REMS: Rover Environmental Monitoring Station

Chapter 3 3.1. Instrument investigation

Usage time DAN can perform passive neutron detection continuously, without using DAN/PNG, but can also operate in an active mode generating 13.4 million (∼ 10⁷) neutrons with each pulse, lasting 2−3 microseconds. Additionally, the pulsing frequency is between 1 Hz (single pulse per second) to 10 Hz. Experiments with the DAN instrument are limited by two factors: the DAN/PNG has a warranted lifetime of 10 million pulses, so the MSL rover can expect to per-form approximately 1000 typical active observations over its lifetime; moreover the DAN/PNG has a clock time limit of approximately 3 years after launch, because the helium generated by the pulses and the tritium target decay ruin the vacuum inside the ion accelerator.

DAN - Power consumption DAN/PNG

Idling operating mode: 0.1 W;

Power in enabling mode (ready to pulse): 1.4 W;

Power in pulsing mode: 13 W;

DAN/DE

Power at standby mode: 2.8 W (22 V) − 3.5 W (36 V);

Power in passive mode: 3.7 W (22 V) − 4.5 W (36 V);

REMS consists of four units: two booms (Boom 1 and Boom 2) attached to the rover Remote Sensing Mast (RSM), the Ultraviolet Sensor (UVS) mounted on the rover top deck, and the electronics box, Instrument Control Unit (ICU), located inside the rover body (Figure 3.35).

Figure 3.35: REMS components: the electronics box contains a pressure sensor. Both Boom 1 and Boom 2 have Wind and Air Temperature Sensors, however while Boom 1 has a Ground Temperature Sensor, Boom 2 has a Humidity Sensor; in the Ultraviolet Sensor the labels identify the different photodiodes [8].

Instrument Control Unit The Instrument Control Unit (ICU) performs multiple functions:

• Provides the interface with the rover in terms of data, telemetry and power;

• Powers the Sensor Front-End (SFE) electronics to receive the digital data to be managed and analyzed;

• Processes data from the pressure, humidity and ultraviolet sensor, because they are not connected to the SFE.

The Sensor Front-End (SFE) ASIC³⁰, in turn, provides the sensor electronic interface for the air temperature, wind and ground sensor (Figure 3.36), so it receives all the low level analogue

30Application-Specific Integrated Circuit.

Chapter 3 3.1. Instrument investigation

signals generated on the sensors and converts them into digital signals ready to be transmitted to the ICU where can be processed and stored.

The ICU also monitors the operating temperature range of each ASICS SFE through a heater and a thermistor: the heater is powered on when thermistor readings are lower than −55 °C and the ASIC is powered on when its temperature is higher than −70 °C, becoming functional from

−52 °C.

Figure 3.36: REMS block diagram: both booms include their associated SFE ASICS electron-ics. In Boom 1, the ASIC electronic manage the WS, GTS and ATS signals, while in Boom 2 it is only responsible for the WS signal, because the HS is connected directly to the ICU.

Moreover, ASIC-ICU communication are digital to minimize external noise effects [43].

Booms The booms are approximately 1.5 m above ground level. Boom length is similar to the RSM diameter and they are fixed on a part of the mast that does not rotate, so they cannot change orientation. The two booms are separated in azimuth by 120° to make sure that at least one would always experience wind and records clean data for any given wind direction.

Wind Sensor The Wind Sensors (WS) are based on hot film anemometry and they are com-posed of three measuring points around each Boom, which are based, in turn, on tiny titanium resistors that to keep them at a constant temperature it is essential to supply a certain amount of power, then an algorithm combines the data from all the six recording points to determine the true wind speed and direction. The booms are located in different position for two reasons:

resistors strongly depends on the temperature of the instrument, so it is better to have them separated from the hot rover; it would be more convenient not to obstruct the wind flow.

Air Temperature Sensor The Air Temperature Sensors (ATS) are mounted on both booms, attached to the lower edge of the SFE ASIC housing, protruding in the direction of the boom front-end and consist of small rods manufactured with low thermal conductivity material with two thermistors: one at the tip to measure the ambient temperature and one at the middle to avoid data contamination due to the rod heating produced by the boom heating itself. The ATS is capable to measure the air temperature near the booms over the range of 150–300 K with a resolution of 0.1 K and an accuracy of 5 K [43], with the Wind Sensor switched off, but the electronic circuits located at the base of each boom, must be kept above −70 °C and when a sensor detects that the temperature falls below, it turns on its heater.

Ground Temperature Sensor The Ground Temperature Sensor (GTS) uses three thermo-piles to record the infrared (IR) brightness temperature of the Martian surface. The GTS is located on Boom 1 and it is connected to the Sensor Front-End electronics (Figure 3.36). The thermopiles look downwards, pointing at the ground to the right of the rover and covering an area of about 100 m. The sensor includes an active self-calibration system to compensate po-tential degradation during the mission (e.g. dust amassing on the sensor). The GTS is capable to record the ground brightness temperature over the range of 150–300 K with a resolution of 2 K, by averaging 1 minute reading, and an accuracy of 10 K.

Humidity Sensor The Humidity Sensor (HS) consist of a polymer film whose electrical prop-erties change as temperature and humidity change. The polymer film constantly responds to changes in the environment, but humidity can only be read when the sensor is powered, this implies a warming up of the sensor itself, which compromises the humidity measurements: the most accurate are the ones made immediately after it has been powered on.

The Humidity sensor is calibrated to measure from 0 to 100% RH and can survive even if the ambient temperature is −135 °C, but below −70 °C the dynamic range becomes too small for practical humidity observations. Typical sensor accuracy is ±2% RH at 0 °C, ±4% RH at

−40 °C, ±8% RH at −70 °C. Mars has little water in its air, but at night the temperature drops low enough that relative humidity can reach as high as 70%

UV Sensor The Ultraviolet Sensor (UVS) is located on the rover’s deck, has six photodiodes (Figure 3.35) and UVS sensor signals are transmitted directly to the ICU (Figure 3.36). Due to its location and direction, the upward-pointed light sensor is exposed to dust deposition and design constraints ruled out any active protection system. Nevertheless, to mitigate the dust degradation effect, each photodiode is surrounded by a ring-shaped magnet that creates a mag-netic field that deflects Martian dust and prevents it from falling in its center. In addition, the sensor are covered about 10% of the time by shadows (mostly from the rover mast), then these collected data are simply removed from the dataset. The resolution of the sensor is 0.5% and

Chapter 3 3.1. Instrument investigation

the accuracy better than 5% of the maximum measurable irradiance.

After calibration the measurement range is:

• UVA: 320–380 nm;

• UVB: 280–320 nm;

• UVC: 200–280 nm;

• UVD: 230–290 nm;

• UVE: 300–350 nm;

• Total Dose (UVABC): 200–380 nm.

The spectral response³¹of the REMS UV photodiodes is shown in Figure 3.37.

Figure 3.37: Spectral response of the REMS UV photodiodes [44].

Pressure Sensor The Pressure Sensor (PS) is located inside the REMS electronics box (Fig-ure 3.36) and it is calibrated for Martian press(Fig-ure range of 4–12 hPa and operational temperat-ure over the range −45–55 °C. The sensor has two transducers, one highly stable and the other highly reactive to pressure changes, both of which record useful measurements for this reason

31On the ordinate axis it is represented the spectral responsivity, which is the ratio of the generated photocurrent to incident light power, expressed in A/W, when the photodiode is used in photoconductive mode.

they can be used in turn, thus providing some redundancy and improved reliability.

Its measurement range goes from 1–1150 Pa with an end-of-life accuracy of 20 Pa, a resolu-tion of 0.5 Pa and a response time of the overall pressure measurement system of 1 second. In addition, the measurements of the last few minutes are taken into account, because the sensor provide better readings after warming up.

Usage time REMS performs regular observations for approximately 5 minutes of every hour, all the time, gathering data at a sampling rate of (1 Hz) or better, depending on the objective.

The sensors become operational each hour and after monitoring and storing data, they go into standby mode regardless of the rover’s operations. REMS operation is designed assuming a total of three hours of operation (limited by power availability) each day, two hours for 5-minute hourly observations, and the third hour can be pre-scheduled and programmed as a continuous block. Another option implemented in the REMS flight software is a simple algorithm which provides the opportunity of managing some of the regular observations autonomously when an atmospheric event is detected.

REMS - Power consumption Stand by: 402 mW;

Start-up idle: 4620 mW;

ASIC heating: 8744 mW;

Humidity regeneration: 5479 mW;

GTS calibration: 5174 mW;

Data download: 4760 mW;

All sensors measuring (without ASIC heating): 5432 mW;

All sensors measuring (with ASIC heating): 10 082 mW.