Thermal design of the Air Temperature Sensor (ATS) and the

47th International Conference on Environmental Systems ICES-2017-193 16-20 July 2017, Charleston, South Carolina

Thermal design of the Air Temperature Sensor (ATS) and

the Thermal InfraRed Sensor (TIRS) of the Mars

Environmental Dynamics Analyzer (MEDA) for Mars 2020

I. Pérez Grande1, L. Peinado2, A. Chamorro3, I. Torralbo4, G. Alonso5

Universidad Politécnica de Madrid, Madrid, 28040, Spain

and

José Antonio Rodríguez Manfredi6, Alain Lepinette7, Eduardo Sebastián8

Centro de Astrobiología, Torrejón de Ardoz, Madrid, 28850, Spain

The Mars Environmental Dynamics Analyzer (MEDA) is a suite of sensors that will

operate on the NASA Mars 2020 Rover. Two of the sensors are the Air Temperature Sensor

(ATS) and the Thermal InfraRed Sensor (TIRS).

The main aim of ATS is to measure the atmosphere temperature. It consists of five units

located in different parts of the rover, each including three chromel-constantan

thermocouples. The thermocouples’ wires are subjected to free and natural convection, Sun

illumination and receive infrared radiation from the planet’s surface. The prediction of

accurate temperatures of the wires is crucial for the verification of the correct performance

of the sensor. The main goal of TIRS is to measure the radiative flux emitted by the planet

surface, the sky and the surrounding CO2 atmosphere. It consists of a set of five sensors,

assembled in a common case, working in four wavelength bands. TIRS has a quite strict

thermal requirement in terms of thermal gradients in the sensors, which cannot exceed

24 mK. TIRS will be located in the Remote Sensing Mast (RSM) of the Mars 2020 Rover.

Both sensors are very sensitive from a thermal point of view and require detailed thermal

analysis. Solar, albedo and planetary infrared radiation was considered in the analysis,

together with the convective heat transfer, either natural or forced. For the solar

illumination the effect of the dust was taken into account, including the opacity factor and

the scattering of light. In this work, the results of these thermal analyses are presented. In

particular, the temperatures obtained for the worst case scenarios are given, as well as the

heat fluxes between the relevant elements. Both steady-state and transient cases were

analyzed.

Nomenclature

ATS = Air Temperature Sensor

CAD = Computer Aided Design

GL = Conductive Coupling

GMM = Geometrical Mathematical Model

GR = Radiative Coupling

I/F = Interface

MCRT = Monte Carlo Ray Tracing

1 Full Professor, Instituto de Microgravedad “Ignacio Da Riva”, [email protected].

ORCID: 000-0002-7145-2835. 2 Aerospace Engineer, Instituto de Microgravedad “Ignacio Da Riva”, [email protected]. 3 Thermal Engineer, Instituto de Microgravedad “Ignacio Da Riva”, [email protected]. 4 Thermal Engineer, Instituto de Microgravedad “Ignacio Da Riva”, [email protected]. 5 Associate Professor, Instituto de Microgravedad “Ignacio Da Riva”, [email protected]. 6 Researcher, Centro de Astrobiología (CSIC-INTA), [email protected] 7 Researcher, Centro de Astrobiología (CSIC-INTA), [email protected] 8 Researcher, Centro de Astrobiología (CSIC-INTA), [email protected]

mailto:[email protected]

International Conference on Environmental Systems

2

MMRTG = Multi-Mission Radioisotope Thermoelectric Generator

PCB = Printed Circuit Board

PI = Proportional-integral

RSM = Remote Sensing Mast

TIRS = Thermal Infrared Sensor

TMM = Thermal Mathematical Model

WCC = Worst cold case

WHC = Worst hot case

α = Absorptance [dimensionless]

β = Coefficient of thermal expansion [K-1]

C = Constant for Hilpert correlation [dimensionless]

D = Diameter [m]

ε = Emissivity [dimensionless]

Gr = Grashof number [dimensionless]

gM = Mars gravity [m/s2]

h = Heat transfer coefficient [W/m2K]

hc = Thermal contact conductance coefficient [W/m2K]

kCO2 = CO2 thermal conductivity [W/m·K]

Lc = Characteristic length [m]

m = Constant for Hilpert correlation [dimensionless]

ν = Kinematic viscosity [s/m2]

Nu = Nusselt number [dimensionless]

NuD = Nusselt number based on diameter [dimensionless]

Pr = Prandtl number [dimensionless]

Ra = Raleigh number [dimensionless]

Re = Reynolds number [dimensionless]

Tfilm = Film temperature [K]

Ts = Surface temperature [K]

T∞ = Flow temperature [K]

τ = Optical depth or dust opacity [dimensionless]

I. Introduction

HE Mars 2020 mission is a future rover

designed to investigate the habitability of Mars.

The mission is part of NASA's Mars Exploration

Program, a long-term effort of robotic

exploration of the Red Planet. Mars 2020 rover

will be launched in of July 2020 and will land on

Mars in February of 2021. The landing place is

not selected yet, therefore, the worst scenario

among the landing site candidates is used for the

design. The rover will investigate the geology of

the site, assess its habitability and look for signs

of ancient life in Mars. In order to achieve these

scientific goals the rover is equipped with 7

instruments1.

The Mars Environmental Dynamics Analyzer

(MEDA) is one of the Mars 2020 rover

instruments currently being developed for the

mission. This paper is focused on the thermal

design and analysis of two of the sensors of

MEDA instrument. These sensors are the Air

Temperature Sensor (ATS) and the Thermal

InfraRed Sensor (TIRS).

I. T

RSM

TIRS (75°)

X

Figure 1. ATS and TIRS locations on rover. ATS1, ATS2,

ATS3 and TIRS are mounted in the Remote Sensing Mast (RSM)

at orientations 50º, 155º, 290º and 75º respectively (clockwise

angles from rover chassis X axis) and ATS4 and ATS5 are

assembled on left and right chassis corners.


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II. MEDA Instrument: ATS and TIRS

MEDA instrument is a suite of sensors that will monitor the environmental conditions of the landing site. It is an

evolution of the Rover Environmental Monitoring Station (REMS) of the Mars Science Laboratory (MSL) mission2.

MEDA sensors will measure the Mars near-surface atmosphere and ground temperatures, wind speed and direction,

pressure and relative humidity. A camera pointing to sky is assembled in MEDA and also a set of photo-detectors

for sky imaging and measurement of ultra-violet, visible and near infrared irradiations at several bands that will

allow characterization of the atmospheric dust. ATS and TIRS are externally mounted on the Mars 2020 rover as it

can be observed in Figure 1.

a. ATS

The objective of ATS is to determine

the temperature of Martian atmosphere

close to the rover. There are 5 units that

will be externally accommodated on the

Mars 2020 rover. Three of them are

mounted on the Remote Sensing Mast

(RSM) of the rover, and another two

sensors are mounted on vertical surfaces

of the rover chassis corners.

ATS is a passive instrument, based on

3 redundant thermocouples mounted in

parallel and vertically in a FR4 structure,

as can be seen in Figure 2. The

thermocouple thin-wires are constantan-

chromel (E-type) of 0.075 mm diameter with butt-welded junction. The reference junctions are the junctures of the

thermocouple wires to copper wires. Those junction blocks (cold junctions) consist of two alumina pieces housed

into an isothermal block of aluminum located in the base support of ATS whose temperature is measured with a

Pt1000 thermistor. Each sensor is within a volume of 23.9 mm x 57.4 mm x 65 mm.

Since the main goal of ATS is to properly measure the temperature of the Martian atmosphere the thermal design

must guarantee that the thermocouple wires are as close as possible to the temperature of the air. This objective is

carried out by thermally insulating the sensor from the rover, by reducing the absorbed external radiation and by

exposing the thermocouple wires directly to the atmosphere to allow proper convection in the wires.

The structure of ATS is made of FR4 composite which has low thermal conductivity. The external FR4 structure

of ATS is painted white (except for the FR4 back cover), as well as TIRS and the rover chassis, to minimize the

solar radiation absorption.

b. TIRS

The purpose of TIRS is to measure the

thermal infrared radiation emitted by the

ground, sky and atmosphere, as well as

solar reflected radiation on the ground.

TIRS consists of an aluminium

structure (case, back plate and insulation

plate) that protects the components of the

instrument, that is, five thermopiles

placed on an aluminium support plate, an

aluminium calibration plate and two

PCBs, made of FR4, for connections, as

can be seen in Figure 3. As has been said, the thermopiles are accommodated in a support plate which is

conductively insulated from the case using low thermal conductivity spacers. The thermopiles are protected by an

aluminum support plate cover. The calibration plate is used for in-flight calibration of thermopiles during the

mission. This calibration consists on heating up the calibration plate to induce a known signal in the sensor. It is

attached to the insulation plate, which is a plate that closes the case and protects the instrument. The dimensions of

TIRS are 57 mm x 62.4 mm x 57 mm.

Figure 2. Air Temperature Sensor (ATS) parts.

Figure 3. Thermal Infrared Sensor (TIRS) parts.


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TIRS will be placed on the RSM of the Mars 2020 rover. The position of TIRS has been selected in order to

reduce the perturbation produced by the Multi-Mission Radioisotope Thermoelectric Generator (MMRTG) footprint

in the measurements.

The thermal design of TIRS has the goal of providing stable conditions for the thermopiles. The uncertainty of

the measurements of the thermopiles is affected by the spatial and temporal gradients3, so the thermal design of

TIRS is focused on the reduction of such gradients. According to the requirements of TIRS, the spatial gradient in

the thermopiles shall be lower than 24 mK and the temporal gradients in the thermopile shall be lower than 9 mK/s.

These requirements were extracted from the error budget of the instrument, taking into account all the sources of

uncertainty in the measurements. The thermal gradients in the calibration plate shall be lower than 226 mK for the

Area 1 and lower than 700 mK for the Area 2, these areas are defined in Figure 13. This requirement is needed to

guarantee the validity of the assumption used during the calibration (the temperature difference between the data to

be provided by the Pt1000 of the calibration plate and the areas that obstruct partially the FOV of the sensors is

considered negligible).

The design for fulfilling the stability requirements is based on insulating the support plate from the case. The

conductive coupling is reduced by using FR4 spacers, which have low thermal conductivity. The internal surfaces of

the instrument have low ε and low α finish, which is characteristic of the Alodine 1200 treatment. Using this surface

finish the radiative coupling between the support plate and the rest of the instrument is also reduced. The external

surfaces of TIRS are painted white, using the same paint as the rover, to reduce the solar heat load.

Finally, in order to reduce the power needed to heat the calibration plate, it is attached to the insulation plate. The

insulation plate is made of FR4, therefore the conductive coupling between the calibration plate and the instrument

is small.

III. Thermal design cases

Taking into account that the landing site has not been selected at the time this paper is being written, it is needed

to consider the worst scenario to assess the thermal design. Among the different options, Holden Crater area has

been considered as the worst thermal scenario for both cold case and hot case. In the following subsections the

thermal environment corresponding to Holden Crater is described.

a. Mission scenarios

The main parameters that define

the worst case scenarios for thermal

analysis, Worst Cold Case (WCC) and

Worst Hot Case (WHC) are strongly

influenced by the season. These

parameters are:

1) Solar constant and albedo

coefficient.

2) Planetary infrared and sky

temperature.

3) Rover temperature.

4) Atmospheric temperature,

pressure, and wind speed (if

any).

5) Thermo-optical properties,

either degraded due to dust or

pristine.

6) Rover orientation.

Table 1. Definition of worst thermal scenarios (steady-state analysis).

Winter and summer temperatures are defined in Table 2. For convection

details see Section IV.

Boundary condition

& thermal loads WCC WHC

Solar and albedo loads Night time (no solar

and albedo loads)

Summer solar loads

Albedo 0.13

Planetary infrared, Sky

and air temperatures

εground 0.9

Winter temperatures

εground 0.9

Summer temperatures

Conductive & radiative

rover interfaces Winter temperatures Summer temperatures

Convective cooling Free convection

Pressure 1066.6 Pa

Free convection

Pressure 666.6 Pa

Thermo-optical

properties Pristine Degraded

Radiative case Night time Maximum solar

exposure


5

Dust deposition effect is to

increase the solar absorptance of the

surface, while its effect in the

emissivity is negligible. Therefore, in

order to define the worst case

scenarios, the absorptance value for

pristine thermo-optical properties

shall be applied in Worst Cold Case

(WCC) whereas the value degraded

by dust deposition shall be used in the

Worst Hot Case (WHC).

On the basis of defining the above

points, it is possible to draw up two

worst case scenarios that are

summarized in Table 1.

b. Solar loads

The solar heat fluxes applied in

thermal cases corresponds to the most

extreme scenario. Holden Crater, with

latitude of 26.6º S, is the farthest

location from the equator within the

candidate locations. In this place there will be the largest variations in solar loads between winter and summer. The

solar fluxes corresponding to winter will be the lowest ones. And for summer the solar irradiance will be the highest

since the inclination of the axis is 25.29º and Sun rays are almost perpendicular to Holden Crater in that season.

Due to airborne dust, the solar loads that reach the planet surface are smaller than at the top of the atmosphere.

Another effect of the atmospheric opacity is that a portion of the incident solar flux is scattered, therefore there is a

diffuse solar load incident in the external surfaces of the instrument.

As boundary condition for both summer and winter, it is applied an optical depth τ of 0.2. For albedo flux load it

has been considered for both winter and summer an albedo value of 0.13. Since the steady-state WCC occurs during

night neither solar nor albedo loads are applied, although they are taken into account for winter day transient

analysis. Figure 4 shows the evolution of all solar loads in both summer and winter over a whole Martian day of

88775 s, note that the period of time for daytime is according to the season. The extreme values have been used for

steady-state calculations.

c. Boundary conditions

Temperatures for both winter and summer

have been set for the environment (sky, ground

and atmosphere) and rover interfaces. These

temperatures vary during a whole day cycle and

their maximum and minimum values are

presented in Table 2. For steady-state analyses,

the minimum temperatures of a winter day are

considered for WCC and the maximum

temperatures of a summer day for WHC.

d. Power

Since ATS is a passive sensor, there is no power dissipation inside of the instrument whereas TIRS is a more

complex instrument with significant power consumption.

TIRS is equipped with four heaters. Two of them are placed in the support plate and two are placed in the

calibration plate. The maximum power of these heaters is 0.4 W, hence the maximum power of the instrument is 1.6

W. The thermopiles are passive sensors and the PCB is only used for electrical connections, therefore the heaters are

the only source of heat dissipation. These heaters will be used for in-flight calibration. The heaters will be operated

with a Proportional Integral (PI) control scheme in order to keep the temperature either of the support plate or the

calibration plate at a certain value.

0

100

200

300

400

500

600

700

0 20,000 40,000 60,000 80,000

He

at f

lux

[W/m

2]

Time [s]

Winter and summer solar direct, solar diffuse and albedo heat fluxes [W/m2]

Summer direct solar

Summer diffuse solar

Summer albedo

Winter direct solar

Winter diffuse solar

Winter albedo

Figure 4. Winter and summer direct solar, diffuse solar and albedo

loads at Holden Crater.

Table 2. Maximum and minimum temperature boundary

conditions of a day.

Boundary conditions

Temperature [ºC]

Winter Summer

Min Max Min Max

Ground -103.2 -47.8 -65.4 32.2

Sky -152.4 -137.4 -114.5 -85.4

Atmosphere -101.6 -62.3 -63.1 9.5

Rover top deck -107.9 -68.3 -34.7 43.3

Rover RSM -107.9 -68.8 -62.3 9.4

Rover chassis corners -107.9 -67.9 -55.7 14.9


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IV. Geometrical Mathematical Model and Thermal Mathematical Model

The Geometrical Mathematical Models (GMM) of ATS and TIRS contain the main elements the instruments

consist of. The software tool for the analysis is ESATAN-TMS r7 and 2017 releases. Radiative calculations have

been performed with Monte Carlo Ray Tracing (MCRT) method. The details of each model are provided in this

section.

A reduced model of the rover shown in Figure 5, has been used in order to represent with a higher fidelity the

radiative environment of the instrument during its operational phase. In addition, this reduced model includes the

effect of the shades produced by the rover, which can be an important effect for the temporal gradients, due to the

sudden change in the solar loads.

Taking as baseline the GMMs of the

instruments, a Thermal Mathematical Model

(TMM) has been created for each instrument.

Some of the internal conductive couplings

have been automatically calculated by the

software, while most of the conductive

couplings have been calculated by hand. In

order to make these calculations, the

interfaces between the components of the

instruments have been analyzed and the

thermal contact resistance has been taken into

account.

The external convective heat transfer between the instruments and the atmosphere has been calculated taking

into account the dependency between the heat exchange, the geometry and the fluid properties. Therefore, the heat

transfer coefficient h has been computed taking into account: the geometry of the surfaces, the surface orientation

and the atmosphere environment (temperature, wind, pressure). These parameters will define if free (0 m/s wind

speed) or forced convection occurs (15 m/s wind speed for forced convection). In this document analyses for forced

convection are only presented for ATS.

The transport coefficients such as viscosity, thermal conductivity and Prandtl number of the atmosphere have

been evaluated at the film temperature Tfilm, which is defined as the mean temperature of the surface Ts and the air

flow temperature T∞. Since Mars atmosphere transport coefficients need to be obtained for carbon dioxide at low

pressures and very low temperatures, Oszceski4 proposes regression equations for obtaining Prandtl number, thermal

conductivity and kinematic viscosity.

For free convection, the Grashof number Gr and Rayleigh number Ra are calculated by means of equations (1)

and (2).

3

2

·Gr

M s cg T T L

(1)

Ra Gr·Pr (2)

where 1/ T denotes the coefficient of thermal expansion, gM is the Mars gravity, Lc represents the

characteristic length of the surface and ν is the kinematic viscosity.

The correlations used for both natural and forced convection for flat geometries at different orientations were

extracted from Incropera5. They have been applied for vertical plates and horizontal plates with air below and above.

And in forced convection, correlations for plates parallel to the flow, plates perpendicular to the flow and cylinders

in cross flow are computed.

ATS thermocouples represent an example of the relevance of the convective heat transfer in the Martian

environment. It is necessary to validate the thermal performance of the thermocouples that will be used to measure

the atmospheric temperature. ATS is mounted in the rover, with such an orientation that the thermocouple wires

stand in vertical position, with chromel at the top and constantan at the bottom (non-twisted thermocouples) in all of

the five locations. In this orientation, in case of still air, the thermocouple wires will be subjected to free convection

Figure 5. Rover and ATS4 GMM.


7

along their length. In case of incident wind, it is only considered wind

parallel to ground, which means that the thermocouple wires are always in

cross flow.

The correlation for vertical thin cylinders in free convection obtained

from Kyte6, in equation (3), is specially selected for the thermocouples

wires since they are the most sensitive part of ATS. In forced convection

for cylinders in cross flow, Hilpert correlation7 in equation (4), is used

with the adequate constants of the Table 3.

D

0.26

D

2Nu

4.47ln 1

Gr ·Prc

D

L

(3)

1 3

D DNu ·Re ·PrmC (4)

Apart from the external convection, both ATS and TIRS have small cavities in their structure, where the internal

parts are allocated. The effect of the Martian atmosphere in the interior of the instruments is not negligible. The

internal cavities of ATS and TIRS have a characteristic longitude lower than 40 mm. Therefore, an analysis of the

convection parameters determines that the conduction through the CO2 gaps is the dominant phenomenon. Internal

conduction through CO2 gap shall be computed between internal surfaces of the cavities, taking into account that the

conductivity of the CO2 is dependent on the temperature. In addition, since the sensors are mounted with stand-offs,

there is a small gap between instrument back cover and rover in which thermal conduction through CO2 instead of

convection has been also computed.

Some of the internal conductive couplings between FR4 nodes have been manually calculated taking into

account the two different thermal conductivities of this material, either longitudinal or crosswise. Conductive

couplings in bolted unions, simple contact, contact by glue, etc. have been estimated by similarity. The thermal

interface between the instruments and rover consists of four contact regions, each one with a bolted union with M4

screw and washer. The bolted unions have not been represented geometrically in the thermal model but they have

been taken into account for the thermal conductance with the rover interface, either the RSM or the chassis corners.

In these contact regions no radiative exchange has been computed. This conductive coupling has been estimated by

similarity as well. Subsequently, uncertainty and sensibility analyses have been performed for these estimated

calculations.

The TMM has been completed with the addition of the solar loads, the convective exchange factor calculation

and the boundary conditions described in previous sections. The details of the TMM of each instrument are provided

in this section.

a. ATS

The geometrical model has been set up based on

the mechanical design, hence ATS structure and

inner parts have been represented geometrically with

the exception of the cold junctions and sensor

Pt1000 that have been taken into account

mathematically as non-geometrical thermal nodes.

ATS GMM is shown in Figure 6. In order to increase

the accuracy in the radiative exchange factors, some

surfaces of ATS have been considered as critical.

These include the thermocouple wires and the

surfaces with more reflections from rover. Radiative

calculations have been performed with minimum

rays per non-critical faces equal to 50,000 and

5,000,000 for critical faces.

Figure 6. ATS GMM.

Table 3. Constants of equation (4)

for circular cylinder in cross flow.7

ReD C m

0.4 – 4 0.989 0.330

4 – 40 0.911 0.385

40 – 4,000 0.683 0.466

4,000 – 40,000 0.193 0.618

40,000 – 400,000 0.027 0.805


8

The Thermal Mathematical Model of ATS consists of 306 nodes, 6 of them

are non-geometrical thermal nodes. These nodes include the rover model and

environment. In order to get enough accuracy in the thermocouple wires, they

have been discretized in 20 nodes per wire, note that each node is 1.25 mm long.

Conductive coupling between nodes of the same piece of material and butt-

welded unions of thermocouples have been calculated as ‘fused’ type. Other

conductive couplings such as bolted unions, thermal conduction through wires,

glued parts or simple contact have been estimated by similarity taking into

account a thermal contact conductance coefficient hc.

The thermal capacitances automatically calculated by ESATAN-TMS in

the TMM have been adjusted to the real value according to the volumes and

materials of the CAD model. The total capacitance of ATS is 39.3 J/K.

b. TIRS

The Geometrical Mathematical Model of TIRS is shown on Figure 7. Due to

the thermal requirements of the instrument a detailed representation of the

support plate and the calibration plate is included. The need to verify the

thermal gradients in these elements leads to a higher discretization of them,

despite the high conductivity of those components. Radiative calculations have

been performed with minimum rays per face equal to 50,000.

The Thermal Mathematical Model of TIRS consists of 823 thermal nodes,

820 of them are shell nodes and 3 are non-geometrical nodes. This amount of

nodes includes the nodes representing the conductive and radiative boundaries

of the instrument.

The assumptions made for the calculations have been:

1) Support plate: component where the two dimensional assumption is not

valid. Therefore, it has been modeled as a 2D ‘box’, and the conductive

couplings between the nodes have been calculated following the

method exposed by Garmendia8.

2) The conductive couplings through the bolted joints have been adjusted

taking into account the results from a dedicated test performed in a CO2

pressurized chamber. The pressure of the chamber in the test is

representative of the pressure of the Martian atmosphere in the Holden

Crater location.

3) According to the results of that test, the effect of the conductive coupling

between the calibration plate and the instrument which is provided by

the harness of the heaters and the Pt1000 placed in that plate is not

negligible. This conductive coupling has been estimated taking into

account the length and diameter of the wires and it has been applied to

the nodes where the connections between the heaters and the wire are

located.

The thermal capacitance of the model has been calculated following the

mass budget of the instrument and the materials used. The thermal capacity of

the model is 70.1 J/K.

V. Results

All cases described in section III have been analyzed. However, for simplicity reasons only some of them are

presented in this document.

a. ATS results

Although analyses for all ATS locations are performed, only ATS4 results (analogous to ATS5) are presented in

order not to overly extend this document. ATS4 and ATS5, located in chassis corners of the rover, experience WHC

conditions more extreme than the ones assembled on RSM. The following results are steady-state analyses for WCC

and WHC, with free convection and forced convection with frontal wind incident to ATS as well as transient worst

cases. Note that on the contrary of what could be presumed for WCC, forced convection is not the extreme condition

Figure 7. TIRS GMM.


9

since the air turns out to be warmer than radiative boundary conditions for winter. In forced convection analyses,

forced convection has been computed in surfaces with incident wind whereas for faces behind the incident flow the

heat exchange computed is by means of free convection.

The temperatures extracted from the analysis belong to ATS parts indicated in Figure 8.

First analyses WCC and WHC with free convection, give as a result the most extreme temperatures predicted in

the instrument. All temperatures, including a margin of uncertainty of ±10 ºC9 (which is the usual margin in the

current phase of the project) are within the required range of temperatures, from -128 ºC to 50 ºC. The expected

temperatures with no uncertainty margin can be observed in Table 4 and in Figure 9.

As can be observed in Figure 9, there is a temperature gradient from the interface with the rover to the edge of

the sensor. This is partly because of the low conductivity of the FR4 structure. There are no temperature variations

between the internal parts of ATS, cold junctions and Pt1000 sensor since they are assembled into the aluminum

isothermal block which maintains an homogeneous temperature.

The conductive heat flow exchanged through the I/F

between ATS and rover has a value of –0.10 W in WHC,

which means ATS loses heat, whereas for WCC there is

0.003 W of heat flow through the I/F, for free convection

steady-state analyses.

In both cases, WCC and WHC, hot junction

temperatures are closer to Martian air temperature than

other ATS parts due to its higher value of convective heat

transfer coefficient than other parts.

Unfortunately, because of the high sensitivity of the

thermocouples to heat transfer radiation by means of their

absorptance and emissivity, neither in hot case nor in cold

case, the hot junction temperature matches exactly the air

temperature.

In case of sun light exposure

the thermocouples reach higher

temperature than air, up to 4.1 ºC

hotter than atmosphere, whereas

during night they could be up to

0.6 ºC colder than the

atmosphere.

In forced convection analyses

this problem reduces

significantly, the maximum

temperature difference being

1.2 ºC in WHC (hot junction

hotter that air) and 0.1 ºC in

WCC (hot junction colder than

air).

Figure 8. ATS temperatures of interest. Note that the temperatures presented for the thermocouples are an

average temperature of the three hot junctions, the unions of chromel and constantan in the middle of the wires.

Table 4. Expected temperatures of ATS parts for

WCC and WHC in natural convection.

Temperatures with no uncertainty margins.

ATS parts Temperature [ºC]

WCC WHC

Base (Box) -108.3 27.9

Middle -108.4 29.2

Upper -108.9 30.5

Lower -108.2 31.5

Pt1000 -108.3 28.6

Cold Junction Chromel -108.3 28.5

Cold Junction Constantan -108.3 28.5

Hot Junction Thermocouples -102.1 13.3

Figure 9. ATS temperature distribution [ºC] for WCC (left) and WHC

(right) in free convection. Temperatures with no uncertainty margins.


10

Forced convection analyses have been performed by

introducing a wind speed of 15 m/s in frontal direction,

which means the air flows from the edge of the sensor to

the interface with rover chassis. It can be observed in

Table 5 and in Figure 10 that, thanks to heat exchange

with the atmosphere, ATS temperatures in WCC are less

extreme than in free convection analysis, and for WHC in

forced convection the temperatures are lower than in

natural convection. In forced convection steady-state

analyses the heat flow exchanged through the interface

with rover is –0.01 W for WHC and for WCC is –0.06 W,

in both cases ATS loses heat.

In order to assess the thermal behavior of the

instrument, uncertainty analyses have been performed

over physical properties as

FR4 thermal conductivity and

absorptance and emissivity of

all the surface finishings. Also,

the contact thermal

conductances estimated by

similarity have been varied to

evaluate their effect in the

thermal behavior. The

calculation of the thermal

uncertainty has been done

following recommendations on

ECSS Thermal Analysis

Handbook9. After these

analyses, it has been estimated

an uncertainty for steady-state WCC with an offset of –0.1 ºC in ATS parts temperatures, thermocouples included.

For steady-state WHC the temperature offset due to uncertainty has an estimated value of +5.3 ºC for ATS parts

temperatures except for the thermocouples which have an offset of +0.3 ºC. Negative offset for WCC temperatures

and positive offset for WHC, make the temperatures in both cases more extreme.

Other analyses performed on thermal model of ATS are the transient ones, computing the heat exchanges

through a whole Martian day in both winter and summer. The results presented correspond to transient analyses with

natural convection since the extreme results in steady-state are with natural convection. The transient analyses have

been computed with such orientations of the rover that for winter day, ATS is exposed to sun light less than four

hours since sunrise, at the coldest hours of the day. On the other hand, for summer day the sensor is exposed to solar

radiation more than seven hours after midday, during the hottest hours of the day.

Figure 10.ATS temperature distribution [ºC] for WCC (left) and WHC (right)

in forced convection. Temperatures with no uncertainty margins.

Table 5. Predicted temperatures of ATS parts for

WCC and WHC in forced convection.

Temperatures with no uncertainty margins.

ATS parts Temperature [ºC]

WCC WHC

Base (Box) -103.4 17.9

Middle -102.9 17.5

Upper -102.8 18.0

Lower -102.6 18.4

Pt1000 -103.3 18.2

Cold Junction Chromel -103.3 18.3

Cold Junction Constantan -103.3 18.3

Hot Junction Thermocouples -101.6 10.4

-120

-100

-80

-60

-40

-20

0

0 20,000 40,000 60,000 80,000

Tem

pe

ratu

re [

°C]

Time [s]

Transient winter day - ATS temperatures [ºC]

Pt1000Hot JunctionBase (box)MiddleUpperLowerAir

-80

-60

-40

-20

0

20

40

0 20,000 40,000 60,000 80,000

Tem

pe

ratu

re [

ºC]

Time [s]

Transient summer day - ATS temperatures [ºC]

Pt1000Hot JunctionBase (box)MiddleUpperLowerAir

Figure 11. Winter (left) and summer (right) transient ATS temperatures with free convection. Note that it is

assumed that Pt1000 temperature corresponds also to chromel and constantan cold junctions temperature.


11

It can be observed in Figure 11 the effect of the solar incidence over ATS in the sudden increase of temperature

of some parts in the summer analysis. The heat flow between ATS and rover varies between –0.09 W and +0.04 W

in case of transient WHC, whereas for transient WCC it varies between –0.01 W and +0.02 W.

b. TIRS results

The results presented in this document correspond to the steady-state analyses for WHC and WCC, both in free

convection regime. Taking into account that the instrument has several operation modes with different power

consumption, the operation modes which provide the most extreme temperatures are shown. Therefore, the cases

shown in this section are the WCC with no power consumption and the WHC with the heaters of the calibration

plate consuming 0.8 W in total. Although the maximum power of TIRS is 1.6 W (0.8 W in the calibration plate

heaters and 0.8 W in the support plate heaters), the heaters of the calibration plate and the heaters of the support

plate will not be activated simultaneously. Figure 12 shows the temperature map for the instrument in the WHC. The

heat fluxes of the instrument are shown in Table 6.

Temperature gradients in the package of the thermopiles are a source of noise in the measurements, as has been

said in subsection II.b. Temperature gradient in the thermopiles, defined as the temperature difference between the

top and the bottom of the thermopile, in this case is lower than 7 mK.

A detailed temperature map of the calibration

plate is shown in Figure 13, and the maximum

gradients for both areas are shown in Table 7.

Another thermal requirement of the

instrument is a maximum allowable value for the

temperature gradients between the Pt1000 of the

support plate and the surfaces where thermopiles

are located. Figure 14 shows that the maximum

gradient between the Pt1000 and the mounting

surfaces of the thermopiles shall be less than

Figure 12. TIRS temperature distribution [ºC] for WHC. Power: 0.8 W. Temperatures with no uncertainty margins.

Table 7. Temperature gradients in the Calibration plate.

Temperature and gradient results of the Calibration plate

Part T (ºC) Max ΔT

Pt1000 34.66 -

A1 34.57 -0.08

A2 34.82 0.16

Table 6. Heat fluxes in WHC.

Heat fluxes (W)

Qheaters 0.80

QI/F -1.06

QSdirect 0.96

QSdiffuse 0.18

QAlbedo 0.13

QIR Mars 2.04

Qconvection -0.02

Qradiative -3.04

Figure 13. Temperature gradients in the calibration plate in the WHC.


12

124 mK. The WHC where the calibration plate heaters are ON is not the most demanding case for this requirement

because in the case, where the support plate heaters are ON, gradients will be higher. Despite of this fact, the

requirement is fulfilled in all the cases analyzed. This is due to the thickness and thermal conductivity of the support

plate, which make the conductive coupling between the points where the gradients are assessed large enough to

prevent the presence of gradients out of the allowable values. Table 8 shows the results for these gradients for the

WHC where support plate heaters are ON and consuming 0.8 W in total.

Figure 15 shows the temperature map for the

instrument in the WCC. The heat fluxes for the

WCC are shown in Table 9. Due to the absence of

solar heat inputs and power consumption in the

heaters, the gradients in the instrument are

reduced. Temperatures of all the instrument are

inside a range of less than 1 ºC.

Thermal requirements in this case are also

fulfilled, because the gradient in the thermopiles

is lower than 24 mK and the gradients in the

calibration plate and in the support plate are also

lower than their maximum values due to the

temperature uniformity in the instrument.

VI. Conclusions

Thermal models of the sensors ATS and TIRS of the MEDA instrument of the mission Mars 2020 have been set

up. The main aspects of the modellization of the instruments and the Martian environment have been exposed and

the obtained results have been shown.

ATS does not exceed the temperature range from -128 ºC to +50 ºC even with an uncertainty margin of ±10 ºC.

It has been demonstrated that a proper thermal design is determinant on ATS performance, since the temperature of

the hot junctions need to be as closer as possible of the outside atmosphere temperature. Furthermore, it is absolutely

Figure 15. TIRS temperature distribution [ºC] for WCC. Power: 0.0W. Temperatures with no uncertainty margins.

Table 8. Heat fluxes in WCC

Heat fluxes (W)

Qheaters 0.00

QI/F 0.06

QSdirect 0.00

QSdiffuse 0.00

QAlbedo 0.00

QIR Mars 0.19

Qconvection 0.02

Qradiative − 0.27

Figure 14. Definition of gradients in the support plate.

Table 9. Temperature gradients in the support plate for WHC

with the support plate heaters ON.

Temperature and gradient results of the support plate

Part T (ºC) ΔT

Pt1000 − 89.47 -

Mounting surface 1 − 89.50 − 0.02

Mounting surface 2 − 89.39 0.08





13

necessary to set up an accurate thermal modelization of the instrument in order to know the temperature offset

between the hot junctions and the atmosphere.

TIRS is also inside its allowable flight temperature (-128 ºC to +50 ºC) in all the scenarios. In addition, it has

been shown that the temperature gradients requirements are also fulfilled. Therefore, results of the thermal analyses

support the thermal design of the instruments. The results of TIRS thermal analyses have been correlated with tests

conducted in a thermal chamber with a CO2 atmosphere with controlled pressure.

Finally, these thermal models will be correlated with flight data in order to improve the results and the

knowledge about the models and they will be used to support the operations of the instrument during the flight.

Acknowledgments

The MEDA TIRS and ATS are designed and built under funding of Instituto Nacional de Técnica Aeroespacial

(INTA), Plan estatal de I+D+I (ESP2014-54256-C4-1-R, ESP2015-68281-C4-1-R, ESP2016-79612-C3-1-R) and

CDTI.

References

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45th Int Conf Environ Syst. 2015;134(July). https://ttu-ir.tdl.org/ttu-ir/handle/2346/64407.

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rover. Space Sci Rev. 2012;170(1-4):583-640. doi:10.1007/s11214-012-9921-1.

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presence of case thermal gradients. Infrared Phys Technol. 2011;54(2):75-83. doi:10.1016/j.infrared.2010.12.038.

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doi:10.1016/j.applthermaleng.2011.03.022.

6. Kyte JR, Madden AJ, Piret EL. Natural-convection heat transfer at reduced pressure. Chem Eng Prog. 1953;49(12):653-

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7. Hilpert R. Forsch. Geb. Ingenieurwes. 4. 1933:215.

8. Garmendia I, Anglada E, Vallejo H, Seco M. Accurate calculation of conductive conductances in complex geometries

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Documents

Thermal design of the Air Temperature Sensor (ATS) and the