In United States aviation, a terminal radar service area (TRSA) is a delimited airspace in which radar and air traffic control services are made available to pilots flying under instrument flight rules or (optionally) visual flight rules for the purposes of maintaining aircraft separation.
TRSAs are most often encountered surrounding busy U.S. airports. In recent years many of them have gradually been replaced by Class B or Class C airspace.
Terminal Radar Service Area was established as part of a program to create Terminal Radar stations at selected airports. Because these were never subject to the rulemaking process of 14 CFR Part 91, they do not actually fit into any of the existing U.S. classifications of airspace and have been classified as non-part 71 airspaces. While operating in these airspaces, a pilot who chooses to participate will receive radar services, but participation is not required.
TRSAs will encompass a primary airport with a class "D" designation and the TRSA will be above other controlled airspace (Typically Class E Airspace) with a typical floor of 700 feet or 1,200 feet AGL (Above Ground Level).
TRSAs are shown on VFR (Visual Flight Rules) "Sectional" charts as a solid black/gray (Depending on the print quality of your map). See Gulfport, Biloxi (Mississippi) for an example.
Sectional chart example of a TRSA surrounding Tom Ridge International at Erie, Pennsylvania FAA: Section 5. Other Airspace Areas Federal Aviation Regulations/Aeronautical Information Manual 2010(FAR/AIM) Section 4-1-18