Accelerometers and gyroscopes are sourceless inertial
sensors, usually part of hybrid tracking systems, that do not
require prepared environments. Timed measurements can
provide a practical dead-reckoning method to estimate position
when combined with accurate heading information. To
minimise errors due to drift, the estimates must periodically
be updated with accurate measurements. The act of taking a
step can also be measured, i.e. they can function as pedometers.
Currently micro-electromechanical (MEM) accelerometers
and gyroscopes are already making their way
into mobile phones to allow „writing‟ of phone numbers in
the air and other gesture-based interaction