SPHERICAL NEAR FIELD RADOME TEST FACILITY FOR NOSE-MOUNTED RADOMES OF AIRCRAFT

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SPHERICAL NEAR FIELD RADOME TEST FACILITY FOR NOSE-MOUNTED RADOMES OF COMMERCIAL TRAFFIC AIRCRAFT Marcel Boumans marcelb@orbitfr.de Ulrike Wagner ulrikew@orbitfr.de ORBIT/FR-Europe GmbH Johann-Sebastian-Bach-Str. 11, 85591 Vaterstetten, Germany ABSTRACT Typically radome tests are performed on outdoor far field ranges or compact ranges. ORBIT/FR has designed, build and qualified a unique spherical near-field radome test facility for the nose-mounted radomes of commercial traffic aircraft for the so-called “after repair” tests according to the international standard RTCA/DO-213, as well as the aircraft manufacturers Component Maintenance Manuals. The facility is extremely compact (chamber size 5.7 m x 5.2 m x 3.2 + 0.7 m, L x W x H), can handle radomes as small as used on the Canadair and as large as used on the Airbus-380 and can be installed directly in the repair workshop for such radomes. The tests performed are transmission efficiency and side lobe level increase. The system is completely automated, so that a workshop technician can operate the facility. Utmost attention has been paid to operational aspects and both operator and equipment safety. After the measurements are done, a test report is fully automatically generated according to RTCA requirements and classifications. The facility is equipped to test all standard Airbus, Boeing, Canadair and Dash nose radomes. Keywords: Radome Measurements, Transmission Efficiency, Sidelobe Increase, Spherical Near Field 1. Introduction RTCA Inc. is a not-for-profit corporation formed to advance the art and science of aviation and aviation electronic systems for the benefit of the public. The organization functions as a Federal Advisory Committee and develops consensus based recommendations on contemporary aviation issues. The RTCA/DO-213 standard is defined in the documents “Minimum Operational Performance Standards for Nose- Mounted Radomes” dated Jan 14, 1993, with “Change No. 1 to RTCA/DO-213 Minimum Operational Performance Standards for Nose-Mounted Radomes”, dated Jun 23, 1995. The presented test facility allows RF testing compliant to the RTCA/DO-213 standard, in particular for Transmission Efficiency and Side Lobe Level Increase (the so-called “after repair” test requirements). Software modifications to also include Incident Reflection, Beam Deflection and Beam Width are under development. This facility is not intended for any of the environmental tests also defined in document RTCA/DO-213. In addition the aircraft manufacturers requirements, which sometimes define deviations from the standard RTCA/DO-213, are taken into account. In particular the Airbus Component Maintenance Manual (CMM) requires sidelobe measurements throughout the entire predictive windshear (PWS) electromagnetic window, where RTCA/DO-213 requires this only in the forward direction. The facility design is unique in that it is based on antenna near field technology. This allows to reduce the size of the facility significantly, so that it can be build indoor, and can be environmentally controlled. This reduces infrastructure cost and increases availability and accuracy. The acquired data is transformed to the far field, so that the data analysis is based on highly accurate far-field data. It can be used for Qualification/Preproduction Testing, Quality Assurance/Production Testing and so called After Repair Testing. The facility is designed for use with a multitude of radomes, for easy and safe handling of these radomes, and for fast and efficient testing. In particular for production and after repair testing, the facility is highly automated, so that workshop personnel not familiar with RF testing can easily do RTCA/DO-213 compliant testing. Test reports are generated fully automatic. Since the design is based on standard antenna / radome test technologies, the advanced user can perform a multitude of tests beyond what is defined in the RTCA/DO-213. Thus this facility also presents a sound investment for future standards and applications. 2. Facility Overview Figure 1. shows the ORBIT/FR near field radome test facility for nose-mounted radomes. The near field probing surface is spherical. The positioning is realized with a fast rotating azimuth positioner (30 rpm), which carries the Device Under Test (radome, radar antenna on gimbal positioner, aircraft bulkhead, where applicable), and an elevation arm. Note that the orientation of the facility is such that the aircraft “flies” to the zenith of the facility. Figure 1. General overview of test facility The test facility chamber outside dimensions are approx. 5.7 m x 5.2 m x 3.9 m (W x L x H), with a pit for the azimuth positioner of 0.7 m deep. Thus it can be easily installed right in the radome workshop. Access inside the facility is through a large double leaved door of 4.0 m x 3.3 m (W x H). Typically the chamber is build as a shielded, self supporting structure, so that there is no interaction / disturbance from the radar signals inside the facility with the outside world and vice versa. The inside of the chamber is installed with 200 mm pyramidal absorbers everywhere. In those areas of the floor where people have to be for installation purposes, so-called walk-on absorbers are installed. Figure 2. The facility “as built” All the instrumentation is mounted in a rack inside the test chamber, right next to the elevation positioner. The system is controlled through Fiber Optic LAN by either a computer directly next to the facility, or optionally any other computer connected to the same network. The RF system is based on a network analyzer. For Production testing and After Repair Testing, typically a single frequency is used (e.g. 9.35 GHz), for Qualification Testing multiple frequencies between 9.3 and 9.5 GHz are used. The RF probe on the elevation arm is a dual linear polarized single choke circular probe. Figure 3. Radar antenna under the radome Facility temperature control is through one inlet into and one outlet out of the test chamber, and can either be connected to the building air conditioning supply or a separate unit just for this facility. Note that the absolute temperature does not need to be as strictly controlled as the temperature variation during one measurement (+17 to +28 deg C versus ± 2 deg). The radar antenna is mounted on the gimbal positioner, which has a travel of ± 88 deg in azimuth and ± 33 deg in elevation (relative to the aircraft). An automated linear translation of approx. 8 mm (representing ¼ λ in X-band) is installed between the radar antenna and the gimbal. Note that RTCA/DO-213 requires the transmission measurements to be done in two antenna positions separated by ¼ λ in the boresight direction. The entire gimbal unit is installed on a vertical slide which allows to position the unit at the correct position inside the radome. In addition this allows to lower the radar antenna below the radome mounting surface, so that it is protected while a new radome is being installed. Figure 4. Radar antenna in low position for easy radome mounting The facility is designed to be able to handle a multitude of different radomes with a minimum of operator adjustments. For this purpose, each different size of radome is mounted on its own support plate, made out of a light weight fibre reinforced plastic honeycomb sandwich structure. Thus it can easily be adapted to include the aircraft bulkhead or to include the real aircrafts interface (hinges, latches, screws), or any other means to position and hold the radome. Since the radomes are supported by these plates and point upright, the loads on the interfaces are relatively small. The plates are coded such that it is not possible to move the antenna inside the radome to a position which would allow the antenna to hit the radome. Also the interface between the support plate and the azimuth positioner is designed such that the radome is automatically mounted correctly in radial and axial directions. In this way most radomes can be mounted and dismounted by just two or three people who carry the support plate with radome into and out of the facility. This action only takes a few minutes. 3. Facility Control The operation of the facility is highly automated for the measurement types described in the RTCA/DO-213, and minimum antenna and RF knowledge is necessary to perform these tests according to the RTCA/DO-213 requirements. The measurements can typically be executed from fully automated batches, provided by ORBIT/FR. On the other hand, there is still a lot of flexibility in regard to variation from the pre-defined measurements. Such variation can consist of selecting a sub-set of the gimbal angles (e.g. to concentrate on an area of the radome where improvement or repair is necessary) and of selecting reduced RF sampling ranges (e.g. to speed up measurement time for indicative measurements). 3.1 Test facility and test preparation Presuming the facility is powered off, first the test facility needs to be powered on, temperature stabilize and the applicable radar antenna needs to be mounted (if there is a large variation in the sizes of the radomes to be tested, also the radar antennas need to be available in different diameters). Then data is entered in regard to the test radome (e.g. Type and Serial Number). Detailed instructions for the operator appear on the computer monitor. 3.2 Measurement selection and execution First the test standard is selected (RTCA/DO-213 resp. CMM or customer defined “Fast Check”), the measurement types are selected (Transmission Efficiency, Side Lobe Levels and Beamwidth, Incident Reflection and / or Beam Deflection). Note that for “After Repair Testing” only Transmission Efficiency and, in some cases, Side Lobe Level Increase are required. Then the gimbal angles are selected (full set, as defined in RTCA/DO-213 resp. CMM, or partial set). Before measurements can be started, the temperature of the facility needs to be stabilized to within ± 2 deg of the reference temperature (see also Chapter 2), each time the large doors have been opened. Note that if the reference temperature is close to the temperature outside of the test chamber, no waiting times would be required. Now a calibration (reference) measurement is made of the radar antenna without the radome. Then the radome is mounted and, after the temperature is stabilized again, the measurements can be performed fully automated. At the end of all measurements the radome is removed again, and a second reference of the radar antenna without radome is performed to verify that the facility was stable during the measurements. 3.3 Data Analysis and Reporting After the measurements are finished, test reports are generated automatically. For “Fast Check” or “Partial Check” the statement at the lower end of the report will then state that these measurements were NOT executed in accordance to RTCA/DO-213 and / or CMM. 4. Safety Many safety aspects have been included in the design and implementation of the Radome Test Facility. These aspects relate to both equipment safety (test facility including radar antenna and radomes) as well as personnel safety. 4.1 Equipment Safety The equipment safety relates to the risk that any component of the test facility, or the radome under test, can get damaged in operation. In particular, if many different sizes of radomes are to be tested, then such a risk is real: if a large radome was measured, and after that a much smaller radome is to be measured, then the radar antenna could hit the radome wall if the facility would not correctly be set up to handle the much smaller radome. If the system is only used for regular weather radar tests, then it is only operated at frequencies from 9.3 to 9.5 GHz. Thus nothing needs to be changed to the RF instrumentation, including the RF probe, at any time. The RF probe on the elevation arm is normally parked at the ceiling of the facility near the “zenith” position when any person has to work inside the facility. This reduces the risk that the arm and the RF probe are damaged during installation work. Before the radomes are installed in the facility, they are fitted to support plates unique to the radomes, with handles or other devices to carry the plate / radome assembly. The radomes are mounted upright on the azimuth positioner (the aircraft is “flying” to the zenith of the facility). This makes the assemblies easy to carry and install. It will be difficult to install a radome on the wrong support plate by virtue of clear indications and instructions on the plates, the size of the plates relative to the radomes, or the interfaces between the plates and the radomes. The support plates are fitted with hardware coding which allows to install the plate to the azimuth positioner in only the correct way, and which give information to the test facility about the position of the radar antenna inside the radome. The installation platform is 0.8 m above the chamber inside floor level, so that installation personnel has best handling control of both the size and the weight of the radomes. The radar antenna is lowered below the support plate interface frame before a new radome is installed. Thus it is virtually impossible to damage the radar antenna with the radome during installation. Lifting and lowering of the gimbal positioner is done automatically by the software. The radar antenna can only be lowered below the interface frame when the gimbal is set close to zero azimuth and zero elevation because of a limit switch interlock mechanism included in the gimbal positioner / vertical lift, thus preventing that the radar antenna hits the interface frame. The radar antenna cannot be lifted to a height which would allow the antenna to hit the radome wall, because the support plate includes limit switch information for the vertical lift. 4.2 Personnel Safety Personnel safety relates to the risk that an operator of the facility, or any other person, gets hurt by this facility. This relates to mechanical, electrical and RF aspects. In a machine with moving parts, there is always the risk that somebody gets hit or squeezed by any of these parts. In this facility, with the 30 rpm azimuth positioner which can carry very large radomes, the gimbal positioner on which radar antennas need to be mounted, etc., such risks need to be carefully considered. Through design, instructions and training this risk needs to be minimized. Several hardware measures have been included in this facility to increase the personnel safety. Emergency stop buttons are included in three locations: one near the elevation positioner, one near the door and one outside the chamber, typically near the control computer. Also a switch is connected to the door, which disables any positioner movement as soon as the door is opened. This will prevent that anybody not familiar with the facility, who could enter the facility unintended, cannot get hurt by any moving parts. Further a shielded window is installed in one of the facility walls, as well as a video network camera inside the facility. Thus the operator can easily make a final check before starting the measurement. The camera can be accessed through any computer on the network, so that one can also have a regular look inside the facility for long unattended measurements. From the operational and health point of view carrying and handling heavy and large objects, the facility is optimized by having a very large door of 4 m wide, a single step of only 5 cm from the outside to the inside of the facility (walk-on absorbers) and by having the installation platform at 80 cm height. In regard to the electrical safety, the standard national and international electrical wiring and installation standards are followed. In regard to RF safety, the transmitted power is very low (less than 50 mW). In addition, the facility is typically build inside a shielded chamber. This is primarily to prevent interference from weather radar signals from real aircraft in the vicinity of the facility, or to prevent the signals from the facility to interfere with weather radar signals from real aircraft, but also lowers the radiation outside the facility to undetectable levels. 5. Test Results First the facility has been tested on stability and reproducibility. This means that all 45 antenna / gimbal combinations have been measured repeatedly, and variation in the angles of the beam peak as well as in the maximum power level have been compared. Repeatability in beam peak angle has been shown to be better than ± 0.1 deg. Note that the power variation of the beam peak within this angular range is only ± 0.02 dB, or less than ± 0.5 %. Repeatability in power level at the beam peak has been shown to be better than ± 0.05 dB, or ± 1 % between the reference measurements with which a radome test is started and the final reference measurement after all radome measurements are finished. Note that up to 14 hours can be between these two measurements for a large radome. These stability and repeatability results are well within the requirements stated in RTCA/DO-213. The following tables and plots show some typical test results for both a typical Kevlar (Class C) and a high quality quartz glass (Class A) radome. Note that the shown tables and graphs are generated fully automated. Customer specific logos, test object information and signature fields can be added according to the customer requirements. Figure 5. Transmission Loss of typical Kevlar radome Figure 6. Sidelobe Increase of typical Kevlar radome Figure 7. Transmission Loss of high quality quartz glass radome Figure 8. Sidelobe Increase of high quality quartz glass radome 6. Measurement times In the spherical NF measurement system, the phi-axis rotates at 30 rpm, thus makes one revolution in 2 seconds. The theta axis, which is the step axis, needs approx. 2 seconds to move to the next theta position. A large radome, like on a Boeing 747, requires an acquisition step angle of 0.72 deg. This presumes that any part of the radome can reflect some radiation (eg. reflection lobes), which is the worst case assumption. For a full characterisation of the radome / antenna system the theta axis needs to be scanned from 0 (zenith) to slightly beyond 90 degrees (horizon), resulting in approx. 9 minutes for a single measurement. A fully compliant RTCA “after repair” test requires 45 gimbal positions at 2 “quarter wavelength” positions of the antenna, plus two reference measurements, or a total of 92 measurements. Thus the total measurement time for a B-747 radome is in the order of 14 hours. A smaller radome, like used on a DASH, requires 1.44 deg increments, and results in total measurements times in the order of 8 hours. These test times have been confirmed in practice. Although the test times for the largest types of radomes are quite long, it must be realized that the facility operates fully automated, and no operator actions are required from after the radome is mounted until all 2 x 45 gimbal angles are measured and the radome can be removed again. Thus, in practice, one to two radomes can be measured per day, which is typically more than sufficient for a radome repair workshop. If higher measurement speeds are required, then more probes can be mounted on the elevation arm. 7. Conclusions A very compact RTCA/DO-213 compliant “after repair” RF test facility has been build and qualified, but also Qualification / Preproduction and Quality Assurance / Production Testing defined in RTCA/DO-213 can be tested with this facility. It can be installed in the repair workshop and can be operated by workshop personnel. Tests are made and evaluated fully automated. Since this avoids packing for and transportation to an external RF test facility, significantly reduced repair times can be realized. 8. REFERENCES “Minimum Operational Performance Standards for Nose-Mounted Radomes” dated Jan 14, 1993, with “Change No. 1 to RTCA/DO-213 Minimum Operational Performance Standards for Nose-Mounted Radomes”, dated Jun 23, 1995, “CMM Component Maintenance Manual”, various Aircraft models, Airbus Industrie,

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SPHERICAL NEAR FIELD RADOME TEST FACILITY FOR NOSE-MOUNTED RADOMES OF AIRCRAFT

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