Recently i came across lot of terms such as IGS, GNSS, GPS,PPP, WAAS, EGNOS, GLONASS, CORS and many more ... I know some of them I can group them such as GPS, GLONASS, GALILEO . And then there is SBAS ...but can any one explain and provide useful links ? What is hierarchy can some draw it ?
GNSS is the generic word for getting a position on Earth based on orbital satellites.
Different countries have developped such spatial systems.
1) USA : GPS (the first one to be operational) 2) Russia : GLONASS 3) Europe : Galileo (not yet operational) 4) China : Beidou
Then you have systems that can be used to improve the precision of the measures by sending additional information to the receivers
1) EGNOS 2) WAAS 3) CORS
And you have methods used to improve the positioning precision
1)DGPS 2)PPP 3)RTK
Although the accepted answer only shortly addresses it, I'd like to provide more detail.
GNSS: Global Navigation Satellite System. This is a satellite constellation that provides a global positioning, navigation and timing (PNT) service. As of 2015 we have GPS (USA), GLONASS (Russia) and in development are Galileo (Europe) and BDS (China, also referred to as BeiDou).
RNSS: Regional Navigation Satellite System. Similar to a GNSS (it is also a satellite constellation) but the satellites only allow for regional positioning (often on top of what GNSS already provides). As of 2015 we have BeiDou (China), and in development are IRNSS (India) and QZSS (Japan).
SBAS: Satellite Based Augmentation System. These are geostationary satellites that submit augmentation signals that provide integrity services to GNSS, and allow for some accuracy improvement. As of 2015 we have WAAS (USA), EGNOS (Europe), MSAS (Japan), GAGAN (India) and SDCM (Russia). Except for SDCM, which augments GLONASS and GPS, all others only augment GPS.
DGNSS: Differential-GNSS. This is a technique by which one or more reference base stations are used to provide corrections to a measurement station so that high accuracy positioning can be obtained. DGPS is often used when you only use the GPS constellation. RTK is real-time kinematic, in which a communication channel is present so you get real-time result (compared to non-RTK methods which only focus on post-processing of the data). A continuous operating reference station (CORS) network is a DGNSS technique.
PPP: Precise-Point Positioning. Although you can obtain similar accuracies as DGNSS, it is not a DGNSS technique as it does not use one or more reference base stations. Instead, it uses correction messages from (for example) services available on the internet (developed for example by the International GNSS Service, IGS).
This is of course just a short summary and many details are omitted. If you like to know more, there is plenty of information online. A good source of information is Navipedia.