stands for attitude and heading reference system. An AHRS uses accelerometers,
gyroscopes and magnetometers combined in a mathematical algorithm to provide orientation.
Orientation consists of the three body angles roll, pitch and heading.
2.5

The Sensor Co-ordinate Frame

Inertial sensors have 3 different axes: X, Y and Z and these determine the directions around which
angles and accelerations are measured. It is very important to align the axes correctly in
installation otherwise the system won't work correctly. These axes are marked on the top of the
device as shown in Illustration 1 below with the X axis pointing in the direction of the connectors,
the Z axis pointing down through the base of the unit and the Y axis pointing off to the right.
Spatial OEM Reference Manual
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Version 2.7