**2. Spaceborne SAR antenna subsystem technical features**

Spaceborne SAR system usually consists of operation loads and function loads. The former mainly serves the spaceborne platform operation, such as the attitude and orbit control subsystem, telemetry control subsystem, environmental control subsystem, power subsystem, and so on. The function loads directly related to SAR imaging processing are usually composed of the following subsystems:

• SAR antenna subsystem

It can radiate the radar pulse signal generated by the front-end subsystem and receive the corresponding radar echo signal. The antenna needs to be able to meet the strict requirements of beam scanning and shaping for spaceborne SAR.

• Radio-frequency signal generating and processing subsystem

According to the task requirements, it generates the modulated pulse signal transmitted by the SAR antenna and down converts as well as samplings the received original radar echo signal. After analog-to-digital conversion, the original imaging

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

data are obtained, which is handed over to the rear terminal system for imaging processing.

• Signal processing and imaging subsystem

It completes the imaging and image information compression processing of the original data in a high-performance data processor.

• Data handling and transmission subsystem

It stores the image information as well as other relevant data and transmits them to the ground receiving station.

**Figure 1** shows the photo of Canada's Radarsat-2 satellite and deployable 12 m 3 m lightweight SAR membrane antenna. The long strip structure covered with gold foil below is C-band SAR Antenna.

The SAR antenna subsystem which occupies a considerable amount of mass as well as volume and consumes the most power during operation is the most difficult, timeconsuming, and costly part, due to the stringent requirements on antenna performance. In general, spaceborne SAR mainly has the following technical features:

• Wide observation strip and large observation area

The commonly observed swath width can reach 40 km or more and the length of continuous imaging area can reach thousands of kilometers. Through antenna beam scanning and satellite platform maneuver, the width of the observable area can reach 500–600 km, including more than 10 common observation strips [7].

• Far slant range and high signal transmission path loss

For example, for a spaceborne SAR system with a center frequency of 9.6 GHz, an orbit height of 600 km, and an antenna look angle of 25°–45°, the slant range

between the antenna to the observation area is 670–890 km and the two-way path loss is 168–170 dB.

• Long revisit cycle period

The satellite platform runs along the preplanned orbit and can only observe the same area once in a revisit cycle. Even if the orbit planning is reasonable and the antenna beam has good adjustment ability, the typical revisit cycle of spaceborne SAR still ranges from 2 to 3 days to more than 10 days [7, 8]. If forced orbit change is required to shorten the revisit period, additional satellite fuel will be consumed and the system life and mission duration will be shortened.

• Serious ambiguity signal interference

Spaceborne SAR system faces inherent ambiguity signal problems, including range ambiguity and azimuth ambiguity. The intensity and distribution of the ambiguous signal are affected by the width of the imaging area, pulse repeat frequency, pulse width, pulse bandwidth, look angle, backscattering coefficient, and other parameters [12]. Most of the above parameters restrict each other and are difficult to control independently. In order to reduce the ambiguity signal ratio to meet the system index requirements, it is necessary to decrease the SAR antenna pattern sidelobe to the source direction of the ambiguity signal.

• Various working modes

Currently, spaceborne SAR systems in operation or under development can generally work in Stripmap mode, ScanSAR mode, Spotlight mode, etc. Each mode has different requirements for antenna beam characteristics [13]. **Figure 2** displays the corresponding beam diagrams of five spaceborne SAR operating modes given from left to right, including huge region ScanSAR mode, wide region ScanSAR, himage Stripmap mode, polarimetric Stripmap mode, and spotlight mode. Generally

**Figure 2.** *Various working modes for spaceborne SAR.*

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

speaking, according to different task requirements, the scanning beam ranges from several degrees to tens of degrees in the range direction while the azimuth beam scans within several degrees.

### • Wide frequency band

In order to meet different mission requirements, the commonly used spaceborne SAR operating frequency band spans from P-band to X-band. Electromagnetic waves with different frequency bands have different penetrating abilities. Generally speaking, the higher the frequency, the wider the bandwidth and the higher the resolution, but the weaker the penetration ability to vegetation and other covers. Ku, K, and Ka band spaceborne SAR satellites with higher resolution belong to the next generation system under research have not been widely used at present.

### • Full polarimetric signal processing

In addition, with the development of SAR target polarimetric information processing technology, more and more spaceborne SAR systems have the ability of full polarimetric signal processing.

According to the above technical features of the spaceborne SAR system, a series of strict performance requirements of array antenna is determined as follows:

### • Narrow beam and high gain

In order to compensate for the extremely high path loss, the spaceborne SAR antenna needs to have a very high gain. Taking the X-band TerraSAR-X system launched in 2007 as an example [8], its SAR antenna in **Figure 3** size is 4.8 0.75 m and the antenna normal gain exceeds 46 dBi. At the same time, in order to suppress the ambiguous signal and improve the imaging quality, the spaceborne SAR antenna has strict restrictions on the maximum width of the main beam for range and azimuth. In addition, COSMO-Skymed's, which was launched in 2007 and works in X-band [14], antenna size is 5.7 1.4 m. The average width of the range beam is 1.5° and the average width of the azimuth beam is only 0.3°.

**Figure 3.** *Diagram of antenna structure for TerraSAR-X.*

• Beamforming and sidelobe suppression

The sidelobe level in the source direction of the ambiguity signal needs to be especially suppressed to ensure that the ambiguity signal ratio meets the system index requirements. Compared with TX pattern, which takes the maximization of effective isotropic radiation power (EIPR) as the design guide and does not particularly consider sidelobe suppression, the RX pattern in **Figure 4** has to carry out a wide range of sidelobe suppression in the main range direction of ambiguity signal source. At the same time, in order to ensure that the echo signal level in the imaging area does not exceed a certain range to ensure the imaging quality, the main beam lobe needs to be shaped and adjusted, which makes the amplitude of the pattern point to the far end for the observation area appropriately higher than that pointing to the near end, so as to compensate for echo signal attenuation caused by larger path loss and smaller ground friction angle [15].

• High EIRP of TX antenna and high G/T value of RX antenna

In order to ensure that the spaceborne SAR system can successfully complete the imaging mission, the actually received signal power and signal-to-noise ratio of the data acquisition device need to meet the minimum requirements, which involves two factors that, first, the EIPR of the TX antenna should be high enough, that is, in addition to the high gain, the actual transmitting power of the antenna should also reach a high level. On the other hand, in order to achieve a sufficient signal-to-noise ratio of the received signal, the RX antenna is required to have a sufficiently high G/T value.

• Full polarimetric mode

**Figure 4.** *Range beam pattern of typical SAR Antenna.*
