Directional spectra comparisons between HF radar and a wave model

Abstract

Abstract-Directional spectra measurements using HF radio frequencies signal to noise is often too low for reliable radar are compared with model data to confirm measurement and the approximations used in the inversion limitations of the currently available theory that underpins begin to lose validity. In high seas at high radio frequencies these measurements. In high seas, waveheight is again the approximations become invalid and in extreme overestimated but it is demonstrated that there is no clear conditions the wave and current parts of the radar signal impact on the shape of the spectrum which is in reasonable cannot be separated. It is this limitation that will be addressed agreement with the model. The need for increased in this paper. Towards the edges of the radar coverage region, averaging before inversion is discussed.away from the buoy, the radar wave measurement are sometimes noisier and larger in amplitude but without additional information it has been difficult to separate possible I. INTRODUCTION real spatial variation in the wave field from errors in the radar HF radar systems located on the coast have been used to measurement. Wave model data will be used here to get some measure surface currents and the ocean wave directional insight into the accuracy of wave measurements at long range. spectrum simultaneously from close to the coast to more than 100km offshore. Measurements can be made from every 10 II. THE DIRECTIONAL SPECTRUM minutes to every hour and with spatial resolutions of 1 to 15km as needed. HF radar current measurement is now a well accepted technology and there many systems of different types problem of the scattering of electromagnetic waves at HF in operation around the world. Wave measurement is a more frequencies from a moving ocean surface was developed in complex process and has not yet gained the same level of [3], [4], [5] and this forms the basis of the measurement acceptance. A higher signal to noise is required and the process discussed here. The solution is in the form of a nonmeasurement, although numerically complex, is much more linear integral equation that has to be inverted numerically to straightforward with phased array systems which are only provide a measurement of the directional wave spectrum. The [2] (developed from a University of measurements at hundreds of locations in near real-time. Birmingham, UK, prototype and available from Neptune Wave parameters such as significant waveheight, peak Radar Ltd). See www.seaviewsensing om and follow ocean direction and period can be determined from the directional data links for more information and access to data from some spectrum using standard wave analysis techniques. Barrick's of these deployments, theory is based on a perturbation analysis with respect to the product of the radio wavenumber and ocean wave amplitudeTo date all our comparisons with wave buoy data have been which has to be small. This approaches one in the high seaat locations at close enough range to ensure good quality radar state, high rad...

Comparison of HF Radar Fields of Directional Wave Spectra Against In Situ Measurements at Multiple Locations

“…The focus of this work is the inversion method presented in [23], which is distributed as a software package by Seaview Sensing Ltd., Sheffield, UK (Seaview hereafter). Its results have been previously examined using datasets collected during different experiments (e.g., [7,[24][25][26]), which have revealed the accuracy and limitations of the method and the HF radar technique itself. Reported inaccuracies have been found to originate predominantly from two different sources: limitations in the theory describing the relation between Doppler and ocean wave spectra [27], and the use of short averaging periods to derive wave directional spectra [28].…”

“…Reported inaccuracies have been found to originate predominantly from two different sources: limitations in the theory describing the relation between Doppler and ocean wave spectra [27], and the use of short averaging periods to derive wave directional spectra [28]. The main consequence of the former is an overestimation of high sea states ( [24,25]), while the latter has been identified as the cause of noise in the measurements ( [26,29,30]).…”