Remote Sensing John Wilkin jwilkin rutgers edu IMCS Building Room 214C ph 848 932 3366 Active microwave systems Dunes of sand and seaweed Bahamas Oct 29 2000 Mapper plus ETM instrument on Landsat 7 CoastalEnhanced HF Thematic Radar http landsat gsfc nasa gov earthasart bahamas html http www environmentalgraffiti com ecology 30 most incredible abstract satellite images of earth Scatterometers satellite borne ocean surface vectors winds Incorporated into ECMWF meteorological analysis Synthetic Aperture Radar SAR satellite and aircraft high spatial resolution tens of meters image ocean surface wave field and by inference processes that modulate the surface waves CODAR Coastal Ocean Dynamics Application Radar land based HF radar system ocean surface currents and waves All these systems exploit the resonant Bragg scattering of centimeter to decameter wavelength microwave radiation from ocean surface roughness due to short waves The interference of scattering from many different parts of the surface results in a reinforcement of scattering from periodic structures in the surface roughness which have suitable wave lengths and destructive interference of all other reflections sin sin n HF Radar is 3 50 MHz 100 m to 6 m wavelength http www codaros com CODAR High frequency HF radio in the band 3 50 MHz has wavelengths 6 m to 100 m The ocean surface is rough surface with water waves of many different periods Bragg scattering occurs for narrow band of wavelengths depending on CODAR frequency 25 MHz transmission 12m EM wave 6 m ocean wave 12 MHz transmission 25m EM wave 12 5 m ocean wave 5 MHz transmission 60m EM wave 30 m ocean wave Radar return is greatest for waves traveling directly toward or away from the antenna Therefore we know wave direction and wavelength http www encora eu coastalwiki Use of ground based radar in hydrography From surface gravity wave dispersion theory we know the wave period and hence wave speed The phase speed C of these resonant ocean waves is given by the dispersion relationship g c tanhkh k k kh c g k In deep water Where g is the acceleration due to gravity is the ocean wave length and h is the water depth Wave speed creates a Doppler frequency shift in the radar return From surface gravity wave dispersion theory we know the wave period and hence wave speed Wave speed creates a Doppler frequency shift in the radar return In the absence of ocean currents the Doppler frequency shift would always arrive at a known position in the frequency spectrum Observed Doppler frequency shift includes the theoretical speed of the speed of the wave PLUS the influence of the underlying ocean current on the wave velocity in a radial path away from or towards the radar Once the known theoretical wave speed is subtracted from the Doppler information a radial velocity component of surface current is determined The effective depth of the ocean current influence on these waves depends upon the wave period and wave length The current influencing the Bragg waves falls within the upper meter of the water column By looking at the same patch of water using radars located at two or more different viewing angles the total surface current velocity vector can be resolved Doppler shift due to waves when no current is present Added Doppler shift due to current Secondary scattering peaks contain information about wave height A SeaSonde HF radar unit has one transmitting antenna and one receiving antenna The transmitting antenna is omni directional it radiates a signal in all directions The receive antenna unit consists of three colocated antennas oriented with respect to each other on the x y and z axes like the sensors on a pitch and roll buoy It is able to receive and separate returning signals in all 360 degrees For mapping currents the radar needs to determine three pieces of information bearing of the scattering source the target range of the target speed of the target A SeaSonde HF radar unit has one transmitting antenna and one receiving antenna The transmitting antenna is omni directional it radiates a signal in all directions The receive unit consists of three co located antennas oriented with respect to each other on the x y and z axes like the sensors on a pitch and roll buoy It is able to receive and separate returning signals in all 360 degrees For mapping currents the radar needs to determine three pieces of information bearing of the scattering source the target range of the target speed of the target The first determination is Range to target The SeaSonde modulates the transmitted signal with a sweptfrequency signal and demodulates this in the receiver the time delay is converted to a large scale frequency shift in the echo signal Therefore the first digital spectral analysis of the signal extracts the range distance to the sea surface scatterers and sorts it into range bins typically 5 km the frequency shift of transmitted minus received signal contains time lag and range information The frequency shift of transmitted minus received signal contains time lag and range information The second determination is Speed of the target the signal is processed for 4 minutes to produce an average spectrum from which the Doppler shift is calculated this gives speed accuracy of 4 cm s The third determination is Bearing of target the receive antenna has 2 directional loop antennas and 1 omnidirectional whip antenna the loop antenna patterns receive power differently from the same incoming direction processing the signal difference from the 2 loop antennas normalized by the omni directional antenna performs the direction finding Rectangular radiation plot Polar radiation plot side lobes Ideal antenna pattern Measured antenna pattern The measured pattern is distorted from the ideal pattern because the antenna interacts with nearby conductive material The vector velocity of the water is determined from 2 or more estimates of the radial velocity in a given cell from 2 different antennas http marine rutgers edu cool codar html Spatial Maps 10 16 2002 0700 GMT RUC Wind and Pressure Analysis 1002 mb Contour resolution 1 mb CODAR Surface Currents 10 16 2002 1500 GMT RUC Wind and Pressure Analysis L 991 mb Contour resolution 1 mb CODAR Surface Currents L 10 16 2002 1800 GMT RUC Wind and Pressure Analysis L989 mb Contour resolution 1 mb CODAR Surface Currents L 10 17 2002 0000 GMT RUC Wind and Pressure Analysis L 992 mb Contour resolution 1 mb CODAR Surface Currents L http ifmaxp1 ifm uni hamburg de