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Mars Pathfinder meteorology sensor configuration

Three of the meteorology experiments temperature sensors are located on the 1.1 meter tall mast, and, along with the wind sensor at the top, and the pressure sensor, help characterize the atmospheric surface layer. During cruise, they are inside the lander in the vacuum of space and their temperature is governed by conduction to the supports and thermal radiation from within the lander. Since the temperature of the spacecraft varies due to solar heating, radiative cooling to space and spacecraft thermal control, the sensors indicate different temperatures. The configuration of the Atmospheric Structures/Meteorology experiment and the factors most important to the temperatures are described in this link and below.

The meteorology mast is 1.1 meters tall, located on the edge of one of the three petals of the lander, (Figure 6), and below. The mast base is located on the end of a lander petal, as far away as possible from the bulk of the lander, to minimize temperature and wind measurement errors due to flow distortion and heating of the atmosphere by the lander: we hope to compare these effects with those of Viking. The mast is seen at the bottom of the photograph, which also shows the solar cells. After the petals are folded, the bottom thermocouple ends up at the top of the lander, i.e., the peak of the tetrahedron.

The top of the mast image below, shows the thermocouple temperature sensor, positioned to measure atmospheric temperature during descent, the wind sensor, and the top of the other three thermocouples. The wind sensor height is 1.1 meters, while the temperature sensors are located 0.25, 0.5 and 1.0 meters above the base of the mast. "Wind Socks", the small cone shaped objects, with the counterweight on the other side of the pivot, are photographed by the camera to estimate wind speed and direction.

The Base of the lander sits in the entry Aero shell, immediately above its heat shield. The heat shield is the brown cone at the bottom of this figure and the cut away drawing. The Cruise stage is the ring above the Entry/Lander vehicle, and the solar panels are on the top, out of view. During Cruise, the solar panels point toward the sun, making that end of the spacecraft the warmest, while the heat shield is shaded by the solar array and views the cold of deep space. Consequently, the lander is warmed by conduction from the cruise stage, and cooled by radiative heat loss to space. The net result is that the "bottom" thermocouple is physically, and thermally, closest to the warmer cruise stage while the "top" thermocouple is closest to the colder heat shield. Active thermal control also affects the temperatures.

Check here and in Resources for more details about Viking Meteorology and other measurements which will be added in the near future.

Thermocouple temperature sensors are almost uniquely well suited for measuring atmospheric temperature. Typical research applications use them as very fine wires, such as illustrated here. Although they produce very small electrical signals, their value lies in that they can be made quite small, (from 1/3 to the diameter of human hair), their response to temperature does not change with size as long as they are not broken, and their calibrations are essentially identical, allowing them to be interchanged without affecting the result. Fine wire resistance temperature sensors also can be made small, but their response dependens on their length, diameter and strain: all of these change if they are stretched or abraded. Measurement of atmospheric temperature with a fine wire sensor is subject to errors, among which are the electronics, calibration, and conduction and radiation errors. Heating by the sun and infrared radiation, and cooling by infrared radiation, both produce errors. For the present purpose, its enough to know that both radiation and conduction errors decrease as the wires become smaller. For this reason, fine wire thermocouples are desirable in that their diameter can be reduced to the limit imposed by reliability without affecting the calibration.

Thermocouples take advantage of the fact that if two different metals are joined, they produce a voltage proportional to the temperature. Different material combinations produce different output voltages with temperature, and are appropriate for different applications. The chromel -- constantan combination used for Pathfinder, and previously for Viking, produce an output voltage which changes by approximately 60 millionths of a volt for every 1 °C temperature change. Although small, this can be measured to an accuracy of 0.1 °C with good design and electronics. Future developments will include constructing simple thermocouple systems for accurate, fast temperature measurements and to investigate the errors encountered trying to measure air temperature.

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