A Novel and Innovative Near Field System for Testing Radomes of Commercial Aircrafts

Transcription

1 A Nvel and Innvative Near Field System fr Testing Radmes f Cmmercial Aircrafts Marc Le Gff, Niclas Adnet, Niclas Grss, Luc Duchesne, Arnaud Gandis, Ludvic Durand MVG Industries 17 Avenue de Nrvège, Villebn-sur-Yvette, France niclas.adnet@mvg-wrld.cm Abstract Current techniques fr testing f aircraft radmes are time cnsuming, nt as precise as desired, and requires dedicated far field test range, cmpact test range, r anechic chamber, r ther specific systems fr measuring ne r ther f the parameters. T vercme all these limitatins, and t btain a unique system fr a cmplete diagnsis, a nvel technique f radme characterizatin is prpsed and detailed in this article. Based n a Near- Field netwrk f prbes placed in the vicinity f the radme under test, this innvative Near-Field measurement system is fully cmpliant with the updated standard RTCA/DO213A and leads t accurate evaluatin f many parameters such as transmissin efficiency and beamwidth, while reducing the measurement time required. - Evaluating the deflectin f the plarizatin; Sidelbe level and beamwidth measurements can be perfrmed in Far-Field distance [1]. An example f typical test set-up in Far-Field is presented in Figure 1. fr testing f the radme parameters. I. INTRODUCTION The maintenance f aircraft radmes is f particular imprtance fr the cmmercial aviatin industry due t the necessity t ensure the crrect functining f the radar antenna, hused within such prtective enclsures. Given that the radar cmpnent prvides weather assessment, as well as guidance and navigatin functins (turbulence avidance, efficiency f rute planning in case f strms, etc.), it is imperative that every repaired radme be tested with accuracy and reliability t ensure that the enclsed weather radar keeps n perating in accrdance with the after-repair test requirements f the RTCA/DO-213 (Radi Technical Cmmissin fr Aernautics) [1]. II. LIMITATIONS OF THE CURRENT TECHNIQUES Radme tests target a reliable valuatin f the disturbances a radme might have n the signals crssing it. This test must be made accrding t the standard that was established by the RTCA.. The varius evaluatins made during the test f radmes cnsist n [2], [3], [4]: - Evaluating the lsses due t the absrptin and the reflectin (it crrespnds t the transmissin efficiency); - Evaluating the radiatin pattern distrtins deflectin f the main beam; half pwer main beam width; level f the secndary lbes; Figure 1. Typical transmissin, sidelbe level and beam width measurement set-up under Far-Field cnditins accrding t RTCA/DO-213 standard [1]. Cncerning the measures f transparency and phase shift, the slutin used at present is a pint-t-pint measure such as presented in Figure 2., where a technician uses a prtable test equipment. This methd fr testing f radmes is based n the duble transmissin [5]. These ne implements a hrn, and a metallic sheet placed n the internal surface f the radme. This methd f test is very lng and is nt very reliable in terms f results. This peratin intrduces an uncertainty, because f the manual character f the measure, and the radme is nt characterized in situatin f functining with the assciated radar antenna. In additin, the discrete nature f the measure results in a lack f infrmatin that makes it difficult t have a cmprehensive diagnsis and accurate parameter extractin. In shrt, this technique is lng and des nt allw t detect with a great precisin the zne f damage. 154

2 Initially the RTCA/DO-213 des nt plan the use f measurement systems in Near-Field fr characterizatin f radmes f cmmercial planes. Only cmpact antenna test ranges and Far-Field systems are mentined in this standard. Nevertheless, sme custmer needs have led t develp a dedicated Near-Field system with a gantry arm that allws cmplete 3D radme surface measurement with a single prbe. A Near-Field t Far-Field transfrmatin was used t btain the fields cmpliant with the RTCA standard [6]. diagnse all parameters in an exhaustive manner with mre precisin and speed. Figure 2. Use f a prtable device fr the diagnsis f radmes in repair shp [5]. Hwever, ne majr drawback f this system lies in the measurement time which is very lng, abut 15 hurs fr a radme. This is a result f perfrming 92 measurement cnfiguratins recmmended by the RTCA / DO-213. Indeed, there are 2 reference measurements f the atmspheric radar antenna withut radme, t which it is necessary t add 45 measurements f the atmspheric radar antenna with radme as well as 45 measurements f the atmspheric radar antenna shifted f lambda/4 frm its nminal statin under the radme. III. NEW RTCA/DO-213A STANDARD EVOLUTION Recently, RTCA/DO-213 quality standard was updated and published under the name RTCA/DO-213A [7], establishing mre stringent measurement requirements and incrprating the pssibility f measuring radmes using Near-Field systems, as presented in Figure 3. It intrduces the pssibility f wrking, after NF t FF transfrm, with a cmplete 3D Far-Field (instead f restricting main cuts) t better calculate the levels f the secndary lbes, the width f beam, and its deviatin. In the case f Far Field ranges, it intrduces the criterin f Fraunhfer (2D 2 / ) instead f Fresnel (D 2 /2 ) which is admitted mre generally as reference distance in the radifrequency dmain. The effect f the Far-Field measurement distance is visible mainly n the characterizatin f the secndary lbes. T summarize, the RTCA DO / 213A pens the dr t the use f Near-Field measurement systems that can be used t Figure 3. Typical transmissin sidelbe level and beam width measurement set-up under Near-Field cnditins intrduced by RTCA/DO-213A standard update [7]. IV. AEROLAB CONCEPT T vercme the time-cnsuming limitatin f single prbe NF systems with gantry arm, and taking int accunt the custmer needs t measure at least 2 radmes a day, MVG intends t accelerate Near-Field measurements, by using a netwrk f prbes. Cnsequently, a cmpliant multi-prbe Near-Field system cncept AerLab has been specifically designed t measure cmmercial aircraft nse-radmes, in rder t meet the new standard requirements. Furthermre, this slutin remains much mre reliable fr the measurement f the electrmagnetic transparency f the radme, with regard t the tests made by means f hrns in a general way. A. Sizing the multi-prbe architecture In the case f the Near Field measurement systems, the time required fr the characterizatin is a functin f the physical dimensins f the radme. The number f prbes shall be cnsistent t meet the Nyquist criterin which sets the spatial step t be respected s that sampling in Near Field is sufficient. The Nyquist criterin (1), defines the spacing between the prbes at less than half a wavelength and is a functin f the frequency and size f the device under test, and gives the minimum quantity f prbes n radians. Nπ = k r (1) 155

3 Fr the example f the Airbus A400M radme (radius f m), which is ne f the largest sizes we can meet tday fr cmmercial aircrafts, this leads t 258 prbes n at a frequency f 9.4 GHz, which represent a lt f prbes. T reduce the number f prbes withut increasing t much the ttal measurement time, the use f a quarter f an arch cmbined with the use f an versampling feature is a smart slutin. In this case, the use f a netwrk f 16 prbes ver 95 degrees requires 9 versampling psitins with a ttal measurement time f 2h20 fr the A400M radme with respect t RTCA/DO-213A. Accrding t this calculatin, the target f reducing the measurement time t less than 4 hurs is met. The quarter-arch slutin is interesting fr btaining a lw-prfile system which can be mved inside a repair shp. The measurement prbes are specially develped t cver GHz band with tw rthgnal plarizatin prts. The lw-prfile prbe antennas were characterized with a MVG StarLab Near-Field measurement system (see Figure 6. ). The final ptimizatin f the prbes was perfrmed by taking int accunt its perating envirnment, i.e. by including the absrbers materials as depicted in Figure 7. B. Overview f AerLab mesurement system AerLab measurement system is mainly cmpsed f an arch which cntains the prbe netwrk as presented in Figure 4. This arch embeds an accurate versampling mechanism, that displaces the netwrk f prbes alng the required curvature. Sme fam absrbers encmpass the arch, in rder t avid interactins between the DUT (Device Under Test) and any ther structural cmpnents f the arch. An uter shell is als integrated fr prtecting all elements f the arch.. Under the Aircraft s radme, the weather antenna is psitined by a specific gimbal assciated with a vertical translatin axis psitiner. The gimbal enables accurate antenna psitining with respect t the specified spherical crdinates. The vertical psitiner has bth the rle f retracting the radar antenna and the gimbal when installing the radme in rder t facilitate this peratin, and set the crrect vertical statin f the radar antenna nce measurements are ready t start. Finally, a primary equipment bay is lcated at the bttm f the arch. Figure 5. RF blck diagram f AerLab Near-Field measurement system. Figure 6. RF measurement prbe characterizatin in MVG StarLab Near-Field measurement system. Figure 4. View f the AerLab with its sub-assies. The RF blck diagram f the AerLab system is sketched in Figure 5. The main elements are the synthesizers, amplificatin and mixer units that enable the IF signal t be prcessed with the NPAC (Numerical Prbe Array Cntrller). Finally, the psitiner cntrller synchrnizes the required mechanical mtins. All these devices are driven by a data acquisitin and prcessing platfrm. Figure 7. Optimizatin f RF measurement prbe in its perating envirnment with absrber materials. 156

4 The measured radiatin patterns f every plarizatin prt is presented in Figure 8. in case f embedded prbe at 9.5 GHz frequency. The natural axial plarizatin discriminatin is nearby 45 db. Figure 8. Radiatin patterns in directivity (prts 1 and 2) fr embedded 9.5 GHz. The whle AerLab system can be cntained within a 4m x 4m x 5m shielded anechic chamber, with a single r duble leaved dr (see in Figure 9. ). C. Cartgraphy f the radme Once the measurements are perfrmed, the Near-Field data are used fr recnstructing the fields ver the radme surface. T build a radme cartgraphy, varius techniques exist and take Near-Field data as input: - Integral equatin techniques - Spherical back-prpagatin - 3D hlgraphy The integral equatin techniques are time cnsuming, which represent a strng drawback in ur case, since the calculatin time is prprtinal t the number f measurements. The technique f spherical back-prpagatin is an apprximatin which gives better results when the surface f the radme is similar t a sphere. Otherwise, the results d nt lead t a satisfying diagnsis. The technique f 3D hlgraphy slves the prblem f previus bth techniques. With this technique, it is pssible t cmpute the fields n the radme surface withut being timecnsuming. The technique f hlgraphic recnstructin takes int accunt the cntributin f every equivalent surce relying n the sphere f Near-Field measurements as presented in Figure 10. The sum f these cntributins f each equivalent surce n the pints f the radme surface enables t cmpute the superficial fields n this surface. Figure 9. Glbal verview f the measurement system inside shielded anechic chamber. AerLab perfrms Near-Field measurements. Near-Field t Far-Field transfrmatins are then applied t the acquired data. Such a Near-Field system allws the test range t be mre cmpact than traditinal Far-field test ranges, and as a result, be independent frm the updated Far-Field distance which has prgressed frm D²/2 t 2D²/ in the new standard RTCA/DO-213A. AerLab enables the measurement f the transmissin efficiency and beamwidth. It als allws accurate characterizatin f side lbe levels by prviding imprved visualizatin f the main sectinal views extracted frm 3D patterns. In additin t, depending n the weather radar system within the radme under test, tw distinct kinematics f scanning sequences can nw be taken int accunt: "elevatin ver azimuth" and " azimuth ver elevatin ".AerLab emulates bth f these mtin sequences thrugh a mnlithic gimbal. Figure 10. Cntributin f every equivalent surce fr the recnstructin f the field n every pint f the radme. The cartgraphy f the radme is divided int 45 sectins that crrespnds t the 45 measured angular psitins f the radar antenna. Figure 11. shws the final cartgraphy f the radme with these 45 measurement sectins. Aerlab acts as a nn-destructive cntrl system dedicated t the diagnsis f aircraft radmes. An example f 3D hlgraphy recnstructin result is prpsed in Figure 12. and Figure 13. fr amplitude and phase differences. The measurements with and withut radme are cmpared t btain these results. We can see that the existing inserts in the radme structure are detected due t the 157

5 inhmgeneities they are causing in the material. These default values are slightly visible in amplitude and mre clearly detected in phase. Additinal tests perfrmed n a planar radme, shw that, using the technique f 3D hlgraphy, it is pssible t detect the presence f dielectric patches, f a minimal size x x 0.1. These additinal results shw better perfrmance f detectin in the case f the cartgraphy f the phase difference. Figure 13. 3D cartgraphy recnstructin result f the surface f the radme phase difference fr phi cmpnent. Figure 11. Visualizatin f the cartgraphy f the radme with 45 measurement sectins. This multi-sensr system fr Near-Field tests f cmmercial aircraft radmes (Aerlab) is dedicated t repair shps and makes it pssible t cmpletely diagnse, in a single measurement peratin, the entire radme accrding t the RTCA/DO213A standard. The AerLab system, which saves measurement time and prvides diagnstic with accuracy, can be accmmdated in a limited vlume within the repair shp. REFERENCES [1] Minimum peratinal perfrmance standards fr nse munted radmes, Dcument RTCA/DO-213, RTCA, Washingtn, D.C., June 23, [2] G. Matthew, M. Shantnu, An autmated cylindrical near-field measurement and analysis system fr radme characterizatin, AMTA 2004, Octber 17-22, [3] D. R. Smith, Recent prgress in metamaterial and transfrmatin ptical design, NAVAIR Nan/Meta Wrkshp, p.17, February 2-3, [4] D. W. Hess, R. Luna, J. McKenna, Electrmagnetic radme measurements: a review f autmated systems, January [5] FAST, Airbus Industrie, Prtable equipment fr testing the radmes, Nb 26, p. 3-8, September 2000 ( [6] M. Bumans, U. Wagner, Spherical near field radme test facility fr nse-munted radmes f cmmercial traffic aircraft, AMTA 2006, Octber 22-27, [7] Minimum peratinal perfrmance standards fr nse munted radmes, Dcument RTCA/DO-213A, RTCA, Washingtn, D.C., March 17, Figure 12. 3D cartgraphy recnstructin result f the surface f the radme amplitude difference n ttal field. 158