A Bistatic HF Radar for Current Mapping and Robust Ship Tracking D. B. Trizna Imaging Science Research, Inc. 6103B Virgo Court Burke, VA, 22015 USA Abstract- A bistatic HF radar has been developed for application to ocean current mapping and ship vector tracking. The radar can operate in a multi-frequency mode, so that it can map ocean current vertical shear and can provide more robust ship tracks than single frequency HF radars. This tracking robustness is achieved by avoiding target fading due to echo nulls from frequency and azimuthal variations in ship radar cross section that occur using a single radar frequency. The radar is fully digital in frequency generation and reception, and has no RF receiver components because the received antenna signals are digitized at the HF frequency directly. We use A/D conversion rates sufficiently high to maintain the 2 to 1 frequency ratio required for the highest radar frequency of interest to avoid frequency aliasing. The newly developed radar acquisition code provides real-time range compression, so that data files that are stored are in-phase and quadrature (I/Q) samples, at a much less dense rate than the original digitized signal time series. The bistatic capability is based on accurate system timing and radar frequency. These are provided at each of two or more radar sites by rubidium clocks and GPS timing, accurate for the first pulse to 50-ns to initiate data acquisition in the bistatic mode. Once acquisition is initiated, the rubidium clocks at each site maintain much more accurate frequency and time stability to allow Doppler velocity measurements accurate to 2 millihertz at 25 MHz operating frequency. The primary site requires an 8- or 16-element receive array, and both primary and satellite bistatic-illuminator sites have a modest 2 or 4-element monopole transmit antenna pair. This bistatic approach reduces the coastal space requirements because of the need for just one receive-antenna array per radar system. These systems can be operated with a pair of bistatic transmitters, either side of the receive site, to expand the spatial coverage. Using such an approach, these units could be staggered to create a system of radars, providing continuous coverage along a coastline, alternating transmit and receive sites. This type of arrangement could be used to provide robust ship tracking along a country s coastline, and a modest estimate of type and tonnage of all vessel traffic based on target echo strength. Due to its digital approach, the cost of these radars is substantially less than that of existing coastal HF radars, none of which have a multi-frequency capability.. I. INTROUDUCTION We have developed a bistatic HF radar for robust ship tracking and ocean current mapping based on purely digital receiver and transmission technology. The bistatic capability represents an upgrade from our previous system, and has been delivered to the U.S. Navy for research applications. This bistatic system requires use of accurate system clocks and absolute time referencing in order to operate in a bistatic mode. These are provided by GPS timing and rubidium clocks. We have comprehensively tested the system in the VHF band initially and present some of those results here. The VHF band allows for shorter range and wider bandwidth usage than can be achieved at HF frequency. The technology transfers directly to HF frequencies, with slower analogue-to-digital (A/D) conversion rates required due to lower radar frequencies and corresponding lower Nyquist frequencies. A layout of the bistatic radar is shown in Figure 1 for a 2-site system, with the master site on the left and a bistatic transmitter shown on the right. The heart of the radar is the Octopus transceiver card, which has both a programmable pulsing capability and eight receive channels per card. For a sixteen-element array, a second OctRec card provides eight additional channels of receive capability. At the bistatic transmit-only site, an Exciter card is used to generate the pulsed waveforms. Typically, the radar is operated with up to a 20% duty cycle pulse compressed waveform, fully programmable by the operator. For long range operation, for example, a 100-μs pulse is transmitted (forcing a 15-km blind area around the area), which is compressed to 10-μs, achieving a 20-dB pulse compression gain. This allows a 250-watt peak power pulse to be compressed to the equivalent of a 5 kilowatt pulse using frequency-modulated pulse waveforms. The receive array at the master site receives both monostatic echoes from pulses transmitted at the master site, as well as bistatic echoes from pulses transmitted at the slave site. The pulses can either be interleaved on a ping-pong like time sequence, or can be transmitted simultaneously at two different radar frequencies with non-overlapping frequency content. Additionally, one could use orthogonal phase modulated waveforms simultaneously for optimal use of frequency-time bandwidth. In the case of multi-frequency operation, more complicated antennas at both the transmit and receive end must be used to accommodate the desired bandwidth. We are developing a low-cost single frequency system that will use off the shelf antenna components to minimize cost and space requirements.
Fig. 1 System layout of a typical bistatic two-site HF radar. A multi-frequency system covering the full 3-30 MHz HF band requires wide band antennas and switchable narrow band filters to assure good signal to noise ratios for reliable ship tracking. The long-periodic array is most suitable for such a wide operating bandwidth. Operation over a smaller fraction of the HF band, as might be required for long range application in order to minimize radar propagation losses in the surface wave mode, might require only a 3-10 MHz bandwidth, for example. One can achieve this by using transmit antennas in a short two or four element transmit array, with trap antennas or multi-mode elements with several resonances that have low reflection and good standing wave ratios at several frequencies in the desired band. For receiving antennas for a multi-frequency radar, loop antennas provide good bandwidth and some directivity to minimize reception of the transmitted pulse. A photo of a multi-frequency radar using a log periodic antenna is shown below in Figure 2. Fig. 2 A wide-band monostatic radar site using a log-periodic array of monopoles of various types.
II BISTATIC SHIP RADAR CROSS SECTIONS A. Small Boat Monostatic Radar Cross section vs. Radar Frequency Experiments conducted in the 1970 s by an NRL Radar Division group that included the author demonstrated that ship and small boat targets have a RCS spectroscopic fingerprint, i.e., a unique template of RCS versus radar frequency across the 3-30 MHz HF band. Figure 3a shows an example of data from a report (Bogle & Trizna, [1]) using a commercial fishing boat target. The experiments were an attempt to assess the potential for using HF radar to detect small fishing and pleasure craft for range safety applications to missile test ranges of the west coast of the U.S.. The results were not classified, but at the time were not considered sufficiently significant for publication. However, they now provide an interesting example of a potential small-boat classification method using multi-frequency HF radar. In addition to the bulkhead, the other metal structures on the boat were the mast and fishing leader lines hanging vertically from the stowed fishing gear which could act as vertical monopole scattering elements. The mast was the tallest element at 16.6 m high, corresponding to a quarter wavelength monopole resonant radar frequency, F R, equal to 4.5 MHz. Over a perfect ground plane that the ocean surface presents at HF, an electrical image is generated so that the mast behaves as a dipole in free space. Figure 3b shows a plot of the RCS of a monopole 7.5-m high. The low frequency behavior rises as F 8-2 R, in the so-called Rayleigh region. At higher frequency, the RCS falls as F R, with additional peaks at odd integer times one quarter wavelength. For longer length monopoles, the RCS rises with the F -2 R, asymptote. Fig. 3a Small fishing boat radar cross section, bowillumination, shows a sharp resonance at 4.5 MHz due to main mast. Fig. 3b Radar cross section of a fat monopole 7.5 m high over a perfectly conducting ground plane. Other scattering elements, such as the cabin and fishing lines, will contribute to the RCS in a complicated fashion that depends upon the spacing of each possible element. For this case, the fishing leader lines were positioned on either side of the boat, and with the mast were aligned at the corners of an equilateral triangle. As this lateral spacing is not known, we do not attempt to model this specific target, but an example of the behavior of a two-element target provides an interesting comparison that illustrates the radar frequency dependence of multi-element targets. B. Bistatic RCS Model using Monopole Elements Figure 4 shows the geometry for a bistatic scattering condition for two monopoles spaced by a distance, L, seen in plan view from above. Each has a respective scattering cross section σ 1 and σ 2, and each is a function of radar frequency. The illuminator generates a surface mode plane wave along a propagation direction separated from the scattered direction to the receive array by bistatic angle, Φ B. The aspect of the target s course heading relative to the receive array is Φ A. Fig. 4 Bistatic scattering geometry, target at lower right with transmit and receive sites indicated.
Since the monopole RCS is omni-directional, the scattered fields from the two elements will sum with a phase difference, k (L 1 + L 2 ), where k is the radar wavenumber, λ/2π. A simple two-mast model based on monopoles resonant at 8 and 12 MHz and spaced 7.5 m apart was used to generate the bistatic received power as a function of bistatic angle. This results in a complex RCS as a function of bistatic angle, an example of which is shown in Figure 5. Further details on the calculation can be found in (2). Fig. 5 Bistatic RCS using 2 monopole model, resonant at 8 and 12 MHz and spaced by 7.5 m creates interference peaks and nulls with frequency and bistatic angle III. BISTATIC VHF EXPERIMENTAL RESULTS An experiment was conducted at VHF frequencies between 50 and 88 MHz to demonstrate the bistatic technology. The geometry for the radar sites setup relative to the moving target is shown below in Figure 6. Log periodic antennas were used for transmitting, and 8-element arrays of monopoles were used for reception. Range cells of 30-m length were achieved with the 5- MHz chirp bandwidth used. Fig. 6 Geometry of bistatic radar setup for VHF demonstration experiment, the target being a small metal boat. The lower radar site was located at the end of the FRF pier, while the upper radar site was on a dune 500 m north of pier.
For this experiment, each of the two transmission sites used different radar frequencies, each with 5 MHz of bandwidth. These were 78 and 85 MHz. A high pass filter with 50 MHz cutoff was used to eliminate the HF band and up to 50 MHz, and a 100 MHz A/D rate was used, resulting in frequency aliasing. Thus, 50 to 100 MHz responses plot backward as shown below. The recorded reference function for the chirped signal is shown in red, while the received spectra for both transmitter sites are shown in black. Vertical polarization was used to minimize interference to and from FM radio stations and the lower TV bands. FM spectra are shown below as narrow band spectral peaks Fig. 7 Transmitted radar spectra are shown in black, one for each of the two sites above, and other users show as narrow spectral peaks. The waveform transmitted to the power amplifier and used for pulse compression reference is in red. A Doppler spectrum for the case of the boat running at 45 deg angle to the layout in Figure 6 above produced the strongest radar echo for all aspects that were used (0, 30, 45, 60 and 90 deg). The boat used was a metal amphibious landing craft with no real mast, but a wide plate hull surface as the dominant scattering element, so little to no resonance effects were seen for several frequencies in the 50-88 MHz band. For such a target, the flat plate hull is expected to produce its strongest echo for the 45 deg course, as specular scatter occurs for this case. Fig. 8 Doppler spectra for one frequency, target receding on left and approaching on right. Note the bistatic Bragg lines, which skew inward with shorter range and extreme bistatic angles, due to longer ocean waves Bragg resonance. The Bragg lines for bistatic scatter are due to ocean wave components with wavenumber, K, given by: K = 2k cos(φ Β ) (1) k is the radar wavenumber, and Φ Β is the bistatic angle from Figure 4 above. For long ranges, the bistatic echo approaches the monostatic value. For short ranges, the bistatic angle opens up and longer ocean waves are responsible for the Bragg echo, causing the Doppler shift to decrease, as is particularly evident in the right hand spectrum of Figure 8. For HF frequencies and very short ranges, the Bragg echo is expected to drop significantly, as the ocean waves required become longer than typical ocean wave spectra support (20 100 sec wave periods). The figures above confirm this expected behavior.
Plans are in place to begin testing bistatic scatter using HF frequencies at the U.S. Army Field Research facility pictured in Figure 2 above. We plan to confirm the bistatic behavior of the Bragg echoes, and will attempt to measure ocean currents with the system. We also plan to map local shipping traffic with the system, and ultimately use multiple radar frequencies to begin investigations of bistatic ship echoes and the classification potential of such a system. The same digital radars will be used as were used for the VHF experiments, as the radars can transmit and digitize up to 100 MHz in frequency. Only antennas need be changed to support HF operation. IV. SUMMARY We have provided a description of a bistatic fully digital HF radar that can operate between 3 and 100 MHz. At HF the radar utilizes loop receive antennas and a log periodic transmit antenna for broad band operation. Rubidium clocks provide sufficient frequency stabilization to allow 2 mhz Doppler shift accuracy. GPS timing allows two sites to start operation to the typically 50-ns accuracy of GPS for the first pulse of operation, but then utilizes the rubidium clocks at each site to provide timing for bistatic operation at both HF and VHF frequencies. We have successfully demonstrated operation of a pair of these radars operating at VHF in a bistatic mode. Doppler spectra sea echoes show the expected change in Doppler shift of the Bragg lines with radar range, as longer ocean waves are Bragg resonant for short ranges and bistatic angles approaching 90 deg. Experiments conducted with a amphibious landing craft show strongest echoes at a course of 45 deg relative to both sites, at which angle specular scatter from the flat plat structures of the craft provide the dominant scattering mechanism. A simplified bistatic scattering model is presented for resonance effects from a pair of monopoles, typical of what one might expect from small fishing boats. The model does not apply for the landing craft target that was used in these experiments, so no resonance effects were observed as flat plate elements provided the strongest scattering source. Future tests are planned at HF that will attempt to track local commercial shipping traffic off the North Carolina coast. Results will be presented at our web site (http://isr-sensing.com) as well as the U.S. Army Field Research Facility web site (http://frf.usace.army.mil). REFERENCES [1] R.W Bogle, D.B. Trizna, Small boat HF radar cross sections, Naval Research Laboratory Memorandum Report 3322, July 1976. [2] D.B. Trizna, L. Xu, Target Classification and Remote Sensing of Ocean Current Shear Using A Dual-Use Multi-Frequency HF Radar, J.Ocean Eng., Vol 4, pp. 904 918, 2006.