MET Pressure and Temperature (PT)
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.
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.
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.
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.
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.
The pressure sensor is accurate to 10 Pa between 7 and 11 hPa, and is sampled with 0.1 Pa resolution.
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.
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.