Atmospheric Structure Instrument / Meteorology Package
Instrument Description

Table of Contents

Instrument Overview

The Atmospheric Structure Instrument and Meteorology Package was an engineering subsystem of the Mars Pathfinder spacecraft which provided data for scientific analysis. It was implemented as a facility experiment in an in-house mode at NASA's Jet Propulsion Laboratory, taking advantage of the heritage provided by the Viking mission.

ASI/MET data were collected during different phases of the mission by accelerometers and wind, temperature, and pressure sensors mounted in various places on the spacecraft.

Data acquired during the entry, descent, and landing (EDL) phase of the mission permitted the reconstruction of profiles of atmospheric density, temperature and pressure from altitudes in excess of 100 km to the surface. After the landing, day to day variations in temperature, pressure, and wind speed were monitored.

Scientific Objectives

The Mars Pathfinder mission has provided a third set of in-situ measurements of the Martian atmosphere, after the two Viking Landers 20 years ago. The ASI/MET package was designed to collect density, temperature, and pressure information about the structure of the Martian atmosphere during the spacecraft's passage through the atmosphere, from about 150 km altitude down to the surface. Once on the surface, it was also designed to regularly monitor the temperature, pressure, and wind speed and direction at the landing site for the duration of the mission.

Combined with information collected from the Imager for Mars Pathfinder about atmospheric aerosols and dust, the amount of water vapor at the landing site, and wind information derived from observation of the MET mast-mounted wind socks, these data should contribute to a better understanding of the overall structure of the Martian atmosphere, and its variation over time. A further comparison with data collected by the Viking Lander spacecraft can also reveal information about the longer-term evolution of the atmosphere.

For more information about the scientific objectives of the ASI/MET investigation, please see [SEIFFETAL1997].

Location

The lander is located in an ancient flood plain in the Ares Vallis region of Chryse Planitia at 19.17°N, 33.21°W with respect to the U.S. Geological Survey cartographic network. The landing occurred on July 4, 1997, at which time it was late northern summer (Ls = 143°) on Mars. See [GOLOMBEKETAL1997B] for further details.

Subsystems, Detectors, and Platform Mounting Descriptions

The ASI/MET experiment is performed by accelerometer and meteorology (MET) instruments using hardware distributed throughout the Pathfinder lander. The accelerometer instrument consists of three science and three engineering accelerometers, with supporting electronics, mounted on two boards in the Pathfinder Integrated Electronics Module (IEM). The MET instrument consists of pressure, temperature and wind sensors and a single IEM electronics board. The pressure sensor is mounted outside the IEM, but within the lander thermal enclosure and is connected to an aperture in the lander vehicle via a pitôt tube. All the temperature sensors and the wind sensor are mounted on a meteorological mast 1 cm in diameter and 1.1 m high, which is deployed after landing. The mast base is located on the end of a lander petal to minimize the thermal contamination of temperature and wind measurements by the spacecraft. In addition to pressure, temperature, and wind sensor electronics, the MET board also carries the signal processing electronics for the temperature sensors of the Aeroshell Instrumentation Package (AIP).

Accelerometers

All six science and engineering accelerometers are Allied Signal QA-3000-003 units. The three science accelerometers are mutually orthogonal and are oriented parallel to the lander X, Y, and Z axes, whereas the three orthogonal engineering accelerometers are oriented in the X direction and at ±45° to the Z-axis (+YZ and -YZ). Because of limitations imposed by the Pathfinder system design, some accelerometers are displaced significantly from the entry vehicle center of mass.

The primary function of the engineering accelerometers is to monitor and contribute to the timing of important events during entry and descent, such as parachute deployment. They are also used after landing to right the spacecraft and point the lander high gain antenna accurately. Science accelerometer observations focus on measuring atmospheric accelerations throughout entry and descent with maximum resolution, although the optimum measurement strategy has been modified to provide redundancy for the engineering accelerometers. Data from all six accelerometers are sampled at the same rate and will be available to ASI/MET investigators.

Pressure Sensor

The ASI/MET pressure sensor is a Tavis variable reluctance diaphragm device similar to the pressure sensors flown on the Viking mission. This device has proven long-term stability and reliability over four martian years of operation. It is designed to make measurements of dynamic pressure during entry and static pressure on the surface, and is connected to the outside world by a pitôt tube approximately 1 m long and 2 mm inside diameter. Its response time is expected to vary from 1.5 seconds at 3 mbar to 0.45 seconds at 10 mbar. The tube entry port lies in the plane of the aperture between the lander instrument shelf and two petals, and is oriented perpendicular to the anticipated airflow during descent. Because no objects were allowed to extend beyond the lander profile during descent the entry port location is not ideal.

Temperature Sensors

Atmospheric temperatures during the descent and landed phases of the mission are measured by fine wire chromel-constantan thermocouple sensors mounted on the mast. It consists of three thermocouples wired in parallel in a fiberglass support structure. The thermocouple wires are 75 um in diameter and the junction diameter is approximately 200 um. The fine wires provide conductive isolation for the junctions, and ensure that they reach equilibrium with the tenuous martian atmosphere fairly rapidly, although radiative corrections will have to be modeled to obtain the best results from these sensors. Three wires provide redundancy against damage, either during descent, or due to dust storms during the landed mission.

A single thermocouple is assigned to temperature measurements during the descent phase of the mission, when the entry vehicle is falling by parachute after the heat shield has been jettisoned. The descent sensor is located near the top of the mast, immediately below the wind sensor. Its thermocouple wires are approximately 4.0 cm from the mast axis and are oriented perpendicularly to it. During descent the mast is stowed and the sensor is close to the center of an aperture in the corner of the lander, although it is set back significantly due to concerns over interference with the lander airbags. Accurate descent temperature measurements rely on undisturbed air flowing through the aperture and over the descent sensor. Wind tunnel measurements on models have shown that this flow is highly disturbed, so that descent temperature measurements are likely to be unreliable. Currently a flexible duct linking the external flow to the sensor is planned to improve the quality of the data.

Three thermocouples are designed for temperature measurements during the landed phase of the mission when the mast is deployed. These sensors are located on the mast 0.273, 0.508 and 1.038 m above the plane of the petal. For each sensor, the thermocouple wires are oriented vertically approximately 2.6 cm from the mast axis. All three sensors and mounts are identical and are clocked at the same angle on the mast. Temperature measurements are subject to thermal contamination when the mast is downwind from the spacecraft or the sensor wires are downwind from the mast. The sensors are therefore oriented away from the spacecraft when the mast is deployed so that these conditions coincide, minimizing the range of unfavorable wind directions. The orientation on Mars is 130° east of north (pointing roughly southeast).

Wind Sensor

ASI/MET uses hot wire resistance thermometers to measure wind speed and direction. The sensor consists of six 50 um diameter platinum- iridium wire elements arranged symmetrically around a cylindrical core approximately 2.6 cm in diameter. Each element consists of a 20 cm wire wound around insulating formers to give eight closely spaced lengths oriented parallel to the axis of the mast roughly 3 mm from the core. The sensor is mounted at the top of the mast, at a height of 1.096 m above the lander petals. All six sensor elements are connected in series and are heated by a constant current source. In still air at typical martian surface pressures, the elements are heated to approximately 40°C above ambient. Wind blowing round the core of the sensor cools the elements, and the cooling produced at an individual element depends on its position relative to the core and the wind direction. Overall cooling is dependent on wind speed and the pattern of cooling for the six elements is a indicator of wind direction. Sensitivity is greatest for low winds and varies roughly inversely with wind speed.

A single chromel-constantan thermocouple junction is mounted within the core of the wind sensor. Its function is to provide a temperature boundary condition measurement needed to interpret the wind measurements.

Wind Socks

Also mounted on the MET mast were three aluminum wind socks, used to determine wind speed and direction. Since data collection from these wind socks was done with the Imager for Mars Pathfinder, they were not technically considered to be part of the ASI/MET package. A separate instrument description exists for them, under the name Mars Pathfinder IMP Windsocks. (A copy of that document, WINDINST.CAT, is included on the ASI/MET CD.)

Operational Modes and Measured Parameters

The sampling frequency of all the sensors during the entry and descent portion of the mission was governed by the vertical rate of descent through the atmosphere. After landing, pressure and temperature measurements were made to establish the diurnal variations and the day-to-day variations over the operating life of the mission. Details are provided in the individual data set descriptions.

Accelerometers

The accelerometers had several gain states to cover the wide dynamic range from the micro-g accelerations experienced upon entering the atmosphere to the 18g peak experienced during deceleration and landing. The gain states provided dynamic ranges of 16 mg, 800 mg, and 40 g. (The accelerometer calibration was done relative to the gravitational force on a concrete plinth in building 150 at the Jet Propulsion Laboratory. The acceleration due to gravity at this location (1 g) is equal to 9.795433 m/s**2.) With 14 bit digitization of the signals, these ranges had respective resolutions of 2 micro-g, 100 micro-g, and 5 mg. The noise levels of each gain state have been measured at 1 to 2 counts or less, implying that accelerations of 10 micro-g should be significant with 20% uncertainty. Low pass filters in the accelerometer electronics were used to attenuate signal frequencies above 5 Hz. This was done to suppress the effects of noise and spacecraft dynamic motion.

Pressure Sensor

The pressure sensor obtains data in two ranges simultaneously; 0 - 12 mbar for descent and 6 - 10 mbar for surface observations. Signal digitization to 14 bits provides respectively 0.75 ubar and 0.25 ubar resolution over these ranges. Again, system noise levels were measured at 1 to 2 counts or less. As Tavis pressure sensor measurements are temperature dependent, the temperature of the device is monitored in flight by an accurate platinum resistance thermometer (PRT). The PRT covers the 200-335 K temperature range with a resolution of 0.01K.

Temperature Sensors

All ASI/MET thermocouples are referenced to 'cold' junctions mounted within a small isothermal block located inside the lander petal close to the base of the mast. The temperature of the block is monitored by an accurate PRT with a range of 140 to 330 K and a resolution of 0.01 K. Each thermocouple has a dynamic range of roughly ±100 K about the PRT temperature, and a resolution of 0.01 K. On the surface of Mars, the thermocouple junction time constant is of order 4-5 seconds.

Wind Sensor

The wind sensor has two operating modes, high and low power. In the high power mode, a current of 51.5 mA flows continuously through the sensor producing the overheat required for wind measurements. In the low power mode, 20.6 mA flows for 3 msecs whilst element temperature is being sampled. Low power produces insignificant wire heating and provides an accurate measure of ambient temperature at the sensor. The resolution of the wind sensor element temperature measurement is 0.11 Kelvin for the low power and 0.04 Kelvin for the high power setting. Under martian conditions, the sensor responds to power changes with a short term time constant of 1-2 seconds.

Calibration

All the ASI/MET sensors have been subjected to a series of calibration measurements. In many cases these are satisfactory but in some cases resource and schedule limitations in the hardware development effort left significant gaps. Preliminary analysis of the calibration data has been completed, but some work remains to be done by ASI/MET science team members. A summary of the measurements is given below.

Accelerometers

Accelerometer calibration measurements can be divided into two categories; end-to-end response calibration for each sensor, and alignment and location calibration relative to the lander spacecraft coordinate system.

End-to-end response measurements using the assembled flight accelerometer board on a precision dividing head have established the linearity, gain and offset of all six accelerometers, at each gain setting over a temperature range of -5°C to +40°C. All the sensors are highly linear, and gains and offsets vary linearly with temperature. Calibration measurements are limited to 1g on the 40g gain setting, and have not covered the full temperature range expected for the IEM electronics, so that some extrapolation is required. These measurements have also established the relative orientation of the accelerometer heads in the flight board coordinate system to a few hundredths of a degree.

Further optical measurements on the spacecraft in its cruise configuration have established the shape of the heat shield and the orientation of its symmetry axis in the spacecraft coordinate system with similar accuracy. The relative orientation of these two coordinate systems will not be measured, but they are specified to be coincident to better than 0.25° and are expected to be considerably better in practice. During the spin balancing of the spacecraft, the location of each head relative to the entry vehicle C of M will be determined to about 2 mm in the X & Y directions. In the Z direction the location of the C of M will not be measured and will be uncertain to approximately 2.5 cm.

Detailed data conversion expressions used to calibrate the accelerometer data are provided in [SCHOFIELD1996A] and [SCHOFIELD1997A].

Pressure Sensor

The calibration of the ASI/MET pressure sensor used in [SCHOFIELDETAL1997] has been revised significantly on this CD-ROM, for the reasons described below.

The Tavis pressure sensors used by ASI/MET are sensitive to temperature and must be calibrated to correct for this dependence. During pre-launch calibration, both the flight and flight spare pressure sensors were inadvertently exposed to temperatures 30 K below their design limits. This produced changes in sensor offset of several tenths of a mbar, and increased the variation of offset with temperature by a factor of 7 relative to the original calibration. These changes were not noticed until spacecraft thermal vacuum testing and by the time they were understood, it was too late to procure new sensors. It was therefore decided to fly the flight sensor and use cruise phase health check measurements to calibrate the temperature dependence of offset.

As sensor gain calibration was not possible during the mission, it was assumed the gain had not changed. Limited pre-flight measurements and more extensive calibration of the flight spare sensor suggested that the effects of gain changes on pressure measurements at 7 mbar were 6-8 times smaller than those of offset changes.

During the nominal landed mission, the ASI/MET pressure sensor experienced diurnal temperature variations of 265-300 K. These introduced large, spurious, components to the diurnal pressure cycle, due to the temperature variation of sensor offset, which had to be removed using calibration data. In cruise, it was only possible to calibrate flight sensor pressure offset in the 270-280 K temperature range. These data indicated a temperature dependence of -0.019 mbar/K. This was the temperature dependence assumed in [SCHOFIELDETAL1997].

However, after the mission, the offset calibration was revisited. It was found that the cruise offset measurements were influenced by long term changes (over 6 months) as well as temperature dependent changes. Offset measurements over the 270-280 K range made during the 90 minutes before entry eliminated the long term drift and indicated a temperature dependence of -0.0135 mbar/K - a smaller dependence than the cruise offset measurements.

Late in the landed mission when the spacecraft was off overnight and took data only during the day, the sensor temperature and measured pressure cycles were observed to be very similar from day to day (with the exception of the slow seasonal increase in pressure). However, during the four 24 hour, 4 second sampling sequences (sols 32, 38, 55, and 68), pressure sensor temperatures often differed by 15 K for the same time of day. This is because at the beginning of the sequence the spacecraft had been off all night, while at the end of the sequence it had been on all night. The -0.019 mbar/K temperature coefficient did not produce repeatable pressure cycles under these conditions but the -0.0135 mbar/K coefficient did, providing additional support for the pre-entry result. The pressure cycle data also suggested that this offset coefficient was valid over the 260-310 K temperature range.

Temperature Sensor

Both the thermocouple and PRT temperature sensor channels have undergone end-to-end temperature calibration. In addition, independent sensor calibration data are available, and the gains and offsets of the electronics have been measured over a range of -40°C to 50°C.

End-to-end temperature calibration was performed with the sensors in a nitrogen atmosphere within an isothermal copper chamber. At a pressure of one atmosphere, chamber temperatures were moved between stable plateaus covering the range 140 to 360 K in 20 K steps whilst data were logged. Spatial temperature variations within the chamber were less than a few hundredths of a degree when data were taken. The thermocouple isothermal block was located outside the chamber at a temperature different from the sensors, to provide representative thermocouple voltages. Both block and chamber temperatures were monitored by National Institute of Standards and Technology (NIST) calibrated PRT thermometers, accurate to 0.01 K.

In addition to the end-to-end calibrations, accurate calibration curves are available for the individual PRT sensors used by ASI/MET. The temperature vs voltage characteristics of chromel-constantan thermocouples are also well documented and are expected to vary little from thermocouple to thermocouple.

The end-to-end temperature calibration was performed with the flight electronics at ambient temperature. To correct for electronics board temperature variations expected during the mission, the response of each channel to fixed input voltages was measured over the -40°C to 50°C temperature range, allowing accurate gain and offset temperature coefficients to be derived. For the thermocouple channels, gain is also dependent on lead resistance. This effect was present in the end-to-end calibrations but was not part of the electronics gain measurements. Lead resistance for each thermocouple was therefore measured at ambient temperature using an AC technique to eliminate errors produced by thermocouple voltages.

Wind Sensor

The wind sensor is effectively an array of PRT temperature sensors and its calibration can be divided into two parts; the calibration of the temperature sensors and the calibration of the temperature response of these sensors to wind speed and direction (wind calibration). In addition the geometrical properties of the sensor elements must be measured accurately.

The temperature calibration of the flight wind sensors is very similar to, and was conducted at the same time as, the thermocouple and PRT temperature calibration discussed above. It consisted of end-to-end temperature calibration from 140 to 360 K and flight electronics gain and offset calibration over the range -40°C to 50°C. In addition the constant currents supplied by the flight electronics to heat the wind sensor windings were measured accurately in both low and high power modes at ambient temperature. In one wind sensor channel, only ambient temperature gain and offset data have been obtained. This is not a serious problem as the temperature dependence of gain and offset variations is very similar for all six channels.

Three nominally identical wind sensors were constructed and subjected to end-to-end temperature calibration measurements. The first was mounted on the flight mast, the second was held in storage as a flight spare, and the third was delivered to the AMES research center as a flight test unit for wind calibration investigations. The wind calibration was conducted in the Mars wind tunnel at AMES using nitrogen at pressures of 8, 12, and 18 torr, for wind speeds in the range 0 - 50 m/s and wind directions covering 360° in steps of 30° and 60° in steps of 5°. Wind direction changes were simulated by rotating the sensor in the wind tunnel. Wind calibration was not performed directly on the flight sensor to avoid contamination by the fine dust used to measure wind speed in the tunnel experiment chamber. The assumption underlying the wind calibration is that it can be transferred from the flight test unit to the flight unit given accurate temperature calibrations and geometrical measurements for both units.

As of the time of writing (Oct. 1998), reliable wind speeds and directions had not yet been derived from the data received from the Martian surface. Re-calibration of the wind sensor continues, and the wind results should be released soon.

Aeroshell Instrumentation Package

The Aeroshell Instrumentation Package (AIP) data were recorded via the ASI/MET data stream for a portion of the descent through the Martian atmosphere. The data calibration expressions for these data are provided in [SCHOFIELD1996B].

Operational Considerations

The sensor test masses of the three science accelerometers were a few centimeters away from the lander's center of gravity and could therefore receive contributions from angular accelerations due to the lander's pitch and yaw.

The science accelerometers first detected the Martian upper atmosphere 160 km above the surface. The descent rockets fired 5.2 minutes later at an altitude of 0.1 km, ending the direct measurement of aerodynamic decelerations.

Data collected from the engineering accelerometers were not used in the generation of atmospheric profiles. These accelerometers were kept in their least sensitive 40g measurement range because they were being used to control the deployment of the parachute.

The EDL phase pressure sensor wasn't able to begin unobstructed measurements of the atmosphere until the heat shield separated from the lander 3.4 minutes after entering the atmosphere. The spacecraft was 7.4 km above the surface at this time. The measurements continued for 1.7 minutes until at 0.3 km above the surface, the inflation of the airbags again obstructed the pressure measurements.

Related Information

The following links provide some additional information about topics related to the Mars Pathfinder mission. You must be connected to the Internet for most of these links to work, since they are located at the Central Node of the Planetary Data System.

Mission

Mars Pathfinder

Targets

PDS Welcome to the Planets: Mars
PDS High Level Catalog: Mars

Instrument Host

Mars Pathfinder Lander

Instrument

Mars Pathfinder IMP Windsocks

Data Set Descriptions

EDL Raw and Calibrated Data
EDL Derived Profiles
Surface MET Raw Data
Surface MET Calibrated Data

References

GOLOMBEKETAL1997B
MEYER1995
MEYER1996
SCHOFIELD1996A
SCHOFIELD1996B
SCHOFIELD1997A
SCHOFIELDETAL1997
SEIFFETAL1997