Propagation and scattering in MF/HF groundwave radar

Abstract

Vertically polarised MF/HF groundwave radar has been used over the frequency range 2-30 MHz for sea-state sensing, surface-current measurement and ship tracking. The paper reviews the propagation factors and noise environment which control the performance of such systems. The various definitions of antenna gain, propagation loss and sea backscattering coefficient are discussed and related to physical concepts. The various first-order and higher-order mechanisms of scattering are reviewed and related to the relevant electromagnetic and hydrodynamic processes involved. These processes, linked with the velocities of the water waves responsible, result in a Doppler spectrum of the radar echoes which may be used to infer the wave height and wave directional spectra on the sea surface. Particular attention is directed towards remaining problems in understanding these processes. In evolving radars for ship tracking, detectability depends on the knowledge of the Doppler spectra of the sea clutter and, in particular, on limitations in directional determination of ship echoes with practical antenna systems located on available coastal sites. In conclusion, experimental measurements of sea echo, ship tracks and antenna-array characterisations are compared with theoretical considerations. List of principal symbols A(d)h F,F r ,F r ,F R = one-way attenuation over lossy ground relative to perfectly conducting ground = effective absorbing aperture of receiving antenna = propagation distance =; EMF in transmitting antenna = electric field incident on target = reradiated electric field from target as received at transmitter = wave frequency, Doppler frequency, Bragg frequency = incident power flux (field intensity) on target = reradiated power flux from target as received at transmitter = Norton attenuation coefficients 9 9, 9T > 0 L Pi > P R » PRR = acceleration due to gravity = gain factors of transmitting/receiving antennas relative to isotropic = modified gains in presence of ground G' r , G' R = gains expressed in dB = directional pattern of wind-waves as a function of angle, 6, relative to mean wind = heights of transmitting and receiving antennas above ground plane = lengths of transmitting, receiving whip antennas = radio wave number 2n/X = sea wave number 2n/L = effective length of antenna = basic transmission loss factor between isotropics in free space = transmission loss factor between antennas in free space = transmission loss factor between antennas in reflective space = transmission loss factor between whips = sea wave length = transmitted, received, reradiated power A A T , A R = radiation resistance of antenna in free space -2s is exponent of cos 9 in spreading function for wind waves = reactance of transmitting, receiving antenna -(1 + A) is factor by which radiation resistance of antenna is increased in proximity of ground plane = values of A for transmitting, receiving antennas = proportion of sea-wave energy travelling in opposite direction to wind = characteristic impedance of free space, 377 ohm/square = an...

Wave Height Estimation from First-Order Backscatter of a Dual-Frequency High Frequency Radar

“…Model fitting concentrates on the overall scatter relationship between Bragg power ratio and wave height, therefore, the selected spectral points are directly taken into account. The model coefficients, (a, b, c, d, e) in Equation (15), are calculated through Least mean square (LMS) fitting. However, for wave height estimation, spatial average and median filtering are required to reduce the fluctuation of η.…”

“…For a given wave height h s , a corresponding ratio η can be obtained from Equation (15). The noise is added by replacing 10 log 10 η with 10 log 10 η + ∆n · n(t), t = 1, 2, · · · , L, where n(t) ∼ U(−1, 1) is a uniformly distributed random variable with a value within −1 to +1.…”

“…The wave height data of two buoys, B1 and B2, are strongly correlated. Note that at least three buoys located at different distances are required for the fitting of the model in Equation (15). By considering the scalable difference caused by the water depth, it is reasonable to obtain the wave height at the middle distance (i.e., 40 km) via interpolation.…”

“…It is found that the Bragg wave power ratio of dual-frequency does vary with h s , and the fitting correlation coefficient is 0.70. The second step is to conduct the three-dimensional fitting with respect to h s and R, using the model given in Equation (15). The fitting result is shown in Figure 9a, and the fitting coefficients as well as the confidence bound are listed in Table 3.…”

An estimation and verification of vessel radar-cross-sections for HF surface wave radar

“…The length of the ship is 63 m and the breadth is 14.2 m. To calculate the RCS of the model, the grid is reflected in the horizontal plane, and then NEC calculates the cross section of the entire structure. Hence, the results here refer everywhere to the "net" RCS, not to the equivalent free-space RCS which is made 12 dB smaller in order to account for the gains of antennas operating over a conducting surface [7]. To test the effect of grid spacing, two Teleost models were generated, first with 3.5-m grid spacing and 0.5-m wire radius and then with 2.2-m grid spacing and 0.3-m wire radius.…”