1 Experimental Evaluation of the Range-Doppler Coupling on HF Surface Wave Radars Luigi Bruno, Paolo Braca, Jochen Horstmann and Michele Vespe Abstract—High-frequency surface wave radar (HFSWR) is used in oceanography to monitor surface wind waves and cur- rents and, more recently, to detect ships in maritime surveillance. The radar accuracy is affected by range-Doppler coupling, which yields a displacement in the measured range proportional to the target radial velocity, i.e. the Doppler shift in the returned pulse. Although in oceanography this effect is usually not accounted for, its relevance grows in ship detection. In this paper we present the results of two experimental datasets showing displacements in the HFSWR range measurements of up to 300 meters and confirming the theoretical analysis. Furthermore, we show that the correction based on theoretical arguments, achieved by the statistical correlation between the range and Doppler measure- ments, provides remarkable improvement in the radar accuracy. Index Terms—High-frequency surface wave radar (HFSWR), range-Doppler coupling, ocean remote sensing, ship detection, over the horizon (OTH) radar, frequency-modulated continuous waves (FMCW) waveform I. I NTRODUCTION H IGH-frequency surface wave radar (HFSWR) is a tech- nology that is being used on an operational basis to monitor ocean surface currents [1], [2], surface waves [3], winds [4] and monitoring of tsunamis is also under study [5]. In the last 3 years the potential of using the HFSWR for maritime surveillance applications has been demonstrated [6]– [10]. In particular radar’s low transmission power (< 50 W) and low cost make it an interesting instrument for monitoring the coast up to a distance of approximately 100 km (depending on the utilized frequency). To obtain a large range with sufficient resolutions HFSWRs typically use a linear chirp as waveform to overcome the power constraints. It is well known that linear chirps are subject to significant range-Doppler coupling [11]. This coupling leads to a Doppler shift in the return pulse, due to a nonzero range rate or radial velocity (with respect to the radar), which in turn will introduce an increment in the time delay. Unfortunately, this effect cannot be distinguished from the changes due to range. For oceanographic applications such as retrieval of ocean surface wind, waves and currents, these effects can be neglected as the velocities of scatterers are small (typically ∼ 0.5 m/s) and therefore only leading to small displacements (∼ 15 m). However, in the case of measuring surface vessel velocities the range is altered by an error of the order of hundreds of meters, which, with respect to ship detection, is no longer negligible. In addition to the range-Doppler coupling, which The authors are with the Research Department, Centre for Maritime Research and Experimentation, via San Bartolomeo 400, La Spezia, SP, 19126 Italy; E-mails: {bruno,braca,horstmann,vespe}@cmre.nato.int. can be compensated [11], there is a correlation between range and range-rate measurements, which in turn leads to small range displacements that cannot be compensated. When using HFSWRs for ship detection, comparisons to the reports from the Automatic Identification System (AIS) have shown that HFSWR results suffer from high clutter and a significant amount of miss detections [6]–[10]. To improve the HFSWRs performance with respect to maritime surveillance, proper target tracking algorithms have to be incorporated [7]. For any tracking algorithm the knowledge of the correlation coefficient between range and range rate is a crucial aspect in the design of the tracker and therefore has to be considered [12], [13]. In this paper we are investigating the range-Doppler coupling effect on ship detections retrieved from HFWSR data compared to the associated AIS data. We used the HFWSR data and AIS data collected during an experiment off the Ligurian coast of Italy, which took place between May and December 2009. The HFWSR ship detections were retrieved using algorithms from the University of Hamburg, Germany [9]. Comparison between the HFWSR ship detections and AIS data clearly shows the range shifts due to range-Doppler effects, for which a compensation can then be made. The range-Doppler coupling effect due to the transmission of a linear chirp is presented in Section II, followed by a description of the Maritime Surveillance HF-Radar experiment as well as the collected data in Section III. In Section IV the range Doppler coupling effects are analyzed and interpreted with respect to the collected experimental data. Finally, in Section V conclusions are presented. II. RANGE-DOPPLER COUPLING It is well known that Doppler radars, which employ linear chirp signals, are affected by a strong coupling between the measurements of target range and range rate [11]–[13]. The linear chirp is a continuous wave signal of duration t, during which its frequency Ω sweeps linearly between the known values of Ω 1 and Ω 2 , around the center band frequency Ω 0 . Note that both cases Ω 2 > Ω 1 (upsweep chirp) and Ω 2 < Ω 1 (downsweep chirp) are feasible. Since they are orthogonal signals, they are often used to reduce interferences when two radars scan the same area. Range measurements, r b , are retrieved from the measured time delay for the signal to propagate back and forth to the receiver, Δt. Moreover, the matched filter on the receiving side adds a further delay dependent on chirp parameters [11] r b = c 2 Δt = c 2 Δt 0 - c 2 tΩ r Ω 2 - Ω 1 , (1)