The big advantage of HF radar over other coastal metocean measurement systems is the ability to map parameters over a wide area. For example, the figures below show wave height and peak wave direction (on the left) and surface current (on the right) measured with the Pisces HF radar. The measurement locations are from 30 to 200km offshore.
To provide maps of this type, HF radar signals have to be processed to separate them into contributions from different ranges and from different directions relative to each of the radar stations.
Range measurement is achieved using time or frequency differences, depending on the radar signal modulation, between the transmitted and received signal. For FM(I)CW (frequency modulated (interrupted) continuous wave) modulation this is usually achieved using an FFT. Range resolution depends on the bandwidth of the transmitted signal for FM or pulsed systems.
Direction measurement is achieved using two different approaches depending on the type of antenna that is used. The starting point is a Doppler spectrum of signal at each the range usually obtained with an FFT.
Surface current measurements are obtained by measuring the Doppler frequency of the peaks in the backscattered power spectrum. The measurement is actually that of the speed of ocean waves of half the radio wavelength travelling towards and away from the radar. To first order, this speed is made up of the intrinsic wave speed (determined from the well-know dispersion relationship for ocean surface waves) plus the component of surface current in the direction of wave propagation.
There are two main types of HF radar; compact antenna systems that use direction finding techniques and phased array systems which can be used with Seaview software.
With compact antenna systems the Doppler spectra contain information from all directions and thus a number of peaks corresponding to current components from different directions. These are resolved by measuring the phase differences at each frequency in the backscattered signal spectrum at each antenna together with geometrical considerations or more commonly now by other methods e.g. Music processing, which analyses the covariance properties of the backscattered signal spectrum at each antenna at each frequency. These approaches are called direction-finding and unambiguous directions with good directional resolution can be achieved with three or more antennas.
The phased-array systems are so called because phase differences, which are prescribed for each required look direction, are added to the signal at each antenna and these modified signals are summed. The Doppler spectrum is obtained by processing the differently phased summations separately. As a result current measurement is straightforward since each backscattered power spectrum is from a single range-direction cell. The phase differences can be dealt with digitally (as in the WERA system), in which case the radar can effectively look in all directions at the same time and provide good spatial and temporal coverage (see below, such measurements were obtained every 10 minutes), or using different cable lengths which have to be switched (as in the Pisces system leading to a 1 hour measurement period). Neptune Radar have now developed a multiple receive channel capability for Pisces which can provide good spatial and temporal coverage.
In the direction finding case with three antennas there can be limitations if surface current patterns are such that there are more than two locations within one range where the components in the direction of the radar happen to be equal. It usually takes longer (an hour or so) to gather current data covering the whole area of interest with direction finding methods. This can be a problem in rapidly varying environments. For good angular resolution with a phased array system a large number (16 is often used) of individual antenna elements are required which means that the area of land required to site such a system can be large especially when operating at low HF frequencies. It is possible to use direction finding techniques with antenna arrays of course.
Surface wave measurements are obtained by applying numerical inverse methods to the backscattered power spectrum. This process, although numerically complex for the full direction spectrum, is again easier when the Doppler spectrum is from a single range-direction cell when, as shown for Pisces below, wave parameters can be mapped with a similar spatial and temporal resolution as currents. Directional spectra for three of these locations are shown underneath the map. Parameters on the map were obtained from the directional spectra. Wave measurements cannot be obtained to the same maximum ranges as currents because they need a higher signal to noise.
|Wave height||Wave period||Surface current|
|Short range spectrum||Mid range spectrum||Long range spectrum|
For compact antenna systems the range resolved Doppler spectrum is a convolution of narrow beam spectra from all directions with the antenna beam pattern. Unlike the current signal, the wave signal from each direction is spread over many Doppler frequencies and direction finding techniques are not useful. To date wave measurements from such systems require that the wave spectrum is homogeneous (i.e. does not vary) over each range and normally just one range is used providing a single measurement of the wave spectrum rather than a spatial map. This is probably not a problem at a range close to an offshore platform in deep water where it is often reasonable to assume that the wave field is more or less the same in all directions, but nearer the coast waves are affected by changes in water depth, by wave-current interaction, by dissipation due to e.g. wave breaking and by local generation e.g. in the case of offshore winds. If the wave field far offshore is uniform and refraction due to changes in depth is the only inshore influence changing the spectrum, it is theoretically possible to relate the local wave field in any look direction from the radar to the offshore field and then to solve the inversion problem for the offshore field.
In the phased array case, provided there is sufficient signal to noise and the depth is known, the local spectrum is determined at all locations in the overlap area of two (or more) radars and can be compared with other local measurements directly.
|Phased-array with beam forming||Compact antenna with direction finding|
|Real-time data — update rate of typically 10 minutes for digital beam forming methods||Often requires long data collection period to get full coverage — update rate of around 60 minutes|
|Extensive validations of wave and current measurements have been published||Extensive validations of wave and current measurements have been published|
|Requires long array of antennas for good directional accuracy —can limit location choice||Compact antenna system increases ease of installation|
|Current measurement is not limited by seastate except at high radio frequencies in very high seas.||Requires lower HF freq in high seas for current measurements because the current and wave signals are not separable. . Antenna pattern measurement is essential|
|Wave measurement — provides the local wave directional spectrum at gridpoints in both deep and finite depth water||Wave measurement — Can only estimate the spectrum in deep water offshore conditions. In finite depth water, estimates are of the equivalent deep water wave spectrum assuming no local generation/dissipation, no wave-current interaction|
|Full wave directional spectrum is calculated for each grid point in the area covered and therefore actual data may be mapped.||Wave data for mapping across the area may only be obtained by use of a physical refraction model.|
|Accuracy of wave data is dependant on the radar system, in particular it may be limited by poor antenna side-lobes. Good antenna design and installation and measurement of the antenna pattern is recommended.||Accuracy of wave data is dependant on the model assumptions in relation to that particular installation. Antenna pattern measurement is essential.|