The spaceborne wide-area surveillance mode, achieved by a modified TOPSAR mode, is proposed for maritime monitoring. In the proposed mode, the requirements of detecting targets from different aspect angles with a high revisit rate can be satisfied by modifying the TOPSAR mode which steers antenna in the azimuth direction. The feasibility as well as its application in maritime monitoring are validated by theoretical analysis and simulation experiments.
One important task of the radar system is to monitor the moving targets on the wide area at any time and in all weather conditions. In order to surveil a wider area with respect to conventional SAR/GMTI mode, the airborne wide-area surveillance mode has been developed and implemented in many real systems, such as phased array multifunctional imaging radar (PAMIR) [
Spaceborne SAR takes the advantage of large coverage and therefore promises a bright application future in maritime monitoring [
Since the coverage of the spaceborne wide-area surveillance mode is much larger than that of the airborne wide-area surveillance mode, maritime targets on broad sea area can be surveilled in a shorter time. What is more, as the radar cross section of targets and the signal-to-noise ratio (SNR) vary with different incidence angles, the capability of target detection can be improved because the targets are illuminated from different aspect angles in the spaceborne wide-area surveillance mode. In addition, the tracking of targets can also be utilized to reduce the false alarm probability.
The paper is organized as follows. Section
In this section, we introduce the airborne wide-area surveillance mode and then extend it to the spaceborne case for maritime monitoring. The system analysis of the spaceborne wide-area surveillance mode is also presented.
The airborne wide-area surveillance (WAS) mode is an effective technique to monitor wide areas using a narrow antenna beam in a scanning operation. This is achieved by steering the antenna in the azimuth direction in a cyclic way after a certain time span. Thus, different parts of the ground are scanned successively, and then a wide area can be monitored. Such a scan operation can be fulfilled efficiently with a phased-array antenna [
Although airborne wide-area surveillance mode performs well in ground traffic monitoring and battlefield surveillance, the extension to the surveillance of broader area is restricted because of its limited coverage. Therefore, the support of surveillance provided by satellites which are more able than aircrafts to cover widely dispersed areas is firstly taken into consideration to realize maritime monitoring in the paper.
In this paper, the case of revisiting the same area by antenna steering in modified TOPSAR mode is referred to as the spaceborne wide-area surveillance mode. Two manners can be considered to realize the revisit of the same area. In one manner, the beam illuminates the same area several times during a burst, and then the beam switches to another subswath; afterwards, the procedure repeats cyclically. In the other manner, the beam covers all the subswaths first, and then it revisits the same area by means of steering the antenna in azimuth (as shown in Figure
Scanning in one of the subswaths in spaceborne WAS mode with three revisits.
In the proposed spaceborne wide-area surveillance mode, the requirements discussed in Section
In this section, the system analysis is presented: first, the timeline of the proposed mode is analyzed; second, the system performance including the effect of potential grating lobes and the target detectability of the proposed mode is discussed briefly.
The basic system parameters and requirements used in the system analysis are shown in Table
The basic system parameters and the system requirements.
Parameter | Value |
---|---|
Wavelength (cm) | 3.125 |
Platform velocity (m/s) | 7558 |
Antenna length (m) | 4 |
Height (km) | 600 |
Azimuth resolution (m) | 20 |
Range resolution (m) | 10 |
Number of antenna columns | 32 |
Transmit peak power (W) | 3200 |
Receiver noise level (dB) | 4 |
Swath requirement (km) | >100 |
Number of subswaths | 3 |
Illumination repetition times | 3 |
Before the analysis of the timeline, the PRF selection, which is critical to optimize the system performance, is presented. The choice is constrained by nadir returns and transmit events, to be avoided within the sampling window time. These constraints are shown in the diamond diagram in Figure
Diamond diagram showing the setting of PRF.
To ensure that the strip coverage that can be revisited
One subswath of spaceborne WAS mode with three revisits.
Based on the previous analysis, the spaceborne WAS acquisition in slow-time/frequency domain (TFD) is represented in Figure
TFD properties of spaceborne WAS mode.
From the TFD of the proposed mode, it can be seen that the continuous strip coverage is achieved by imposing that the last target processed in the
Afterwards, in order to simplify the analysis, the switching time is ignored, and the preliminary burst length can be calculated by
The rotation rate and the shrinking factor are defined as
Therefore, the azimuth resolution can be calculated by
The implementation of a spaceborne WAS scheme requires to steer the antenna azimuth pattern (AAP) in the azimuth direction, and the sweeping is implemented by an electronic steering. In fact, the update rate demands for a phased array to be formed with a discrete number of rows/columns. The beam forming introduces grating lobes and antenna gain loss that become relevant at the large steering angles, which impacts on AASR and NESZ significantly.
In Figure
(a) AAP, assuming a steering to 3°, (b) Antenna gain loss, (c) AASR, and (d) NESZ for different number of antenna columns.
In the application of maritime target detection, the ambiguous signals may lead to false alarms, which will degrade the detectability of targets. For the spaceborne WAS mode, large steering angle, introducing grating lobes, makes the impact of ambiguous signal more severe (the worst case is shown in Figure
The impact of grating lobes with different PRF.
Based on the theory of phased-array antenna and the principle of azimuth ambiguity, the location scopes of the ambiguous signals and grating lobes can be expressed by (
Therefore, the PRF should be optimized to avoid the overlapping of
In addition, the minimum detectable target radar cross section, which is impacted by the transmit power and the receiver noise level, should be calculated to analyze the detectability of targets. The calculation is based on the radar equation:
It can be seen from (
The detection probability (a) and minimum detectable radar cross section (b) as functions of SNR.
Since spaceborne SAR is prevailing to airborne SAR in the aspects of coverage, the spaceborne wide-area surveillance mode has a great advantage in maritime monitoring. Under this mode the same area can be revisited several times by antenna steering; then targets can be detected from different aspect angles and successfully tracked. As fine resolution images can also be obtained, the candidates for maritime targets can be selected by CFAR detection, and the targets are verified by the tracking of candidates. Finally, the motion parameters of maritime targets are obtained by estimating their displacement vectors.
The motion parameter estimation will be done for each target individually. As no target has been detected so far, target candidates are selected by a CFAR processor first.
As is shown in Figure
The procedure of maritime monitoring.
The tracking of candidates.
Here, the block matching algorithm is used to obtain the displacement of parts of the image between successive images
Finally, the motion parameters of the targets that have been successfully tracked can be estimated by their displacements between successive images of the same area.
Since every target occupies several pixels, the estimation of motion parameters is different from that of point target. The block matching algorithm is utilized to make a coarse estimation first. Then the extracted targets are interpolated to obtain more accurate estimation results, and the NCFF criterion in one direction is used in the range direction and the azimuth direction, respectively.
Afterwards, the weighted average method that can get an accurate and robust estimation is utilized for the motion parameter calculation:
In the spaceborne wide-area surveillance mode, multiple images of the same area are utilized. Although the collection of multiple images reduces the azimuth resolution, the target detection capability of each image does not degrade a lot because large targets are considered in this paper. Simultaneously, compared to conventional burst modes, the proposed mode based on TOPSAR can obtain better image quality with small scalloping effect. From another aspect, the capability of detecting the same target from different aspect angles and target tracking improves the detection performance. Actually, target tracking is also an achievement of the proposed maritime monitoring method which is not only utilized to reduce the false alarm probability.
In this part, simulations are presented to validate that the system performance of spaceborne wide-area surveillance mode is acceptable without rigorous hardware requirements. The basic system parameters and requirements used in the simulations have been given in Table
Swath-dependent parameters and timeline optimized for the proposed mode.
Parameter | Symbol | IW1 | IW2 | IW3 |
---|---|---|---|---|
PRF | 4570 | 4470 | 5025 | |
Look angle (°) | 26 | 28.3 | 30.6 | |
Slant range (km) |
|
667.5612 | 681.4485 | 697.0730 |
Burst length |
|
1.1819 | 1.2070 | 1.2354 |
Echoes/burst | 5401 | 5396 | 6208 | |
AAP sweep rate (°/s) |
|
0.0886 | 0.0868 | 0.0848 |
The revisit time and the azimuth resolution degradation (since these two parameters of different subswaths vary a little, only the result of one subswath is shown) obtained with different steering angles in the critical case are shown in Figure
The revisit time (a) and the resolution degradation (b) with different revisit times.
It can be found that the performance, such as the azimuth resolution, degrades as the revisit times or the number of subswaths increases. But more subswaths are required to obtain broader coverage; meanwhile, more revisit times are needed to track the targets. The analysis above shows that an antenna with more columns can be utilized to upgrade the performances, which causes the increase of the cost and the power consumption. Thus, the selection of system parameters in spaceborne WAS mode must be compromised.
From the simulation results, it can be seen that the system requirements are possible to be achieved in practice, and the performance with corresponding system parameters, such as the minimum detectable target radar cross section, is also acceptable. Therefore, the proposed spaceborne wide-area surveillance mode is feasible and reasonable.
In the simulation, three targets with different velocities are set and other parameters are the same with the simulation above. The simulation flowchart is shown in Figure
The simulation flowchart.
As described in Section
Motion estimation results.
Azimuth velocity | Range velocity | |||
---|---|---|---|---|
Actual value (m/s) | Estimated value (m/s) | Actual value (m/s) | Estimated value (m/s) | |
Target 1 | −15 | −14.89 | 5 | 5.30 |
Target 2 | 10 | 9.63 | 6 | 6.25 |
Target 3 | 5 | 5.24 | 3 | 3.15 |
In this paper, wide area surveillance from space has been realized based on modified TOPSAR, referred to as spaceborne wide-area surveillance mode. Its application in maritime monitoring is also discussed. The capability of maritime target detection can be improved because the targets are illuminated from different aspect angles in spaceborne wide-area surveillance mode, and the tracking of targets is utilized to reduce the false alarm probability, and it also improves the performance. To verify the theoretical conclusions and demonstrate the efficiency of the proposed idea, simulations have been carried out. The results show that the spaceborne wide-area surveillance mode is feasible, and it has a big application potential in maritime monitoring.