FEATURE INTRODUCTION TO HIGH-FREQUENCY RADAR: REALITY AND MYTI-I By Jeffrey D. Paduan and Hans C. Graber THE CONCEPT OF USING high-frequency (HF) electromagnetic spectrum. Figure 2 illustrates sev- radio pulses to remotely probe the ocean surface eral of these remote sensing techniques used to ex- has been around for decades. In this paper and the tract information about the ocean surface. The fig- companion paper by Teague et al. (1997) we ure is adapted from the review by Shearman strive to introduce this technique to a broad ocean- (1981) and it contrasts space-borne systems, such ographic audience. Teague et al. (1997) provides as altimeters and scatterometers, which use mi- the historical context plus an outline of different crowave frequencies, with shore-based systems, system configurations, whereas we focus on the which use a range of frequencies depending on the measurements of primary interest to coastal ocean- application. (Not shown are aircraft-borne sys- ographers, i.e., maps of near-surface currents, tems, which also operate in the microwave band.) • . we focus on the wave heights, and wind direction. Another goal of The figure also illustrates the different types of measurements of pri- this paper and, indeed, this entire issue is to pres- transmission paths, including true line-of-sight ent a realistic assessment of the state-of-the-art in paths, "sky wave" paths, which reflect off the ioni- mary interest to HF radar techniques vis-h-vis coastal oceanogra- sphere, and "ground wave" paths, which exploit phy. When evaluating any new measurement tech- coupling of the radiowaves with the conducting coastal oceanogra- nique, it is important to separate issues related to ocean water to achieve extended ranges. For HF phers... system design from fundamental limitations of the radars, instruments that operate using sky wave technique. The former are engineering shortcom- transmissions are often referred to as over-the- ings, which are subject to continuous improve- horizon (OTH) radars (e.g_ Georges, 1980), al- ment. The latter are real limitations in the use of though HF ground wave radars, which are the the particular geophysical signal in the presence of major focus of this issue, also achieve beyond-the- realistic noise• Most of the "myths" about HF horizon ranges. radar measurements, in our view, stem from the Reflection (or backscatter) of electromagnetic confusion of these two issues. energy from the sea surface can be expected to One common misconception about HF radar produce an energy "spectrum" at the receiver, stems from the word "'radar" itself. A more de- even if the energy source was single-frequency, scriptive name would be HF "radio," as the HF because of the complicated shape and motion of portion of the electromagnetic spectrum is within the sea surface. Interpreting these spectral returns the radio bands. Figure 1 shows a broad range of for various transmit frequencies is the key to ex- the electromagnetic spectrum, including the tracting information about the ocean. Many instru- nomenclature commonly applied to different por- ments rely on a resonant backscatter phenomenon tions of the spectrum. The HF band, with frequen- known as "Bragg scattering," which results from cies of -3-30 MHz and wavelengths of-10-100 coherent reflection of the transmitted energy by m, sits between the spectral bands used for televi- ocean surface waves whose wavelength is exactly sion and (AM) radio transmissions. Often, the term one-half as long as the transmitted radar waves. radar is applied to instruments operating in the mi- The inset in Figure 2 attempts to illustrate this crowave portion of the spectrum, for which wave- process by showing how energy reflected at one lengths are measured in millimeters or centimeters. wave crest is precisely in phase with other energy Throughout oceanography, many different in- that traveled ~I wavelength down and ~[ wavelength struments exploit many different portions of the back to reflect from the next wave crest. These co- herent reflections result in a strong peak in the backscatter spectrum. Scatterometers exploit Jeffrey D. Paduan. Code OC/Pd, Naval Postgraduate School, Monterey, CA 93943. USA- Hans C. Graber, Rosen- Bragg scattering from capillary waves (-1 cm) to stiel School of Marine and Atmospheric Science. University of obtain information about winds. HF radars, on the Miami, Miami, FL 33149-1049, USA. other hand, exploit Bragg scattering from surface 36 OCEANOGRAPHY'VoI.10, NO. 2"1997 gravity waves (-10 m) to obtain information about Wavelength • currents (and winds). 1 / l I I I I I I I I 1 1 I [ I I Measuring Currents 1020 1018 1016 S 1014 1012 1010 I(~ 106 104 102 gamma I t ~ehf~ shf ~uhf vhf, hf mf~ If The history of HF backscatter measurements is --ray--J I ~ / Lrad~--radi°'a--bands ' l audio ' x-ray 1 t...microwave...a better outlined by Teague et al. (1997). We point Nomenclature t'ultra-e-near~mid~far'a "~'-=" Frequency (Hz) vmiet.JM., infrared to the work of Crombie (1955) as the first to iden- vlbgyor tify strong sea echoes in the HF band with reso- nant Bragg scattering. Bragg waves in the HF Fig. 1: Electromagnetic spectrum showing the HF band relative to other radio wave band happen to be "short" surface gravity waves, bands and the broader spectrum. which can be assumed to be traveling as deep- water waves, except in very shallow depths of a few meters or less. This is important because it al- metric dilution of precision (GDOP) in the global lows information contained in the Doppler shift of positioning system (Chapman and Graber, 1997). If Bragg peaks to be used to estimate ocean currents. currents are assumed to be constant over several ra- Figure 3 illustrates the Doppler technique for dial bins, it is also possible to estimate velocities ocean current determination from HF radar using a single radar site as was done by Bjorkstedt backscatter. It shows an actual spectrum from the and Roughgarden (1997), although the GDOP-re- Ocean Surface Current Radar (OSCR) system. The lated errors will be relatively large in this case. spectrum contains obvious Bragg peaks due to the The current measurement by HF radars is close presence of Bragg waves traveling toward and to a "true" surface current measurement. Because away from the receiver. The frequencies of these radar pulses scatter off ocean waves, the derived peaks are offset from that of the transmitted en- currents represent an integral over a depth that is ergy for the two following reasons: 1) the Bragg proportional to the radar wavelength. Stewart and waves are moving with the deep-water phase Joy (1974) show this depth to be -d = X/87r. Be- speed given by c = ~(gX/47r), where k is the wave cause wavelength depends on the radar frequency, length of the transmitted energy and g is the gravi- it is feasible to use multifrequency HF radars to tational acceleration and 2) the Bragg waves are estimate vertical shear in the top two meters of the moved by the underlying ocean current. Because ocean. the expected Doppler shift due to the Bragg waves Present system and coverage capabilities of HF The current mea- is known, any additional Doppler shift is attributed radars are quite impressive. Measurements can be to the current as shown in Figure 3. made in range as short as 1 km and as long as 150 surement by HF It is important to keep in mind the following km from the shore at a resolution of -0.3-3 km points about HF radar-derived currents: 1) a single along a radial beam. Radio interference or high sea radars is close to a radar site is capable of detecting only the compo- states can limit the actual range at times as well as "true" surface current nent of flow traveling toward or away from the site for a given look angle; 2) the effective depth measurement. of the measurement depends on the depth of influ- ence of the Bragg waves and is quite shallow (-1 m); 3) stable estimates require scattering from hundreds of wave crests plus ensemble averaging of the spectral returns, which sets the space-time resolution of the instruments; 4) the precision is limited by the frequency resolution of the Doppler spectrum and is typically 2-5 cm s '; and 5) the accuracy is controlled by numerous factors, such as signal-to-noise ratios, pointing errors, and geometry. Because a single radar station measures only the component of flow along a radial beam emanating from the site, "radial" currents from two or more sites should be combined to form vector surface current estimates. Figure 4 illustrates this principle using radial data from two radar sites. It also illus- trates the "baseline problem" that occurs where both radar sites measure the same (or nearly the Fig. 2: Schematic representation of various remote same) component of velocity, such as along the sensing methods exploiting signals backscattered from baseline between the sites or at great distances from the sea surface (adapted fronl Shearman, 1981). The both sites. Generally two radials must have an angle inset illustrates the resonant Bragg scattering process >30 ° and <150 ° to resolve the current vector. This that occurs due to reflection from waves whose wave- geometric sensitivity is similar to the familiar geo- length"ts ~ as long as that of the incident energy. OCEANOGRAPHY*VoI. 10, NO. 2"1997 37 -115 i i the ground conditions in the vicinity of the receive __ ~.._/__f a.._£_f antennas• Wet and moist sandy soils enhance the -120 ground wave propagation, whereas dry and rocky Bragg Peak from Bragg Peak from -125 Receding Waves Advancing Waves grounds reduce signal strengths• Typical azimuthal resolutions are -5 °.