Three years after NASA's Viking spacecraft landed on Mars scientists at Jet Propulsion Laboratory are still receiving data from one orbiter and two landers.

       The Viking mission has gone into fourth major stage, called the Survey Mission, and the first data from that period has been received at JPL. The Survey Mission is scheduled to continue through 1990, more than 15 years after the spacecraft were launched.

       Both Viking landers and the remaining orbiter are operating. The landers have been placed in an automatic condition that allows them to function unattended; Lander 1 transmits its information to Earth once week.

       Viking Orbiter 1 is taking high-resolution pictures of the Martian surface with clarity not obtained before, since the Martian atmosphere has become unusually clear. (Viking Orbiter 2 was shut down July 24, 1978, after it ran out of attitude-control gas; Viking Orbiter 1 is expected to cease operations sometime in 1980.)

       Landers 1 and 2 continue to take pictures and to collect weather data.

       Lander 2's cameras have revealed new layer of water frost on the Martian surface at the Utopia Plains landing site. It is Martian winter again, and thin layer of frost can easily be seen in the photos.

       The new frost layer poses scientific puzzle to members of the Viking team: In September 1977, Viking Lander 2 found frost on the surface during the Martian northern winter. (That was one Martian year or almost two Earth years ago.) Scientists associated that frost collection with major dust storm that had obscured the planet's surface before and during that period.

       But recent observations have shown no dust storms on Mars this year -- in fact, the atmosphere is clearer than scientists have seen it since Viking arrived in 1976. So no one is certain just what triggers the appearance of frost.

       This much is believed: Dust particles in the atmosphere pick up bits of solid water (ice). That combination is not heavy enough to settle to the ground. But carbon dioxide, which makes up 95 per cent of the Martian atmosphere, freezes and adheres to the particles and they become heavy enough to sink. Warmed by the Sun, the surface evaporates the carbon dioxide and returns it to bhe atmosphere, leaving behind the water and dust. The resulting frost layer may be only onethousandth of an inch thick.

       Viking 1 was launched Aug. 20. l975, and arrived in Mars orbit June l9, 1976. Viking Lander 1 touched down on the Chryse Plains July 20, 1976.

       Viking 2 was launched Sept. 9, 1975. It reached Mars Aug. 7, 1976, and Lander 2 dropped to the surface Sept. 3, 1976. The planned lifetime for the spacecraft was 90 days after landing.

       Viking is managed and controlled for NASA's Office of Space Science by the Jet Propulsion Laboratory, California Institute of Technology. Kermit Watkins is Viking project manager; Dr. Conway Snyder is Viking project scientist.