Rover Environmental Monitoring Station (REMS)
The Rover Environmental Monitoring Station (REMS) instrument is intended to provide in situ near-surface measurements of air and ground temperatures, wind speed and direction, pressure, humidity and ultraviolet radiation. Its objective is to characterize the Martian climate and to study Mars habitability.
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.
REMS operation is designed assuming a minimum of three hours of operation each day, primarily constrained by power availability. 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. Depending on resources available during the mission, and the science team wishes, REMS can operate more than three hours per day.
REMS instrument is formed by four main subsets:
- ICU (instrument Control Unit), allowing power conditioning and distribution, analog acquisition and control of some of the sensor, thermal control loops, communications with the Booms and data processing and communications with the RCE. The ICU is composed of three boards: CPU, Analog and a DC/DC Converter. The Analog board also integrates the Pressure Sensor (PS).
- Ultraviolet Sensor (UV), consisting of 6 photodiodes, each one devoted to a different measurement band.
- Boom 1, hosting an Air Temperature Sensor (ATS), a Wind Sensor (WS), the Ground Temperature Sensor (GTS) and the front-end ASIC that processes all the received analog inputs from the transducers and sends the data to the ICU via a transmission link.
- Boom 2, hosting another Air Temperature Sensor, another Wind Sensor, the Humidity Sensor (HS) and other front-end ASIC that also processes all the received analog inputs from the transducers and sends the data to the ICU via a transmission link.
Booms are to be located orthogonal to the rover mast, with boom 1 looking to the side and slightly to the rear of the rover, while boom 2 points in the driving direction of the rover. The UVS is going to be placed on top of the Rover Deck and the ICU is to be located inside the warm body of the rover.
An extensive description of the REMS instrument can be found in [GOMEZELVIRAETAL2012].
The five main science objectives of REMS are:
- Microscale atmospheric Dynamics. Characterization of the near-surface meteorological environment at Diurnal and Seasonal Scales.
- Mesoscale atmospheric Dynamics. It includes flows forced by interaction of solar heating and large-scale winds with topography or other surface inhomogeneity such as sharp contrast in surface inhomogeneity (10-1000 km).
- Synoptic atmospheric Dynamics. A fundamental scientific objective is the characterization of the atmospheric global waves (> 1000 km) through the pressure fluctuations records.
- The Local Water Cycle. The fluctuations of the diurnal water vapour concentration will increase our knowledge about the Mars water cycle at the boundary layer.
- The Local UV Environment at the surface. To evaluate the role of the UV radiation environment in the local chemical and biological processes.
The Calibration for the REMS instrument is provided in the REMS Calibration Plan (DOCUMENT/REMS_CALIB_PLAN.PDF of the archive volume).
Boom Thermal Management
Mixed ASIC inside each Boom needs to be warmed-up to temperatures over -55C before they are used. This is performed automatically be a hardware (software-assisted) control-loop implemented into the ICU for any operation that requires Booms ASIC use (science data acquisition, engineering acquisition, ground temperature sensor calibration).
When the Booms ambient temperature is too low, the ICU will switch-on the Booms' ASIC heater at temperatures below approx. -54C nominally. On reaching -70C (this is a default but modifiable parameter) the ASICs will be switched on and start operation. The heater will be switched-off when the temperature reaches approx -50C nominally. Any operation requiring Booms operation will be automatically delayed until the Booms reaches operation temperatures. This is performed automatically by instrument and need just to be taken into account in the operations planning. The ICU includes a 15 minutes warming-up timeout. In case one ASIC does not reach operation temperature before that timeout expires the ASIC is declared failed and the operation continues with the other available sensors.
It is occasionally necessary to perform sensors management procedures to allow correct operation and to improve the performance of the sensors (e.g. by obtaining calibration data).
Three sensor management operations are presently defined: HS regeneration, HS defrosting and GTS calibration.
HS Regeneration shall be done about once a month by ground command when the ambient temperature is at its highest. Nominal regeneration time target is between 3 and 10 minutes, being this time a changeable parameter (via a System Parameter). Regeneration temperature target is between +135C and 150C. This restores Humicap calibration by removing contamination from the sensor surface. HS regeneration is performed by sending the corresponding Instrument Command or by setting the equivalent entry into the Schedule Table when operating in Auto mode. Please note that REMS software includes an enabling ambient temperature window array that will be checked automatically by the software before proceeding with the regeneration. Regeneration will then only be performed if the ambient temperature as measured by the own HS channel is between the upper and lower limits programmed in such table (as System Parameters).
HS Defrosting procedure is similar to the regeneration one. It requires no more than +100C as H-PRT temperature, thus removing frost in the sensor heads. Defrosting shall be done every night at coldest night hours (ambient temperature usually below -50C, but depends on ICU H-PRT secondary voltage level at that time) by pre-defined timeline commanding or by direct ground command. Nominal defrosting time is between 3 and 5 minutes, but this time shall be a changeable by a System Parameter.
HS Defrosting is performed by sending the corresponding I-cmd or by setting the equivalent entry into the Schedule Table when operating in Auto mode. As with the regeneration, REMS software includes an enabling ambient temperature window array that will be checked automatically by the software before proceeding with the defrosting. Defrosting will only be performed in case the ambient temperature as measured by the own HS channel is inside the upper and lower limits programmed in such table (as System Parameters).
Humidity Sensor should be calibrated at night, when the temperature is so low that the humidity reaches its saturation level. Data for calibration will be taken through a standard measurement cycle, not requiring any specific sensor operation.
For calibration of the GTS sensor thermopiles, a heater placed in front of the thermopiles is implemented (intercepting part of the thermopiles field of view) and heated to a known power (~330mW, typical) during the calibration mode of the instrument. In this way, it will provide a thermal gradient of at least 30C between the calibration plate and the thermopiles reference temperature, thus giving a good thermopile output response. This operation should be performed when ambient temperatures are as stable as possible (and consequently, there are calm winds), to minimize error contributions.
Wind speed and direction will be derived based on information provided by three two-dimensional wind sensors on each of the booms. The basic concepts in which the two-dimensional wind sensors are based can be found in [DOMINGUEZETAL2009]. 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.
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. Please see [SEBASTIANETAL2010] for a further information about the GTS sensor.
Air temperature will be recorded at both booms with two PT1000-type sensors placed on a small rod long enough to be outside the mast and boom thermal boundary layers. The information provided by these two sensors alongside knowledge of the temperature at the base of the rod (boom temperature) shall be used to estimate the temperature of the fluid around it. 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.
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.
More information about each sensor can also be found in the REMS Calibration Plan.
REMS electronics consist of the ICU and the ASICs on both booms. The intra-instrument harness connecting these elements is provided by JPL.
The development of an ASIC for data conditioning is motivated by two requirements: 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. The ASIC must survive a -130C to +70C temperature range and minimize power consumption for operation.
The Booms' sensors signals (exception made of the Humidity Sensor) are acquired through the Mixed Analog/Digital ASIC, which are accommodated inside the booms housing. These ASICs communicates via serial links with the Instrument Control Unit down in the Rover warm body. There will be a master-slave communication protocol, being the ICU the master of the communications; ICU will sent commands to the ASICs and wait for reply from them (ASICs cannot initiate communications), There will be always an answer for every command sent. In addition, the ICU can not send a command while a previous answer is pending. Humidity Sensor's signals (Boom 2) are directly connected -via harness- to the ICU for its control and data acquisition.
On board software runs inside the ICU microprocessor. There are two on-board REMS software products:
- Start Up SW (SUSW): Stored in PROM, it can not be changed during the mission. This product will be in charge of the booting tasks, initial instrument initialization/testing and patching of APSW. SUSW will start the execution of APSW.
- Application SW (APSW): Stored in EEPROM, it may be changed during the mission. This product will perform the instrument management operations and the scientific data acquisition/downloading. EEPROM contains also a Memory Management Parameters are (MMP) that defines important software configuration parameters such as base pointers and sizes of Application Software, REMS Data Product buffer, Schedule Table, System parameters area, etc.
Science and Engineering data is stored into a Flash EEPROM (as RDP_Data_Product) to be later on downloaded for telemetry. This Flash Memory is used to store non volatile data structures that need to be kept through REMS sleep/wake cycles but at the same time they need to be often read/modified by REMS SUSW and APSW during nominal operation.
The following data structures are stored in the Flash memory:
- Schedule Table, contains the specification of tasks to be done in APSW Auto mode. This table needs to be loaded periodically into the instrument.
- REMS Data Product, scientific and operational data gathered by the instrument. This data has to be downloaded periodically.
- System Parameters (SP), persistent but modifiable information needed for nominal instrument operation (e.g. Booms' ASICs default power-on configurations, sensors gains, sensors management limits and operational durations, Event mode parameters, etc.
- Persistent data, SW internal variables that need to be kept between resets, for example ST and RDP management information (e.g. internal pointers to flash structures, measurements averages for Event mode calculations, OBT value before going to standby, etc.). The allocation and sizes for these structures are specified inside the MMP stored in EEPROM.
REMS can operate either:
- Autonomously, in base of a given 'Schedule Table' that is uploaded to the instrument,
- Or supervised, by means of Instrument commands (I-cmds) sent directly from the rover. This will be used mainly to load the Schedule Table or to handle special situations.
The baseline operations scheme has the instrument waking up every hour to take 5 minutes of observations, with each sensor collecting samples at 1 Hz. This scheme will provide 2 hours of data per sol, and will be implemented using the Schedule Table.
In addition to the baseline hourly observation cadence, REMS can operate continuously for longer periods, be either in one single block or into several smaller blocks distributed along the sol. It is foreseen a total extra time of one hour each day, but can be more depending on mission resources and the wishes of the science team.
The additional observations can be handled in several ways. One is by examining phenomena of known interest (e.g. atmospheric conditions at dawn or dusk). Other one is by distributing several observations along each sol and then shifting them the following sols. That way, after several days, a full 24 hour cycle of measurements can been covered.
There is also the possibility of using the extra observation time by activating the REMS event mode. In that mode, REMS will make real time comparisons between data taken during the hourly 5 minutes periods and an expected trend kept in memory. These comparisons may trigger additional observations (up to a predefined maximum) immediately following the scheduled hourly periods. These triggers are based on observations statistics, such as absolute values or unusual temporal variations, and try to detect any ongoing transitory atmospheric event.