LAAS/GBAS Ground Reference Antenna With Enhanced Mitigation of Ground Multipath

Transcription

1 LAAS/BAS ound Refeence Antenna With Enhanced Mitigation of ound Multipath Alfed R Lopez, BAE Systems BIORAPHY Alfed R Lopez is a Life Fellow of the IEEE He is a Hazeltine Fellow and Engineeing Fellow with BAE Systems, eenlawn, NY He stated his caee at Wheele Laboatoies in 958 as an antenna design specialist He has made contibutions to the theoy and pactice of electonic scanned antennas Fom 969 to 99 he was involved with the development of the Micowave Landing System He has published extensively in IEEE publications, has been issued 6 US Patents, and has eceived seveal IEEE and BAE Systems awads He is the invento of the ARL-9 ound Refeence Antenna ABSTRACT The concept fo the ARL-9 LAAS/BAS gound efeence antenna was fist descibed at the ION 2 National Technical Meeting This antenna has the following featues: One-pot cicula-polaization L-L2-L5 coveage of the uppe hemisphee Shap antenna patten cutoff on the hoizon fo mitigation of gound multipath at low elevation 3 db sidelobes in the lowe hemisphee fo mitigation of gound multipath at highe elevation Vey high quality caie delay (antenna phase cente) and code (goup) delay chaacteistics Cicula polaization at low elevation fo mitigation of lateal multipath This antenna has been unde development since 999, and is now enteing the initial phase of field deployment The pototype ARL-9 antenna was built unde contact to the FAA Afte successful testing by the FAA, in Decembe of 25, 7 additional antennas wee built and tested Ten additional antennas ae cuently being built This pape eviews the gound multipath issue and descibes a means fo enhanced mitigation of the gound multipath poblem The vaiability of the gound eflectivity is quantified and its impact on the possible vaiation of the gound multipath eo is descibed Siting ecommendations ae made which enhance the multipath pefomance INTRODUCTION The ARL-9 gound efeence antenna is shown in Figue installed at the William J Hughes FAA Technical Cente, Atlantic City, NJ It is a 9-element collinea aay antenna designed specifically fo the LAAS/BAS application Figue 2 shows the aay antenna with the adome emoved ARL-9 Diffeential PS ound Refeence Antenna Figue COMPUTER SIMULATION MEASUREMENT

2 ARL-9 Antenna Radome Removed 9 Element Aay 9 Excited Elements Paasitic Elements High-Accuacy Aay Excitation (±25 db, ±2º) Figue 2 The aay antenna compises 9 adiating elements and a 9-way-powe-divide and cabling assembly as shown in Figue 2 The cabling assembly has 9 equal-length coaxial cables that connect to (counting fom the bottom) elements numbes 3, 5, 7, 9,,, 3, 5, and 7 These elements ae excited diectly; the emaining elements ae excited paasitically (indiectly via mutual coupling)the ARL-9 is a vey simple but a vey high pecision antenna The key featues of the ARL-9 antenna ae: One-pot, cicula polaization, L-L2-L5 coveage of the uppe hemisphee o One pot one eceive, no satellite handove poblem o Full NSS band, 5-6 MHz Shap antenna patten cutoff on the hoizon fo mitigation of gound multipath at low elevation o 25 db/º slope on hoizon o 3 db antenna Up/Down gain atio down to 6º elevation 3 db sidelobes in the lowe hemisphee fo mitigation of gound multipath at highe elevation o 3 db antenna Up/Down gain atio Vey high quality caie delay (antenna phase cente) and code (goup) delay chaacteistics o Caie delay vaiation ±cm (unit-to-unit ±mm) o Code delay vaiation ±25cm (unit-to-unit ±3mm) Cicula polaization at low elevation fo mitigation of lateal multipath [5-6] o Taxing aicaft, hanges, etc Unit-to-unit de-coelated sidelobes in the lowe hemisphee o Enhances gound multipath de-coelation amongst gound efeence eceives Integal ai teminal fo lightning potection o Non-scatteing ai teminal configuation, no pefomance degadation The initial concept fo the ARL-9 was developed in 993 and the fist patent was issued in 996 [] The initial concept was vey naow band and thee wee poblems with the implementation An impoved wideband vesion was developed in 999 with an associated patent issued in 2 [2] This second vesion had the same basic achitectue as the ARL-9 [3] but it had one significant poblem, excessive code delay vaiation with azimuth angle at high elevation [] The solution to the poblem associated with the second vesion became appaent in 23 The ARL-9 antenna is the embodiment of this solution Measuements in Decembe of 25, at the William J Hughes FAA Technical Cente [7] (see Figue 3) demonstated excellent pefomance fo the ARL-9 antenna FAA Measuement Atlantic City Decembe 7, 25 ARL-9 Antenna Seial No Figue 3 RANDOM SIDELOBES ENHANCE MULTIPATH DE-CORRELATION AMONST REFERENCE RECIEVER STATIONS The equiement fo coveage of the uppe hemisphee esults in an inheently low-gain antenna Ideally, with no antenna dissipative loss, the highest gain possible is 3 dbi It is vey difficult to design a low-gain antenna with low sidelobes (3 db) The design stategy fo achieving low sidelobes fo a low gain antenna was pesented in [3] The basic concept is to set the systematic (zeo aayexcitation eo) peak sidelobe level substantially below the desied peak sidelobe level (3 db), and then to specify the eo toleance on the aay excitation such that the combined 2-sigma value fo the peak systematic and eo sidelobes is less than 3 db Figue pesents the basic appoach fo the design of the ARL-9 antenna The aay excitation toleances of ± 2 db and ± 2º equie high pecision components fo the collinea aay antenna The elevation pattens fo 2 andom-eo tials pesented in Figue show the pedominant andom natue of the unit-to-unit sidelobe levels

3 Random sidelobe levels with 2 db amplitude and 2 phase eos 2-sigma up/down gain atio equals 296 db Figue ARL-9 Random Sidelobe level Elevation Angle (Degees) Zeo Eo Peak Sidelobe Level One significant benefit fo the ARL-9 antenna is povided by the andom natue of the unit-to-unit sidelobe level Fou efeence eceive stations ae typically utilized to suppess multipath The fou stations ae located with espect to each othe to povide decoelation of the multipath signals The andom natue of the ARL-9 antenna unit-to-unit sidelobe level enhances the de-coelation of the gound multipath signals ROUND MULTIPATH CHARACTERISTICS The basic geomety fo gound multipath is shown in Figue 5 At the antenna phase cente the diect signal and a gound eflected (multipath) signal ae combined The polaization of the gound eflected signal expeiences a tansfomation upon eflection The Fesnel fomulas quantify this tansfomation and ae pesented below Antenna Phase Cente H Figue 5 ound Multipath eomety Delay 2 H sin( θ ) Diect ound Reflected (Fesnel Fomulas) The Fesnel fomulas fo the eflection facto fo vetical polaization,, and fo hoizontal polaization,, ae given by: θ sin sin + Fee space dielectic constant Relative dielectic constant Conductivity ω Radian fequency θ Elevation angle Smooth Sea Vey Dy ound Figue 6 Reflection Facto (Voltage Ratio) ound Type Fesh Wate Wet ound ω ω sin sin + ω ω ound Paametes Relative Dielectic Constant, ω ω cos cos cos cos Conductivity, (S/m) 2 ound Reflection Facto Linea Polaization Figue Smooth Sea Vetical Polaization Smooth Sea Hoizontal Polaization Vey Wet ound Vetical Polaization Vey Wet ound Hoizontal Polaization Vey Dy ound Vetical Polaization Vey Dy ound Hoizontal Polaization Fequency 575 MHz Bewste Angle: Elevation angle whee the vetical polaization component is a minimum value The gound eflection chaacteistics ae quantified by two paametes in the Fesnel fomulas, the elative dielectic constant, and the conductivity, Figue 6 pesents a table that lists these paametes fo fou diffeent types of gounds anging fom a smooth sea to a vey dy gound

4 Cicula Pol Reflected Powe (db) ound Reflection Facto Cicula Polaization Figue 8 H 3m, Antenna Up/Down ain Ratio 3 db 9 δ 6 cm Elevation Angle (Degees) Smooth Sea Cicula Polaization Vey Wet ound Cicula Polaization Vey Dy ound Cicula Polaization δ 5 cm Multipath Eo δ cm 8 db The eflection facto magnitude vesus elevation angle fo vetical and hoizontal linea polaization components of an incident signal ae pesented in Figue 7 The Bewste angle is defined as the elevation angle whee the magnitude of the vetically polaized component is a minimum The eflection phase fo the vetically polaized component vaies 8º acoss the Bewste angle The handedness fo a eflected ciculaly polaized incident signal eveses acoss the Bewste angle On the zenith side of the Bewste angle an incident ight-hand ciculaly polaized incident signal is eflected with lefthand polaization Figue 8 pesents the magnitude of the eflection facto fo an incident ciculaly polaized signal At nea the hoizon ight hand cicula polaization is eflected as ight-hand cicula polaization At the Bewste angle the eflected signal is essentially hoizontally polaized At nea zenith, an incident signal with ight-hand cicula polaization is eflected with left-hand cicula polaization (Thee have been cases whee the antenna gain patten has been measued with espect to ight hand cicula polaization, and the atio of the gain in the zenith diection to the gain in the nadi diection has been epoted as the desied-to-undesied atio fo the zenith diection This, of couse, was in eo since the antenna has, inheently, left-hand cicula polaization in the nadi diection) Figue 8 shows that fo elevation below º the magnitude of the eflection facto is essentially independent of the type of gound Fo elevation above 3º thee is substantial vaiation of the eflection facto magnitude In the zenith diection thee can be an 8 db diffeence in the eflection magnitude fo smooth sea and a vey dy gound The coesponding vaiation in the multipath eo could ange fom 5 cm fo the case of a smooth sea to 6 cm fo the case of vey dy gound DEFINITION OF DESIRED-TO-UNDESIRED (D/U) SINAL RATIO Fo a given satellite elevation angle and assuming a flat hoizontal gound: The D/U atio is defined as the atio of the eceived diect signal to the eceived multipath (indiect) signal An appoximate fomula fo D/U is given by: D Up / Down ain Ratio ( θ ) () U ( ( ( ( Antenna gain with espect to an isotopic antenna (dbi, total adiated powe) ( Magnitude of gound eflection facto θ Elevation angle Equation () assumes polaization match fo both the diect and indiect signals (The magnitude of the diect signal is popotional to the total powe in the diect signal; the magnitude of the indiect signal is popotional to the total powe in the eflected signal) It is a consevative estimate fo the D/U atio An exact fomula fo D/U is given by: D U Antenna gain with espect to vetical linea polaization Antenna gain with espect to hoizontal linea polaization Fesnel Fesnel ( + j + j ( fomula fo vetical polaization fomula fo hoizontal polaization All the factos in Equation (2) ae complex voltages MULTIPATH ERROR VERSUS ELEVATION ANLE The multipath eo, δ(, vesus elevation angle is given by the appoximate fomula [8]: πh δ( 2H sin sin sin D / U λ ( πh δ( ( 2H sin sin sin λ It is assumed that the gound is flat and hoizontal Equation (3) is valid fo H < 75 m and D/U >> (2) (3)

5 Code Delay Multiptah Eo (Metes) 2 Multipath Eo Vesus Elevation Angle Elevation Angle (Degees) H 3 m D/U( 36 db Antenna Up/DownainRatio/( Antenna Up/DownainRatio 3 db ( - 6 db Figue 9 Multipath eo vesus elevation angle is pesented in Figue 9 The intefeence patten shown in Figue 9 is simply that of a souce located 3 m above the gound suface and its image 3 m below the suface It is noted that if the antenna wee located above a pool of wate, the peak eo would be nea 5 m ENERAL SITIN CONSIDERATIONS As indicated in Equation (3), the peak multipath eo is diectly elated to the antenna height above the local gound One would tend to locate the antenna as close to gound as possible, but it is soon ecognized that this location would be ovely sensitive to local tansient objects that can poduce substantial multipath effects Expeience to date has indicated that an antenna phase cente height of 3 m is suitable fo aipot envionments It is ecommended that the phase cente height should not be less than 2 m The vaiability of the peak multipath eo associated with the wate content of the local gound leads to the following ecommendations: Avoid a site with a local bowl-shaped (concave) teain featue Concave gound can amplify the gound eflection facto Concave gound can also accumulate ain wate at the base of the antenna Location at the apex of a convex suface is the ideal location Stabilization consideations ade the local site ( m adius) to minimize the possibility of a wate satuated local gound o pools of wate nea the antenna base Add a laye of cushed stone (o othe suitable mateials) to povide a stable electically-ough suface with a eflection facto anging between -6 and -9 db SUMMARY This pape: Pesented an oveview of the ARL-9 gound efeence antenna pefomance and histoy This antenna is unique with nea ideal chaacteistics Pesented a detailed desciption of gound multipath chaacteistics, which highlighted the possible vaiability of the local gound eflectivity This impacts the multipath pefomance, especially at high elevation Made ecommendations fo the siting of the antenna and possible teatment of the local gound suface fo contol of the vaiability of the local gound eflectivity ACKNOWLEDEMENTS Many thanks ae given to M John Wabuton (William J Hughes FAA Technical Cente) fo his help ove the many yeas with the development of the ARL-9 antenna The successful development of the ARL-9 antenna is cedited, in geat pat, to the technical leadeship of M ay A Schay (BAE Systems) and the manageial leadeship of M ay T Nolan (BAE Systems) REFERENCES [] AR Lopez, US Patent No 5,53,882, PS Antenna Systems, Jul 9, 996 [2] AR Lopez, RJ Kumpfbeck, EM Newman, US Patent No 6,2,5, Self-Contained Pogessive- Phase PS Elements and Antennas, Ma 3, 2 [3] AR Lopez, PS ound Station Antenna fo Local Aea Augmentation System, LAAS, ION NTM 2, Januay 2, Anaheim, CA [] AR Lopez, Calibation of LAAS Refeence Antennas, ION PS 2, - Septembe 2, Salt Lake City, UT [5] AR Lopez, LAAS Refeence Antennas Cicula Polaization Mitigates Multipath Effects, ION 59 th Annual Meeting/CITF 22 nd uidance Test Symposium, June 23, Albuqueque, NM [6] AR Lopez, LAAS Refeence Antennas Key Siting Consideations, ION NSS 7 th Intenational Meeting of the Satellite Division, 2-2 Sept 2, Long Beach, CA [7] FAA Navigation/LAAS T&E Team, Local Aea Augmentation System Pefomance Analysis / Activities Repot, Repot #9, Repoting Peiod: Januay to Mach 3, 26, Apil 3, 26, pp 3-5 [8] CC Counselman, Multipath-Rejecting PS Antennas, Poceedings of the IEEE, Vol 87, No, Jan 999