gps signal degradation modeling

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  • GPS Signal Degradation Modeling

    Gong

    umria

    J. , S

    BI ChaGeorecEng(XiEle(Berece GyuEleKorfromworrecevehusinnetwreceinte Mr.Depof Engcom Dr.Deprespposinv198 ScoEledeg

    SwdCPNdyeneceenmU GGMththem1bfoSSvanT Changlin Ma, Gyu-In Jee, Glenn Mac

    Department of Geomatics Engineeri

    S. BloebaEricsson Inc, Research T

    G. Cox, L. Garin,

    SiRF Technology Inc.

    OGRAPHY

    nglin Ma is a graduate student of the department of matics Engineering at the University of Calgary. He

    eived his B.S.(1992) and M.S.(1995) in Electronics ineering from Northwestern Polytechnical University

    an, China), and his first Ph.D.(1998) also in ctronics Engineering from Tsinghua University ijing, China). His current research is focused on GPS iver technology.

    -In Jee is a Professor in the Department of ctronics Engineering at Konkuk University in Seoul, ea. He received his Ph.D. in Systems Engineering Case Western Reserve University on 1989. He has

    ked on several GPS related research projects; GPS iver software design, GPS/INS integration for land icle, DGPS system development, GPS engine design g the Mitel chip sets, wireless location in CDMA ork, etc. His research interests include software GPS iver, wireless positioning for E911, and GPS/INS gration for personal navigation.

    Glenn MacGougan is a MSc. student in the artment of Geomatics Engineering at the University

    Calgary. In 2000 he completed a BSc. in Geomatics ineering at the University of Calgary. He will plete his second degree in September 2002. Gerard Lachapelle is a Professor and Head of the artment of Geomatics Engineering where he is onsible for teaching and research related to

    itioning, navigation, and hydrography. He has been olved with GPS developments and applications since 0.

    tt Bloebaum received the B.S. and M.S. degrees in ctrical Engineering from Virginia Tech, and the Ph.D. ree in Electrical Engineering from North Carolina

    A LtoteAE JoEB

    882ugan, and Gerard Lachapelle , the University of Calgary

    ngle Park, N.C.

    Shewfelt an Jose, CA

    tate University. He has been with Ericsson since 1990, here he has held a variety of roles in research and evelopment for cellular phones and networks. urrently, he is a Technical Manager in Ericsson Mobile latforms and Technologies in Research Triangle Park, C where he is responsible for positioning technology evelopment. He has been involved with GPS for four ars and played a key role in setting GSM standards for twork-assisted GPS. His technical interests are hybrid llular-satellite positioning technology, speech hancement and compression, and multicarrier odulation and transmission systems. He holds four .S. patents.

    eoffrey F. Cox received his B.A. degree in eology/Chemistry and Mathematics at the University of aine in 1992, M. Eng. in Geomatics Engineering from e University of Calgary in 1996. His area of study at at time was GPS Positioning and Navigation with phasis on the Foliage Effects on GPS Signals. Since

    996, Mr. Cox has worked in many engineering and usiness capacities ranging from WADGPS development r Terrestrial and Aviation Precision Agricultural

    ystems, Commercial RTK Survey and Mapping ystems. In beginning of 2000, Mr. Cox consulted for arious companies by providing GPS related engineering d marketing services. Mr. Cox joined SiRF

    echnology, Inc. in the fall of 2000 as Senior

    pplications Engineer.

    ionel Garin is Lead Architect at SiRF Technology. Prior joining SiRF he worked on Multipath Rejection chniques, survey quality GPS and Glonass receivers at shtech and Sagem. Mr. Garin holds a MSEE from coleNationale Suprieure des Tlcommunications.

    hn L. Shewfelt received a B.Sc. in Electrical ngineering from the University of California Santa arbara in 1981. Since that time Mr. Shewfelt has been

  • involved in design, development, test and integration of complex avionics and guidance systems for various types of aircraft and naval platforms, including working with mimMInthpr A Thfageefefsist(Uredithinth IN Aprpesmdeacarpo Threwviw OthmprpratFiopcenobu20

    in this case, thus requiring a GPS receiver to be able to acquire and track weak signals. Secondly, GPS signals in these serious situations can contain serious multipath signals, which can degrade positioning accuracy sig Toknprdoto ThrefaofarShminfitth Thchthexcoth G Wreasatmoben icrowave radars and receivers, Jammers, UV and IR aging systems, and GPS/INS guidance and control. In arch of 2000, Mr. Shewfelt joined SiRF Technology c. as Applications Engineering Manager to facilitate e integration of GPS technology into embedded oducts and platforms.

    BSTRACT

    is paper attempts to provide some insight into the ding properties of GPS signals. When a GPS signal ts to an antenna, it suffers from masking and blocking fects from surrounding objects. With respect to these fects, GPS signals can be divided into clear LOS gnals, shadowed signals, and blocked signals. A atistic model, Urban Three-State Fade Model TSFM), is discussed in this paper. Experimental

    sults show that this model can describe the fading stribution of GPS signals very well. After model fitting, e model parameters can indicate the composition of the coming signals in terms of the relative magnitude of e three signal types.

    TRODUCTION

    lthough GPS was first designed as a military system to ovide real time position, it is becoming a necessity in oples daily life. GPS receivers are now being made aller and smaller and can be integrated into many vices to provide both position and time with high curacy. Its application has already extended to many eas, such as, earthquake detection, cellular phone sitioning, so on and so forth.

    ese new applications impose more serious quirements on GPS itself. Traditionally, a GPS receiver as required to function in an open area with a clear ew of the sky, but in new applications it is required to ork in degraded signal environments.

    ne typical example is the use of GPS in cellular phones at are required to be location Aware for the E911 andate in the near future ( FCC 2001). GPS is a omising solution to this requirement since it can ovide position autonomously. However, when looking this problem in detail, there are many issues to address. rst, cellular phones are used in many places, not only in en areas. This means the GPS receiver built in a llular phone must work well in places where there is t much open sky, such as, urban canyons or inside a ilding. From previous a study (Frank van Diggelen, 01), it was shown that GPS signals become very weak

    883

    nificantly.

    understand and perhaps solve these problems owledge about the GPS signal channel is obviously a erequisite. Unfortunately, not much research has been ne in this area. Thus, the motivation for this paper was further such research.

    is paper examines the GPS signal channel near ceiver antennas. Specifically, it focuses on the signal ding distribution due to masking and blocking effects surrounding objects. To do so incoming GPS signals e first divided into three categories: Clear LOS signals, adowed signals, and Blocked signals. A statistic

    odel, Urban Three-State Fade Model (UTSFM), is troduced to fit the fading histogram of real data. The ting results describe the signal composition based on e data.

    e outline of this paper is a follows: the GPS signal annel is discussed, a signal classification is presented, e Urban Three State Model is explained, and finally perimental data and model fitting is discussed. Some nclusions and discussion of ensuing research conclude e paper.

    PS SIGNAL CHANNEL

    hen a GPS signal propagates from a satellite to a ceiver antenna, it suffers from degradation effects, such , free space loss, refraction and absorption from the

    osphere, reflection and masking from surrounding jects such as trees and buildings, jamming, and vironmental noise.

    Figure 1: GPS signal propagation

  • This paper focused on signal strength fading due to reflection and masking by surrounding objects such as trees and buildings. Huygens principle tells us that waves are propagated by wabloreade

    NofiranFig

    Thellof ellpolonACneanob

    SIGNAL CLASSIFICATION With respect to fading effects, GPS signals can be divided into three categories: Clgetthelos Shprothrtre BloFrerecref UR ThstuGodesthethrexpandof Cleandcan

    whvomuBe If sigfadwhsig

    velets on sequential wave fronts. When an object cks some wavelets, waves of other wavelets can still ch the shadowed region, but the signal strength is

    creased, as shown in Figure 2.

    Figure 2: Huygens Principle

    rmally, the concept of the Fresnel zone especially the st Fresnel zone is used to characterize the shadowing d blocking effects (Barry McLarmon), as shown in ure 3.

    Figure 3: Fresnel Zone

    e Fresnel zone is the volume of space enclosed by an ipsoid, which has the two antennas A and B at the ends a radio link as its foci. The first Fresnel zone is an

    ipsoid defined such that the distance summation of a int C on the ellipsoid to A and B is one wavelength ger than the direct distance between A and B, i.e. +CB = AB + . From experience, the fading effect is

    gligible if there are no objects in the first Fresnel zone, d the fading effect is thought serious if there are jects in this region.

    884

    ear line-of-sight (LOS) signal: This kind of signal s to the receiver antenna directly without any object in way of propagation. Fading is only due to free space s and atmosphere absorption.

    adowed signal: For this kind of signal, the pagation takes place over the first Fresnel zone ough a medium that just attenuates the s

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