High Accuracy Horizontal Scanner operating in X band for the Measurement of a spaceborn SAR Antenna

High Accuracy Horizontal Scanner operating in X band for the Measurement of a spaceborn Synthetic Aperture Radar (SAR) AntennaShai Dreizin Orbit/Fr-Israel P.O. Box 8657 Netanya 42504 Israel, shaid@orbit-ltd.co.il Dr. Yuri Sharay ELTA Systems Ltd P.O. Box 330, Ashdod 77102 Israel ysharay@elta.co.ilABSTRACT ELTA is now in the process of designing and building a new spaceborn SAR “TECSAR” – Israel Synthetic Aperture Radar (SAR) X-Band lightweight satellite. TECSAR contains an ultra-light weight high accuracy Paraboloid deployable reflector antenna. TECSAR’s electronic beam steering capability is achieved by using a feed array in the focal plane. For future testing at ELTA, Israel, an horizontal Planar near-field antenna test range (7m x 8m scan) has recently been completed by ORBIT/FR to allow testing of large fully integrated space antennas as stand alone as well as integrated with a satellite The paper will describe: o Short TECSAR SAR antenna description o The special requirements of the measurement system o System design and measured performance Keywords: Antenna, measurements, SAR, Horizontal scanner, measurement accuracy, Gain, MiDAS, RTC Figure 1 SAR “TECSAR” 2.0 Introduction to the SAR and its test’s requirements 1.0 Introduction Horizontal planar near-field antenna test range has recently been completed by ORBIT/FR at Elta Systems Ltd (subsidiary of IAI) in ISRAEL to allow for the testing of large deployable antenna of a new Israel space SAR “TECSAR” – X-Band Synthetic Aperture Radar (SAR) placed in a lightweight satellite. The new horizontal scanner was required to provide wide scan area (7m x 8m), simultaneous measurement of a set of channels in 12 frequencies and 2 polarizations with high absolute gain accuracy < 0.2 dB and side lobe level accuracy 3.0 dB at level of 40 dB below main beam. This paper will present a short description of the SAR antenna and some special requirements of measurement system. Mechanical and system design are described in details together with final system results. 2.1. SAR components TECSAR configuration [1] is shown on figure 2.1. The TECSAR payload is composed of five major subsystems: Radar Signaling and Control system, Multi-Tube Transmitter, On-Board Recorder, Data-Link Transmission Unit and deployable Paraboloid mesh antenna with electronic beam steering. The last capability is achieved by using antenna feed array in focal plane. The payload modules occupy a section close to the antenna. The payload section is separated from the bus so that only cables and wires connect the two. This separation makes the integration of the bus simpler, easier and shorter.Feed ArraySAR AntennaPayload modulesFigure 2.2. Antenna and the payload of TECSAR satellite The best-fit Paraboloid computed for ribs and dish together is used for antenna surface accuracy assessment. Having targets on the mesh surface it is also possible to measure accuracy of cylindrical gore surface and pillow effect influence. An example of measurement is shown on figure 2.3Star TrackerBUS modules Propulsion systemFigure 2.1. TECSAR Satellite view 2.2. SAR antenna configuration and evaluation TECSAR antenna is a deployable umbrella reflector with a rigid CFRP central dish and a set of knitted mesh gores stretched by skeleton - ribs. Ribs lie on the surface of a parent Paraboloid and each gore surface has a Paraboloid cylinder contour. Configuration of the antenna is shown in figure 2.2. High Paraboloid reflector surface accuracy is achieved by ribs position adjustment after mesh gores mounting. High precision measurements of the reflector surface were performed by stereo photogrammetry method [2]. Small targets with diameter 10 mm and height 0.1 – 0,9 mm are glued on ribs and dish surface. From 4 till 6 photo images were processed together for receiving 3D coordinates of each target. Achieved accuracy of the method was 0.1 mm. Then new designed program was used for computing of the best-fit Paraboloid with given and calculated optimal focal length and defining of RMS errors of real antenna surface. The best-fit Paraboloid can be computed for rigid disk only for adjusting of ribs position.Figure 2.3 Deflection of antenna surface from the best-fit Paraboloid Numerically determined RMS was 0.4 mm for ribs surface. Antenna measurement in horizontal near field range has shown that gore related loss together with loss due to random surface errors and pillow effect were 0.1 dB. Gore related losses are close to predicted value [3]. Measured reflector surface loss was 0.8 dB. Mesh transparency was measured in lab condition previously and was from –17 dB till -23 dB for two orthogonal polarizations. This transparency causes loss no more than 0.1 dB. Calculated spill over was loss 0.5 dB. So, losses due to non-perfect contacts of the wire junction was 0.2 db that coincide with predicted value of losses in mesh with perfect contact [4].2.3. Measuring SAR antenna with Horizontal scanner Due to Earth gravity action extremely lightweight ribs can change their predefined profile, so anti-G compensation is needed for accurate antenna pattern measurement. This compensation can be performed only in horizontal antenna position. High accuracy of the SAR antenna gain in the near field is important for effective optimization of ribs position, ribs curvature and ribs mounting. Required absolute accuracy is 0.2 dB, repeatability is 0.1dB. The special feature of the new near field is a possibility of multi-beam antenna pattern tests in transmitting mode at the SAR payload unit level. In this case antenna transmits full RF power in one channel (beam). This mode is needed for verification antenna patterns measured in receiving mode when all receiving channels are terminated by matched load. During transmitting all channels are active and one selected beam pattern is measured. For this special test SAR transmitter works as an amplifier of near field synthesizer signals being synchronized by SAR controller. Scheme of integration is shown on figure 2.4. Here, the same software of beam forming which was already tested at SAR unit level is used during near field test.SARSAR Controller-High stability of the long term measurements Simultaneous measurement of multi-beam near field at different frequencies Range size and height that gives opportunity to test antenna in assembly with payload or payload and satellite Integration of SAR and near fields hardware during near field measurement in transmitting mode 2.5 SAR within the clean roomTECSAR antenna within the near field range is shown in figure 2.5. Special support structure is used for right location of the antenna at the dedicated distance from the near-field probe aperture plane. This support structure is thus able to adapt for any height of the antenna: whenever reflector used stand alone or with payload or as a part of satellite. The support provides also alignment of the reflector in the horizontal plane by using air bearing and its rotation relative Z-axis.Beam forming and TransmitterTriggerGates RF CWAntennaNEAR FIELDCONTROLMIDASRTCDATAMeasure equipmemtProbeFigure 2.4 Antenna test and integration block diagram. 2.4 SAR antenna measurement system requirements The main requirements of a near field range proceeding from above mentioned tasks are: - Conformity of the near field with clean room conditions - Horizontal position of a scanner - High accuracy of the gain measurement Figure 2.5. TECSAR antenna within the near field range 3. Horizontal scanner mechanical design The Scanner was designed and constructed using high strength truss structure. The basic construction techniquetogether with the drive and encoder system yields an inherently accurate mechanical scanner with very high repeatability. This inherent mechanical repeatability allows the use of a laser system to map the scanner and create correction tables for use by the software. An optical Jig-Transit was used during installation to accurately align the scanner axes. After the scanner has been accurately installed and mechanically aligned, an external Spinning Diode Laser system was used to calibrate the scanner planarity. The X axis was built from a set of high precision low friction linear bearings rails, mounted on the longitudinal beams. These linear bearings support the transverse beam and enable it to slide smoothly along the rails for the entire length of the required travel. The linear rails and rack gears are installed on the top surface of the two longitudinal beams. The X axis drive mechanism consists of a dual drive rack & pinion. The drive system is located at the center transverse beam. Rotation of the drive pinions is be synchronized by means of a stiff composite material drive shaft spanning the transverse beam. The data take off is be provided by a rotary encoder. The Y-axis tower is constructed from one accurately machined steel subassembly. Linear bearing guides are attached by means of bolts to these surfaces as well as to the racks, mechanical stops, alignment devices, etc. Accurate linear bearings are used. The axis is equipped with an encoder assembly, which transmits the Y-axis position in digital form to the computer control PC-DSP. The drive mechanism for the Y-axis is also rack and pinion type. Alongside the Y tower structure, a support wall is mounted to support the Y-axis beam tower. The Y-Axis beam tower carries a moving table on which the Z-Roll unit is mounted. The Z-Roll unit slide is on the tower left side. Orbit performed finite analysis models in order to optimize the scanner design. The analysis is compared between four alternative versions of the Horizontal Planar Scanner structure. The comparison criterion is the minimal natural frequency of the structure. This frequency should be maximal. The carriage was placed in the middle position on the Y beam. The Y beam was placed in middle position between two adjacent columns. FEA models of the structure versions were built using ALGOR software. Following Drawings 1-4 show general view of the models:Figure 3.1. Structure Model #1Figure 3.2. Structure Model #2Figure 3.3. Structure Model #3Figure 3.4. Structure Model #4The finite elements models show that the fourth option of the structure has maximum stiffness in Y direction and sufficient stiffness in X direction.The RTC is responsible for all the synchronization part of: o Beam control o RF switching o Receiver Event trigger o RF Pulse control and synchronization o Communication with SAR MiDAS analysis package is uniquely capable to efficiently process and plot the vast amount of data collected in the acquisition mode. Data presentation and analysis can run on a second computer, while the acquisition process is running on another platform. The data presentation includes: Cartesian plots, Polar plots, Data listing, Contour plots, Intensity plots and 3D plots (including polar 3D). Specific Analysis parameters which are to be displayed: Gain, Directivity, Beam Width, Sidelobe levels, Cross Polarization, Overlays, Electrical Boresight and circular polarization Analysis.Figure 3.5. Mode shape of the 1 st, 2nd and forth natural frequencies of the structure 4. System Design The measurement system uses a powerful computational subsystem to support the real-time and math intensive operations required by the near-field measurement process. The System is controlled via MiDAS Software. MiDAS software is designed to add full automation to antenna measurement system. It can operate under Microsoft Windows 95/98/NT/2000/XP operating system. It provides all of the standard features expected of a completely automated antenna measurement system as well as many unique and custom tailored functions. The MiDAS Acquisition controls: o o o o Motion of the scanner axes Laser system RF Measurement Equipment RTC – Real Time Controller (real-time measurement sequencer) Figure 4.1 3D Polar Plot 4.1 System Results Planarity: 0.02mm RMSFigure 4.2 Planarity Results after correctionFigure 5.3 Calibrated (TICRA) Probe and SGH Gain Error budget – Direct method: 0.18dB Gain Error budget – indirect method: 0.2dB Beam Pointing – ±160 microRadians RMS SideLobe level – ±3.0 dB with a probability of 90% at a level of 40 dB below the main beam peak 4.2 FFT Size As part of the system validation, it was found that the FFT size required to obtain the high accuracy Gain (Due to Straddling Loss), was to perform min FFT size of 1024. A higher size FFT (2048) did not improve the results. 5. Additional PicturesFigure 5.4 Calibrated SGHFigure 5.4 Scanner Figure 5.1 Reflector on Air Bearing Mount 6. REFERENCES [1] U. Naftaly, “TECSAR – Performance, Design and Status” – Proceedings of EUSAR2004 Conference, May 2004. [2] H. Alperovich, D. Giller, Y. Sharay “High precision method of surface accuracy measurement – application for space SAR deployable antenna measurement and calibration” 44th Israel Annual Conference on Aerospace Science Feb 2004 [3] W. Rusch, R. Wanselow, “Boresight gain loss and gore related sidelobes of an umbrella reflector”, IEEE Trans,AP-30, No1, Jan.1982 [4] W. Imbriale, V. Galindo-Israel, Y. Rahmat-Samii, “On the reflectivity of complex mesh surface”, IEEE Trans, AP-39, No9, Sep.1991Figure 5.2 Control Room。