An advance party of the USAF 6010th Tactical Group arrived at Don Muang Royal Thai Air Force Base at the request of the Thai government to establish an aircraft warning system.[18] On 20 April six F-100 Super Sabres from the 510th Tactical Fighter Squadron based at Clark Air Base deployed to Don Muang in Operation Bell Tone.[19] On 17 April USAF pilots began flying an RT-33 on reconnaissance missions over Laos under the code-name Operation Field Goal.