**3. Technical features and development of spaceborne SAR planar phased array antenna**

Planar phased array antenna has been widely used in spaceborne SAR systems and its superior performance has been fully demonstrated and verified. **Table 1** summarizes the features and technical parameters of some spaceborne SAR antennas using planar phased array antennas.

Planar phased array antenna uses a medium-scale line array as the basic unit to form a full-scale area array. Since the scanning angle of the azimuth beam is very small, according to the basic principle of the phased array antenna, the size of the array element in this direction or the phase center spacing of the adjacent array elements can be expanded to achieve the purpose of reducing the number of control channels without reducing the number of control channels. Grating lobes appear when the beam is scanned. The scanning angle of the distance beam is relatively large, and it is necessary to ensure that the phase center spacing of adjacent units is less than 0.8 times the wavelength of the center frequency to ensure that the sidelobe performance does not deteriorate excessively during beam scanning. Therefore, the current mainstream spaceborne SAR phased array antennas usually use 10–20 minimum radiating elements (microstrip patches or waveguide slots) to form a linear array, and the excitation of each element inside the linear array cannot be controlled independently. After that, the linear array is used as the basic unit of the full-size antenna to control the amplitude and phase of each linear array. Generally speaking, there are 12–


**Table 1.** *The features and technical parameters*

 *of spaceborne*

 *SAR antennas.*

*The Present Situation and Development for Spaceborne Synthetic Aperture Radar Antenna… DOI: http://dx.doi.org/10.5772/intechopen.106040*

#### **Figure 7.**

*Conceptual schematic diagram of COSMO-Skymed phased array antenna.*

20 line arrays along the azimuth direction, and 30–64 line arrays along the distance direction. The entire array can have 300 to more than 1000 line arrays and corresponding amplitude and phase control channels, and more than 10,000 actual minimum radiation unit (microstrip patch or waveguide slot). **Figure 7** shows a conceptual schematic diagram of the COSMO-Skymed phased array antenna structure. The antenna consists of 64 (elevation plane) 20 (azimuth plane) a total of 1280 line arrays, each line array contains 12 basic radiating elements. All line arrays are arranged in the horizontal (azimuth plane) direction. Note that in order to improve the grating lobe performance, a staggered arrangement is used between adjacent linear arrays on the pitch plane, but in principle, it is still a planar linear array.

In addition, its relative bandwidth is not large. At present, the relative bandwidth of the transmitted pulses of spaceborne SAR systems using phased array antennas usually does not exceed 5%. For example, COSMO-Skymed has a working center frequency of 9.6 GHz, a maximum bandwidth of 400 MHz and a relative bandwidth of 4.2%. Sentinel-1 has a working center frequency of 5.405 GHz, a maximum bandwidth of 100 MHz and a relative bandwidth of 1.85% [4, 19]. In fact, this is also an inevitable limitation brought by the use of medium-scale linear arrays as the minimum amplitude and phase control unit system. For example, the bandwidth of a waveguide slot antenna with more than 15 slots is difficult to exceed 6% [24]. The Nova-SAR with a smaller antenna size (3 6 linear arrays, each with 24 patch elements) has the widest working bandwidth among spaceborne SAR systems equipped with microstrip patch phased array antennas and its working center frequency is 3.2 GHz. The maximum bandwidth is 200 MHz and the relative bandwidth is 6.25% [3].

However, the traditional spaceborne SAR periodic array phased array antenna still has the following shortcomings:

### • Expensive

The total price of a large number of high-performance RF active devices is so considerable that some spaceborne SAR users with urgent needs and limited budgets can only use other antenna solutions [5].

*The Present Situation and Development for Spaceborne Synthetic Aperture Radar Antenna… DOI: http://dx.doi.org/10.5772/intechopen.106040*

• Excessive total system weight and widely distributed mass

This aspect occupies a large proportion of the launch payload, but also makes the SAR satellites have a large moment of inertia and a high fuel consumption rate for maneuvering and attitude control during in-orbit operation [25–27].

• High Power consumption

When working at full power, the phased array antenna will consume most of the power supply of the satellite platform, and the peak power consumption far exceeds the power provided by the solar panels on the satellite. For this reason, SAR satellites are usually equipped with large-capacity batteries. The issue of power consumption has become one of the main factors limiting the working time of continuous imaging of spaceborne SAR. Taking the COSMO-Skymed system, which began to operate in orbit in 2007, as an example, the peak power required for SAR imaging observation in spotlight mode is 17.3 kW, and most of the energy is consumed by the phased array antenna (especially the transmitter); while the solar panels onboard the satellite can only provide a maximum of 4.5 kW (at the initial stage of the mission) to 3.5 kW (at the end of the mission). To make up for this gap, a lithium-ion battery with a capacity of 336 Ah is onboard the satellite. The battery weighs 136 kg, and the mass of the entire satellite at launch is only about 1700 kg (including fuel and propellant). Even with this power supply configuration, the COSMO-Skymed only lasts about 10 s of continuous imaging when operating in the most power-hungry spotlight mode, and only about 10 s when operating in the lower-power stripe mode. Minutes limit the information acquisition capability of spaceborne SAR to a large extent. One of the important reasons for this problem is that the transmit-receive power transition efficiency (TRPE) of the current mainstream spaceborne SAR periodic array phased array antenna is relatively low.

### • Redundant system performance

As far as the general spaceborne SAR system needs to scan and shape the antenna beam, the periodic array phased array antenna is actually in a state of excess performance. Specifically, phased array antennas that achieve the smallest antenna aperture area of spaceborne SAR are of considerable size, usually containing more than 300, or even more than 1000 independent transceiver control channels. For a phased array antenna of this size, even if up to 10% of the transceiver units fail, the excitation of the remaining units can still be adjusted to roughly maintain the main radiation performance indicators of the antenna, such as beam deflection angle, main lobe width and the sidelobe level in the ambiguous area, which is equivalent to a certain amount of redundancy in the antenna performance. In the traditional concept of spaceborne SAR engineering practice, it is generally believed that the performance of some antenna unit components will gradually degrade or fail during the operation of the mission. At this time, it is necessary to release the redundant performance of the array to ensure that the SAR antenna function can still be properly used at the end of the mission cycle [28, 29]. Function properly. However, in recent years, with the development of advanced solid-state active RF circuit technologies such as Monolithic Microwave Integrated Circuit (MMIC), Low-Temperature Co-fired Ceramics (LTCC), and spaceborne SAR systems. With the improvement of orbital operation management experience, the reliability of phased array antenna units and radio frequency components used in spaceborne SAR is increasing day by day. The number

**Figure 8.** *Phase error test results of T/R components for TerraSAR-X (2016).*

of failed units during the full mission cycle is often lower than expected during mission planning [30–33]. For example, the CosmoSky-Med (No. 1 Star) and TerraSAR-X systems [8], they were launched by ESA in 2007 and 2008 and then started operating, respectively. The original mission periods of the two satellites are only until 2012 [34, 35]. However, as of the end of 2016, it is still operating reliably in orbit, and the phased array SAR antenna carried is still in good condition. **Figure 8** shows the phase error test results of all 384 T/R components of TerraSAR-X in the eighth year (2016) in orbit. The phase errors of all channels are far less than the tolerance range of 10° (significant difference). Significant difference. It can be seen that the scale of the traditional spaceborne SAR phased array antenna has a certain compressible space under the condition of maintaining the system performance generally unchanged.

The deficiencies of large phased array antennas in the above four aspects have restricted the development and application of spaceborne SAR systems to a certain extent. Studies have shown that the above problems are expected to be improved when applying Uniform Amplitude/Equal Amplitude or Quantized Amplitude aperiodic array phased array antennas in spaceborne SAR systems [36–39]. The spacing between the elements of an Aperiodic Phased Array Antenna is usually not uniform [36, 38, 40, 41]. This is the most significant difference from the periodic array phased array. The number of aperiodic and periodic array elements of the same aperture is not necessarily the same. Generally speaking, the number of practical aperiodic array elements is less than or equal to that of the periodic array of the same aperture. When the number of aperiodic array elements is smaller than that of periodic arrays with the same aperture, it can also be called a thinned array or a sparse array. For sparse arrays, the element spacing is an integer multiple of a certain greatest common divisor; while for sparse arrays, the spacings of array elements are randomly distributed and have no greatest common divisor.

A series of studies have discussed the advantages of aperiodic array phased array antennas in reducing the number of active devices and improving energy utilization efficiency when applied to spaceborne SAR [36, 39, 42, 43] and synchronous satellite communication systems [37, 38, 44–52]. For these two types of applications, there are three things in common: (1) high-gain beams; (2) strict sidelobe control; and (3) beam scanning requirements over a relatively small angular range. In order to achieve

### *The Present Situation and Development for Spaceborne Synthetic Aperture Radar Antenna… DOI: http://dx.doi.org/10.5772/intechopen.106040*

high-gain beams, the array antenna must have a larger aperture and a larger number of elements; strict sidelobe control means that the excitation amplitude of the antenna array must be significantly tapered; the beam scanning requirement of a smaller angle makes the restrictions on the maximum spacing of the antenna elements in the medium are looser. The above three characteristics make it possible to design aperiodic array antennas excited by equal amplitude [36] or quantized amplitude [38, 53] on the basis of tapered amplitude excitation periodic array antennas. Such aperiodic array antennas naturally have the advantages of reducing the number of active devices and improving energy utilization efficiency.

A widely studied and applied aperiodic array antenna design is the density tapering (Density Tapering) constant amplitude excitation aperiodic array antenna [39, 44, 54, 55]. Compared with the Amplitude Tapering periodic array antenna used as a design reference, a well-designed density-tapered equal-amplitude excited aperiodic array antenna has roughly equivalent gain and sidelobe performance, and can be used as an active transmit antenna. All high-power amplifiers are made to work at the saturation operating point of the highest efficiency so that the DC-RF Power Transition Efficiency (DC-RF Power Transition Efficiency) is higher than that of the traditional periodic array antenna whose excitation amplitude is tapered. **Figure 9** presents the results of a typical constant-amplitude excitation aperiodic array design [39]. Among them, the upper left is the comparison of the aperiodic array element position, excitation amplitude, and the reference excitation amplitude used as a reference to taper the periodic array; the upper right is the pattern of the aperiodic and periodic array antennas, especially the sidelobe performance comparison, it can be seen that

**Figure 9.** *Example of density tapered aperiodic array antenna design results.*

#### **Figure 10.**

*Diagram of amplitude taper periodic array phased array and equal-amplitude aperiodic array phased array structure.*

the two are basically equivalent; the lower left is the variation of the DC power consumption of the aperiodic array with the number of elements, and compared with the case of the 20-element periodic array, the lower right is the variation of the Equivalent Isotropic Radiation Power (EIRP) of the aperiodic array with the number of elements and is compared with the case of the 20-element periodic array. Compared with the 20-element periodic array, it can be seen that when the number of elements is between 14 and 18, the aperiodic array consumes less DC power than the 20-element periodic array and obtains a higher EIRP.

Compared with periodic array antennas, aperiodic array antennas have very different characteristics. As shown in the right of **Figure 10** [39], firstly, the analysis of the array characteristics can no longer be simplified to the processing of one element in the periodic boundary; secondly, the characteristics of each element in the aperiodic array are quite different; finally, the processing and Test work is also much more difficult and complex than periodic arrays. Despite the above difficulties, the research on aperiodic array antennas has still received extensive attention. In addition to the aforementioned advantages of reducing the number of radiation units or control units and improving the DC-RF conversion efficiency, the aperiodic array actually increases the degree of freedom in the design of the array antenna, so it is expected to obtain a more ideal array design result. Characteristics are also one of the reasons why it is studied.

In recent years, a group of European research institutions funded by ESA has carried out research on spaceborne aperiodic phased array antenna experimental systems mainly serving satellite communication systems. In 2010, a team of researchers from Naples University, Università di Cassino and Space Engineering S.P. A. reported the results of their preliminary development of an experimental system for satellite communications aperiodic array antennas. The designed aperiodic array antenna is composed of various aperture units, as shown in the left in **Figure 11**. The test results prove that the antenna design has high aperture efficiency and global beam coverage. The same group of researchers also completed the design of a multi-beam aperiodic dielectric lens antenna based on a similar theoretical design, as shown on the right in **Figure 11**. The experimental test results show that the performance of the designed aperiodic array antenna is basically the same as that of the periodic array antenna used for comparison, and the number of control devices is significantly reduced [37].

*The Present Situation and Development for Spaceborne Synthetic Aperture Radar Antenna… DOI: http://dx.doi.org/10.5772/intechopen.106040*

#### **Figure 11.**

*ESA satellite communication aperiodic array antenna experimental system hybrid unit direct radiation array and test results (left), aperiodic reflection array and test results (right).*

**Figure 12.** *Aperiodic sparse array phased array antenna experimental system for satellite communication on the ground.*

In 2014, Maria Carolina Viganó of ViaSat Antenna Systems in Switzerland and others completed the research and development of the ground-side aperiodic sparse array phased array antenna experimental system for satellite communication with the special support of ESA [45]. The system works in the Ka-band and adopts 1-bit quantization phase control, and its design is based on a novel sunflower arrangement, as shown in **Figure 12** [56]. The left picture shows the array topology and the distribution of the feeding network, and the right picture shows the finished experimental sample. The test results show that the antenna has reached the basic design index and has good development potential.

The application of aperiodic array phased array antenna in spaceborne SAR systems began in the 21st century. ESA and the National Aeronautics and Space Administration (NASA) have funded a number of research institutions to carry out research on Spaceborne SAR sparse array or aperiodic array phased array antenna, and have made some research progress in array synthesis technology, aperiodic array imaging technology, and hardware implementation scheme.

In terms of aperiodic array synthesis technology for Spaceborne SAR requirements, ESA and Cristian luison and Stefano selleri of the University of Florence in Italy published an important research paper on the application of aperiodic array phased array antenna to spaceborne SAR in 2012 [36]. This paper presents a practical method for Spaceborne SAR aperiodic array synthesis, which is divided into three steps: firstly, according to the task requirements of spaceborne SAR, the synthesis of a single excitation amplitude distribution pattern based on the traditional periodic array is completed; secondly, according to the results of the amplitude distribution of periodic array, the aperiodic sparse array is obtained by amplitude density taper transformation; finally, the element position and excitation phase of the sparse array are optimized to make the final design results meet the beam performance requirements. As shown in **Figure 13**, the upper part of the figure shows the three-step design process and the schematic diagram of array element distribution obtained in each step. Starting from the initial 64-element periodic array, a 48-element linear array with constant amplitude excitation is obtained after the second step of thinning, and then the final aperiodic array unit position and excitation phase distribution are obtained through the third step of array element position optimization. The lower part of **Figure 13** shows the patterns generated by the designed aperiodic array to meet the beam requirements of two different spaceborne SAR. Both patterns are generated by

*The Present Situation and Development for Spaceborne Synthetic Aperture Radar Antenna… DOI: http://dx.doi.org/10.5772/intechopen.106040*

**Figure 13.** *Results of spaceborne SAR aperiodic array antenna.*

the same aperiodic array with equal amplitude excitation, and only the excitation phase distribution is different.

Although it can significantly improve the system performance in theory, the engineering realization of aperiodic array antenna is very difficult, and the development of related technologies is not mature. Therefore, from the perspective of ensuring system reliability and avoiding technical risks, the conditions for applying aperiodic array phased array antennas in spaceborne SAR systems with high R&D costs and a small number are not yet mature.
