SCM 330 Ocean Discovery through Technology Area F GE

Sensors - Physical Physical Sensors: Temperature Salinity Pressure Acoustics Active Passive Radar Theory Application Sensor

Radar Provide a comprehensive overview of RADAR, RADAR Elements and RADAR Functionality concepts by observing the behavior of a RADAR Signal (PULSE) as it is Generated and Propagated, Interacts with Targets Recaptured and Processed to provide useful information for its intended purposed

RADAR: RAdio Detection And Ranging RADAR is an INSTRUMENT (a measuring device) that uses the transmission and reflection of Radio (i.e. Electro-Magnetic) waves to - (Traditionally) Detect and Track - (Now) Characterize, Classify/Discriminate Targets at “long” distances. RADAR has several important applications Missile Defense Radio Astronomy Air Defense Missile Testing Space Defense Navigation Intelligence Data Gathering Weather Air Traffic Control Fishing Remote Sensing/Mapping Ornithology Imaging Law Enforcement Wave/Current Mapping

The key physical characteristic of a RADAR (and hence its Pulses) is its carrier Frequency, or, Correspondingly, its Wavelength Frequency = (Speed of Light)/(Wavelength) Speed of Light = c = 300,000,000 m/s RADAR Frequency is a Key RADAR Parameter Some common Frequency-Wavelength RADAR types are UHF 400 MHz m BMEWS, ALTAIR, UEWR L 1.0 GHz m COBRA DANE, AWACS S 3.0 GHz m COBRA JUDY - S, NTW C 5.0 GHz m ALCOR, PAC X 10 GHZ m GBR-P, CJ-X, LRIR, TR W 94 GHz m Seekers HF RADAR 3-30 MHz

The distance traveled by the RADAR wave (Pulse) to the Target is called the Range (R) The Time-of-Travel to the Target = R/c Time-of-Travel of reflected wave back to the Targets also = R/c Total Time-of-Travel = 2R/c

High frequency (HF) radar is used by ocean researchers to measure surface current velocity fields near the coast. A HR-radar system can measure surface currents averaged over 15 minutes as far offshore as 160 km. The resulting surface plots provide a much higher resolution in space than previous techniques like current meter arrays. With a HF-radar system an entire current field can be generated every fifteen minutes. These vector plots allow mesoscale features, like coastal eddies, to be resolved with much more accuracy than an array of current meters.

I.Current Velocity of Target The basic mechanics of a Hf-radar system is the analysis of a backscattered radio wave. The CODAR system works very much like a radio station in that it emits a radio signal. While a radio station does not monitor the signal that is scattered back to the station, a CODAR site uses this backscattered radio wave to calculate surface currents. If the ocean were completely flat, no signals would be backscattered. Since the ocean is not flat, it scatters the radio signal in many different directions. In order to amplify the portion of the scattered signal that is directed back to the receiver antennae, the CODAR system utilizes the principles of "Bragg Scattering".

Resonant Bragg Scattering Since the ocean surface scatters a signal in many different directions, some mechanism must be used to maximize the signal directed back to a CODAR receiver. Resonant Bragg Scattering basically amplifies the scattered signal directed toward the receiver using resonant theory. Resonance will only occur for certain signal wavelengths:

The signal scattered off a wave and back toward the antenna will be in phase with a signal that traveled to the next wave (1/2 transmit wavelength further) and returned to the original wave (another 1/2 transmit wavelength). Therefore the signal that traveled a whole wavelength further will line up the first signal. When all of the scattered signals directed toward the receiver are in lined up, each signal is added to the other and results in a stronger signal. All the CODAR system has to do is send out a signal that is twice the wavelength of an ocean wave and the scattered signal directed back to the receiver will be amplified.

So how is this signal used to calculate surface currents? All of the previous equations assume that the surface waves are not moving. In fact the waves are moving and a moving wave will change the frequency of the return signal. This phenomenon is known as the Doppler Shift. The frequency of a signal scattered by a moving wave will be shifted depending on the velocity of the surface wave. If the wave is approaching the receiver, the return frequency increases. On the other hand, a wave moving away from the receiver will return a lower frequency. Therefore the shift will be positive if the wave is moving toward the receiver and negative if the wave is moving away from the receiver. The following equation is used to measure the magnitude of the frequency shift:

An example of a return signal is shown in the figure below. Notice how the size of the two peaks are amplified by the Bragg Scattering. The relative size of the peaks tells us which way most of the waves are moving. In the figure the negatively shifted peak is larger and therefore it can be said that the wind is forcing most of the waves offshore (i.e. an offshore breeze is present).

The Doppler shift calculated above is assuming that there is no surface currents changing the motion of the waves. So the current can be calculated by measuring the frequency shift from the original Doppler shift caused by the wave motion. If there is no current then the Doppler shift caused by the surface wave motion will not be changed. If however, the surface current is not zero, the frequency will be shifted further depending on the magnitude and direction of the current. The Doppler equation is used again to calculate the velocity of the target using the frequency shift measured by the receiver antenna. Note that the velocity calculated is only the component of the velocity moving toward or away from the receiver (radial velocity component). CODAR must use radial components from at least one other site to determine the total current vector at a given point. Using this system, CODAR can calculate surface currents with an error of less than 4 cm/s.

II. Range to Target Most conventional radar systems measure the distance to a target by measuring the time delay of the return signal. If the speed of the signal and the time is known, then the total distance traveled can be calculated. The range to the target would then be half the total distance. The problem with this method is that CODAR system needs to be resolved to very fine grid points (about 1 km). Since it does not take very long for a signal travelling at the speed of light to move 1 km, a very sensitive watch is needed. CODAR overcomes this problem by sending out a frequency modulated (fm) signal. The frequency of an fm signal increases linearly with time.

The time delay can therefore be measured by subtracting the return signal (b) from the transmitted signal (a). The difference (c) will be equal when both signals are present since they both increases at the same rate. So the higher the frequency of the horizontal line, the further away the target. This time delay is then used to determine the range to the target.

III. Angular Direction of Target The direction of the target is determined using the signal received by three different antennas. The three antennas include two loop antennas and a monopole. Each antenna has a different beam pattern. The monopole receives the same signal independent of the incoming direction, omnidirectional. Signal information received by the monopole can therefore be used to normalize the information collected by the two loop antennas.

The signal received by the two loop antennas is dependent upon the incoming direction. They are oriented ninety degrees to each other so that they can be used in combination to determine the incoming direction of the signal. When information from the two loop antennas are normalized with the monopole signal, the arctangent function is used to determine the direction of the signal. This process is referred to as Direction Finding and allows a CODAR system to have a directional resolution of one degree.

Role of Antenna Patterns in Signal Direction Determination Loop 1 Loop 2

Measured vs. Ideal Antenna Patterns 4 ft Antenna Elements

4 ft Ideal Patterns Northern Site Radial Coverage km 4 ft Measured Patterns 25 km Percent Coverage

Radial Velocities 25 cm/s CODAR Site Moored ADCP 25 km

CODAR Total Vector Calculation CODAR Central Site Kilometers CODAR North CODAR South LEO-15 Great Bay Little Egg Harbor A T L A N T I C O C E A N Atlantic City ?

RUC Wind and Pressure Analysis CODAR Surface Currents Spatial Maps 10/16/ GMT 1002 mb Contour resolution – 1 mb

L RUC Wind and Pressure Analysis L CODAR Surface Currents 10/16/ GMT 991 mb Contour resolution – 1 mb

L RUC Wind and Pressure Analysis L CODAR Surface Currents 10/16/ GMT 989 mb Contour resolution – 1 mb

L RUC Wind and Pressure Analysis L CODAR Surface Currents 10/17/ GMT Contour resolution – 1 mb 992 mb

74:25 74:20 74:15 74:10 74:05 74:00 73:55 73:50 BeforeDuringAfter The inner-shelf response to tropical storm Floyd

CODAR System Antennas Receive Antenna Transmit Antenna 25 MHz and 13 MHz 5 MHz

Long Range HF Radar Networks

Data Distribution Route Map UConn URI UMass, Dart. UNC, Chapel Hill Johns Hopkins SIO UCSB NPS OceanTemp.com Rutgers USCG R&D Applied Math CODAR

Site locations for 1km high resolution CODAR systems operating with ~ 40km range green: historical site blue: existing UCSB orange/yellow: existing SDCOOS light blue: proposed new sites