Motus Reference Manual
Page 12 of 146
Version 1.1
29/11/2019
5
Foundation Knowledge
This chapter is a learning reference that briefly covers knowledge essential to
understanding Motus and the following chapters. It explains the concepts in simple
terms so that people unfamiliar with the technology may understand it.
5.1
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 BeiDou is in the process of becoming operational and the European
Union’s GALILEO should be operational within five years.
GNSS is excellent for navigational purposes and provides fairly accurate position (2.5
metres) and velocity (0.05 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.
5.2
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.
5.3
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.
5.4
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.
