High frequency radar
High frequency (HF) radar, which is based on surface electromagnetic wave propagation, provides a unique capability to detect targets far beyond the conventional microwave radar coverage. HF radars use the frequency band of 3-30 MHz to provide a large coverage that could extend to 200NM in range. This large range of values is of great interest, and appears as a consequence of the United Nations Convention on the Law of the Sea, which established 200NM as the Exclusive Economic Zone (EEZ).
Regular maritime surveillance of activity within a nation’s EEZ is a key question in protecting national security. HF radar systems recently became an operational tool in coastal monitoring worldwide for many applications including ship detection, tracking, and guidance; as well as search and rescue, distribution of pollutants, fishery and research in oceanography.
WERA (WEllen RAdar) HF radar system
The WERA (WEllen RAdar) HF radar system was developed at the University of Hamburg, Germany in 1996. WERA allows a wide range of working frequencies, spatial resolution, and antenna configurations in order to operate as a low-power oceanographic radar, providing simultaneous wide-area measurements of sea surface currents, waves and wind parameters. The WERA system transmits signal using an average low power of 30 Watts, but can achieve a detection range of 110NM, which is far beyond conventional microwave radar coverage of up to 30NM.
The HF radar uses a frequency modulated continuous wave mode for range resolution, hence the transmitter and the receiver are operated simultaneously. This mode allows a blind range in front of the radar to be avoided, simplifying the radar range resolution modification, and reducing the impact of radio interference. The azimuth angle covered by WERA is ±60° perpendicular to the linear array of antenna receivers, which consists of 16 antenna elements located linearly along the coast as shown in Figure 1.
Selecting the appropriate radar frequency and bandwidth
The attenuation of the electromagnetic wave traveling along the sea surface depends on the radar frequency, and on the conductivity (salinity) of the water. These factors determine how large the observable area is. However, the radar frequency is selected taking into account the use of the radio spectrum by other communication services. The values for the highest possible range resolution of the radar system are limited by the available chirp signal bandwidth (the width of the gaps in the radio spectrum). To find the optimum radar operating frequency and bandwidth, frequency scans are started regularly. For example, the operating frequency band of the WERA system is 8 MHz with a typical bandwidth of 100 kHz; it gives a 1.5-km range resolution cell. Every hour the operating frequency is selected adaptively according to the scan of an HF spectrum load. Depending on the radar operating frequency, the total HF radar coverage area occupies from 10,000 to 80,000km2 of sea surface.
Useful implementations of a WERA system
WERA is based on a modular design that can be easily adopted to the requirements of an actual application. Most of the signal processing steps are implemented in software and thus the system can be adapted to different needs in a simple way – for example, simultaneous oceanographic research, ship traffic monitoring, tsunami warning, and so on.
The dominant contribution for HF radar echoes is due to scattering from the sea surface. Since sea waves are moving targets, they cause Doppler frequency shifts while moving towards and away from the radar site. That is why this radar system is a tool for synoptic online mapping of sea surface current fields and spatial distribution of the wave directional spectrum simultaneously.
Using the WERA system to monitor ship traffic
This paper describes a new approach to utilize oceanographic low-power HF radar in order to monitor ship traffic over long distances. For ship detection and tracking procedures the sea clutter can be considered to be an unwanted, self-generated interference. This interference level can ultimately limit the detection capability of the radar system. To evaluate the quality of radar detection, a data set of GPS-acquired ship locations, provided by the Automatic Identification System (AIS), was recorded for the same period of time.
Due to external noise, radio frequency interference and different kinds of clutter, special techniques of ship detection using the WERA system have to be applied. In the case of ship monitoring, the main interference contribution for the WERA system is due to the sea surface echo signals. In a range- Doppler frequency map the sea surface reflections are observed permanently, and limit the target detection performance. The technical challenge is to detect targets in this strong interference environment and to control the false alarm probability.