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Rover Environmental Monitoring Station (REMS)
PI: Javier Gómez-Elvira, Centro de Astrobiología, Spain
REMS has been designed to record six atmospheric parameters: wind speed/direction, pressure, relative humidity, air temperature, ground temperature, and ultraviolet radiation. All sensors are located around three elements: two booms attached to the rover Remote Sensing Mast (RSM), the Ultraviolet Sensor (UVS) assembly located on the rover top deck, and the Instrument Control Unit (ICU) inside the rover body.
The booms are approximately 1.5 m above ground level. Boom length is similar to the RSM diameter, and therefore the wind flow perturbation by the RSM may reach the boom tip where the wind sensor is located. The two booms are separated in azimuth by 120 degrees to help insure that at least one of them will record clean wind data for any given wind direction. The figure below shows the booms’ relative position. There is a 50 mm height difference to minimize mutual wind perturbation.
Boom 2, which points in the driving direction of the rover, has wind sensors and the relative humidity sensor. Boom 1, which looks to the side and slightly to the rear of the rover, hosts another set of wind sensors and the ground temperature sensor. Both booms have an air temperature sensor.
Wind speed and direction will be derived based on information provided by three two-dimensional wind sensors on each of the booms. The three sensors are located 120 degrees apart around the boom axis. Each of them will record local speed and direction in the plane of the sensor. The convolution of the 12 data points will be enough to determine wind speed as well as pitch and yaw angle of each boom relative to the flow direction. The requirement is to determine horizontal wind speed with 1 m/sec accuracy in the range of 0 to 70 m/sec, with a resolution of 0.5 m/sec. The directional accuracy is expected to be better than 30 degrees. For vertical wind the range is 0 to 10 m/sec, and the accuracy and resolution are the same as for horizontal wind.
As mentioned previously, the wind field at the booms will be perturbed by the RSM and by the rover itself. Calibration will be done via a variety of wind tunnel tests under Mars conditions as well as numerical analysis. Simulations will be used to obtain results where tests conditions cannot be reproduced on Earth.
Ground temperature will be recorded with a thermopile on Boom 1 that views the Martian surface to the side of the rover through a filter with a passband of 8 to 14 microns. The requirement is to measure ground brightness temperature over the range from 150 to 300 K with a resolution of 2 K and an accuracy of 10 K.
Air temperature will be recorded at both booms with a PT1000-type sensor placed on a small rod long enough to be outside the mast and boom thermal boundary layers. Its measurement range is 150 to 300 K. It has an accuracy of 5 K and a resolution of 0.1 K.
Boom 2 houses the humidity sensor, which is located inside a protective cylinder. That sensor will measure relative humidity with an accuracy of 10% in the 200-323 K range and with a resolution of 1%. A dust filter protects it from dust deposition.
Two of the main constraints on the REMS instrument design are the need for the booms to survive and operate in a broad range of temperatures, and for the entire instrument to have a mass less than 1.3 kg. Both conditions have required the development of an ASIC for data conditioning which must survive a -130 °C to +70 °C temperature range and minimize power consumption for operation.
The UV sensor will be located on the rover deck and is composed of six photodiodes in the following ranges: 315-370 nm (UVA), 280-320 nm (UVB), 220-280 nm (UVC), 200-370 nm (total dose), 230-290 nm (UVD), and 300-350 nm (UVE), with an accuracy better than 8% of the full range for each channel, computed based on Mars radiation levels and minimum dust opacity. The photodiodes face the zenith direction and have a field of view of 60 degrees. The sensor will be placed on the rover deck without any dust protection. To mitigate dust degradation, a magnetic ring has been placed around each photodiode with the aim of maximizing their operational time. Nevertheless, to evaluate dust deposition degradation, images of the sensor will be recorded periodically. Comparison of these images with laboratory measurements will permit evaluation of the level of dust absorption.
The pressure sensor will be located inside the rover body and connected to the external atmosphere via a tube. The tube exits the rover body through a small opening with protection against dust deposition. Its measurement range goes from 1 to 1150 Pa with an end-of-life accuracy of 20 Pa (calibration tests give values around 3 Pa) and a resolution of 0.5 Pa. As this component will be in contact with the atmosphere, a HEPA filter will be placed on the tube inlet to avoid contaminating the Mars environment.
Systematic measurement is the main driver for REMS operation. Each hour, every sol, REMS will record 5 minutes of data at 1 Hz for all sensors. This strategy will be implemented based on a high degree of autonomy in REMS operations. The instrument will wake itself up each hour and after recording and storing data, will go to sleep independently of rover operations. REMS will record data whether the rover is awake or not, and both day and night. It is expected that under certain conditions, the ground temperature and humidity sensor measurements will require the integration of multiple measurement samples within the 5-minute interval in order to meet their science requirements.
REMS operation is designed assuming an integrated total of three hours of operation each day, primarily constrained by power availability. Nevertheless, the REMS science team will have the capability to define additional prescheduled observation periods with durations longer than 5 minutes and located at any time during the day. Since the hourly observations will use a total of two hours of operational time, the third hour can be scheduled as a continuous block, for example. Another option that has been implemented in REMS flight software is a simple algorithm to lengthen some of the regular observations autonomously when an atmospheric event is detected.
The main science objectives that the science team will focus on are:
- Signature of the Martian general circulation and mesoscale phenomena near the surface (e.g., fronts, jets)
- Microscale weather systems (e.g., boundary layer turbulence, heat fluxes, dust devils)
- Local hydrological cycle (e.g., spatial and temporal variability, diffusive transport from regolith)
- Destructive potential of UV radiation, dust UV optical properties, photolysis rates, and oxidant production
- Subsurface habitability based on ground-atmosphere interaction
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