Persistent Surveillance - of the 200 Nautical Mile Exclusive Economic Zone Using Raytheon's Land-based High Frequency Surface Wave Radar
Protecting a nation's sovereignty and effectively managing the natural resources around its coasts place new demands on the organizations tasked with surveillance and enforcement. As a general principle, the level of surveillance required at any given time depends on the perceived threat. The ideal surveillance system must be capable of normal, day-to-day operation for the lowest possible cost, yet have the integrity to allow enforcement agencies to respond decisively and economically when required.
Until recently, such a surveillance system did not exist. Now, however, shore-based high frequency surface wave radars (HFSWR) developed by Raytheon can provide persistent (continuous), cost-effective, over-the-horizon surveillance up to and beyond the 200 nautical mile limit of the exclusive economic zone (EEZ*). Surface wave radar developed by Raytheon Canada Limited, a leader in surveillance and navigation systems, is the only land-based radar with this capability.
The need for persistent surveillance is illustrated by an incident that occurred on July 31, 2011 when a 1,000 ton, general cargo vessel, the Panama-flagged MV Pavit, grounded on Juhu beach in Mumbai, India (Figure 1). It was later determined that the vessel was abandoned on June 29 near Oman. The vessel drifted into the Arabian Sea and entered India's EEZ. The MV Pavit passed undetected through one of the world's busiest shipping lanes during a heightened level of alert following the Mumbai terrorist attacks. In doing so, it passed through three tiers of security — the Navy, the Coast Guard and the Coastal Wing of the Mumbai Police — before eventually grounding. Had an HFSWR been installed, this vessel could have been observed as it entered the Indian EEZ and appropriate action could have been taken.
Surface wave radars operate in the high frequency (HF) portion of the radio frequency (RF) spectrum. As illustrated in Figure 2 the lower the frequency of operation the greater the detection range. Unlike traditional microwave radars whose detection range is limited to the line-of-sight horizon, HFSWR signals follow the curvature of the Earth due to ionospheric refraction at the frequencies in which they operate. This enables the detection of surface vessels at significantly greater ranges.
HFSWRs operate in a pulse-Doppler mode, emitting a coherent pulse train where the phase of the signal is precisely controlled from pulse to pulse. The radar's area of coverage is symmetrical around the radar's boresight, where the boresight is perpendicular to the axis of the array. Returns from beyond +/- 60 degrees in azimuth are typically not displayed due to a rapid falloff in system performance. The echoes from objects within the area illuminated by the radar are received by a linear array of antennas (Figure 3) and are digitally processed to enhance detection of the wanted echoes.
The returning echoes are sorted according to range, velocity (Doppler) and bearing. The echoes are then compared against a detection threshold chosen to achieve a certain value of constant-false-alarm-rate (CFAR). If the magnitude of an echo exceeds the threshold, it is declared a detection. A tracking algorithm associates successive detections to form a track. The ability to form tracks enables the radar sensitivity to be increased by lowering the detection threshold (thereby increasing the false alarm rate), since only those detections that are consistent with the established track are displayed. In this way, HFSWR conveys to the user only those tracks corresponding to real vessels. The track history also allows the operator to visualize the activity of the vessel and highlight anomalous or suspicious behavior.
The maximum detection range of HFSWR depends on many factors. These include the transmit power, radar frequency, radar cross section of the target, target range, background noise, and interference level as well as sea-state. Of these, only the transmit power and transmit frequency are under the control of the radar. Both the noise and interference levels are dependent on the geographic location of the radar site, time-of-day and season, and also on the level of sunspot activity.
Generally, at the lower end of the HF-band the noise level increases at dusk and remains high through the hours of darkness. Also, during the hours of darkness there are high levels of co-channel communication interference from distant HF users. As a result, the radar operates with a reduced range at night. Raytheon's HFSWR system, however, is equipped with a number of patented signal processing functions that mitigate the effects of some of these interferences. Typical performance is illustrated in Figure 4.
Since HFSWRs are tracking radars, object classification based on track history is possible. From this track data, anomalistic vessel behavior can be observed and used to heighten awareness of suspicious activity. Examples of anomalistic behavior observed by the radar are shown in Figure 5. In the first example, the vessel track was dropped and subsequently re-acquired. Analysis of the distance and time between the termination of one track and the initiation of the new track indicated that the vessel rapidly slowed down and remained stationary for approximately 10 minutes before re-embarking on its journey. The system highlighted this to the operator as being an anomaly. This behavior was indicative of a vessel that had transferred cargo to or from another local vessel. In the second example, the radar tracks a vessel that left port heading east-north-east. At a range of 70 nm from shore, the vessel, now well outside the range of traditional coastal surveillance radars, made an abrupt 90 degree course change and began to head in a north-westerly direction, which in this case was a maneuver that raised security concerns. An intercept of the vessel was undertaken.
The high frequency surface wave radar was developed by Raytheon to meet the need for persistent surveillance of the strategically significant EEZ. It was designed to convey to the user high-quality tracks corresponding to real vessels, allowing anomalous or suspicious vessel behavior to be readily identified and patrols dispatched to investigate further.
Tony Ponsford, Rick McKerracher, Adeeb Khawja
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