In order to provide location information for indoor
applications and context-aware computing, a lot of research is
being done since last decade for development of real-time Indoor
location system. In this paper, we have investigated indoor
location concepts and have focused two major technologies used
in many indoor location systems i.e. RF and ultrasonic. An
overview of various RF systems that use different RF properties
for location estimation has been given. Ultrasonic systems have
been reviewed in detail as they provide low cost fine grained
location systems. A few well known ultrasonic location systems
have been investigated with a comparison of the system based on
performance, accuracy and limitations.