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Pennies from Heaven: a retrospective on the use of wireless sensor networks for planetary exploration
Wireless sensor networks are finding many applications in terrestrial sensing. It seems natural to propose their use for planetary exploration. A previous study (the Mars daisy) has put forward a scenario using thousands of millimeter scale wireless sensor nodes to undertake a complete survey of an area of a planet. This paper revisits that scenario, in the light of some of the discussions surrounding its presentation. The practicality of some of the ideas put forward is examined again, and an updated design sketched out. It is concluded that the updated design could be produced using currently available technology.
Pennies from Heaven: a retrospective on the use of wireless sensor networks for planetary exploration
Pennies From Heaven: a retrospective on the use ofwireless sensor networks for planetary exploration Robert Newman, Mohammad Hammoudeh School of Computing and IT University of Wolverhampton Wolverhampton, UK R.M.Newman@coventry.ac.ukAbstract—Wireless sensor networks are finding many II. THE MARS DAISYapplications in terrestrial sensing. It seems natural topropose their use for planetary exploration. A previousstudy (the Mars daisy) has put forward a scenario usingthousands of millimeter scale wireless sensor nodes toundertake a complete survey of an area of a planet. Thispaper revisits that scenario, in the light of some of thediscussions surrounding its presentation. The practicality ofsome of the ideas put forward is examined again, and anupdated design sketched out. It is concluded that theupdated design could be produced using currently availabletechnology. Keywords-component; wireless sensor network, planetaryexploration, autonomous systems. I. INTRODUCTION Figure 1: The original mars daisy This paper is a further visit to a concept whichreceived some attention two years ago. The concept was The Mars daisy (Figure 1) was a speculative nano-a case study of the exploration of Mars using a wireless probe first presented in 2005 [1,2]. Its origins were assensor network, in place of more conventional probes part of the storyline for a short film designed to presentsuch as landers and rovers. Each probe was in itself a the potential of wireless sensor networks in a way that‘nano-lander’, being scaled somewhat smaller than any would capture the imagination of teenagers. The filmother probes proposed before, in the scale of millimters. presented a planetary exploration mission to Mars.It was proposed that acting together, a host of these Rather than a conventional lander, the mission used alanders could perform many of the funstions associated number (9000 20g probes would have the same mass astraditionally with rovers, and cover a larger area in more the Spirit and Opportunity rovers) of nano-landers., eachdetail than could a rover. It should be stated that the 40mm long by 7mm in diameter. The nano-landersproposers are computer scientists, not planetary would be deployed within atmospheric entry vehicles,explorers, and many of the assumptions made were and released at low altitude from where they scatteredniaive. Nonetheless, the scenario did create some and became impaled in the regolith using a spike,resonance amongst real plantary scientists. containing the major mass (the battery). This paper represents an attempt to revisit the design The probe design was envisaged to be based around aof the scenario, and move a step closer to practicality. It stack of chips, forming an opto-electrical-mechanicalis organised as follows. In the next section, the original system. In addition to a processor and memory,idea is reprised. additional chips provided power regulation, optical sensing, chemical analysis, imaging, seismographs and Section III discusses the perceieved advantages and communication, both between probes and with a relaydisadvantages of a mission based on wireless sensor satellite.networks. Sections IV to VIII discuss various instrumentsto be expected to be deployed on such a probe in more The precise function of the chips in the stack isdetail. Section IX discusses some issues of wireless described below.sensor network design in this context. Section X A 7W semiconductor laser providesproposes the sketch of a revised design, more a ‘penny’ communications functions. Opticalthan a ‘daisy’. communication was selected in order to provide a
sufficiently narrow beam width to allow reported. However, the details of how the actuators for communication with the satellite using the very the petals would work and how regolith samples would small (40mm) antenna aperture dictated by the find their way from the spike to the chromatograph were tiny size of the probe. The laser chip also provided not resolved. In particular, the petal design called for optical energisation for the instruments on a chip, selective application of ‘unobtainium’. as well as energy for vapourisation of soil samples. III. SENSOR NETWORKS VERSUS CONVENTIONAL PROBES A ‘lab’ on a chip, containing a number of Although much of the discussion around the original instruments, mostly operating on the Fabry-Perot Daisy centred on the design of the individual probe, the principal. These instruments included a core concept is that of a sensor network as a planetary chromatograph for chemical analysis and exploration instrument. Naturally, the previous work accelerometers for seismic analysis. emphasised the advantages of the approach, the major ones of which which were seen to be: The optical sensor chip included image sensors (cameras) to capture the images formed by the A mission with much greater robustness than one lenses in the ‘insect eye’ at the top of the daisy. using a ‘rover’, due to the massive redundancy of This chip also provided sensing for the Fabry- indiviual micro probes. Perot instruments and the receivers for the optical communication. Coverage of a much larger area than a rover based mission for the same payload. A power management chip provided would provide power management functions as well as Discussion resulting from the publication of the driver circuitry for the actuators, mostly steering previous papers, some with people with experience of mirrors supporting the various optical functions. planetary exploration, brought to light a number of ways that a sensor network would be inferior to traditional The other major component was formed by the instruments, namely: ‘petals’ of the daisy. This was a multipurpose component, serving as a steerable antenna, a solar Because of the tiny size of the probes, the function concentrator and also a movable aerodynamic of the instruments carried was severely limited. surface, during the descent. The probes were fixed, and so could not investigate After the daisy’s first appearance in the film, certain any site other than the very immediate vicinity ofaspects of its design became detailed and a mechanism the landin point.for packing and disseminating the daisies via a tape wasdevised, which also provided for their charging and Conventional surface probes can be equipped withprogramming before delivery into the Martian tools to allow them to take deep core samples, oratmosphere. The scenario also provided a case study for drill into rocks which have been selected as being ofthe design of autonomous networked sensor systems, interest. By contrast, a nano-probe is restricted toincluding protocols, information retrieval mechanisms surface sampling where it lands.and methods for constructing maps from fields ofsensors, such as the daisies. IV. GEOGRAPHICAL MAPPING The systems work described above differed markedly The geographic mapping function of the daisyfrom most other nano-probe studies, in that it envisaged a network was in fact the place where the thoughtflat, peer to peer network, as opposed to a lander which experiment started. At the time, sensor node licalisationoperated as a base station, used in combination with a was one of the main research themes of the groupswarm of nano-landers. The major reason for this concerned, and the question arose as to whetheremphasis was the direction taken in terrestrial sensor localisation information from a sensor network could benetworks. This is heading towards a vision of global, ad used to make a detailed 3-D map of the terrain on whichhoc, sensor webs, perhaps best exemplified by Nokia’s the sensors were placed. Since the original aim of the‘Sensor Planet’ . At the hardware level, this had the scenario was popular science dissemination, a highlydisadvantage of requiring every node to communicate visual application was desired, the one selected being thewith the host satellite. The advantage is a much more use of 3-D maps derived from the nodes’ localisation torobust and adaptable system, which is not vulnerable to support a virtual presence application, along with use ofloss of a single specialized resource, such as a ground the sensed data to provide environmental feedback (in thestation. real world, the virtual presence scenario would be precluded by the 4-20 minute transmission delay between Since it was never intended as a serious contribution Earth and Mars).to the canon of planetary exploration, little further workwas done on the daisy itself, although a brief feasibility The importance of sensor localisation lies in thestudy was published. The conclusion was that some parts mapping of sensed data. It is obviously imperative towere feasible, at least in operating principle, some know the location within the map of an individualweren’t. It appeared that such a probe would have sensor’s data point. In terrestrial applications, it is notsufficient processing and memory to undertake the task always possible to determine location easily. For mostrequired, the power budget balanced, and successful existing large sensor systems, sensor locations areimplementations of the instruments envisaged had been determined as a result of a site survey. This is expensive,
and often problematic since the area under investigation size of the daisy. Blain, Cruz and Flemming  haveis not always accessible. While GPS is a possible produced a useful survey om micro-miniturised masssolution to localising individual sensors, it has its spectrometers, and conclude that the ion-trap principle ofproblems. Firstly, it is sensitive to many environmental operation shows the most promise for miniaturisation.problems, operation under tree canopies being one. Such an instrument could be at a scale of a fewSecondly, the addition of a complete system in each centimetres, physically constructed from a stack of chipssensor node, simply for localiseation, is likely to add or wafers. There remains the problem of how tosubstantially to the cost of the mission, since each extra introduce a sample into the instrument. If a probe is staticitem of node cost is multiplied several thousand times. once landed, there is one sample opportunity, and the useFor this reason, a strong trend in sensor network of imact energy to excavate and move the sample seemslocalisation research has been the use of the existing to be an attractive option.radio communication resources for localisation, using avariety of techniques including time of flight, relative Another element of the field sensor network scenariophase and signal strength. that was proposed as an advantage is the ability to make detailed maps of such parameters as soil chemistry. This For extraterrestrial exploration, it is likely that is indeed an advantage in terrestrial applications, insystems will be provided within the overall mission for which the regolith will often be obscured from overheadsurface mapping, such as the use of synthetic aperture observation by foliage or artificial structures. When thisradar (SAR) by an orbiter. This can provide terestrial is not the case, satellite imaging spectroscopy producesmapping to a resolution of a few metres. Against this, excellent soil chemistry maps. As an example of thetrilateration localisation techniques operating in the quality obtainable, and what might be expected from amicrowave transmission region can achieve resolutions sensor network the following maps, derived from a seriesof a few centimetres, but operate only where the sensor presented by Clarke and Swaze  are presented.nodes are positioned. The end result is a rather sparsemap of very precisely located points. One opportunity is The first is derived directly from these images andthe use of mult-modal systems, in which the location of shows distribution of iron minerals around Cuprite,the sensor nodes is used to add precision to a SAR map. Nevada.Precisely localised nodes, equipped with cameras, couldalso locate precisely features on the surface of a planet,using optical triangulation. When used in combination,these techniques do provide an opportunity for veryprecise surface mapping, if that is required. VI. REGOLITH CHEMISTRY Regolith chemistry, in some form, is one of theprimary investigation carried out by most planetaryexploration vehicles. Since publication of the originalwork, the authors have gained practical experience ofdesigning sensor networks for the investigation of soilchemistry, but unfortunately these do not provide a goodguide as to what might be done with an extraterresrialprobe. Firstly, for reasons of timeliness and cost, thedevices is constrained to use off the shelfinstrumentation. The most commonly available low costdevices depend on electrochemistry, and sense chemicalsin aqueous solution. Typically, these networks use ionsensitive electrodes to sense particular analytes. In the absence of water, the mechanical arrangementsfor conveying the sample to the sensor become Figure 2: Fe distribution around Cuprite, NV.considerably more complex. The scenario for the MarsDaisy envisaged the use of the landing impact to force a The image represents an area 2 km on a side, andsmall sample into a tube in the spike of the probe, provides a very detailed account of the mineralwhereby it would be vapourised and ionised using the disribution in that area. This image has been used as thelaser and analysed using a MEMS scanning Fabry-Perot basis for a simulation of the results that might bespectrometer, integrated onto the instrumentation chip. expected from a sensor network, sensing for evidence ofAll the parts of this scenario have been reported, but a the same chemicals. Using the Dingo simulator , acomplete instrument has not been designed, stll less network of sensors were randomly distributed over theproduced and evaluated, so it is difficult to say with any 2km square, and the value of the image in Figure 2 usedconfidence that such an approach is feasible. as the output of the sensing device at that point. Mass spectrometry is commonly used for chemicalanalysis in planetary landers. The smallest suchinstruments, which use MEMS technology, are still not ata scale that could be accommodated within a probe of the
The simulated sensor network was programmed to Trebling the number of sensors has clearly made aproduce a map, using the Shepard interpolation method, considerable improvement to the image quality, but it isas has been reported previously . Figure 3 still considerably poorer than the Figure 5: Isopleths added to the 1000 sensor map.Figure 3: A map produced using 1000 sensor original. Finally, Figure 5 shows the image from thenodes. 1000 node network, with an overlaid contour map. shows the map produced by a 1000 node network. Although this is a difference of presentation only, it doesWhile this is recognisably the same area, the map is make the map more readable.clearly of much poorer quality than the original, which These maps use a relatively primitive method ofcould have been obtained using an orbiting image interpolation. Improved interpolation methods, togetherspecrometer. with multivariate and model based techniques, might Figure 4 shows the same image, but now produced improve the interpolated maps somewhat, but the satelliteusing a 3000 node network. spectroscopy is a hard target to hit. In conclusion to this section, it can be seen that a sensor network, unless very densely populated, cannot compete in terms of detail with image spectrography. However it may well be a useful adjunct to it, allowing calibration of the satellite images, and determination of some species with more accuracy than is possible. Sensor networks would also come into their own in situations where spectroscopy was impracticable, for instance impenetrable atmospheres. VII. IMAGING The ‘insect eye’ on the daisy was not, in truth, a practical solution to a requirement for visual imaging. The objective of the design was to gain a 360º field of vision and, given the arbitarily determined weight and size limitations, to do it without moving parts. In practice, it would be difficult to make lenses with the required characteristics, and distribution of the imaging arrays on the chip would be difficult. In a more realistic scenario, there would be a number of requirements forFigure 4: A map produced using 3000 sensor visual imaging. One was mentioned in Section IV,nodes. precision mapping by sterioscopic imaging. Using widely spaced sensor nodes could provide very accurate localisation of objects using triangulation, provided that the nodes could be accurately localised and angles of incidence could be accurately determined. The ‘insect eye’ would not perform this function well. A better solution would be a camera which could be panned. A larger chip would allow a very high resolution image
sensor, which could allow detailed inspection of objects, started with the goal of the deployment of a 10,000 nodeeven at some distance. sensor network and built the worlds largest deployed WSN, some 1,200 nodes, installed over an area of 1.3km VII. SEISMOLOGY by 300m. The network was designed to detect and track intruders using acoustic methods. ExScal remains the The original Daisy included a accelerometer. The most thoroughly researched and documented massivelymotivation was simply that seismology is of potential plural network, and thus is a reference point for futureimportance in planetary investigations, and applications.accelerometers are a well understood component ofwireless sensor networks. Subsequently, it has become Bapat et.al., in their summary of the results of theeveident that field sensing of seismic events Ithat is, ExScal project  cite the problems expected to bemonitoring the same even over a large area) provides an encoutered in the design of a very large network. Theyimportant additional capability – to undertake seismic are:tomography . This means that, provided seismic Failure of sensor network protocols to scale:activity was present, it would be possible to use the The main reason for this given is inability ofsensor network to make an image of the subterranian protocols, which work at a small scale, to deal withstructure. Even in cases in which there was little seismic node failure in the large scale.activity, releasing an object to impact the planet at speedcould provide an event which could be used to probe Complexity of integration: here the issue isunderneath the surface. the interaction of the multiple protocols which deal with issues such as medium access, reliable VIII. METEOROLOGY communication, sensing, and time synchronization Another capability of sensor networks, forseen in the Lack of sufficient fault data: given theoriginal scenario, is the abilty to make meteorologic susceptibility of networks to faults, it was arguedmeasurements and map them precisely over an area. In that there was a need for more real fault data in athe agricultural work, the ability to understand working context.microclimates is very important. Whether or notmicroclimates are as important in extraterrestrial Unpredictability of network behavior:applications is a question for meteorologists. Certainly, a Essentially the consequence of the above – little issensor network can make accurate maps, of a comparable known about behaviour of networks on this scale,quality to those in figures 4 and 5, of temperature, and it is not clear that scalable solutions have beenatmospheric pressure and wind speed and direction. In validated in real-life use.terrestrial applications other factors such as CO2 and The design approach used to address these concernsH20 content are often sensed also. Since electronic gas was one of use of a planned architecture, the impositionsensing is a relatively well developed art, it would be of various design constraints to simplify the operationalpossible to detect a number of gases using well complexity of the system. Nonetheless, it would seemunderstood and mature technology. If the microscale when the results are studied, that ExScal was at the limitmass spectrometer discussed in section VI were of network size feasible with such an approach.available, it coulds be turned also to atmospheric sensing,and would provide a flexible, general purpose To simplify the localisation of nodes and theinstrument. interpretation of the data from them, the system was laid out on a rectangular grid, with nodes being located on IX DISTRIBUTED SYSTEM DESIGN installation using a hand held GPS device. Nonetheless, 11.4% of the nodes were incorrectly located. Most of the sensing modalities proposed above comeinto the category of field sensing – that is the use of an To ensure reliable data transmission, a three-levelarray of sensing devices to determine the value of some hierarchical architecture was adopted, with differentmeasurand over a surface or volume. There have been a specialised hardware at each level. The end to endnumber of terrestrial examples of such networks. For reliability achieved using this architecture was 85.61%example, in the Microclimate Sensor and Image for the best traffic type (low bandwidth) and 55.14% forAcquisition Networks system reported by the Center for the worst type of traffic (high bandwidth)Embedded Network Sensors (CENS) at UCLA , multi A conservative attitude to hardware specification wasfunction sensor nodes have been deployed to allow, adopted. Despite this 6% of the nodes were non-amongst other functions, maps to be made of climate functional after the 15 day trial. Loss of a second tierdata. node caused loss of a complete section of the network. A The promise of field sensing, with many data points, significant amount of node malfunction (7%) wasusing a massive plurality of sensors, is to produce a associated with their reprogramming.detailed map directly from the sensed data, using Without doubt, completion of the ExScal networkthousands of nodes, as depicted in the simulations in was a considerable achievement, but it has notsection VI. The scale of networks illustrated there is established planned architecture as a suitable basis foraround the current state of the art. A major attempt to networks an order of magnitude larger that was achieveddemonstrate a system of similar size is the ExScal (short there.for Extreme Scale Networking) project . This project
Furthermore, the carefully planned and installed as coins are stacked. The stacks would be retained bynetwork of ExScal, and the attrition over a short period, wires, which would be used to cahrge and program thewould not be feasible within the planetary exploration nodes while within the entry vehicle. A possiblescenario put forward. The network must be self arrangement is shown below:configuring and self maintaining. This was the majorreason for the adoption of a flat, peer to peer architecture. (camera ready version will contain diagram)The problem of self-configuration of a network becomes An advantage of the disc form factor is that powersomewhat simpler when all nodes are the same. Various cells are conveniently manufactured in disc form, so aprotocols for self configuration have been published, power cell forms the lowest layer, putting the mass at theusually designed to presever power as much as possible. bottom, to help ensure that the node lands the right wayExamples are Leach  and MuHMR . It should be up.noted that these protocols have been verified insimulation only, to date no-one has produced a network The next component is the stack of wafers that formof autonomous sensors sufficiently large or long lived to the mass specrometer. These would be topped by severaltest in real life. Another point to be noted is that the vast multi chip hybrids, integrating the circuitry andmajority of protocols and simulated results assume embedded sensors (such as accelerometers). Finallyomnidirectional, radio data transmission. The directional, would be another hybrid containing the external sensorsoptical transmission envisaged in the Daisy is completely (image, pressure, temperature, gas sampling valves) andoutside the parameters of these protocols. A safer choice deployment drives. During descent the top surfacewould be to revert to conventional radio transmission, would be covered with a number of layers, which wouldwhich gas been well characterised. For nodes on a very be unfolded by the deployment drives. These could besmall scale, this probably rules out direct communication one or more solar cells, for power, a loop antenna forwith the satellite, due to the lack of directivity at these radio transmission and a steerable mirror for the imagelonger wavelenths. Thus, such a system would probably sensor.use specialised ground stations to communicate back Although larger and heavier than the original Daisy,home. The vulnerability of these could be eliminated by this coin shaped probe is still scaled small enough toreplicating them sufficiently. The sensor network would allow deployment of very many for the weight of anstill operate on an ad-hoc, peer to peer basis, routing existing orbiter. Undoubtedly if a mission is planned inmessages back to one of the ground station sinks. So long earnest, the real design would be very different from this,as one remained operational, and a route from all nodes but as a concept sketch it suggests that a multi modalto it could be found, the network would remain planetary exploration node is feasible with currentoperational. technology. Energy saving is also the reason for localisingprocessing to the nodes as much as is possible. Typically, ACKNOWLEDGMENTin sensor networks, power consumption is dominated bydata transmission. A previous study has shown the The original Mars Daisy scenario was a team effort.potential energy savings attainable by selection of an Contributors included Sarah Mount, Andree Woodcock,appropriate processor, and maximising processing so as John Burns, Jim Tabor, James Shuttleworth and Elenato minimise data transmission . The interpolation Gaura.algorithms which produced the maps in Figures 3, 4, and5 have been designed so as to be readily distributable and REFERENCESto minimise communication used in their construction.  Newman R.M., Gaura, E. Tabor, J. Mount, S., (2005) HardwareThis is important in an application such as planetary Architectual Assessments for Cogent Sensors - Requirementsexploration networks, where the aim must be to minimise derived from a Planetary Exploration Scenario, Proc. of Secondthe necessity to transmit data to the host satellite. Rather International Workshop on Networked Sensing Systems INSSthan every node requiring to communicate raw values, 2005 (IEEE &SICE), June 2005, San Diego, pp. 113-118such maps can be produced within the network, and  Woodcock, A., Burns, J. Gaura, E. Newman, R.M. Mount, S.transmitted by a single node. That node, in posession of (2006), Daisies on Mars: disseminating scientific information tothe complete output, may use effective data compression unmotivated audiences, Proceedings of IEA 2006, 16th Worldtechniques to minimise the actual data transmitted. Congress on Ergonomics, July 2006, Maastricht.  Chen Canfeng, Ma Jian,Yu Ke, Designing Energy-Efficient Wireless Sensor Networks with Mobile Sinks, Workshop on X. ALTERNATIVE NANOPROBE DESIGN World-Sensor-Web (WSW2006), in Proc. SenSys2006. While visually attractive, the daisy no longer seems to  Matthew G. Blain, Dolores Cruz, James G. 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