Atmospheric Structure Experiment

The frictional drag of the atmosphere upon the entry vehicle results in a reduction in the speed of the entry vehicle. This deceleration depends on atmospheric density and can be used to calculate atmospheric structure, including how the atmospheric temperature changes with altitude along the flight path.

The Phoenix Lander had two Inertial Measurement Units (IMUs) that were mounted on the underside of the lander. The IMUs were manufactured by

Honeywell (Clearwater, FL) with model number YG9666BC. No input from scientists was possible on the device selection and location within the lander, which was done by spacecraft engineers for the cancelled 2001 Mars Lander prior to the initiation of the Phoenix project. Consequently, the IMUs were not located at the entry vehicle’s center of mass. The Atmospheric Structure Experiment (ASE) is concerned with optimizing use of the IMU data to calculate the atmospheric structure during entry despite the non-optimum IMU location and performance characteristics such as noise.

On Phoenix, only one IMU returned data while the other was not turned on and merely used as mission-critical backup for entry, descent and landing. The operational IMU gave output of accumulated linear velocity change (the time integral of acceleration) in three Cartesian axes and accumulated angle change (the time integral of angular rate) in three Cartesian axes. These outputs were internally compensated for biases, scale factors and alignments based on coefficients determined by factory calibration tests. For deriving atmospheric structure, linear velocity changes can be numerically differentiated w.r.t.

time in order to derive linear accelerations. Angle changes can be numerically differentiated w.r.t. time in order to derive angular velocities.

Full details of the instrument are described in a PDS catalog file, INST.CAT.

Source: Atmospheric Structure Experiment EDR SIS