Distributed Adaptive Control and Metrology for Large Radar Apertures PI: James Lux Co-Is: Adam Freedman, John Huang, Andy Kissil, Kouji Nishimoto, Farinaz Tehrani Poster No Optical sensor measures coarse (1:500) mechanical position and orientation by looking for LED “stars” Widely available 640x480 sensors provide this level of performance. Camera calibration / photogrammetry algorithms needed are well known. Other Components Antenna Elements CMOS image sensor In-band RF measurement of precise phase shift & path loss to/from beacons. Evaluated two basic approaches: PN ranging (e.g. GPS): acquire and synchronize to high rate code Phase Comparison (e.g. Omega, DECCA, Loran) CW beacon phase measurement and comparison selected Directly gives us the metric we need: phase shifts Usual problem with phase comparison (ambiguity) not an issue because we have mechanical constraints on possible locations. RF measurements also measure gain Measurements must be made in two bands with multiple frequencies per band Bands: working (radar) frequency and link frequency Number of frequencies must be > number of degrees of freedom Measurements feed into microcontroller algorithms executing on each element: Mechanical & Structural Model predict future position of element Beamforming Computations look direction, element position/orientation Element Control compensate for variations Goal: Conceptual design and analysis for novel method to compensate for inevitable movements in large (>100m 2 ) lightweight radar apertures. Conceptual Approach: Electronically scanned phased array using phase and amplitude controls at each element to form the beam and steer it. Each element includes simple hardware to measure its own position and RF properties and an embedded processor compensates for the variations from ideal. The measurements are made by “looking” at beacons in the structure that have “well known” position and characteristics (similar to surveying benchmarks). Quantitative Performance Requirements: Apertures on order of m 2, 50m linear extent Hardware ROM mass near 2kg/m 2 Compensate to <1/20 th wavelength at L band (1.2 GHz, 23 cm) (i.e. 1 cm) Mechanical deviations on order of 1 meter at 1Hz (i.e. few m/s velocities) RF property changes due to adjacent element interaction, aging, temperature Radio Computer Incoming signals arrive staggered in time and phase, depending on direction of arrival. Embedded microcontroller in element commands radio to adjust time delay, phase, and amplitude to compensate for: - Direction of arrival - Physical position - Radio performance variations Adjusted element signals are transmitted by radio to a common receiving point where they are summed. Metrology Radio Computer Metrology Radio Computer Metrology Metrology on each element measures position of element and RF performance Control Wireless links used for control, status, and software loads from central point and to share information with adjacent elements. Radar Processor Beacons: Radiate RF CW signals with well controlled frequency and phase. Have an optical source (LED) that elements can see. Beacon positions are well surveyed and broadcast to all elements. Number of beacons determined by two primary factors: FOV of optical sensors: 4-6 beacons visible Uncertainties in element antenna patterns: RF path to beacon should be reasonably close to path to radar beam direction Relative geometry of beacons is important also FY04 tasks Generate set of quantitative bounding requirements from published literature and proposed future missions Size, performance, structural dynamics Analytical and Simulation models of conceptual design Metrology and calibration approaches Blend of optical and RF techniques Tradeoff of optical FOV, resolution Develop RF metrology requirements and concept Parallelizable Modeling Codes Calculates contribution of each element (or group of elements) and sums electric field components. Ready for insertion of “element implementation”specific models: Microcontroller algorithms Optical sensor error model RF metrology error model Modeling software structure designed to be similar to actual element execution environment. Breadboard Validation of RF Phase Measurement SDR1000 “Digital Radio” VIA EPIA 533MHz C3 5 port Switch a WLAN access point Radar Freq Converter Link Freq Converter RF In/Out time DDS error (ppm) time Residual frequency error (ppb) sigma = 5.02 ppb Simple algorithm (straight line predictor) removes variations from 10 MHz DDS output (  f /f = 5 ppb after removal of linear trend) > 5ppb is approximately 1.8º phase error / 0.1second Measured variation (approx 150 ppb) in f osc over two seconds (linear trend shown in green) Residual variation after using linear predictor {previous 0.3 seconds predicts 0.1 second into future} 4 breadboard elements Metrology and Calibration Concept Measured variation in commercial crystal oscillators in 4 breadboard elements frequency using 2 reference RF tones from a “beacon”.