Target Monitoring Using Small-Aperture Compact High-Frequency Surface Wave Radar Figure 3. R-D-T data. Figure 2. Area target in the R-D image for HFSWR. (a) CFAR detection and peak detection (+) result. (b) Area target and point target. PROPOSED TARGET TRACKING Based on the area target characteristics of moving targets in R-D images, multiple dimensions of range-Doppler-time (R-D-T) data can be constructed when multiple continuous-frame R-D images are taken into account. Figure 3 shows an example of constructed R-D-T data. Both the target spatial position and the echo intensity are continuously distributed over time, whereas the clutter noise is distributed randomly. Target detection for HFSWR can also be extended in the time dimension to realize integrated detection and tracking, as shown in Figure 4. From any two-dimensional section of the R-D-T at a given range or Doppler cell, the target trace can be clearly observed. The range-time (R-T) image can illustrate the range change relative to the radar site. Figure 5 shows an example R-T section at a given Doppler bin with a positive value. The stripes in the image represent target traces in the R-T dimension. Several target traces are clearly visible and show that the targets were approaching the radar station. Long straight stripes indicate that the target maintained a constant velocity for a long time. In most cases, short stripes indicate that the radial velocity of the target changed rather than indicating that target movements were short or broken. In contrast to R-T, Doppler-time (D-T) images can describe the change of the target's radial velocity over time at a given range cell. In a D-T image, the trace appears for a short time and then moves to the previous or next range cell, even if the target radial velocity remains unchanged. Figure 6 shows a D-T image at the 65 range bin. Figure 7 presents R-T and D-T images showing two moving targets. Although they are close in the R-T image, the two targets can be distinguished from each other in the D-T image. Figure 4. Schematic diagram of the integrated monitoring method. 24 IEEE A&E SYSTEMS MAGAZINE MARCH 2018