Great Circle Distance Calculator (GRDC)
Examples:
- KATL KJFK (Atlanta to JFK)
- KATL KSFO XX KSJC KATL (Atlanta to San Francisco - returning San Jose to Atlanta)
- RT KJFK EDDF JFK International Airport to Frankfurt International Airport
- RT JFK FRA JFK International Airport to Frankfurt International Airport
- ATL SFO XX SJC ATL (Atlanta to San Francisco - returning San Jose to Atlanta)