APPLICATION OF OCEAN RADAR ON THE BALTIC, FEATURES AND LIMITATIONS

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1 APPLICATION OF OCEAN RADAR ON THE BALTIC, FEATURES AND LIMITATIONS Thomas Helzel, Matthias Kniephoff, Leif Petersen, Markus Valentin Helzel Messtechnik GmbH Presentation at Hydro 2010 Rostock,Warnemünde, Germany member of and Contents 1. Introduction 2. WERA System Concept 3. Measurements and Features 4. Baltic Specific Restrictions 5. Conclusions 1

2 Introduction WERA is a shore based remote sensing system using the over the horizon radar technology to monitor ocean surface currents, waves and wind direction. A vertical polarised electromagnetic wave is coupled to the conductive ocean surface and will follow the curvature of the earth. The rough ocean surface interacts with the radio wave and due to the Bragg Effect back-scattered signals can be detected from ranges of up to 200 km. Introduction The back-scattered radar signal will be Doppler shifted with a specific frequency offset given by the velocity of the gravity wave that is responsible for the Bragg scattering. These Doppler shifted signals will be symmetrical around the centre frequency as long as the ocean surface does not move. An ocean current will shift these Bragg lines up or down in frequency. 2

3 Introduction 2. Range of WERA Products The WERA systems can be configured in various configurations: 1. For Direction Finding or Beam Forming Mode 2. For Short or Long Ranges 3. In Standard, Compact or Splitted Site Geometry 4. In Single- or Multi-Frequency Mode 5. Individual Software and Service Packages 6. Open Data Interfaces 3

4 2.1 Using BF or DF? Feature Real-time data update rate Siting Currents Waves Phased-array with Beam Forming 5/10 minutes for currents and 10/20 minutes for waves Requires antenna array of 8 to 16 small antennas. Array length of of range High dynamic ocean current structures can be measured down to microscale. Measures the local wave data pixel by pixel. Directional spectra are available as well. Compact antenna with Direction Finding Requires long data collection period to get full coverage. update rate of 20 to 60 minutes Compact antenna system (3 x 3 to 12 x 12 m) increases ease of installation Mesoscale current structures can be measured, but resolution can be limited due to long integration times and low azimuthal accuracy. Can only be measured at one or a few points and requires many assumptions. No access to wave data pixel by pixel. 2.2 Flexible System Concept of WERA The complete system concept is very flexible. It can be configured for short range applications with highest resolution or... Left: e.g. 30 km with 300 m range cells (Norway, 2000)... for long ranges of more than 200 km with range cells of 1 to 3 km. In all configurations a temporal resolution of up to 5 minutes can be achieved to monitor very dynamic features. 220 km with 3 km range cells Display: 1 arrow for 16 pixels (US east coast) 4

5 2.3 Site Geometry Options The Standard Curved Arbitrary Tx Compact Splitted arraysite Array is Site Spaced very Geometry Site compact Site Geometry provides and requires can requires a be allows no a wider placed separation no angular to cable adapt the of connection field of the roof the antenna Rx of of Rx view. and the and Tx Tx position between equipment antenna to arrays the environmental container Rx by but and about will Tx or antenna provide 10 beach conditions wavelengths a arrays. hut. reduced (e.g. The operating Rx the and edge Tx range. stations of a cliff Any can or be (25 attached connected compromise to 150 to m, wireless a between depending fence along over the a on a normal distance foot operating path). and of up the frequency) to compact 3 % of the geometry radar range. is possible. 2.4 Multi-Frequency Option For sensitive applications it is possible to use a Multi-Frequency Option. Multi-Frequency will generally increase the data availability. In particular the data availability of wave data can be improved by increasing the upper or decreasing the lower limits for the wave detection. For Ship Detection and Tracking this option can prevent that ships may hide in a Bragg Line. The effects of the day-to-night range variation can be reduced. 5

6 2.5 Individual Software and Service Turn-Key Systems: Of course customer specific software configurations and required services are available through Helzel and the WERA Group. Local Support: It is our policy to integrate local partner companies to provide optimal service for our customers. Training: User training seminars (1 week) are free for WERA users. Scientific Support: In addition to these professional services the WERA Group supports scientific partners by offering access to source code of oceanographic tools. This scientific group is coordinated by Klaus-Werner Gurgel, University of Hamburg. 3. Dynamic Surface Currents Current map: new data all 12 min spatial resolution: 1.5 km fo = 12.4 MHz 2 x 16 antenna arrays 6

7 3.1 Accurate Currents Validation with ADCP Pro4, located about 30 km offshore Data kindly supplied by Actimar and SHOM from their WERA system 3.2 Wave Parameter 0.3 Directional spectra Map with Wave Height and Direction /01/ :05 radar wind direction Data kindly supplied by Actimar and SHOM from their WERA system 7

8 3.3 Wave Power and Energy Period Radar (black) and buoy (red) comparisons ρg c g ( f ) S( f )df 27MHz at Norwegian coast (1535 samples) Power: correlation: 0.91 rms:13.2 kw/m Energy period: correlation: 0.9 rms: 0.7s Data kindly supplied by SeaView Sensing 3.4 Maps of Wave Power Map of Wave Energy 27MHz at Norwegian coast Data kindly supplied by SeaView Sensing 8

9 3.5 Wind Parameter Map of wind direction and wind speed 1 map per hour 3.6 Application: Current Drift Prediction Simulation of a Search and Rescue case. The prediction of the actual position of a drifting buoy man-over- board was compared with different trajectory methods. WERA based trajectories showed the best accuracy. Data are kindly provided by Actimar. 5 km Drift prediction using WERA HELZEL Messtechnik current GmbH data around Ushant 9

10 3.7 Application: Oil Slick Drift The WERA data can be used for data assimilation into a coastal ocean current model. Such a enhanced model can be used to give a very reliable prediction of drifting oil slick. 4. Restrictions for Baltic Applications For an over the horizon radar system the operation frequency is the most important parameter to provide a defined range. With a decreasing operating frequency the range will increase, but... Range of 1-st order beam versus Frequency 250,00 Tests with different WERA installations all with linear array (12 or 16 antennas) 08th of May 2006 Beamformed Range in km 200,00 150,00 100,00 50,00 y = 1354,5x -0,9781 R 2 = 0,867 0,00 0,00 5,00 10,00 15,00 20,00 25,00 30,00 35,00 40,00 45,00 50,00 Frequency in MHz 10

11 4. Restrictions for Baltic Applications But with a decreasing radar frequency, the according Bragg resonant wave length is increasing. On the Baltic this limits the use of lower frequencies, in particular in regions having typically short wave periods. 25 Bragg W ave Length and Velocity versus Carrier Frequency 20 Bragg Wave Length in m and Velocity in m/s Bragg Wave Length (BWL) BWL = c / (2 * fo) Velocity of Bragg Wave (VBW) VBW = SQR(g * BWL / (2 * 3,14)) 0 5 7, , , , ,5 30 Carrier Frequency in MHz 4. Restrictions for Baltic Applications Another limitation is the low salinity of the Baltic. The range strongly depends on the conductivity of the water which is decreasing with lower salinity and decreasing temperature. 11

12 Range versus WERA Frequency on the Baltic Range in km Skagerak 6 psu 15 psu Kattegat Frequency in MHz 5.1 Conclusions The shore based radar system WERA is a powerful oceanographic instrument giving reliable information about large ocean areas. It is easy to install and flexible for various application making it attractive for scientific experiments as well as for permanent installations for applications like SAR or vessel traffic services. For applications at the Baltic Sea or other smaller ocean areas the specific local boundary conditions needs to be taken into account. 12

13 Acknowledgement We would like to thank: SHOM and Actimar for providing data from SURLITOP project, Seaview Sensing Ltd. for providing their wave analysis data and Klaus-Werner Gurgel (Uni Hamburg) and his team for continuous support of the WERA projects WERA Installations Since 1999, more that 50 WERA systems are installed. Permanent WERA Installation Temporary WERA Installation Planned WERA Installation 13