Calibration of METEO-P pressure and METEO-H relative humidity instruments for ExoMars 2022 surface platform

Abstract

Finnish Meteorological Institute has delivered a pair of miniature meteorological instruments, METEO-P and METEO-H, for the ExoMars 2022 mission. The instruments are part of the METEO package, a set of scientific meteorological instruments included in the payload of the ExoMars 2022 surface platform. METEO-P/H will perform stationary and continuous measurements of the Martian atmospheric pressure and relative humidity.

1 Introduction

METEO-P and METEO-H constitute the latest pressure and relative humidity instrument pair developed by Finnish Meteorological Institute (FMI). The instruments have been built and delivered for the ExoMars 2022 mission of the European Space Agency (ESA) and Roscosmos [1]. METEO-P, a compact pressure instrument, continues FMI’s extensive history of providing pressure instruments for Mars landers. The core technology has heritage from a number of previous missions, such as Mars Phoenix, MSL Curiosity, ExoMars 2016 Schiaparelli and Mars 2020 Perseverance. The relative humidity instrument METEO-H is likewise based on a strong heritage from similar instruments delivered for Curiosity, Schiaparelli and Perseverance.

The instruments are part of the meteorological package METEO, led by the Russian Space Research Institute (IKI) [2]. METEO will operate on board the Russian-led surface platform Kazachok, taking part in the Entry, Descent and Landing of the ExoMars 2022 lander and providing scientific surface observations from a stationary location after landing. METEO-P is located inside the platform’s warm compartment together with the METEO Central Electronics Unit (CEU), while METEO-H is exposed to the ambient environment on top of the meteorological mast.

METEO-P and METEO-H protoflight models (PFM) and flight spares (FS) have been built and calibrated in FMI’s facilities. The FMI calibration laboratory has an in-house developed calibration system for simulating the Martian pressure and humidity environment. Furthermore, the METEO-H ground reference model has gone through testing campaigns in two other laboratories in order to obtain additional data for complementing the relative humidity calibration.

2 Instrument description

Both METEO-P and METEO-H use sensor and measurement technology by Vaisala Inc., Finland. The capacitive Barocap® pressure sensor contained in METEO-P is a micromachined silicon sensor optimized for the Martian pressure range. METEO-P consists of two pressure transducers, each with 8 measurement channels: 2-3 Barocap® channels, 2 capacitive Thermocap® temperature sensor channels and 3-4 reference channels with a constant capacitance. The measurements are controlled by Vaisala proprietary ASIC.

Figure 1: METEO-P pressure instrument protoflight model

METEO-H is based on Vaisala’s capacitive Humicap® sensor technology and the same ASIC as METEO-P. METEO-H has one humidity transducer with 2 Humicap® channels, 2 Thermocap® channels and 4 reference channels. Each of the Humicap® sensor chips include an integrated resistive PT1000 temperature sensor for obtaining the temperature near the sensor. Accurate temperature measurement is essential for the Humicap® calibration as the sensor is sensitive to temperature changes.

Figure 2: METEO-H humidity instrument protoflight model

Both instruments are controlled by an automotive microcontroller (MCU) integrated on the METEO-P board. The controller also handles the power distribution between the pressure and humidity transducers and collects the measurement data. METEO-H is connected to the controller through cabling, while an RS-422 serial interface allows the controller to communicate with the METEO CEU.

3 Calibration

The pressure calibration tests of METEO-P were performed in the FMI calibration laboratory. The PFM and FS were measured simultaneously in different constant temperature and pressure points within the temperature range of -45 to +55 °C and pressure range of 0 to 12 hPa. Measurements were also performed in changing temperature and pressure, as well as in high vacuum for obtaining the zero point. After integration to the METEO CEU, a thermal vacuum test was conducted by IKI, providing data for correcting calibration changes affected by the new environment. The final calibration correction will be obtained from measurement performed during the cruise phase prior to landing to compensate for possible calibration drift.

The relative humidity calibration is based on measurements performed for different models of METEO-H in three laboratories: the FMI calibration laboratory, Michigan Mars Environmental Chamber (MMEC) and DLR Planetary Analog Simulation Laboratory (PASLAB). The FMI calibration system is able to produce two humidity calibration points – dry (0%RH) and near saturation (95-100%RH) – in temperatures down to -70 °C in Martian pressure CO₂. The dry points were measured in temperatures -70 to +22 °C and the saturation points in -70 to -40 °C. All tests at FMI were run simultaneously for METEO-H PFM, FS and the ground reference model.

The calibration was complemented after the delivery of METEO-H PFM by additional measurements for the identical ground reference model in DLR PASLAB. The resulting data set includes a number of calibration points over the full relative humidity scale in temperatures -70 to -30 °C and CO₂ pressures 5.5 to 10 hPa. The calibration results are extrapolated to higher and lower temperatures to cover the entire expected operational range.

4 Conclusions

ExoMars 2022 is scheduled for launch during the launch window in 2022. Following the landing in 2023, METEO-P and METEO-H will make continuous pressure and humidity measurements on the surface of Mars. The past missions have provided valuable knowledge about the calibration and performance of the sensors, allowing this new instrument pair to continue the in-situ investigations of the Martian atmosphere. Currently FMI has two sets of operational pressure and humidity instruments on Mars, one on board the Curiosity rover and the other in the Perseverance rover. If both of these are still operational at the time of ExoMars 2022 landing, METEO-P/H will create the third observation point in the mini-network of FMI’s pressure and relative humidity sensors on Mars.

References
[1] Vago, J. et al.: ESA ExoMars program: The next step in exploring Mars. Sol Syst Res 49, 518-528 (2015).
[2] Rodionov, D. et al.: ExoMars-2020 Landing Platform scientific payload, EGU General Assembly 2020, Online, 4-8 May 2020, EGU2020-21128.