What is the main purpose of ADS-B? How do aircrafts interact with each other using them?
Is it intended to replace the Traffic Collision Avoidance System (TCAS) or ground based Air Traffic Control (ATC)?
Also, why they need the Global Navigation Satellite System (GNSS)? Are there other systems that has a dependency relationship with ADS-B?
For a short introduction on what ADS-B is and why it is called dependent see this answer.
ADS-B is a surveillance technology whereby the aircraft broadcasts information about its identity, position, altitude and velocity of the aircraft to any interesting party. The position and velocity information is derived from GNSS (GPS).
TCAS
ADS-B is not intended to replace TCAS, although in the future it will augment TCAS. The TCAS algorithm currently only uses distance and altitude to calculate whether there is a conflict and to determine the best conflict resolution strategy. With the more accurate ADS-B position available to the system as well, the number of TCAS interrogation can be reduced (the surveillance radio frequencies are getting congested in some areas) and the performance of TCAS can be improved. A new standard for this hybrid approach was published in 2013.
It may also be possible to have a passive TCAS-like system that does not require active interrogation but is purely depending on ADS-B. Currently a technical standard for a new Airborne Collision Avoidance System (ACAS, the generic name for TCAS) is being developed in a joint RTCA / EUROCAE committee (RTCA SC-147 / EUROCAE WG-75), which will take advantage of more data offered by ADS-B. This new standard will eventually replace TCAS II.
A 'TCAS light' for small aviation has also been developed based on ADS-B. It is called Traffic Situation Awareness with Alerts (TSAA), a standard that was published last year. Within the next couple of years we will see manufacturers start offering products based on this standard.
ATC
The purpose of ADS-B is neither to replace ground based ATC. But it will change the way ATC is done. ADS-B IN will improve the situation awareness of pilots; they will have a display of accurate positions of other aircraft. New procedures will allow pilots to maintain visual separation to other aircraft in marginal Visual Meteorological Conditions (VMC), where they currently would often lose sight of other traffic. A more advanced usage of ADS-B will be flight deck based interval management (FIM) where ATC will be able to instruct aircraft to 'follow that plane XX seconds behind for landing on runway YY'. A first technical standard for FIM is being worked on, it is intended to be published before the end of this year.
ADS-B is neither a replacement for all radars, although it will allow the number of radars to be reduced. For remote areas that currently do not have radar coverage because of the associated high costs, ADS-B will be a cost effective alternative. Over the oceans, satellite ADS-B receivers will be able to provide surveillance, changing the way ATC is performed over the high seas.
GNSS
Given the examples above, it is easy to understand the requirements on the reliability of ADS-B are substantial. Not only must the system be able to deliver accurate position data, it must be able to deliver that data with high integrity. That means, the probability that the system transmits inaccurate position data, even when there is a failure in the system, must be extremely small. To achieve this, a positioning systems must be able to do internal cross checks to detect failures and not depend on possible erroneous inputs from humans. GPS can meet the position source requirements using Receiver Autonomous Integrity Monitoring (RAIM).
The ADS-B technical specifications have all been written in such a way that they do not explicitly require GNSS to be the position source, but at the moment it is the only system that meets the requirement effectively. Most certification documents are written with the assumption that a GNSS will be used to provide position data. Theoretically one could put another system forward, but the costs of certifying that would be very high.
ADS-B will eventually replace transponder+secondary radar as the primary means of separation. Transponders+secondary radar will be kept for a long time as a backup. Right now the authorities are saying it won't replace it, because they don't want the pressure to stop having to keep transponders in the long run. ADS-B is a much more sophisticated way to do aircraft separation.
In order to know how far another aircraft is from you, you need to interrogate it. Send out a pulse and time the response. Measure the general direction the response came from. The response contains the altitude of the contact. This allows ATC and TCAS to know where the traffic is (position and altitude). ATC radar compares this information over time to estimate course and speed. ADS-B sends all this information automatically every second. GPS coordinates, altitude, forward speed, vertical speed, course, heading, and desired course/altitude (when its changing). This is leaps and bounds better than transponder+tcas/secondary radar. Also its new technology. We need at least a decade fully operating it worldwide before we can talk about retiring secondary radar and transponders. In my view its not a matter of IF, but WHEN its going to happen. My credentials ? I'm an IFR rated private pilot, a computer technology professional, and a telecommunications specialist with just enough experience in every piece of the operational/technology puzzle that makes the whole thing work. The key fact is that airliners transmit ADS-B over the same frequency as transponder responses flow through. Eventually transponder responses will overwhelm the bandwidth needed for ADS-B traffic, making ADS-B unsafe. At this point ATC will be forced to create large transponder off zones. That will be the beginning of the end of transponders. With ADS-B another aircraft in the area is just one extra message per second. With transponder another aircraft in the area potentially adds another X messages per second where X = number of interrogators asking for a transponder response (including ATC and TCAS systems).
