Atmospheric Structure Experiment (ASE) data sets

Data Set Overview

The DATA directory is divided up into two subdirectories: EDR_DATA and RDR_DATA. In each subdirectory, there will be experiment data records (EDRs) or reduced data records (RDRs) for the Inertial Measurement Unit (IMU) on Phoenix. The location of the IMU within the Phoenix spacecraft with respect to various frames of reference is described in the EDR Software Interface Specification (SIS) with the aid of diagrams.

Data files in the EDR_DATA subdirectory consist of time ordered records of linear velocity changes or 'delta-v' values every 5 milliseconds in three Cartesian axes and angle change or the 'delta-angle' values every 5 milliseconds in three Cartesian axes. Data comes from the IMU measurements during the entry and descent of the Phoenix spacecraft. NASA Level 1A data have been converted to physical units and transformed in space from the local reference frame of the IMU to the mechanical Cartesian frame of the spacecraft described in the EDR SIS.

Data files in the RDR_DATA subdirectory will contain time-ordered altitude, latitude, east longitude, velocity, atmospheric pressure (and uncertainty), atmospheric temperature (and uncertainty), and atmospheric density (and uncertainty).

Parameters

The NASA Level 1A data files in the EDR_DATA subdirectory contain the following measurements:

Measurement	Units
Relative spacecraft event time from start of data acquisition	seconds
Angle change (delta-angle) about the X-axis of the mechanical frame of the spacecraft	radians
Angle change (delta-angle) about the Y-axis of the mechanical frame of the spacecraft	radians
Angle change (delta-angle) about the Z-axis of the mechanical frame of the spacecraft	radians
Velocity change (delta velocity) in the X-axis of the mechanical frame of the spacecraft	m/s
Velocity change (delta velocity) in the Y-axis of the mechanical frame of the spacecraft	m/s
Velocity change (delta velocity) in the Z-axis of the mechanical frame of the spacecraft	m/s

The Level 2 data files in the RDR_DATA subdirectory contain the following data:

Measurement	Units
Spacecraft Clock count	seconds
Relative time from a defined entry state	seconds
The altitude of the spacecraft above the landing site	seconds
Planetocentric latitude of the spacecraft at the time of sample acquisition	degrees
Planetocentric east longitude of the spacecraft at the time of sample acquisition	degrees
Speed of the Phoenix entry probe at the given location	m/s
Derived atmospheric density along the flight path	kg/m**3
Derived atmospheric pressure along the flight path	Pascal
Derived atmospheric temperature along the flight path	Kelvin
The estimated uncertainty of the atmospheric density	kg/m**3
The estimated uncertainty of the atmospheric pressure	Pascal
The estimated uncertainty of the atmospheric temperature	Kelvin

Ancillary quantities are necessary to understand the derivation of RDR data and are also found within this archive volume. The CALIB directory in the DOCUMENT directory will contain a tabulation of three-dimensional force and moment coefficients of the Phoenix spacecraft as a function of atmospheric conditions and spacecraft attitude with respect to the atmosphere-relative velocity vector. INST.CAT contains the entry state of the Phoenix probe, consisting of a position vector and entry attitude at a defined time. INST.CAT also contains the following additional ancillary quantities: the Phoenix entry vehicle mass and its uncertainty: Units = kg The entry vehicle cross-sectional area: Units = m**2

Data

The Phoenix Lander spacecraft had two Inertial Measurement Units (IMUs) that were mounted on the underside of the lander, one as backup only that was never turned on. The IMUs were manufactured by Honeywell (Clearwater, FL) with part number YG9666BC. The IMUs were on the spacecraft as engineering sensors, primarily to assist in Entry, Descent and Landing (EDL). Thus the data were processed and collected for engineering use and their scientific use was not a driver for the selection of the IMU performance characteristics. Consequently, the IMU that collected data was not located close to the entry capsule center of mass or spin axis. Data were output from one IMU and collected at the full instrument rate of 200 Hz. The DN values from the IMU were transmitted back to Earth from the spacecraft and converted to physical values after ground depacketization using factory calibration data. Frames of reference and transformation of the data to the mechanical frame of reference are described in the EDR SIS with the aid of diagrams.

Processing

The 'raw' DN values were stored on the spacecraft at the instrument acquisition rate of 200 Hz. The EDR data presented are those derived from the raw DN transmitted from the spacecraft and processed using the ancillary information and method described in the EDR SIS. The EDR data products are the time-ordered delta-velocity change (the time integral of acceleration) per count in three Cartesian axes and delta-angle change (the time integral of angular rate) per count in three Cartesian axes. The density, pressure and temperature of the atmosphere along the flight path was deduced from this data by methods generally outlined in the RDR SIS.

Data Coverage and Quality

Within the 1 to 93798 data samples recorded every 5 milliseconds in the EDR data set, spurious data was recorded for samples 12031 to 12040 (inclusive) because of a downlink transmission problem. These data points are zero-filled. Because of the 50 millisecond duration of this anomaly, the effect on higher level analysis is likely to be negligible with suitable interpolation in using the data. Another noticeable quality aspect of the EDR data is the comparatively high noise level, which is a function of the way the instrument was selected and configured by spacecraft engineers, as described in INST.CAT.