Accuracy of the relative humidity sensor MEDA HS onboard Perseverance rover

MEDA HS is the relative humidity sensor on the Mars 2020 Perseverance rover provided by the Finnish Meteorological Institute (FMI). The sensor is a part of Mars Environmental Dynamic Analyzer (MEDA), a suite of environmental sensors provided by Centro de Astrobiología in Madrid, Spain.

The accuracy requirement for MEDA HS relative humidity was ±10% RH for temperatures above -70 ºC, and ±20% RH for temperatures between -83...-73 ºC. Dynamic range from 0 to 100% RH shall cover the whole Martian temperature range from -83 ºC to -3 ºC. However it must be noted that during the daytime when the relative humidity drops close to zero, the readings are not scientifically meaningful due to the large relative uncertainty.

MEDA HS flight model was tested and calibrated in Mars-like dry environment at FMI together with flight spare and ground reference models from +22 ºC to -70 ºC and in saturation conditions from -40 ºC down to -70 ºC. Further, the MEDA HS flight model final calibration is complemented by calibration data transferred from an identical ground reference model which has gone through extensive humidity calibration test campaign at DLR PASLAB. The MEDA HS has been calibrated to full relative humidity range between -70 to -40 ºC in CO₂ in the pressure ranges from 5.5 to 9.5 hPa, representative of Martian surface atmospheric pressure, and partial range up to +22 ºC. For lower temperatures the results are extrapolated.

The complex analysis of the MEDA HS measurement uncertainty has now been finalized and the results are presented in the conference. It has been found that the sensor exceeds the design requirements and will provide high accuracy relative humidity measurements from the Martian surface to provide important meteorological observations and to support MEDA and other M2020 investigations.