Radio Frequency Modelling for Future Wireless Sensor Network on Surface of the Moon

Transcription

1 Int. J. Communications, Network an System Sciences, 00, 3, oi:0.436/ijcns Publishe Online April 00 ( Raio Frequency Moelling for Future Wireless Sensor Network on Surface of the Moon Jayesh P. Pabari, Yashwant B. Acharya, Uay B. Desai, Shabbir N. Merchant 3, Barla Gopala Krishna 4 PLANEX, Physical Research Laboratory, Ahmeaba, Inia Inian Institute of Technology, Hyeraba, Inia 3 Inian Institute of Technology Bombay, Mumbai, Inia 4 Space Application Centre, Ahmeaba, Inia jayesh@prl.res.in Receive January 6, 00; revise February 8, 00; accepte March 0, 00 Abstract In orer to stuy lunar regolith properties, wireless sensor network is planne to be eploye on surface of the Moon. This network can be eploye having few wireless sensor noes capable of measuring soil properties an communicating results, as an when reay. Communication scenario on lunar surface is quite ifferent as compare to that on the Earth, as there is no atmosphere an also there are lots of craters as well as various terrain topologies. Since the eployment of sensors on the Moon is a challenging an ifficult task, it is avisable to preict the behaviour of communication channel on lunar surface. However, communication moels like Irregular Terrain Moel use for terrestrial communication networks are not irectly applicable for Unattene Groun Sensor type sensor networks an nee moifications accoring to lunar surface conitions an lunar environment. Efforts have been put to evise a moel of raio frequency environment on the Moon using basic equations governing various physical phenomena occurring uring raio propagation. The moel uses Digital Elevation Moel of four sites of the Moon, measure by Terrain Mapping Camera on boar Chanrayan-, a recent Inian mission to the Moon. Results presente in this paper can provie unerstaning of percentage area coverage for given minimum receive signal strength, potential sites for sensor eployment assuring wireless communication, ecision whether a given sensor noe can work an can provie suggestion for possible path of rover with cluster hea to remain in contact with the noes. Digital Elevation Moel base results presente here can provie more insight in to the communication scenario on the Moon an can be very useful to mission planners. Keywors: Wireless Moel, Moon, DEM, Coverage. Introuction In orer to etect few regolith properties on lunar surface, a Wireless Sensor Network (WSN) is planne to be eploye near lunar South Pole, which utilizes wireless sensors capable of working in harsh environmental conitions. Main property of interest is permittivity obtainable from impeance sensor. Since the eployment of network on the Moon is a challenging an ifficult task, it is avisable to preict the behaviour of communication channel on the lunar surface by some channel moel. There have been various moels for raio communication on the Earth, for example Irregular Terrain Moel (ITM) also known as Longley-Rice moel given by G. Huffor, A. Longley an W. Kissick [] an moel given by Durkin [,3]. ITM is a goo moel for terrestrial communication network an it uses minimum antenna height as 0.5 m for simulation while Durkin s moel oes not consier multipath effect in the simulation. Hata [4] ha propose an empirical formulation of graphical path loss vali from about 50 MHz to 500 MHz an it was extene up to GHz. Walfisch an Bertoni [5] gave a moel to consier impact of rooftops an builing height by using iffraction to preict average signal strength at street level. Alberto Cerpa et al. [6] escribes statistical moel of lossy links in wireless sensor networks. S. Willis an C. J. Kikkert [7] have given raio propagation moel for long range wireless sensor networks. Chirag Patel [8] Copyright 00 SciRes.

2 396 J. P. PABARI ET AL. has escribe wireless channel moeling in his thesis. Vishwanath Chukkala et al. [9] an Aniruh Daga et al. [0] gave moeling an simulation of Raio Frequency (RF) environment of Mars. In case of present application scenario, operating frequency is to be.4 GHz an sensors are to be eploye on lunar surface, where operating conitions are ifferent than that on the Earth. It is known that there is no atmosphere on the Moon an there exists very high vacuum of the orer of 0 - Torr [,]. The communication moels use for terrestrial communication networks are not irectly applicable to Unattene Groun Sensor (UGS) type sensor network, like that planne on the Moon an nee moification accoring to lunar surface conition. This has motivate us to carry out the work presente in this paper. Efforts have been put to erive moeling of raio frequency environment on the Moon, using basic equations governing various physical phenomena occurring uring raio propagation. Our work uses Digital Elevation Moel (DEM) of four sites of the Moon, measure by Terrain Mapping Camera (TMC) on boar Chanrayan-, a recent Inian mission to the Moon. Results presente in this paper can provie unerstaning of percentage area coverage for given minimum receive signal strength, potential sites for sensor eployment assuring wireless communication, ecision whether a given sensor noe coul be use an suggestion for possible path of rover (carrying cluster hea) to remain in contact with the noes. Section presents suitability of existing propagation moels for lunar application, Section 3 escribes physical phenomena which can occur on the Moon an associate path losses, Section 4 gives lunar wireless moel, Section 5 gives etails of DEM of selecte sites on lunar surface, Section 6 shows results an paper ens with conclusion.. Suitability of Existing opagation Moels for Lunar Application It is expecte that the communication woul be better on the Moon as compare to that on the Earth, ue to absence of atmosphere on the Moon. Few existing propagation moels were evelope for communication on the Earth an nee to be reviewe for the Moon in view of their applicability... Irregular Terrain Moel Irregular Terrain Moel (ITM) was suggeste by Rice et al. [3] an is also known as Longley-Rice moel. Huffor [4] informe that it can be use in area preiction moe an point-to-point moe. ITM takes terrain an other parameters as input an prouces output as signal istribution in a given area. There are certain limitations of ITM that it can be use for minimum antenna height of 0.5 metre an minimum istance for communication as km. The Point-To-Point (PTP) moel given by Wong [5] is base on Longley-Rice moel. However, PTP moel escribes metho to obtain iffraction loss. As the wireless sensor network involves eployment of sensors on the groun with very small antenna heights especially at.4 GHz operating frequency, the ITM can t be use for such applications... Two-Ray Moel For a line-of-sight communication, a two-ray moel was given by Neskovic et al. [6]. This theoretical moel incorporates reflection by the reflection coefficient, which is calculate from incience angle, ielectric constant, surface conuctivity an polarization of antenna. The signal strength at the receiver as given in () is shown in Willis [7] using free space loss an reflection. PG t tgr λ exp( jk ) ( ψ) exp( jk) 6 π () where an are lengths of first an secon path respectively. The two ray moel is mostly use for irect an groun reflecte rays. In case of lunar wireless sensor net- work, sensor noes are to be the surface with very small antenna heights an therefore groun reflecte signal is not expecte at the receiver..3. VSS Multipath Moel Signals reflecte from reflectors woul reach the receiver with ifferent strengths an phases with reference to irect signal. At receiver, equivalent signal strength is equal to superposition of varying amplitue an phase signals an it is possible to get overall signal strength as reuce or improve. The multipath channel moel in AWR Visual System Simulator [7] uses Equations () an (3) to obtain signal strength for a sample k. The multipath moel was implemente by Willis [8] in MAT- LAB. N x( k) path( k, ) i i () jπ V fc Cosθi path( k, i) Ai x( k i) exp( ) C (3) where x() k is kth sample, N is number of multipath signals, path( k, i ) is contribution of ith multipath signal etermine by (3), A i is gain of ith path, x( k i ) Copyright 00 SciRes.

3 J. P. PABARI ET AL. 397 is elaye sample associate with path i an exponential term represents oppler shift ue to receiver movement. This moel may be use for moving noes, which is not the case for lunar wireless sensor network..4. Multipath Signal Distribution Instantaneous receiver signal strength (envelope) is represente by either Rayleigh or Rician istribution as shown by Hernano et al. [9]. Rayleigh istribution consiers only multipath components available an oes not take irect path. However, Rician istribution consiers irect as well as multipath components. Communication on lunar surface is expecte to be line-of-sight an also multipath ue to reflections from terrain an therefore, Rician istribution is more suite for lunar application. Rician istribution is shown by Hernano et al. [9] an given below: r r a r a pr () exp[ ] I 0( ) σ σ σ for r 0 where I 0 is moifie 0th orer Bessel function an value of a epens on irect component. The Rician function is usually expresse in terms of carrier-to-multipath ratio or k factor as c a k m σ (5) where c an m are strengths of carrier an multipath components respectively. As k increases, receive signal strength increases an for larger values of k, pr () becomes Gaussian..5. Willis Multipath Moel Willis [8] has given extene version of two ray moel as multipath moel, shown in Equation (6) for terrestrial application on the Earth. L t r PG t G 6 n ( ) n n L ( ) exp( jk exp( jk ) Physical Phenomena an Path Loss on the Lunar Surface Since the Earth base moel cannot be irectly use for lunar applications, we have examine various physical phenomena occurring uring wave propagation in general an those applie to wave propagation on the Moon. Such phenomena an losses associate with them are ) (4) (6) given below an we have combine them to arrive at the path loss given in Section Free Space Loss The plan is to use compact wireless impeance sensors (along with other types of sensors) for etection of electrical properties of lunar Regolith having small size quarter wave antenna (3.5 cm). The iea is to use line of sight propagation technique for small istances of up to few hunre metres. Uner ieal communication conitions, the power raiate from antenna is omni irectional in the plane of interest, which is horizontal here an power is inverse square function of the istance for free space wave propagation given below [9] by Friis formula. t t r PG G λ 6 π where P t an P r are transmitter an receiver powers, Gt an Gr are gains of transmitter an receiver antennas, λ is wavelength of operation an is istance from the transmitter. Free space loss is a basic loss for communication on the lunar surface, since there is a vacuum on the Moon. 3.. Reflection For communication on the Earth, there can be factors like steay an moving reflectors an scatterers, atmospheric absorption etc an can lea to multipath components at the receiver. Due to multipath components, signal strength can vary at the receiver ue to moving scatterers. However, in case of lunar application, there are no moving objects on the surface an therefore there can be multipath signals at the receiver ue to surface topography, but receive signal strength is not expecte to vary ranomly ue to steay terrain, but it can show perioic variations ue to signals travelling in ifferent time urations. Also, there is no atmosphere on the Moon an therefore there are no atmospheric losses, which are present on the Earth. The main possibility of signal getting affecte is that of the surface reflections ue to uneven terrain structure. This can cause multipath propagation an signal reaching at the receiver by irect path will be moifie ue to multipath components. The expecte number of multipath components is few as the mission laner shoul lan on comparatively plane or smooth surface, where rover can move easily. The wavelength of signal is.5 cm at.4 GHz wireless operation an objects shoul be of larger size to cause the reflection. Parsons [0] erive the receive signal strength using reflection coefficient ( ) from the reflector as given below. (7) Copyright 00 SciRes.

4 398 J. P. PABARI ET AL. where PG t tgrλ exp( ) 6 π a Sinψ ( εr jx) cos ψ a Sinψ ( εr jx) cos ψ j (8) is reflection coefficient = phase shift create ue to reflecte signal a = or ( ε r jx) for horizontal or vertical polarization respectively ψ = angle of incience with vertical from transmitter antenna to the reflector ε = relative ielectric constant of the groun r σ x π f ε0 σ = conuctivity of the groun f = frequency of operation ε 0 = free space permittivity It is also possible to calculate receive signal strength by the Equation (9) given by Hernano et al. [9] erive from two ray moel for longer link istances an lower antenna heights at both ens having irect visibility of each other. PG t tgrht hr 4 (9) where ht an hr are heights of transmitter an receiver antennas respectively. However, for sensors being on surface, Equation (9) may not be use. Reflections are expecte from nearby groun terrain on the Moon. Infact the major component for channel faing on the Moon is ue to multipath create by reflections from craters an surface irregularities. Also, transceiver use for groun sensors shoul not use horizontal polarization; otherwise signal woul be attenuate [] very near the transmitter. Instea, vertical polarization shoul be use, which can provie sufficient signal at farther istances Reflection Scattering When signal is reflecte from lunar surface, it is likely that the ray may be scattere ue to isperse signal. Gibson [] has suggeste this specular reflection an state that roughness of a surface can be classifie by Rayleigh criterion given below: λ hc (0) 8cos θ where θ i is angle of incience at the reflector. Gibson [] gave a parameter h, which represents minimum to maximum eviation about mean terrain height. If h h c, i then terrain is consiere as rough terrain an the loss for it is consiere by multiplying reflection coefficient by a scattering loss factor ρ s calculate by Bothias equation as below: πσhcosθi πσhcosθi ρs exp[ 8 ( ) ] I0[8( ) ] λ λ () where σ h is stanar eviation of surface height about the mean surface height an I 0 is the 0 th orer Bessel function of the first kin. On the lunar surface, the regions are mostly rough an may have varying size objects, which can lea to scattering losses Diffraction For line of sight communication, if an obstacle of size comparable to wavelength is present in between transmitter an receiver, then iffraction loss can occur at the ege of an obstacle. The signal may be scattere an attenuate before reaching to the receiver. Diffraction from knife-ege obstacle can cause signal to ben an Wong [5] state that bening of signal ue to knife-ege obstacle causes higher signal strength as compare to that ue to roune. On the lunar surface it is likely to have such obstacles in between transmitter an receiver, which may be consiere only for irect path but the eployment is suppose to be in almost plane terrain for smooth movement of the rover an hence possibility of occurrence of such loss is rare an may be neglecte. 4. Lunar Wireless Moel Equation (6) given by Willis [8] oes not inclue possibility of irect path, but it is expecte to be present in case of wireless sensor networks. Also, iffraction loss given by Wong [5] may be consiere for irect path from transmitter to receiver an not for multipath for operation at.4 GHz frequency. Consiering iffraction loss only for irect path an incluing reflection scattering loss factor, following equation is given for obtaining area coverage by wireless sensor network on the lunar surface: PG t tgr λ L exp( ) jk 6 π ρs ( ψ) e xp( jk) ρs ( ψ) exp( jk)... ρsn n ( ψn ) exp( jkn ) n () Copyright 00 SciRes.

5 J. P. PABARI ET AL. 399 where L is iffraction loss for irect path, is i- rect path istance, k is phase constant, on scattering loss factor of nth multipath, is reflection coefficient of nth multipath, ence at nth reflector an path component. n ψ n sn n ρ is reflecti- is angle of inci- is istance of nth multi- 5. Digital Elevation Moel of Lunar Surface Recent mission to the Moon from Inia, Chanrayan-, ha Terrain Mapping Camera (TMC) on boar for eriving Digital Elevation Moel (DEM) of the Moon an has provie goo quality ata uring the mission time. Four sample sites at various locations on the Moon have been selecte consiering almost plane surface, region with some peaks an region with few craters in orer to stuy lunar raio propagation moel. Table shows etails of site use in eriving communication area coverage. DEM ata for various sites were obtaine uring ifferent orbits an resolutions were ifferent. Sites an have resolution of 56. m in both irections; site 3 has resolution of.67 m for horizontal (longitue) irection an m for vertical (latitue) irection, while site 4 has m for both irections. Figures (a) to () show images of all these four sites respectively, taken by TMC on boar Chanrayan-. 6. Results Lunar wireless moel is given in Equation (), which consiers all possible phenomena an losses for wireless sensor network on the lunar surface. Diffraction loss is expecte to be very low in value as compare to irect path an multipath reflections for targete application. Since, major interest is in establishing irect line-of-sight communication between transmitter an receiver, such loss is neglecte. As reaily available software cannot be use for wireless sensor network for lunar applications, MATLAB coe was evelope an DEM ata were taken as input in the programme along with values shown in Table an results of raio coverage are Site No. Table. Lunar sites for RF moel. Crater Near the Site Location Catalan Baae 3 Zsigmony 4 Moretus Lunar Latitue to to to to Lunar Longitue to to to to Latitue Longitue Latitue (a) Longitue (b).596 Latitue Latitue Longitue (c) Longitue () Figure. (a) Lunar site ; (b) Lunar site ; (c) Lunar site 3; () Lunar site 4. Table. Input parameters for lunar raio frequency moel. Parameter for Moel Value Moon Raius km Transmitter Power 0 Bm Transmitter Antenna Quarter Wave Monopole Receiver Antenna Quarter Wave Monopole Frequency of Operation.4 GHz Lunar Regolith Dielectric Constant [4] 4 Lunar Regolith Conuctivity [5] 0-8 s/m Wave Polarization Vertical Transmitter Location Mile Left Copyright 00 SciRes.

6 400 J. P. PABARI ET AL (Bm) (Bm) (a) (b) (Bm) (Bm) (c) Figure. (a) RF moel for site ; (b) RF moel for site ; (c) RF moel for site 3; () RF moel for site 4. () -0 epicte in Figures (a) to () respectively; for sites,, 3 an 4. For a given area on the Moon, there can be many multipath components epening up on terrain an if all are allowe to contribute in the coe, then computational complexity is increase highly ue to involvement of higher orer matrix. Moreover, it nees highly sophisticate computing facility an it takes very long time for computation. Number of multipath components may, therefore, be restricte to smaller numbers, which is reasonably justifie, as istance travelle by signal along multipath is quite large in most of the regions as compare to that for the irect path an hence the contribution of those multipath components woul be very less as compare to contribution from the irect path. Normally, sensor noe operating in.4 GHz ISM ban has 0 Bm as output power an its receiver has sensitivity of about -00 Bm [3]. We have calculate raio coverage of the sites as a ratio of occupie region having more than -00 Bm receive signal strength to total region uner consieration. Table 3 shows percentage raio coverage of all sites for 50 kbps link, effectively showing useful area for sensor network eployment. Sites an have been selecte with comparatively plane surface having lesser unulations on the surface, while sites 3 an 4 have ifferent profiles, for example, site 4 is at the ege of Moretus crater near lunar South Pole. Due to this, site coverage is very less for sites 3 an 4 as compare to that for sites an, as expecte. From the erive raio coverage at selecte lunar sites, one can know the areas having more than some specific value of receive power, for example, region receiving more than -00 Bm power. Results inicate the percentage coverage of a given site with known topography an also the possibility of checking if a given sensor noe woul be useful on the Moon, as far as power is concerne. Sensor noes shoul preferably be eploye in the region where sufficient amount of power is available (pink in the coverage patterns). In case of lunar wireless sensor network, rover is suppose to carry the cluster Site Table 3. Site coverage. Site Coverage (50 kbps link).7 % 0.0 % 3.73 % 4.0 % Copyright 00 SciRes.

7 J. P. PABARI ET AL. 40 hea an raio coverage patterns can suggest possible path for the rover to move, assuring intact communication link with the noes. 7. Conclusions In this paper, we have investigate that existing raio frequency moels cannot be irectly applie to lunar wireless sensor network an arrive at lunar wireless moel from funamental physical phenomena occurring uring wave propagation on the Moon. We have presente raio signal coverage patterns of four lunar sites by using actual DEM ata of the Moon. We have use 0 Bm transmitter power at.4 GHz frequency with quarter wave antennas for transmitter an receiver. The results show percentage coverage for 50 kbps links on the lunar surface, suggesting possible use of commercially available transceiver in sensor noe as well as possible eployment sites an rover paths to assure wireless connectivity. 8. References [] G. A. Huffor, A. G. Longley an W. A. Kissick, A Guie to the Use of the ITS Irregular Terrain Moel in the Area eiction Moe, National Telecommunication an Information Aministration, Report 8-00, NTIS Document PB8-7977, April 98. [] R. Ewars an J. Durkin, Computer eiction of Service Areas for VHF Mobile Raio Networks, oceeings of IEE, Vol. 6, September 969, pp [3] J. Durkin, Computer eiction of Service Areas for VHF an UHF Lan Mobile Raio Services, IEEE Transactions on Vehicular Technology, November 977, pp [4] M. Hata, Empirical Formula for opagation Loss in Lan Mobile Raio Services, IEEE Transactions on Vehicular Technology, August 980, pp [5] J. Walfisch an H. L. Bertoni, A Theoretical Moel of UHF opagation in Urban Environments, IEEE Transactions on Antennas opagation, Vol. 36, 988, pp [6] C. A. E., Wong, J. L., Kuang, L. Potkonjak an M. D. Estrin, Statistical Moel of Lossy Links in Wireless Sensor Networks, oceeings of the 4th International Symposium on Information ocessing in Sensor Networks, Center for Embee Network Sensing, University of California Los Angeles, item/4s698fs [7] S. L. Willis an C. J. Kikkert, Raio opagation Moel for Long-Range A Hoc Wireless Sensor Network, International Conference on Wireless Networks, Communications an Mobile Computing, Vol., No. 3-6, 005, pp %F039%F330%F pf%3Farnumber% 3D54954&authDecision=-03 [8] C. S. Patel, Wireless Channel Moeling, Simulation, an Estimation, Ph. D. Thesis, School of Electrical an Computer Engineering, Georgia Institute of Technology, May 006. [9] V. Chukkala, P. DeLeon, S. Horan an V. Velusamy, Raio Frequency Channel Moeling for oximity Networks on The Martian Surface, Computer Networks, Vol. 47, 005, pp [0] A. Daga, G. R. Lovelace, D. K. Borah an P. L. De Leon, Terrain-Base Simulation of IEEE 80.a an b Physical Layers on the Martian Surface, IEEE Transactions on Aerospace an Electronic System, Vol. 43, No. 4, October 007, pp [] P. D. Spuis, Introuction to the Moon, Moon 0, NASA Johnson Space Centre, 7 December [] Anonymous, 7 December org/wiki/moon [3] P. L. Rice, A. G. Longley, K. A. Norton an A. P. Barsis, Transmission Loss eictions for Tropospheric Communication Circuits, Technical Note 0, National Bureau of Stanars, Vol. -, 967. [4] G. Huffor, The ITS Irregular Terrain Moel Version.. - The Algorithm, December its.blroc.gov/itm/itm_alg.pf [5] H. Wong, Fiel Strength eiction in Irregular Terrain - The PTP Moel, December gov/oet/fm/ptp/report.pf [6] A. Neskovic, N. Neskovic an G. Paunovic, Moern Approaches in Moeling of Mobile Raio Systems opagation Environment, IEEE Communications Surveys, Vol. 3, No. 3, 000, pp. -. [7] Microwave Office, December awrcorp.com/proucts/mwoffice/ [8] S. L. Willis, Investigation in to Long Range Wireless Sensor Networks, Ph. D. Thesis, James Cook University, Townsville, Australia, December 007. [9] J. M. Hernano an F. Perez-Fontan, Introuction to Mobile Communications Engineering, Artech House, Boston, 999. [0] J. D. Parsons, The Mobile Raio opagation Channel, n Eition, Wiley, New York, 000. [] G. Kenney, Electronic Communication Systems, 3r Eition, McGraw-Hill Book Company, 985. [] J. D. Gibson, The Mobile Communications Hanbook, CRC ess Inc., Floria, 996. [3] Atmel, 7 December proucts/prouct_car.asp?part_i=394 [4] M. G. Buehler, H. Bostic, K. B. Chin, T. McCann, D. Keymeulen, R. C. Anerson, S. Seshari an M. G. Schaap, Electrical operties Cup (EPC) for Charac- Terizing Water Content of Martian an Lunar Soils, IEEE Aerospce Conference, org/xplore/login.jsp?url=http%3a%f%fieeexplore.iee e.org%fiel5%f0%f34697%f pf&au thdecision=-03 [5] D. H. Chung, W. B. Westphal an G. R. Olhofet, Di- Electric operties of Apollo 4 Lunar Samples, oceeings of the Thir Lunar Science Conference, The MIT ess, Cambrige, Vol. 3, 97, pp Copyright 00 SciRes.