Atmosphere Structure Instrument/Meterology Package
The ASI/MET is an engineering subsystem which acquires atmospheric
information during the descent of the lander through the atmosphere and during the entire
landed mission. It is implemented by JPL as a facility experiment, taking advantage of the
heritage provided by the Viking mission experiments.
Data acquired during the entry and descent of the lander permits the
reconstruction of profiles of atmospheric density, temperature and pressure from altitudes
in excess of 100 km to the surface.
The accelerometer portion of the ASI is provided by the Attitude And
Information Management subsystem of the lander. It consists of x-, y- and z-axis sensors.
Several gain states are provided to cover the wide dynamic range from the micro-g
accelerations experienced upon entering the atmosphere to the peak deceleration and
landing events in the range of 30 to 50 g's.
The ASI/MET instrument hardware consists of a set of temperature,
pressure and wind sensors and an electronics board for operating the sensors and
digitizing their output signals. Temperature is measured by thin wire thermocouples
mounted on a meteorological mast that is deployed after landing. The location of one
thermocouple is chosed to measure atmospheric temperature during descent, and three more
monitor atmospheric temperatures 25, 50, and 100 cm above the surface during the landed
mission. Pressure is measured by a Tavis magnetic reluctance diaphragm sensor similar to
that used by Viking, both during descent and after landing. The wind sensor employs six
hot wire elements distributed uniformly around the top of the mast. Wind speed and
direction 100 cm above the surface are derived from the temperatures of these elements.
ASI/MET Temperature And Wind Sensors
During entry and descent, the sampling of acceleration, pressure and
temperature data is optimized to the vertical rate of decent through the atmosphere. After
landing, considerable flexibility in data averaging and sampling is provided to measure
the variation of temperature, surface pressure and wind on short, diurnal, and seasonal
time scales over the life of the mission.
Three wind socks are located at various heights on the meterology mast
to determine the speed and direction of winds at the Pathfinder landing site. The wind
socks will be imaged repeatedly by the IMP. The orientations of the wind socks will be
measured in the images to determine the wind velocity at three heights above the surface.
This information can then be used to estimate the aerodynamic roughness of the surface in
the vicinity of the lander, and to determine the variation in wind speed with height.
Because the Viking landers had wind sensors at only one
height, such a vertical wind profile has never been measured on Mars. This new knowledge
will help to develop and modify theories for how dust and sand particles are lifted into
the martian atmosphere by winds, for example. Because erosion and deposition of wind-blown
materials has been such an important geologic process on the surface
of Mars, the results of the wind sock experiment will be of interest to geologists as
well as atmospheric scientists.