This consist of two sensors. The Temperature Sensor measures air temperatures and temperature differences between levels will be monitored, almost continuously, by three temperature sensors based on fine-wire, butt-welded thermocouples (75 micron diameter, Constantan-Chromel) mounted in C-frames on a 1m mast, coupled with a reference platinum resistance thermometer in an isothermal block containing the 'cold' junctions of the thermocouples. 
The Pressure Sensor, an FMI (Finnish Meteorological Institute) sensor, has 3 pressure sensor heads. One of these is the primary sensor head and the other two will be used for monitoring the condition of the primary sensor head during the mission. During the mission the primary sensor will be read with a sampling interval of 2 s and the other two will be read much less frequently as a check of instrument health. The pressure sensor system has a sophisticated real-time data-processing and calibration algorithm that allows the removal of temperature dependent calibration errors.

Tamppari, L. K. M.D. Smith, D.S. Bass, and A.S. Hale, Water Vapour Behavior in
                   the North Polar Region of Mars as Seen by MGS TES, 38th Lunar and Planetary
                   Science Conference, LPI contribution 1338, 2007.

Instrument Overview
    ===================
 
      Temperature Sensor
      ---------------
        Air temperatures and temperature differences between levels will be
        monitored, almost continuously, by three temperature sensors based on
        fine-wire, butt-welded thermocouples (75 micron diameter, Constantan-
        Chromel) mounted in C-frames on a 1m mast, coupled with a reference
        platinum resistance thermometer in an isothermal block containing the
        'cold' junctions of the thermocouples. Levels on the mast are 0.25,
        0.5 and 1.0 m above the Lander deck, which itself is approximately 1m
        above the ground. There will be three thermocouple junctions in
        parallel in each of the air temperature sensors, providing a degree of
        redundancy.
 
        The micro-voltages generated by the thermocouples are measures at 2-
        second intervals and converted to digital signals with a 16-bit
        analogue to digital converter. Drifts in the read-out electronics are
        calibrated and corrected for at the same interval.
 
      Pressure Sensor
      ---------------
        An FMI (Finnish Meteorological Institute) sensor, based on a silicon
        diaphragm sensor head manufactured by Vaisala Inc., combined with MDA
        data processing electronics will measure pressure. The FMI unit has 3
        pressure sensor heads. One of these is the primary sensor head and the
        other two  will be used for monitoring the condition of the primary
        sensor head during the mission. During the mission the primary sensor
        will be read with a sampling interval of 2 s and the other two will be
        read much less frequently as a check of instrument health.
 
        The pressure sensor system has a sophisticated real-time data-
        processing and calibration algorithm that allows the removal of
        temperature dependent calibration errors.
 
        The time constant of the pressure sensor had not been specified but it
        was originally hoped that it would be short compared to the sampling
        interval of 2 s. In fact, it is a little longer than that,
        approximately 3s due to locational constraints and dust filtering
        requirements, but should still be short enough to detect pressure
        drops associated with the passage of a nearby dust devil.
 
 
    Scientific Objectives
    =====================
 
      In situ measurements of near-surface temperature, pressure, humidity and
      wind are essential to improve our understanding of Mars weather and
      current climate. Phoenix, as the first mission to the high northern
      latitudes, will provide in situ meteorological information from an
      environment with a seasonal maximum of atmospheric water vapour
      [TAMPPARI2007], and an expectation of access to sub-surface ice. Basic
      meteorology measurements will support all aspects of the surface science
      mission and have been included as measurements critical to mission
      success.
 
 
    Location
    ========
 
    The Phoenix Mars Lander arrived at 68.2184N, 234.2487E on May 25, 2008.
 
 
    Operational Modes and Measured Parameters
    =========================================
 
      The temperature and pressure sensors sample a total of 256 data records
      (8.53 min), which are then buffered and can either be stored at full
      resolution in the flash memory of MET, or processed to provide mean,
      standard deviation, maximum and minimum values for storage on the MET
      unit.  As well, the processing can also switch autonomously to the full
      resolution mode based on the magnitude of pressure or temperature
      fluctuations in the data.  The data will thus be presented at 512 sec
      averages, and where possible 2 sec data for each 512 sec interval.
 
      The Pressure and Temperature will operate at all possible times during
      surface operations.
 
    Temperature Sensors
      -------------------
        The three mast thermocouples (250, 500 and 1000 mm from Lander deck)
        measure with an absolute accuracy of +/- 1K, a resolution of 0.5 K and
        the uncertainty between two thermocouples does not exceed 0.3 K.  The
        Isothermal block, to which all thermocouples are referenced, has an
        accuracy of +/- 1K.
 
    Pressure Sensor
      ---------------
        The pressure sensor is accurate to 10 Pa between 7 and 11 hPa, and is
        sampled with 0.1 Pa resolution.
 
 
    Calibration
    ===========
 
      Temperature Sensor
      ------------------
        The accuracy with which the thermocouples can measure temperature
        under ideal conditions is a relatively straightforward issue which was
        addressed by laboratory testing in thermal baths or and dry wells.
        This was undertaken by the prime contractor MDA and polynomial
        calibration curves for individual thermocouples and reference PRTs
        have been obtained. The other issue is how well the thermocouples will
        work in a Mars atmosphere and in particular what will be their time
        constant and how will they respond to solar radiation?
 
        In order to address these issues the York University, CSIL Mars wind
        tunnel facility was developed specifically for testing and
        characterizing the Phoenix MET temperature sensors. Two small wind
        tunnels (Figure 3) have been constructed to fit, side by side, in a
        cylindrical vacuum chamber that can be cooled to Mars temperatures.
        One tunnel is run with relatively warm air while the other is colder.
        A switching device allows us to alter the flow through a test section
        from warm to cold virtually instantaneously and by monitoring the
        thermocouple output we can determine the time constant. Once the wind
        tunnel unit has been installed in the chamber the pressure is reduced
        to near vacuum (0.6 hPa) and the whole wind tunnel unit is cooled by
        pumping LN2 through cooling pipes within it. It is then backfilled
        with carbon dioxide to Mars pressures, and slight heating applied to
        one of the tunnel walls. Tests were conducted in about 8 hPa of CO2 at
        temperatures in the range 200-270 K. A range of wind speeds up to 25
        ms^-1 was used.  The essential point is that in these tests the time
        constant varies from about 0.3s at 25 m/s to 0.5s at 4.5 m/s.
 
        In addition we used the same wind tunnel to evaluate the impact of
        solar radiation on the thermocouples at a range of wind speeds. The
        thermocouples and their surroundings will both absorb and emit
        infra-red radiation. We are assuming that this will have a small
        effect but when solar radiation falls on a thermometer that is not in
        the shade the temperature can rise significantly. With a slow
        response, large heat capacity temperature sensor, e.g. a mercury
        thermometer or large thermocouple, it is customary to shade the sensor
        so that it is not exposed to direct sunshine. For very fine wire
        thermocouples this is not the usual practice and one relies on more
        efficient conduction of heat between the air and the thermocouple to
        reduce the impact of solar heating. In addition the thermocouples are
        moderately reflective.
 
        Simulated solar radiation was generated with a Xenon lamp and filters
        and then directed onto the thermocouples in the C-frame within the
        wind tunnel through an optical fibre. By turning the lamp on and off
        at regular intervals, we were able to detect the temperature increase
        reported by the thermocouples. This also provided another measure of
        the time constant. The tests were repeated at a range of wind speeds.
        Tests were conducted with both clean and dust coated thermocouples.
        Solar radiation intensities were matched to the maximum expected at
        the lander site and tests were run both at Mars and room temperatures.
        Our optical fibers lost transmission capacity at temperatures below
        -40C but we found very similar solar radiation effects for
        -40C < T < 0C as we obtained at room temperature. Radiation effects on
        the thermocouples varied with wind speed but were generally less than
        the 1 degree accuracy requirement. For temperature differences between
        levels it is assumed that both levels will be exposed to the same
        amount of solar radiation.
 
        An additional issue being researched using both physical and numerical
        models are the impacts of flow distortion and thermal contamination
        due to the lander itself. Noting that the lander is a fairly bluff
        body and that the mast is only one meter above a deck which stands
        approximately one meter above the surface, we must expect that the
        flow past the temperature sensors will not necessarily have come from
        the same upstream elevation, and that the measurements of temperature
        may also be affected by thermal diffusion or plumes emanating from the
        lander. Studies of both of these effects are underway in order to help
        interpret temperature and temperature profile measurements to be made
        once the lander has arrived on Mars. Note that on some occasions we
        will have additional air temperature measurements from the TECP
        allowing us to extend the height range of the profiles to include near
        surface measurements.
 
      Pressure Sensor
      ---------------
        Calibration of the engineering and flight model pressure sensors were
        conducted by FMI over pressure and temperature. These calibration
        coefficients were used by the read-out electronics to calculate
        pressure.  System level tests that included the flight read-out and
        data processing electronics were conducted at MDA and verified the FMI
        calibration.
 
        To conduct these tests, the Phoenix and reference pressure sensors
        were inserted in a controlled pressure and temperature enclosure,
        backfilled with CO2. Pressure and temperature was varied over
        appropriate ranges and the Phoenix sensor output compared with a
        calibration standard. There were initial discrepancies associated with
        the calibrator but a thorough investigation and subsequent retest
        satisfactorily confirmed that the sensors could meet project
        requirements.

Phoenix Meteorology Suite, Pressure & Temperature

Description

Identifier (LID)

Version