 PCAS is passive and less expensive than active aircraft detection systems, such as TCAS.TCAS operates with more precision than PCAS but is also more expensive and usually requires 'permanent' in-aircraft installation (requiring, in the United States, an FAA-approved mechanic to install).Class 2 TCAS gives mandatory instructions (called Resolution Advisories) whereas PCAS only alerts the pilot and may give a suggestion as to how to act.