The Russians are operating their Tu-154M aircraft configured for surveillance flights sanctioned under the Open Skies Treaty that allows member countries to conduct surveillance flights over each other's territory relatively unimpeded. The aircraft are equipped with imaging equipment with specific limitations and monitors from the country being surveilled are onboard the flights to make certain the party complies with the parameters of the treaty. This latest series of Russian Open Skies flights are being conducted out of Great Falls, Montana and are covering a slew of strategic points in the western part of the United States, including the highly secure Nellis Test and Training Range (NTTR) in southern Nevada, home of Area 51.
The mid-day flight on March 28th, 2019 appears to have originated out of Travis AFB, located near San Francisco, and continued on something of a highlights tour of American military installations in California and Nevada. It flew south over central California, passing near bases like Naval Air Station Lemoore and headed out over the Channel Islands. It then headed directly over Edwards AFB before meandering around Fort Irwin and on to Naval Air Weapons Station China Lake before hooking a right and heading toward Creech AFB in Nevada. It then headed north, directly into the NTTR—the most secure airspace in the United States along with Washington, D.C.
After it flew over Creech AFB, it headed up to Yucca Flat, where one of America's nuclear weapons assembly plants is located, and a secretive airstrip that specializes in test flights of unmanned aircraft, as well as other sensitive Department Of Energy installations. It then headed over the pockmarked Nevada Test Site. Area 51 sits just to the east of this location. The aircraft's panoramic cameras can collect fairly wide swathes of imagery along the Open Skies aircraft's flight path.