No abstract

A meteorological ST radar operating at a frequency of 47.8 MHz has been used for remote sensing of sea state. General features of the Doppler spectra of the echoes are found to be the same as those obtained at HF; however, space resolution and intrinsic accuracy of the current velocity measurements can be significantly improved. Arguments are presented to suggest a potential range of a few tens of kilometers on the open sea if the full capabilities of ST radars are used. The experiment also shows that, when it is possible, the range could be extended by increasing the altitude of the radar. INTRODUCTION obtained both over a basin of low-salinity water andHigh-frequency (HF) remote sensing of the sea over the open sea, with different altitudes of the anstate is now a well-established technique [e.g., Lipa tennas. They are encouraging and show that such a and Barrick, 1986,]. In coastal areas (up to several device may be useful for some specific studies contens of kilometers offshore) it supplies measurements cerning coastal, estuarine or lagoon circulation. of surface currents and estimates of wave directional 2. EXPERIMENT spectra. There is some interest in increasing radar frequency: the accuracy and the dwell time of the 2.1. The radar measurements can be improved; moreover, the im-The radar used in this experiment is a single freportant traffic in HF range, especially in Europe, pre-quency radar operating atf• = 47.8 MHz. It has been vents to use the short pulses which are necessary to developed for meteorological soundings of low and achieve a detailed coverage of the sea surface in the mid atmosphere (ST radar, e.g., Balsley and Gage observed area. Space resolution can then also be im-[1980,]). An array of 16 vertical whip antennas has proved at VHF. On the other hand, it is well known been used, both for transmitting and receiving. They that the attenuation of electromagnetic surface waves extend along a total distance of 60 meters and the 16 increases with frequency [e.g., Barrick, 1971-], so that elements are connected to form a broadside beam. a compromise has to be defined to keep the maxi-The theoretical two-way 3-dB beam width is about mum range of the radar at a useful value. Some of 3.5 ø. the present ground wave radars do already work in The radar itself is a coherent pulsed radar. Its opthe higher HF range (25-30 MHz) (Codar system, erating parameters (pulse width z and pulse repetie.g., Barrick and Lipa [1979]' or narrow-beam tion rate f•)are adjustable. Typical values during radars, Wolanski et al. [1984] and Prandle and Ryder these experiments were pulse width of 2 or 5 /•s; [1985]). Nevertheless, other possibilities exist at pulse repetition rate of 400, 500 or 625 Hz, corm-VHF: tropospheric ducting and, if the antennas are sponding to an interpulse period (IPP) of 2500, 2000, high enough, free-space propagation may occur and 1600 its, and maximum possible ranges of 375, 300 increase the range over that for ground wave propa-and 240 km. Given the actual range of the soundings gation alone. (a ...

Abstract. We propose and test experimentally a new way of probing vertical shear of ocean surface currents by means of a single frequency VHF radar. The key idea is to use additional information provided by the so called secondary peaks in radar echo spectra which appear due to nonlinearity. Results of the field experiment carried out in the RhSne's delta show that the new method does enable one to probe the vertical shear with a reasonable accuracy and has potential for further development.

An HF monostatic experiment is performed in order to evaluate the relative attenuation with distance of radio waves propagating in the ground wave mode over the sea surface, at 7 and 14 MHz. Clear evidence of the wind speed (causing the sea roughness) influence arises from the data, particularly at 14 MHz. The results are compared to Barrick's theoretical calculations, and a good agreement is found for the values of attenuation rates (in decibels per kilometer) and their variation with the wind speed in the upwind/downwind case. Some discrepancies in the data are partially interpreted as due to a fetch influence upon the radar cross section.

The VHF remote sensing radar technique is applied for mapping the current velocities off a river mouth. The river plume and its front are clearly observed in the data. The method appears to provide the convenient time and space resolutions for investigating the kinematics of these highly variable coastal events such as river plumes, even if it cannot at present achieve azimuthal diversity. The radar observations are felt to be particularly relevant for modelling small‐scale coastal circulation.