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SeaSonde Overview

This overview explores the definition and uses of HF RADAR technology. It explains how HF RADAR systems can observe currents, waves, ionosphere layers, and discrete targets such as ships and ice packs. The principles of HF RADAR are discussed, as well as its operational performance and waveform.

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SeaSonde Overview

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  1. SeaSonde Overview

  2. HF RADAR Definition and Uses • What Is HF RADAR? • RADAR = RAdio Detection And Ranging • HF = High Frequency: 3 - 30 MHz or 100 - 10 m wavelength • VHF = Very High Frequency: 30 - 300 MHz or 10 - 1 m wavelength • What Can Be Observed/Detected? • Currents • Most robust environmental data product from HF RADAR systems • First-order effect - sea echo from Bragg scattering • Waves • Second-order effect • Subject to perturbation theory limits - upper waveheight limitation • Ionosphere Layers • Can cause interference with current measurements • Discrete “Targets” • Ships: dual use w/ current mapping (under development) • Ice Packs/Bergs (work done in 70’s - more being done currently)

  3. What does an HF RADAR consist of? Computer and Monitor Transmitter Transmit Antenna Receiver Receive Antenna receive antenna monopole (A3) loop box (A1 & A2) radial whips electronics loop box loop 2 (A2) loop 1 (A1)

  4. RF Modes of Propagation

  5. Ground Wave Propagation & Depth of Measurement • Requires interface between free space (air) and highly conductive medium (>8 ppt salinity sea water) • Ocean surface exists as a free boundary allowing surface molecules freedom to conduct EM energy, much like a waveguide • Allows vertically polarized EM energy to propagate w/ reduced energy loss for greater distances and beyond horizon • Radar wave does not penetrate surface at all - depth of measurement comes from effective depth-averaged current “felt” by ocean wave • 25 MHz measures to < .5 m, 5 MHz measures to 2 m deep Air is almost like free space D ∝λ Depth of measurement is related to ocean wavelength (Can be linear or logarithmic) Seawater is conductive

  6. Bragg Sea Echo λ λ λ/2 λ/2 A B C SeaSonde Principles SeaSonde Principles

  7. Bragg Sea Echo λ λ λ/2 λ/2 A B C SeaSonde Principles SeaSonde Principles

  8. Bragg Sea Echo λ λ λ/2 λ/2 A B C SeaSonde Principles SeaSonde Principles

  9. Bragg Sea Echo λ λ λ/2 λ/2 A B C SeaSonde Principles SeaSonde Principles

  10. Bragg Sea Echo λ λ λ/2 λ/2 A B C SeaSonde Principles SeaSonde Principles

  11. Bragg Sea Echo λ λ λ/2 λ/2 A B C SeaSonde Principles SeaSonde Principles

  12. Bragg Sea Echo λ λ λ/2 λ/2 A B C SeaSonde Principles SeaSonde Principles

  13. Bragg Sea Echo λ λ λ/2 λ/2 A B C SeaSonde Principles SeaSonde Principles

  14. Doppler Spectrum Echo Strength (dBm) -fB 0 +fB Doppler Frequency (Hz)

  15. Radial Currents 5 4 3 2 1

  16. Radial Currents Echo Strength (dBm) -fB 0 +fB Doppler Frequency (Hz) 5 4 3 2 1

  17. The Doppler Spectrum Negative Bragg peaks (Waves receding) Positive Bragg peaks (Waves approaching) Noise Floor Loop 1 (A1) Loop 2 (A2) Monopole (A3) Negative Doppler: Targets moving away from Antennas Positive Doppler: Targets moving towards Antennas 0 Hz Doppler Offset a.k.a. “DC”

  18. First Order Regions are convolution of spectral energy from all velocities at a given range cell Compare Phase, Amplitude of all three antennas to determine direction of velocity Loop 1 (A1) Loop 2 (A2) Monopole (A3) +30 cm/s 0 cm/s -45 cm/s

  19. What does an HF RADAR consist of? Computer and Monitor Transmitter Transmit Antenna Receiver Receive Antenna receive antenna monopole (A3) loop box (A1 & A2) radial whips electronics loop box loop 2 (A2) loop 1 (A1)

  20. Loop 2 Loop 1

  21. Direction Finding

  22. Direction Finding

  23. Direction Finding

  24. Radial Vector Output of MUSIC Processing Output of MUSIC processing: radial vectors Vectors are in polar coordinate system centered at receive antenna 1 radial map per averaged cross spectra file Typically, seven radial maps “merged” into one hourly map Angular resolutions are 1 - 5˚

  25. SeaSonde Operational Performance vs. Frequency Radar Frequency (MHz) Typical Bandwidth (kHz) Radar Wavelength (m) Typical Resolution3 (km) Ocean Wave Period (s) Ocean Wavelength (m) Depth of Current1 (m) Typical Range2 (km) Upper H1/3 Limit4 (m) 15-30 25-100 50-300 150-600 60 25 12.5 6 175-220 60-75 35-50 15-20 30 12.5 6 3 4.5 2.5 2 1.5 2 1-1.5 .5-1 <.5 6-12 2-5 1-3 0.25-1 25 13 7 3 5 12 25 48 1. Depth averaged current 2. Range based on 40W avg power output. Salinity, wave climate and RF noise may affect this. 3. Based on bandwidth approval only - no system limitations - higher resolution will cause some range loss 4. Significant Waveheight at which 2nd order spectra saturates 1st order and no current measurements possible

  26. SeaSonde Gated FMCW Waveform -- Time DomainEcho Range Determination from 1st FFT • Linear FMCW (frequency-modulated continuous wave) determines: • Range to target • Range resolution • Pulsing Only Used to Protect Receiver During Strong Transmission • 50% duty factor (square wave) is optimal for signal-to-noise ratio • Pulse period determines maximum range and blind zones in coverage

  27. TSweep TPulsePeriod FSweepWidth SeaSonde Waveform

  28. How We Achieve Simultaneous Synchronization via Modulation Multiplexing • Second transmitter's modulation start is shifted to t2 • After demodulation in receiver, signal plus echoes shifted to beyond f2 • First FFT puts these signals plus echoes in distant, unused "rangebins"

  29. Note Sweep Direction Can Either Be Up or Down Figure Shows Upsweep All Three Sandy Hook Radars Are Sweeping Downwards Radiated Waveform Parameters for SeaSondes at Sandy Hook in Their Three HF Operating Bands [Refer to Controller Settings on Next Slide] • 5 MHz Band (4.53 MHz center frequency) • Sweep Period TSweep : 1 second • Sweep Bandwidth FSweepWidth : 25.6 kHz • Pulsing Period TPulsePeriod : 1946 microseconds • 13 MHz Band (13.46 MHz center frequency) • Sweep Period TSweep : 0.5 second • Sweep Bandwidth FSweepWidth : 49.4 kHz • Pulsing Period TPulsePeriod : 669 microseconds • 25 MHz Band (24.65 MHz center frequency) • Sweep Period TSweep : 0.5 second • Sweep Bandwidth FSweepWidth : 101 kHz • Pulsing Period TPulsePeriod : 486 microseconds

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