Near-space slow SAR high-resolution wide swath and sustained imaging technology
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Sustained high-resolution imaging for wide swath area is getting emergence. However, conventional airborne and spaceborne SAR can hardly achieve them simultaneously. Near-space technology draws much attention in this aspect. Current researches of near-space SAR focus on fast platform which encounters contradiction between azimuth high resolution and range wide swath. To solve it and achieve sustained imaging, low-speed SAR is introduced in this article. Joint aperture technology is then proposed to make full use of the redundant pulse repetition frequency and obtain wide area imaging fast. Performance of the example system given at the end of the article verifies the effectiveness of the proposed near-space low-speed SAR high-resolution wide swath and sustained imaging concepts.
KeywordsNear-space Low-speed High-resolution Wide swath Sustained Joint aperture technology
High-resolution wide swath (HRWS) and sustained imaging is of great significance in many fields, like public security, disaster warning, etc. Conventional airborne SAR is capable of high-resolution narrow swath imaging while spaceborne on the contrary[1, 2]. Also, it is impossible of sustained imaging for both of them due to revisiting frequency.
Current researches of near-space SAR focus on high-speed platforms[5, 6, 7, 8]. However, it’s inherent defect of contradiction between azimuth high resolution and rang wide swath cannot be solved by conventional technologies. Multichannel- or multiaperture-based techniques[2, 5, 6, 7, 8, 9, 10, 11] are introduced to solve it. However, multiaperture in platform drives whole system more complex and heavy. And also it has other channel matching problems.
In recent news, a new concept of low-speed platform is coming into sight. From the strategic point of view, low-speed could be sorted into two types: airship and balloon[12, 13, 14]. As Figure 1b shows, typical airship systems are the DARPA’s high-altitude airship (HAA), Integrated Sensor Is Structure (ISIS) Unmanned HALE airship. Also the Blue Devil is developed for battle reconnaissance in Afghanistan. Meanwhile, balloon slow-speed system is also developed. Near-space diameter balloon with 400-ft. structure is on developing in Spain and it could float 36 km high to take a glimpse of the curvature of earth. Even the India’s army took hand on it and developed the Akashdeep Aerostat.
So in this article, we present the design of a near-space low-speed SAR (NSLS-SAR), and its imaging strategy. In general, the NSLS-SAR adopts balloon and airship to balance its own gravity, so energy is saved for devices. Sunlight in near-space could then replenish energy consumption and ensure the sustained imaging.
Most importantly, low-speed brings redundant samplings in spatial domain which can hardly be obtained in high-speed system. Redundant sampling then solves the contradiction between azimuth high-resolution and range wide swath while keeps conventional radar system. However, redundant sampling in spatial domain brings time expansion and takes much longer time for the same azimuth size. This is called time space exchange. Meanwhile, in order to overcome the problem of long synthetic time, new concept of joint aperture technology (JAT) is firstly proposed for fast imaging in this article.
The remainder of this article is organized as follows. Section 2 introduces capability of near-space in high-resolution, wide swath imaging and figures out current researches as well as challenges encountered. Section 3 proposes low-speed SAR and discusses the idea of time space exchange, new issue of long synthetic time. JAT that solves this new issue is then proposed in Section 4. Details of JAT and following assembling process also described. The performance of an example system verifies the effectiveness of the proposed concepts in Section 5. Finally, Section 6 concludes the article.
Current near-space SAR and problems
where c and θ0 denote the speed of light and the incidence angle, respectively. That is to say a low PRF is favorable to unambiguously image a wide swath on the ground.
where υ is the speed of the platform, and B a denotes the doppler bandwidth.
Generally speaking, c/v is nearly constant at 20,000 for spaceborne SARs and typically in range of 3,00,000–750,000 for airborne SARs. In contrast, near-space platforms can fly at a speed ranging from nearly stationary to 1500 m/s, then the corresponding c/v will be greater than 100,000. Thus, compared with spaceborne and airborne SARs, near-space SAR can provide a more flexible choice between azimuth resolution and swath width for satisfy the high-resolution and wide-swath imaging.
where λ is the radar wavelength and R c is the slant range from the radar to the midswath.
In reported materials, researches of near-space SAR focus on high-speed platform[5, 6, 7, 8]. From Equation (4) we can get that, with regard to the high-speed platforms, the minimum antenna area of Aantenna will relatively be large. However, in order to achieve the HRWS imaging, the antenna area should be small. So, as to alleviate the requirements imposed on the minimum antenna area for the high-speed platform, several multichannel- or multiaperture-based techniques, such as displaced phase center antenna technique, digital beamforming technique[5, 6, 7, 8], viz., have been proposed.
Nevertheless, multichannel- or multiaperture on receiver greatly increases the complexity of the whole system both in hardware and software, such as requiring additional complex signal process before imaging process to extract expected signal. This not only complicates the system design, but also brings weight burden for platform.
To address these problems for high-speed platforms, we present the design of NSLS-SAR, and its imaging strategy in the following sections.
Low-speed platforms operate at speed of 0–30 m/s which is dozens of times smaller than high-speed SAR. Because of this special speed, from Equation (4) we can get that NSLS-SAR could adopt energy-efficient platform and solve restrictions above while keeps conventional radar structure. For example, assume the speed of the platform is 30 m/s, the azimuth resolution is 0.5 m, then the Doppler bandwidth is 60 Hz, so the PRF should be larger than 60 Hz. And also we assume the large rang width is 200 km, then we can get the PRF should be smaller than 750 Hz. We can see that the two restrictions are not contradictory. So, NSLS-SAR can avoid the problems for high-speed platforms. Meanwhile, because of the low-speed, it can also achieve the sustained imaging as explained in the following sections.
Low-speed SAR uses airship or balloon as its platform while high-speed SAR uses vehicles. Vehicles are born with defect of short air-staying time while airship and balloon are perfect of suspension in the air. Moreover, sunlight is plenty in near-space which enables energy replenishing. Wind direction is stable for sustained imaging. In this way, low-speed SAR could operate months and even years in certain area. So, NSLS-SAR can achieve the sustained imaging.
Time space exchange
Conventional SAR encounters contradiction between wide swath and azimuth resolution because of PRF limitation. For low-speed SAR, however, possess flexible vehicle speed that ranges from zero to 30 m/s typically. This distinctive feature allows couples times of PRF extension. When the platform speed cuts down to two-thirds as before, the azimuth bandwidth for the same resolution also cut down to two-thirds. Finally, PRF requirement cut down the same percent.
We define a factor G to represent the ratio of reference velocity to actual velocity. Low-speed SAR holds a G value of over 50 with reference of high-speed SAR. Thus, allows over 50 times lower PRF requirement. In this case, conventional contradiction is out of existence and even brings a redundant characteristic in PRF.
This could be called as time space exchange. Redundant sampling in time domain is obtained at the cost of synthetic length in spatial domain. However, new issues short synthetic length means short surveillance coverage in azimuth direction which is greatly adverse to fast imaging and real-time monitoring.
In general, PRF is several times larger than the azimuth bandwidth of NSLS-SAR, viz. the PRF is redundant. In order to deal with the long synthetic time caused by the low-speed, we will make full use of the redundant PRF and propose the JAT.
The main idea of JAT is mapping one aperture to various ground area to accomplish large azimuth area surveillance. In JAT, certain large area is divided into several parts with overlapping area between neighbors, and the azimuth width of the overlapping area is one synthetic aperture length, which will be used to assemble the multi-images. Radar periodically changes its line-of-sight (LOS) angle to illuminate each part of the area. Under this circumstance, multi-imaging areas share the same aperture during synthetic time and large imaging areas are generated fast. After imaging process, register adjacent part and assemble the whole image. We will discuss in more detail of the JAT in the following section.
Target area divide
where rect[·] is the fast-time envelope, T r represents the pulse duration time, τ is the fast time variable, K r is the FM rate, and f r is the carrier frequency.
where t1 ∈ PRI [1 4 7 10⋯] is the azimuth time variable, and. The echo of LOS 1 is squint back-looking mode, and then it will be processed by the squint back-looking SAR processing method.
where t2 ∈ PRI [2 5 8 11⋯], and. The echo of LOS 2 is boresight mode, and then it will be processed by the boresight SAR processing method.
where t3 ∈ PRI [3 6 9 12 ⋯], and. The echo of LOS 3 is squint forward-looking mode, and then it will be processed by the squint forward-looking SAR processing method.
After different LOS echoes are processed by independently, the multi-images will be received.
After the processes above, images from different LOS angle are independently obtained. In order to get whole image of wide swath area, the multi-images assembling technology based on image registration (MIAT-IR) will be proposed in this article. This MIAT-IR includes two main steps.
First, image registration technology will be used in order to assemble the adjacent regions.
Second, remove the overlapping area that shares only part of whole aperture and assemble different parts together. Then the whole image of the large area has been obtained.
Performance of an example system
NSLS-SAR example system parameters
Minimum slant range
Maximum slant range
Azimuth imaging boundary
−45 to 45 km
Number of Joint aperture
Pulse duration time
Range sampling frequency
The cost time to get the echo of the whole imaging area using the proposed JAT is 11 min. While the required time of the traditional stripmap mode is 99 min, which is nine times large as that of the proposed JAT in this article.
In order to achieve the HRWS, sustained imaging, and overcome the inherent defect of contradiction between azimuth high-resolution and rang wide swath for the high-speed platform, NSLS-SAR and its imaging strategy are proposed in this article. The first advantage of the proposed NSLS-SAR is that it can avoid the contradiction for high-speed platforms. At the same time, because of the low speed, it can also achieve the sustained imaging. While low-speed brings out the long synthetic time problem, then in order to deal with the slow imaging speed, the redundant PRF has been made full use of and the JAT is proposed to shorten the time needed for the large area imaging. Performance of the example system has verified the effectiveness of the proposed NSLS-SAR HRWS and sustained imaging concepts.
The study was supported by the Pre-research Fund (no. 9140A07020412DZ02084) and the Pre-research Support (no. 62501023009).
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Open AccessThis article is distributed under the terms of the Creative Commons Attribution 2.0 International License (https://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.