Wind Sensing on the Surface of Mars Eurion Leonard-Pugh

Wind Sensing on the Surface ofMars

Eurion Leonard-Pugh

August 24, 2011

Supervisors: Colin Wilson, Simon Calcutt

Reviewers: Fred Taylor, Pat Irwin

7,200 words

ABSTRACT

This report describes the work done to date and the further research necessary tocomplete a thesis project on the subject of wind sensing on Mars. The project isdivided into two main areas; thin-film thermal anemometry and capacitive ultrasonicanemometry. In addition, the project involves the re-commissioning of the OxfordLow-Density Wind Tunnel (LDWT) which has recently been moved to the new AxisPoint site in Osney.

CONTENTS

Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . i

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.1 The Martian Atmosphere . . . . . . . . . . . . . . . . . . . . . . . . . 11.2 Previous Missions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21.3 Anemometry Techniques . . . . . . . . . . . . . . . . . . . . . . . . . 3

2. Capacitive Ultrasonic Transducers . . . . . . . . . . . . . . . . . . . . . . . 52.1 Principles of Operation . . . . . . . . . . . . . . . . . . . . . . . . . . 52.2 Previous Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62.3 Instrument Design and Requirements . . . . . . . . . . . . . . . . . . 62.4 Hardware Testing and Progress . . . . . . . . . . . . . . . . . . . . . 12

2.4.1 Electronics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122.4.2 Low Pressure Testing . . . . . . . . . . . . . . . . . . . . . . . 162.4.3 Film Thickness . . . . . . . . . . . . . . . . . . . . . . . . . . 182.4.4 Transducer Separation . . . . . . . . . . . . . . . . . . . . . . 202.4.5 Backplanes . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

2.5 Future Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

3. ExoMars MarsWind Instrument . . . . . . . . . . . . . . . . . . . . . . . . 253.1 The ExoMars Programme . . . . . . . . . . . . . . . . . . . . . . . . 253.2 The Beagle 2 Wind Sensor . . . . . . . . . . . . . . . . . . . . . . . . 263.3 Future Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

4. Low-Density Wind Tunnel . . . . . . . . . . . . . . . . . . . . . . . . . . . 294.1 Wind Tunnel Design . . . . . . . . . . . . . . . . . . . . . . . . . . . 294.2 Current Progress . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 304.3 Ongoing Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

5. The Project . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

6. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

Appendix 37

A. Gas Piston Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

B. Electronics Circuit Diagram . . . . . . . . . . . . . . . . . . . . . . . . . . 41

C. Thesis Outline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

1. INTRODUCTION

Named after the Roman god of war, Mars has fascinated scientists since it wasfirst observed by the ancient Egyptians. However despite great advances intechnology and a large number of missions to Mars, there is still much that we donot know about the red planet; in particular it’s atmosphere.

1.1 The Martian Atmosphere

In many ways, Mars is very similar to Earth. Although it is roughly half as largeas the Earth, it has a very similar axial tilt and experiences 24.7 hour days. Onesignificant difference between the two planets are their atmospheres; whilst theEarth’s atmosphere is composed mainly of nitrogen and oxygen, Mars’ is 95%carbon dioxide. As a result, the atmospheric density is much lower on Mars andthe surface pressure is 170 times less than on Earth. Due to the very little amountof water vapour on Mars, the weather variability is primarily driven by dust; notwater like on Earth. Mars has a very thin atmosphere due to the lack of a globalmagnetosphere and atmospheric loss due to impacts. As a result, there are vasttemperature differences between day (300K) and night (140K) due to solarradiation. The substantial solar surface heating can lead to vast dust storms in thesummer months which can engulf the entire planet and create large variations (upto 30m/s) in the planetary boundary layer (up to 4-6km above the surface). It isnot known how these dust storms can grow to be so large or have such longevity(some can last for months). Dust can make data collection on Mars difficult asdust storms can obscure the data from satellites and can coat the solar panels andinstruments on surface landers. Since the global circulation patterns on Mars arevery similar to Earth, Global Circulation Models are being developed to gain abetter understanding of how the circulation is driven and how this will affect theMartian climate in the long term[1]. More data is needed to test the validity ofthese models and improve our understanding of the Martian atmosphere.

1. Introduction

1.2 Previous Missions

Fig. 1.1: Image from the surface of Mars showing the Phoenix Telltale wind sensor

Although there have been 49 Mars missions since the first one in 1960, only 24 havesuccessfully made it to Mars and transmitted data back to Earth[2]. Of these, onlyfour have collected wind speed data from the Martian surface [Table 1.1 below].

Mission Launch Termination Wind Sensor Type

Viking 1 20/08/1975 13/11/1982 3-film hot film

Viking 2 09/09/1975 25/07/1978 3-film hot film

Mars Pathfinder 04/12/1996 27/09/1997 6-wire hot wire, wind socks

Phoenix 04/08/2007 10/11/2008 Telltale

Tab. 1.1: Successful Surface Wind Sensor Missions. Scientists often refer to the largenumber of failed missions to Mars as the ”Mars curse”.

A total of about 250 sols (Martian days) of Mars wind data have been collected;most of which (151 sols) has come from the Phoenix telltale sensor (Fig 1.1). Datacollection for Phoenix involved photographing a suspended mass to determine windspeed and direction. However calibration problems[3] led to inconsistent data rates- between zero and 193 values per sol with an average of 49[4]. The Phoenix datashowed an average wind speed of 5m/s and a maximum of 16m/s; as well as diurnalvariations in both wind speed and direction.The Viking landers contributed the remaining 105 sols of wind data [6].

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1. Introduction

Few absolute wind values were ever published from the Pathfinder mission due tocalibration problems[5]; however the wind socks did provide limited data aboutboundary layer profiles for the first time.

Reliable 3-D wind speed data is vital to characterise aeolian processes and dusttransport on Mars; whilst validated mesoscale atmospheric models will lead toimproved predictability and safety for spacecraft landing. Combininghigh-frequency wind speed data from ultrasonic transducers with that fromcomposition sensors will enable local flux measurements (eg. water vapour,methane) to be made. Direct measurements of water vapor and its flux throughthe boundary layer have never been attempted on Mars. These measurementswould allow for a better understanding of water vapour exchange between thesurface and atmosphere[7].

1.3 Anemometry Techniques

Due to the different atmospheric conditions encountered on Mars, some methodsused to measure wind speed and direction on Earth are unsuitable for use on Mars.Four promising Martian wind speed measurement technologies are illustratedbelow and discussed overleaf.

Fig. 1.2: Potential Mars Anemometry Techniques. Clockwise from top left: PhoenixTelltale (mechanical), Beagle 2 Wind Sensor (thermal), Laser DopplerVelocimeter (optical), Oxford Ultrasonic Transducers (acoustic)

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1. Introduction

Mechanical methods such as windsocks and wind vanes have been previously usedto measure wind speed and direction on Mars; such as on Mars Pathfinder.Though these are relatively simple to manufacture, they are difficult to calibrateand are prone to damage from launch and landing.

Thermal wind sensor designs include hot wire and hot film; both operate on thesame principles and have been used successfully on previous missions. A wire isheated by passing a current through it and the temperature of the wire obtainedby measuring its resistance. The temperature of the wire varies with wind speeddue to convection; comparing this temperature with an independent ambienttemperature reading allows a wind speed to be determined. Though these are areliable, low-mass method for wind speed sensing on Mars, they require accurateindividual calibrations and are prone to erroneous results caused by varyingambient temperatures and other thermal loads. Film designs are typically morerobust and compact than wire designs.

Acoustic ultrasonic transducers are very popular on Earth for turbulencemeasurements due to their full-range accuracy and high measurement frequency.One was originally proposed to be included on the Beagle 2 lander but wasscrapped due to funding difficulties. Two transducers are positioned opposite eachother and the time of flight for the acoustic wave measured over both directions.The difference in transit times gives the wind velocity component parallel to thesensor axis and the average gives a value for the speed of sound; which can be usedto extrapolate the wind speed. Since the speed of sound is only a function oftemperature, ultrasonic transducers can also be used as high-frequencytemperature sensors. Ultrasonic transducers are very promising for use on Marsdue to their high measurement frequency, simple calibration, immunity to drift and3-D wind vector potential. Piezo-electric ultrasonic transducers, commonly usedon Earth, are unsuitable for use on Mars due to their high acoustic impedance.Capacitive ultrasonic transducers are much more suitable as they have acousticimpedances closer to that of the Martian atmosphere; and will be the focus of thisreport. However there are also several obstacles which need to be overcome beforethey can be used on Mars; these are discussed in Section 2.5 (Page 24).

Laser Doppler anemometry is used on Earth for remote, non-invasive flowmeasurements. Two coherent laser beams are focussed onto a point inside theflowing fluid. The light is back-scattered off particles suspended in the fluid flowand the Doppler-shifted frequency of the scattered light intensity can be used todetermine the speed of the particle perpendicular to the beams. Thesemeasurements are highly accurate, free from calibration errors and non-invasivebut cannot determine fluid flow direction and may not represent true fluid velocity.In addition, the sensors are relatively large and require complicated electronicprocessing. If they could be successfully miniaturised and modified to determineflow direction, they could be the most accurate and reliable method of windsensing on Mars[8].

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2. CAPACITIVE ULTRASONIC TRANSDUCERS

The principle research in this project will involve the optimisation of capacitiveultrasonic transducers for use on Mars. Unless otherwise acknowledged, all work ismy own.

2.1 Principles of Operation

These transducers consist of a conducting electrode and backplane separated by aninsulating layer [Figure 2.1 below].

Fig. 2.1: Transducer Schematic Diagram

The metal layer is drawn towards the backplane by a DC bias voltage. In effect, theyare two parallel plates separated by an insulator; and so behave like a capacitor. Inpractice, the insulator and metal layer are provided through the use of a metallisedpolymer film. The backplane is contoured to allow small trapped air pockets to formand allow the film to oscillate. A varying signal is decoupled on top of the DC biasto oscillate the film. The oscillating film causes acoustic waves to be produced; theirfrequency is dependent on the signal frequency.

2. Capacitive Ultrasonic Transducers

2.2 Previous Work

Capacitive transducers rely on the same principles as condenser microphones;invented in 1916 by E.C. Wente. Since then, research has been carried out oncharacterising transducer performance[9] and optimisation for signal transmissionthrough different materials[10]. They have also been shown to work at low gaspressures and in carbon dioxide atmospheres but there is more work to be done tooptimise the transducers for operation on the Martian surface[11]. There has alsobeen research on the different backplane manufacturing techniques includingmachining and etching[14]. In general, the greater the depth of the pits behind thefilm, the lower the resonant frequency of the transducer; lower resonant frequenciescan also be achieved using thicker membranes.

2.3 Instrument Design and Requirements

At the start of the year four transducers were provided along with an experimentaltest circuit. Two of the transducers were manufactured by the University ofWarwick and two were built in-house to be smaller and more ”flight-like” (Figure2.2). The Warwick transducer backplanes were manufactured usingmicro-machining techniques whereas the Oxford ones used conventional machining.Although different methods were used to produce the backplane features, theoperating principles of both transducer types are the same.

Fig. 2.2: Warwick transducer (35mm diameter, right) and Oxford ”flight-like” transducer(14mm diameter, left)

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Fig. 2.3: Exploded diagram showing construction of the Oxford transducers; courtesy ofColin Wilson.

The current generation Oxford transducers (Figure 2.3) were designed and builtbefore I arrived but future improvements will likely be made to optimisetransducer performance. For example, the current design uses a screw-in frontshell to provide tension in the Mylar film. Although this works well in providingvarying amounts of tension, this also twists the film and can cause it to tear.Alternatives are currently being investigated.

In order to determine optimum transducer operating conditions on Mars, progresshas been made in theoretical modelling of the transducer operation. There are twoprinciple models of transducer operation; one treats the trapped air in thebackplane grooves as a mass-spring oscillator (gas piston) and the other takes intoaccount the membrane stiffness. A new model combining both current models iscurrently under development.

See Appendix A (Page 39) for a derivation of the gas piston model.

Gas piston Model[12]:

f0 =12π

√γPa

ρmdmda(2.1)

Membrane Stiffness Model[13]:

f0 =2.405πD

√Tρm(2.2)

Resonant frequency f0, Adiabatic constant γ, Air-gap pressure Pa, Membranedensity ρm, Membrane thickness dm, Air-gap thickness da, Membrane diameter D,Membrane tension T

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Theoretically, larger forces are achieved by larger gas pressures, thinnermembranes, thinner air-gaps and lower density membranes. However these alsoresult in higher resonant frequencies; which would cause greater attenuation on theMartian atmosphere. Losses to the atmosphere are primarily due to beamspreading, impedance mismatch and attenuation.

Beam spreading is the loss of signal due to the directionality of the transmittedsignal. It is less of a problem on Mars as the speed of sound is lower than on Earth[Figure 2.4 below].

Fig. 2.4: Theoretical directionality of transducer signal at a frequency of 50kHz. Valuescalculated using standard diffraction equation and an aperture size of 1cm.

It is clear that the transducers need to be accurately aligned (±5◦) for maximumsignal transmission.

Impedance mismatch losses are the most significant when looking at modifyingultrasonic sensors for use on Mars. This is due to the large impedance differencesbetween the transducers and the atmosphere. Banfield[11] quotes a transducerimpedance of 1000 kg/m2/s; the acoustic impedances on Earth and Mars are 418kg/m2/s and 5 kg/m2/s respectively. These values translate to insertion losses of17% on Earth and 98% on Mars; these losses would be present at both transmitterand receiver. From these figures it is clear that trying to reduce the impedance ofthe transducers will be vital in trying to maximise the received signal amplitude.

The atmospheric attenuation at different frequencies has been well characterisedon both Earth[15] and Mars[16]. In both cases, the attenuation coefficientincreases with frequency. However there is a compromise to be made in theoperating frequency since there is also less beam spreading at higher frequencies.To find the optimum frequency, both effects need to be taken into account. Thisleads to an optimum signal frequency of around 85kHz [Figure 2.5 overleaf].

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Fig. 2.5: Theoretical Losses on Mars due to Beam Spreading and Atmospheric Attenuationfor a 1cm diameter transducer and a separation of 10cm; including total receivedintensity. Attenuation values on Mars courtesy of Andi Petculescu[16].

A similar method can be used to determine the optimum transducer separation.Shorter separations allow for the measurement of smaller eddies but at the sametime require higher resolution timing. Large separations allow for less accuratetiming however they also result in greater atmospheric attenuation and beamspreading losses. This is illustrated graphically in Figure 2.6 overleaf.

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Fig. 2.6: Trade-off between wind speed resolution and signal losses when choosing atransducer separation. Values calculated using a timing resolution of 62.5nsand a signal frequency of 50kHz. Combined values are the product of the signallosses and wind speed resolution at each separation.

This gives an optimal transducer separation of between 5cm and 10cm; though atthe same time the separation is constrained by transducer diameter - theseparation should be at least 5 times the transducer diameter (Oxford - 14mm,Warwick - 35mm) to ensure minimal interference with the flow[11]. This gives anoptimal transducer diameter of less than 2cm. This beam-spreading model usesfar-field diffraction and so is not valid where the Fresnel number <1; thiscorresponds to distances of less than 9cm. However, it was found experimentally tobe in good agreement down to 8cm (Figure 2.16, Page 21).

The transducers can be driven using a continuous sine-wave or several pulses.Pulses are preferred since they require much less power and it is simpler tomeasure signal travel times than signal phase shifts.

Other Mars wind sensors have had masses ranging from 15g (Beagle 2 ThermalWind Sensor) to 300g (MWX proposed sonic anemometer[7]) and powerconsumptions of 250mW to 1W respectively. We will be aiming for mass andpower budgets somewhere in-between these values.

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Typical Earth-based ultrasonic anemometers used for turbulence measurementshave measurement frequencies of up to 200Hz and wind speed resolutions of up to1mm/s. Considering the speed of sound on Mars is less than on Earth, theserepresent maximum values for a Mars-based system. For a separation of 10cm anda wind speed resolution of 1cm/s, a timing accuracy of 10ns (10MHz) would berequired. Different possible timing methods include high-speed clocks andconstant-current capacitor charging. Timing accuracy could be improved byaveraging several pulse timings.

From the information contained in this section, a set of device requirements weredevised:

Specification Required Value

Mass <300g

Power <500mW

Transducer Diameter ≤2cm

Transducer Separation 5-10cm

Wind Speed Resolution <5cm/s

Wind Vector 3-D

Signal Frequency <150kHz

Measurement Frequency >10Hz

Operating Mode Pulsed

Tab. 2.1: Required Instrument Specifications on Mars

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2.4 Hardware Testing and Progress

2.4.1 Electronics

A significant amount of time was spent this year working towards a functioningtest-bench setup for the transducers.

None of the test electronics provided at the start of the year worked as expectedand all of it needed to be re-designed. In December, the electronics andtransducers were taken up to Warwick University, who have a working ultrasonictest setup, to determine what needed improvement or re-design. It was discoveredthat one of the Oxford transducers and one of the Warwick transducers did notwork. The Warwick transducer simply needed a re-filming but the Oxfordtransducer had no electrical continuity between the film and the connecting wire atthe back of the sensor head. This was most likely due to a micro-tear in the verythin (3.5μm) film and could not be fixed without replacing the entire film assemblyback in Oxford. Different tests were performed on the working transducersincluding frequency response and bias voltage tests. It was found that the Oxfordcharge amplification circuit was much worse than the Warwick amplifier(commercial amplifier) and gave an amplified signal of less than 0.5mV; whollyinadequate for sensing on Earth let alone Mars. It was also found that the biasvoltage was more important than signal voltage; above a minimum bias voltage of30V. There was no noticeable difference in received signal when swapping”transmitter” and ”receiver”. The Oxford transducer had a much lower peakfrequency (160kHz) than the Warwick transducers (370kHz).

As a result of the Warwick tests, the Oxford amplification circuit was completelyredesigned; firstly the charge amplifier then further amplification stages wereadded to increase the signal to noise ratio (see Appendix B, Page 41 for anelectronic circuit diagram and test setup block diagram). The drive signal andcharge amplifier output, as measured by an oscilloscope (without averaging), areshown overleaf.

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Fig. 2.7: Transmitted and received signals. The first set of large-amplitude received signalswas electrical pickup from the transmitter amplifiers.

A high-voltage amplifier was specified by me then designed by Johan Fopma(central electronics) and incorporated in the transmitting circuit. This increasedthe transmitted signal voltage from 10V to 140V peak-to-peak. The testsperformed in Warwick were repeated in Oxford with the new electronics [Figure2.8 overleaf]. The input signal for all tests was an 8-pulse burst; the amplifieroutput was measured using an oscilloscope with 512x averaging. The plottedvalues were the peak amplified received pulse amplitudes. All tests to date havebeen at room temperature.

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Fig. 2.8: Variation of Warwick transducer output signal with sensor bias voltage usingboth Warwick (blue) and Oxford (red) amplifiers

The re-designed Oxford electronics was found to give a very similar bias voltageresponse to the Warwick electronics; albeit with a higher gain. However, theOxford electronic setup was also much more noisy than the commercial Warwickelectronics (10mV versus 0.1mV). Whilst this was adequate for signal detection onEarth, further improvements will be required to operate under Martian conditions.Planned noise reduction techniques include better shielding and using lower noiseamplifiers.

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Fig. 2.9: Frequency response of the Warwick transducers as measured by the Oxford (red)and Warwick (blue) electronics

The frequency response of the Warwick transducers was compared using thedifferent electronics setups in Oxford and Warwick [Figure 2.9 above]. The peakfrequency was reduced by approximately 50kHz and the signal dropped off morequickly with frequency due to the Oxford electronics. Since the Oxford electronicsnow gave results similar to those measured in Warwick, the focus shifted to lookingat transducer performance at low pressures. The frequency response of the Oxfordelectronics will soon be measured in order to separate electronic effects from theresults.

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2.4.2 Low Pressure Testing

Fig. 2.10: Photograph of the vacuum chamber

A vacuum chamber (Figure 2.10) was used to test how the Warwick transducersperformed at low pressures in air [Figure 2.11 below].

Fig. 2.11: Plot showing how the amplified received signal amplitude varied with airpressure for the Warwick transducers and Oxford electronics

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As expected, the signal amplitude decreased fairly linearly with pressure until itwas less than the noise; at a pressure of around 20mbar. This is because as moreair is pumped out of the chamber, there are fewer air molecules to transfer theultrasonic signal between transducers and so more signal is lost. In the future, it isplanned to improve operation at low pressures using more sensitive transducerscoupled with lower noise, more selective (pass-band filtered) electronics. Onephenomenon not shown in Figure 2.11 was the increase in received amplitudebetween atmospheric pressure and 100mbar. This was most likely due to thereduced pressure effectively drawing the film towards the backplane; increasing theeffective film tension and therefore received amplitude. Once the film was tightagainst the backplane, much less air would be able to escape from the gaps andthis effect would have a reduced impact; allowing for a linear reduction in receivedamplitude with pressure. This was also observed in the form of hysteresis;temporary larger amplitudes were observed at all pressures following adepressurisation.

The frequency response of the Warwick transducers was compared at roompressure and 100Torr using the Oxford electronics [Figure 2.12 below].

Fig. 2.12: Frequency response of the Warwick transducers at room pressure and 100Torr

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As expected, there was a significant shift in frequency response and attenuationwith pressure. The peak frequency almost halved from around 390kHz at roompressure to around 200kHz at 100Torr. The reduction in received amplitude wasconsistent with the previous results at low pressure. Inserting the peak frequenciesand pressures into Equation 2.1 give surface roughnesses of 2.5 microns and 8.8microns for low pressure and atmospheric pressure respectively.

2.4.3 Film Thickness

The bias voltage, frequency response and pressure attenuation graphs were repeatedusing three different thickness films [Figures 2.13, 2.14 and 2.15].

Fig. 2.13: Comparing the response of different thickness films to varying bias voltages

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Fig. 2.14: Sensitivity of the Warwick transducers using different film thicknesses at lowpressures

As expected, the lower oscillating mass provided by thinner film membranes gavegreater sensitivities at low pressures as well as Earth pressures. In Figure 2.13 thiswas shown by the greater received amplitude at different bias voltages whereas inFigure 2.14 this was demonstrated by the significantly lower pressure at which thesignal was lost. Although aluminised Mylar films were available at 2μm at thetime of data collection, the Warwick transducers stopped working before data couldbe obtained. They are still not fully operational but the Oxford transducers haverecently been fixed and will be used from now on for ease of prototyping.

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Fig. 2.15: Frequency response of the Warwick transducers using different films at roompressure

The thicker films led to a lower frequency, as predicted by theory (Equation 2.1).As expected, the thinner films were also more sensitive. Using Equation 2.1, thetheoretical surface roughness values were calculated to be 8.8 microns, 7.6 micronsand 4.4 microns for the 3.5μm, 5μm and 12μm films respectively. The exactroughnesses of the Warwick MEMS backplanes are not known but these values areperfectly reasonable and directly comparable with the previously calculated valueson Page 18.

2.4.4 Transducer Separation

The signal attenuation with transducer separation was measured and compared withthe theoretical beam spreading and attenuation models [Figure 2.16 overleaf].

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Fig. 2.16: Normalised signal losses with increasing transducer separation

The theoretical values in Figure 2.16 were normalised using the experimental valueat a separation of 9cm; this is where the Fresnel number is approximately equal toone. Below this separation, the model would not be expected to work as it is onlyfor far-field diffraction. However it seems to work well in practice down to 8cm.

2.4.5 Backplanes

Recently, work has begun looking at different backplane machining processes andmethods for measuring backplane surface roughness profiles.

Two sets of backplanes were machined by hand; one kept ”rough” straight off thelathe and one made ”smooth” by the application of scotch-brite to the rotatingsurface.

The Engineering Department (Metrology) has an Alicona Profilomiter which canproduce 3D maps of surface roughness; this may prove very useful in the future forinspecting different surface machining techniques. Some results from one of the”smooth” backplanes are shown overleaf (Figure 2.17).

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Fig. 2.17: 2-D surface profile of the central peak on one of the ”smooth” Oxfordbackplanes. The profilometer measured an average peak-to-trough height of12.8 microns for this backplane.

A variety of tests were performed using the Oxford transducers with differentbackplanes and the 2μm metallised Mylar films. The results are shown below andoverleaf [Figures 2.18 and 2.19].

Fig. 2.18: Received signal amplitude with varying bias voltage for the Oxford transducers

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Fig. 2.19: Received signal amplitude with varying air pressures for the Oxford transducers

These graphs confirmed the findings of Hietanen et al.[9] in that smaller surfacefeatures result in greater sensitivities. In Figure 2.18, one possible explanation forthe graph tailing off at higher bias voltages was that the dielectric strength of theMylar film (17kV/mm) was less than the bias voltage so a small current waspassing through the dielectric; reducing the overall sensitivity of the transducer.Figure 2.19 showed that the ”smooth” surface profile resulted in significantlygreater received amplitudes at low pressures and so smooth backplanes are bettersuited for use on Mars. The third set of tests involved obtaining a frequencyresponse curve for both surface roughnesses in air at atmospheric pressure:

Fig. 2.20: Frequency response of the ”smooth” (red) and ”rough” (blue) backplanes

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It is not known why the smooth surface showed a lower peak frequency than therough surface; opposite to the theory [Equation 2.1, Page 7]. One possibility isthat the film tension was altered when changing the backplanes. However thesmooth peak frequency is in agreement with the theory; using Equation 2.1 gives atheoretical surface roughness of 22 microns compared to the average surfacemeasured value of 12.8 microns. This is a good agreement considering the error inthe Mylar film density alone is 10%.

High-precision CNC machining tools have sub-micron position accuracies but thetool dimensions are much larger than that tolerance. For example, the smallestengraving tool in the Physics workshop has a 50 micron diameter ball tip. As aresult, conventional machining methods may not be able to produce surfacefeatures precise enough for optimising transducer performance since shallower pitsresult in greater sensitivities. Three possible alternate technologies have beenfound:Chemical etching methods involve the use of chemicals to remove unwanted surfacefeatures whilst using a protective layer to protect those to be kept. Using suchtechniques, it is possible to create surface features with 5-micron resolution anddepths as low as 10 microns.Laser-drilling is a promising possibility as it can produce holes as small as 1micron diameter and depths as low as 10 microns in aluminium.Ion beam ablation is a similar process which fires ions at the metal surface toremove material but it only goes down to a feature size of 10 microns.

Backplanes with different surface curvatures have also been manufactured but notyet tested. It is expected that curved backplanes will result in greater sensitivitiessince the metallised films should be closer to the backplane at all points. Anyirregularities such as central ”peaks” would also have less of a negative effect ontransducer performance.

2.5 Future Work

The next steps will involve further investigation into how different machinedbackplanes and film thicknesses affect transducer operation on Earth and underMartian conditions. Thinner films are more sensitive but lead to higher resonantfrequencies and are more prone to tearing. Greater air-gap thicknesses would lowerthe resonant frequency but they could also reduce the sensitivity. This will requirefurther theoretical modelling of these inter-dependent parameters; as well aslooking at practical methods for reducing transducer acoustic impedance. Theoperational trade-offs of separation and resonant frequency need to be verifiedexperimentally. The electronics are suitable for signals on Earth but will requirefurther modification for use under Martian atmospheric conditions. Eventuallymore sensors will be manufactured and positioned to allow for measurement of athree-dimensional wind vector. The whole setup will be made portable for fieldtesting next summer; it is planned to join the Geography department on anexpedition to the Kalahari desert. This will require investigations into signaldetection methods (cross correlation/level crossing), nanosecond timing andpossibly how to reduce the signal and bias voltages (70V and 72V respectively).After this, instrument optimisation will continue in order to try and meet thespecifications outlined on Page 11.

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3. EXOMARS MARSWIND INSTRUMENT

One exciting development since the beginning of the project is the opportunity tobe involved in a flight mission. We will be providing a thermal anemometer similarin design to the Beagle 2 Wind Sensor (B2WS) for the ExoMars Entry, Descent andLanding Demonstrator Module (EDM).

3.1 The ExoMars Programme

ExoMars is a joint NASA-ESA project which consists of two separate launches.The first launch in 2016 will place a Trace Gas Orbiter in orbit around Mars andtest the new EDM system for landing on the surface of Mars. The second launch(2018) will put a large ExoMars rover on the surface of Mars. The EDM isprimarily a technology demonstration mission and will only be operational on thesurface of Mars between 2 and 4 sols; as such there will be limited science goalsfrom the data obtained. The EDM is expected to land in the Meridiani Planum asit is flat and relatively free of rocks; and so is the perfect perfect place to test theproposed airbag landing system.

We have been allocated space on board the EDM; which contains a 3kg sciencepayload on a 600kg lander. Originally, it was planned to house the wind sensor ontop of the camera however since the camera has now been removed from thepayload, it is likely the sensor will be placed on top of a ”stub” since booms arenot allowed on the craft. We have been allocated a mass budget of 30g±5% andwe should have a 180◦ clear field of view. Since all electronics must be housed inthe warm compartment, we will probably not be using hybrid electronics; whichwould have allowed us to incorporate other sensors (eg. dust) into our payload. Interms of electronics interfacing, we have been allocated three data lines and twohousekeeping lines to be read by an external analog-to-digtal converter. The datalines will be sampled at 2s intervals and housekeeping lines at 10s. It is planned touse the housekeeping lines to measure the temperature in the centre of the sensorfor improved accuracy. The absolute accuracy required is ±1m/s in wind speedand ±10◦ in direction for wind speeds >5m/s.

3. ExoMars MarsWind Instrument

3.2 The Beagle 2 Wind Sensor

Fig. 3.1: Beagle 2 flight model wind sensor and electronics

The Beagle 2 wind sensor was a hot-film thermal anemometer developed for theBeagle 2 lander which launched in 2003. It consisted of three platinum filmssputtered onto a Kapton substrate; which was then bonded to a central cylindricalRohacell substrate [Figure 3.1 above]. The sensor had two operating modes; awind sensing mode and a temperature sensing mode. In ”wind” mode, a constantpower was dissipated in each film to heat them up to 80 degrees over ambienttemperature. In ”temp” mode, the resistance of each film was measured to allowfor an ambient temperature reading. The sensor alternated between the two modesperiodically. The electronics consisted of a wheatstone bridge and amplifier; aswell as a timer to switch between the two operating modes. However, since eachfilm and electronic resistor had a slightly different resistance, each flight model andelectronics board needed to be extensively calibrated.

Originally for the Beagle 2 mission, four flight model sensors were manufactured;these were labelled FM1 to FM4. FM4 was the final flight model and is probablysomewhere on the surface on Mars; FM2 was the flight spare. FM3 developed aproblem with one of its films (film 2) shortly before the mission was due to launchand FM1 was never used. The three spare models have been located and theirresistances tested to identify which sensor is which flight model.

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3.3 Future Work

Once the Low Density Wind Tunnel is fully operational again, the spare B2WSflight model will be calibrated by Colin and I using the original B2WS calibrationprocedures. More flight model sensors and electronics will need to bemanufactured; as well as accommodation and qualification models. I willinvestigate several improvements on the original B2WS design; including placing aplatinum resistance thermometer in the centre of the sensor and different filmgeometries. Additionally, to aid planetary protection requirements and improvedust resistance, we will test different sensor coatings such as Parylene and Quartz.

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28

4. LOW-DENSITY WIND TUNNEL

Over the course of the past year, the Engineering Department has been moving windtunnels from the old building on Arthur Street to the new building on the OsneyMead Industrial Estate. This included the Low-Density Wind Tunnel (LDWT);which was used for the calibration of the Beagle 2 Wind Sensors. Since this windtunnel will be required for testing prototype ultrasonic wind sensors, and possiblycalibrating the ExoMars MarsWind Sensors as well, part of the project will involvethe recommissioning and re-calibration of the LDWT.

4.1 Wind Tunnel Design

The Oxford LDWT is unique in that it provides laminar flow under Martianconditions; which is crucial for calibrating Mars wind sensors. Originally designedfor hypersonic flow measurements, it has been adapted for use at low gas pressuresand temperatures.

Fig. 4.1: Operation of the Oxford LDWT[17]

The Oxford LDWT is an open-circuit system; which means that the fluid is notdirectly re-circulated. It achieves a dynamic equilibrium; allowing very rapidchanges in wind speeds and can be run on air or carbon dioxide.

In the previous configuration, a 10,000 litre balloon was filled with carbon dioxideto act as a reservoir to handle the high flow rates. In the new building, the balloonwill likely be replaced by several carbon dioxide cylinders in parallel.The flow control valve allows the mass flow rate to be set; giving a wind speedresolution of 0.03m/s. Downstream of this valve, there is low pressure due to thepumps drawing air through the chamber. Once the mass flow has been set, the

4. Low-Density Wind Tunnel

chamber pressure is altered by adjusting the air intake valve to allow air into thechamber section. This is actuated by a stepper motor, which keeps the pressure at±0.01mbar. The exact wind speed is calculated from the mass flow rate andseveral differential pressure sensors measuring the pressure drop across the orificeplate. The wind speed at different positions along the test section is obtained fromcalibration and modelling of the flow. A custom-built nozzle ensures laminar flowat higher Reynolds numbers[18].Cooling is achieved through a closed-cycle cascade cooler; care must be takenoperating so close to the freezing point of CO2. The wind tunnel can achieve windspeeds of up to 30m/s, at pressures of 5-10mbar and temperatures of 200-300Kwith an absolute accuracy of 3%.

4.2 Current Progress

Fig. 4.2: Current state of the LDWT

Most of the equipment relating to the LDWT has been relocated to the new site;the remainder is in the AOPP space instrumentation lab. As shown in Figure 4.2,the wind tunnel itself has been reconstructed at the new site and the electrics havealso recently been connected by engineering maintenance. However there are stillseveral tasks to be completed before it is ready to be turned on (Section 4.3, P.31).

To allow the sensor to be rotated inside the wind tunnel test chamber whilstoperational, a stepper motor control system was developed. This was housed in a19-inch rack and configured for operation from a computer via RS232 (serial)communications. A program was developed in LabVIEW to control the steppermotor and display the current position of the spindle. This program has beenincorporated into the LabVIEW code which controls the LDWT.

30


The computer used to control the LDWT was over seven years old and, althoughfully functional, it was decided to replace it with one with a newer operatingsystem and hardware. The new computer was setup with LabVIEW andconfigured to use the data acquisition (DAQ) card from the old system. The DAQcard had previously been used to read the wind tunnel sensor outputs and displaythem on the LabVIEW control software. The card was tested using all 32 analogueinputs to measure the voltage of a battery. All inputs gave the same reading of7.66V; the voltage was accurately measured by an oscilloscope to be 7.658V. Thistest confirmed that the DAQ card was correctly calibrated.

The three wind tunnel baratrons (pressure sensors) have been tested andconfirmed to be functioning in a low pressure chamber. The 10Torr baratron wassent off to Chell for calibration against a traceable international standard.

4.3 Ongoing Work

The carbon dioxide cylinders need to be connected up to the wind tunnel. Thecooling system at the new facility has a flow rate three times less than before butat a lower temperature. Depending on the performance of the wind tunnel onceoperational, the cooling system may need to be augmented and the flow in the testsection will need to be re-characterised. Additionally, the oil in the rotary and Rootspumps may need to be replaced. If the tunnel will be used for the calibration of theExoMars MarsWind sensors, traceable calibration will be required so the differentialpressure sensors will need to be calibrated externally.

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5. THE PROJECT

The project can be divided into three main sections: ultrasonic sensors, thermalsensors and wind tunnel. Having several separate aspects to work on allows theproject to move forwards in spite of possible delays in individual tasks. A Ganttchart outlining the individual tasks is shown overleaf (Figure 5.1).

The LDWT computer has already been setup to control the wind tunnel operationand stepper motor control system. The DAQ (ADC) card has been calibrated; aswell as the 10Torr baratron. The temperature sensors and differential pressuresensors still need to be calibrated. It is expected to have the LDWT fullyoperational, including any recalibration and re-characterisation, by the end of thisyear. That will allow sufficient time for the manufacture and testing of theExoMars MarsWind sensors. At the same time, further progress will be made inthe characterisation and testing of the ultrasonic wind sensors.

The Warwick ultrasonic transducers have already been characterised at roompressure and at low pressures down to 20mbar. The next stages will involvemanufacturing different backplanes and films for the Oxford transducers andinvestigating their effects on transducer operation on Earth and under Martianconditions. This will require further theoretical modelling of these factors; as wellas looking at practical methods for reducing transducer impedance. Theelectronics have been optimised for signals on Earth but will require furthermodification for use at Martian atmospheric conditions. Eventually more sensorswill be manufactured and positioned to allow for measurement of athree-dimensional wind vector. The whole setup will be optimised for use on Marsand also prepared for field testing in Q2 2012. This will require investigations intosignal detection methods, nanosecond timing and possibly how to reduce the signaland bias voltages.

All of the MarsWind deadlines have been taken from the ESA E-PIP andDREAMS proposal. By the end of this year, we plan to have the spare B2WSflight model calibrated. After that, tests will be performed on the possible designimprovements outlined in Section 3.3 (Page 27); the sensor design should befinalised by Q3 2012 at the latest. After this, the flight and qualification modelswill be manufactured and calibrated for delivery to ESA in Q2 2013.

In terms of conferences, I will be presenting a poster at EPSC-DPS this year and Iam planning to go to DPS in 2012 as well.

An outline for the final thesis can be found on Page 43.

5. The Project

Fig. 5.1: Project Gantt Chart

34

6. CONCLUSION

Progress has been made in all three areas of the project but there is still muchwork to do in each of the three areas; ultrasonic transducers, ExoMars MarsWindand the wind tunnel.

The operation of capacitive ultrasonic transducers has been demonstrated at roompressure and down to 20mbar. However to operate the transducers on Mars willrequire further optimisation of the backplanes, films and electronics. This willinvolve theoretical modelling of these factors as well as transducer impedance.There are many trade-offs to be considered when choosing ideal transducerseparation and operating frequency. Higher frequencies lead to greater wind speedresolutions but also require high-speed timing. Higher frequencies also lead tolower beam spreading losses but greater losses due to atmospheric attenuation.Smaller transducer separations allow the characterisation of smaller eddies butrequire high-speed timing. Large separations result in greater attenuation andbeam spreading losses. Thinner membranes are more sensitive at all pressures butare more fragile and result in higher resonant frequencies.Eventually more sensors will be manufactured and used in a three-dimensionalconfiguration for field testing on Earth. This will require investigations into signaldetection methods and possibly how to reduce the signal and bias voltages.

The ExoMars MarsWind sensor will be an improved version of the Beagle 2 WindSensor. Possible improvements include placing a platinum resistance thermometerin the centre of the sensor and different film geometries. Additionally, to aidplanetary protection requirements, we could try different sensor coatings such asParylene.

The Low Density Wind Tunnel, used for the calibration of the Beagle 2 flightmodel wind sensors, has recently been relocated to the new engineering building inOsney Mead. Since the facility may be required for the calibration of theMarsWind sensors and testing of the ultrasonic transducers, part of the projectwill involve the re-commissioning of the wind tunnel. The wind tunnel has beenreconstructed at the new site and the electrics installed. A new computer has beenpurchased to control the wind tunnel and this has been integrated with a newstepper motor control system for autonomous movement of the test subject in thetunnel. The 10Torr baratron has been re-calibrated externally but the differentialpressure sensors and temperature sensors still need to be calibrated. Depending onthe performance of the wind tunnel once operational, the cooling system may needto be augmented and the flow in the test section will need to be re-characterised.Additionally, the oil in the rotary and Roots pumps may need to be replaced.

6. Conclusion

36

APPENDIX

A. GAS PISTON MODEL

A. Gas Piston Model

40

B. ELECTRONICS CIRCUIT DIAGRAM

B. Electronics Circuit Diagram

42

C. THESIS OUTLINE

1. Introduction to wind sensing on Mars

• History and Motivation

• Different Technologies

• Ultrasonic Benefits

2. Sensor Design

• Construction

• Metallised Films

• Backplate Machining

3. Electronics Design

• Sensor Biasing

• Input Signals

• Amplification

• Filtering

• Threshold detection

• Timing

4. Sensor Characteristics

• Power Consumption

• Accuracy

• Temperature

• Pressure

• Vibration

5. Comparison of thin-film and ultrasonic anemometers

C. Thesis Outline

44

BIBLIOGRAPHY

[1] Mars General Circulation Modelling Group, NASA Ames Research Centre,http://www-mgcm.arc.nasa.gov/MGCM.html

[2] http://mars.jpl.nasa.gov/programmissions/missions/log/

[3] Telltale Calibration Report,http://an.rsl.wustl.edu/phx/solbrowser/documentation/missionDocs/n tt/Telltale calib report.PDF

[4] Phoenix Telltale data acquired fromhttp://starbrite.jpl.nasa.gov/pds/viewProfile.jsp?dsid=PHX-M-TT-5-WIND-VEL-DIR-V1.0

[5] http://cmex.ihmc.us/data/home/pathfinder/Lessons/weather.htm

[6] Hess, S. L., R. M. Henry, et al. (1977). ”Meteorological Results From the Surfaceof Mars: Viking 1 and 2.” J. Geophys. Res. 82(28):4559-4574.

[7] http://adsabs.harvard.edu/abs/2007LPICo1353.3344B

[8] Merrison, J. P., H. P. Gunnlaugsson, et al. (2004). ”A miniature laseranemometer for measurement of wind speed and dust suspension on Mars.”Planetary and Space Science 52(13): 1177-1186.

[9] Hietanen, J., P. Mattila, et al. (1993). ”Factors Affecting the Sensitivity ofElectrostatic Ultrasonic Transducers.” Measurement Science Technology 4(10):1138-1142.

[10] Grandia, W. A. and C. M. Fortunko (1995). ”NDE applications of air-coupledultrasonic transducers.” 1995 IEEE Ultrasonics Symposium Proceedings, Vols1 and 2: 697-709:1636.

[11] Banfield, D. and R. Dissly (2005). A Martian sonic anemometer. AerospaceConference, 2005 IEEE.

[12] Rafiq, M. and C. Wykes (1991). ”The performance of capacitive ultrasonictransducers using v-grooved backplates.” Measurement Science and Technology2(2): 168.

[13] Suzuki, K., K. Higuchi, et al. (1989). ”A silicon electrostatic ultrasonictransducer.” Ultrasonics, Ferroelectrics and Frequency Control, IEEETransactions on 36(6): 620-627.

[14] Carr, H. and C. Wykes (1993). ”Diagnostic measurements in capacitivetransducers.” Ultrasonics 31(1): 13-20.

[15] http://www.kayelaby.npl.co.uk/general physics/2 4/2 4 1.html

Bibliography

[16] Petculescu, A. and R. M. Lueptow (2007). ”Atmospheric acoustics of Titan,Mars, Venus, and Earth.” Icarus 186(2): 413-419.

[17] Wilson, C. F., A. L. Camilletti, et al. (2008). ”A wind tunnel for the calibrationof Mars wind sensors.” Planetary and Space Science 56(11): 1532-1541.

[18] Wilson, C. (2003). ”Measurement of wind on the surface of Mars.” PHD Thesis.

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Further reading:

Electrostatic Ultrasonic Transducers

Clark, J. A. (1981). ”A matched impedance, electrostatic approach tohydrophone design.” Journal of Sound and Vibration 77(1): 51-59.

Haller, M. I. and B. T. Khuri-Yakub (1996). ”A surface micromachinedelectrostatic ultrasonic air transducer.” Ultrasonics, Ferroelectrics andFrequency Control, IEEE Transactions on 43(1): 1-6.

Ge, L. (1998). ”A theoretical model of electrostatic ultrasonic transducers withmicro air-gap structure.” Chinese Science Bulletin 43(9): 728-731.

Walker, A. J. and A. J. Mulholland (2010). ”A theoretical model of anelectrostatic ultrasonic transducer incorporating resonating conduits.” IMAJournal of Applied Mathematics 75(5): 796-810.

Hutchins, D. A., D. W. Schindel, et al. (1998). ”Advances in ultrasonicelectrostatic transduction.” Ultrasonics 36(1-5): 1-6.

Munro, W. S. H. and C. Wykes (1994). ”Arrays for airborne 100 kHzultrasound.” Ultrasonics 32(1): 57-64.

Anderson, M. J., J. A. Hill, et al. (1995). ”Broadband electrostatic transducers:Modeling and experiments.” The Journal of the Acoustical Society of America97(1): 262-272.

Schindel, D. W. and D. A. Hutchins (1991). Capacitance devices for thecontrolled generation of ultrasonic fields in liquids. Ultrasonics Symposium,1991. Proceedings., IEEE 1991.

Warren, J. E. (1975). ”Capacitance microphone static membrane deflections:Comments and further results.” The Journal of the Acoustical Society ofAmerica 58(3): 733-740.

Mattila, P. and et al. (2000). ”Capacitive ultrasonic transducer with netbackplate.” Measurement Science and Technology 11(8): 1119.

Carr, H., W. S. H. Munro, et al. (1992). ”DEVELOPMENTS IN CAPACITIVETRANSDUCERS.” Nondestructive Testing and Evaluation 10(1): 3 - 13.

Carr, H. and C. Wykes (1993). ”Diagnostic measurements in capacitivetransducers.” Ultrasonics 31(1): 13-20.

Li-Feng, G. (1999). ”Electrostatic airborne ultrasonic transducers: modelingand characterization.” Ultrasonics, Ferroelectrics and Frequency Control,IEEE Transactions on 46(5): 1120-1127.

47

Bibliography

Hietanen, J., P. Mattila, et al. (1993). ”Factors Affecting the Sensitivityof Electrostatic Ultrasonic Transducers.” Measurement Science Technology4(10): 1138-1142.

Bashford, A. G., D. W. Schindel, et al. (1997). ”Field characterizationof an air-coupled micromachined ultrasonic capacitance transducer.” TheJournal of the Acoustical Society of America 101(1): 315-322.

Grandia, W. A. and C. M. Fortunko (1995). ”NDE applications of air-coupledultrasonic transducers.” 1995 Ieee Ultrasonics Symposium Proceedings, Vols 1and 2: 697-709 1636.

Medley, A. P., D. R. Billson, et al. (2006). ”Properties of an electrostatictransducer.” The Journal of the Acoustical Society of America 120(5): 2658-2667.

Schindel, D. W., D. A. Hutchins, et al. (1995). ”The Design andCharacterization of Micromachined Air-Coupled Capacitance Transducers.”Ieee Transactions on Ultrasonics Ferroelectrics and Frequency Control 42(1):42-50.

O’Sullivan, I. J. and W. M. D. Wright (2002). ”Ultrasonic measurement of gasflow using electrostatic transducers.” Ultrasonics 40(1-8): 407-411.

Manthey, W. and et al. (1992). ”Ultrasonic transducers and transducerarrays for applications in air.” Measurement Science and Technology 3(3): 249.

Mars Meteorology

Bass, H. E. and J. P. Chambers (2001). ”Absorption of sound in theMartian atmosphere.” The Journal of the Acoustical Society of America109(6): 3069-3071.

Williams, J.-P. (2001). ”Acoustic environment of the Martian surface.”J. Geophys. Res. 106(E3): 5033-5041.

Sullivan, R., D. Banfield, et al. (2005). ”Aeolian processes at the MarsExploration Rover Meridiani Planum landing site.” Nature 436(7047): 58-61.

Harri, A. M., T. Siili, et al. (1995). ”Aspects of atmospheric science andinstrumentation for martian missions.” Advances in Space Research 16(6):15-22.

Chamberlain, T. E., H. L. Cole, et al. (1976). ”Atmopspheric Measurementson Mars: the Viking Meteorology Experiment.” Bulletin of the AmericanMeteorological Society 57(9): 1094-1104.

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Holstein-Rathlou, C. (2011). ”Wind related evolution of the Martiansurface.” PHD Thesis.

Kahre, M. A., J. L. Hollingsworth, et al. (2011). Coupling Mars’ Dustand Water Cycles: Effects on Dust Lifting Vigor, Spatial Extent andSeasonality. The Mars Atmosphere: Modelling and Observations.

C. Holstein-Rathlou, H. P. G., J. Merrison, P.Taylor, C. Lange, J. Davis, M.Lemmon (2009). ”Winds at the Mars Phoenix landing site.” 40th Lunar andPlanetary Science Conference.

Sutton, J. L., C. B. Leovy, et al. (1978). ”Diurnal Variations of theMartian Surface Layer Meteorological Parameters During the First 45 Solsat Two Viking Lander Sites.” Journal of the Atmospheric Sciences 35(12):2346-2355.

Neakrase, L. D. V., R. Greeley, et al. (2005). Dust devils on Mars: Effectsof surface roughness on particle threshold. 36th Lunar and Planetary ScienceConference. Houston, Texas, USA.

Renno, N. and J. Kok (2008). ”Electrical Activity and Dust Lifting onEarth, Mars, andBeyond.” Space Science Reviews 137(1): 419-434.

Greeley, R. (2002). ”Saltation impact as a means for raising dust onMars.” Planetary and Space Science 50(2): 151-155.

Wurm, G., J. Teiser, et al. (2008). ”Greenhouse and thermophoreticeffects in dust layers: The missing link for lifting of dust on Mars.” Geophys.Res. Lett. 35(10): L10201.

Heavens, N. G., M. I. Richardson, et al. (2008). ”Two aerodynamic roughnessmaps derived from Mars Orbiter Laser Altimeter (MOLA) data and theireffects on boundary layer properties in a Mars general circulation model(GCM).” J. Geophys. Res. 113(E2): E02014.

Kok, J. F. and N. O. Renno (2008). ”Electrostatics in Wind-Blown Sand.”Physical Review Letters 100(1): 014501.

Peter A. Taylor, W. W., Babak Tavakoli Gheynani and P-Y Li (2006).”MARS BOUNDARY-LAYER AND DUST MODELLING FOR THEPHOENIX LANDER LOCATION.” Fourth Mars Polar Science Conference.

Ryan, J. A., R. M. Henry, et al. (1978). ”Mars meteorology: Three seasons atthe surface.” Geophys. Res. Lett. 5(8): 715-718.

Ringrose, T., J. Zarnecki, et al. (2001). Martian Dust Devil Detectionwith the Beagle 2 Wind Sensor. 32nd Lunar and Planetary Science Conference.Houston, Texas, USA.

Spiga, A. and S. R. Lewis (2010). ”Martian mesoscale and microscale

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Merrison, J. P., H. P. Gunnlaugsson, et al. (2007). ”Determination ofthe wind induced detachment threshold for granular material on Mars usingwind tunnel simulations.” Icarus 191(2): 568-580.

Hess, S. L., R. M. Henry, et al. (1977). ”Meteorological Results Fromthe Surface of Mars: Viking 1 and 2.” J. Geophys. Res. 82(28): 4559-4574.

Taylor, P., P. Y. Li, et al. (2007). ”Modelling dust distributions in theatmospheric boundary layer on Mars.” Boundary-Layer Meteorology 125(2):305-328.

Murphy, J. R., C. B. Leovy, et al. (1990). ”Observations of Martian SurfaceWinds at the Viking Lander 1 Site.” J. Geophys. Res. 95(B9): 14555-14576.

Sullivan, R., R. Greeley, et al. (2000). ”Results of the Imager for MarsPathfinder windsock experiment.” J. Geophys. Res. 105(E10): 24547-24562.

Parteli, E. J. R., M. P. Almeida, et al. (2009). ”Sand transport on Mars.”Computer Physics Communications 180(4): 609-611.

Schofield, J. T., J. R. Barnes, et al. (1997). ”The Mars Pathfinder AtmosphericStructure Investigation/Meteorology (ASI/MET) Experiment.” Science278(5344): 1752-1758.

Sullivan, R., R. Arvidson, et al. (2008). ”Wind-driven particle mobilityon Mars: Insights from Mars Exploration Rover observations at 8220;ElDorado8221; and surroundings at Gusev Crater.” J. Geophys. Res. 113(E6):E06S07.

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Documents

Wind Sensing on the Surface of Mars Eurion Leonard-Pugh