**1. Introduction**

In 2018, an English named Ian Wilson claimed that he had found the MH370 passenger plane which had been missing for four and a half years on Google maps and it was located in a dense forest in Cambodia. The news caused a sensation on the global Internet. Changguang Satellite Technology Co., Ltd. immediately maneuvered

all 10 satellites to observe this area one after another. Unfortunately, the first photos were disappointing. The authenticity of the rumor could not be confirmed because the suspected crash site of MH370 was covered by clouds. This dramatic event reminded people of synthetic aperture radar (SAR), because it can make up for the shortcomings of optical earth observation by the capability to pass through all kinds of obstacles such as clouds, rain, snow, fog, sand, dust, and so on and to work under all weather conditions, day and nights [1–3].

Spaceborne SAR equipped on low/medium orbit satellites or other orbital platforms such as space shuttle, the upper stage of the launch vehicle, and so on is a kind of advanced earth observing and imaging system [4–6]. It has played an irreplaceable role in the fields of global military reconnaissance, environmental remote sensing, natural disaster monitoring, and planetary exploration and gradually become a research hotspot in the field of earth observation since the United States launched the first SAR satellite SeaSat in 1978 [7, 8]. Many countries have successively carried out spaceborne SAR technology research and formulated corresponding development plans of the SAR satellite system. In the 21st century, many countries have successively deployed their own spaceborne SAR satellite systems and realized the upgrading of SAR satellites. For example, European Space Agency (ESA) launched Sentinel-1 to replace ENVISAT [9]. In recent 10 years, spaceborne SAR has made great progress in hardware structure, imaging theory, system performance, and application fields. It is worth noting that only the TecSAR system adopts the reflector antenna and the rest are phased array antenna [5]. In fact, an array antenna is the main configuration scheme for most modern high-performance spaceborne SAR, because it is preferred to obtain better performance within the limited mission budget [10, 11]. This chapter focuses on the development trend of array antenna technology of spaceborne SAR and discusses the research status in the future from the aspects of system composition, antenna performance, planar phased array antenna, paraboloid array antenna, and metasurface array antenna.
