# Highly Integrated Wideband Transmit/Receive Module for X-Band SAR Applications
## 1 Introduction  
The current trends in the development of Earth observation systems are driven by a variety of applications involving many scientific research fields [1, 2] such as weather forecast, monitoring of the Earth's surface (i.e., water and air pollution, thickness of the polar ice, etc. ), agricultural purposes, geology, urban mapping, defense, and cartography, just to mention a few of them [3, 4, 5, 6, 7, 8, 9, 10]. This trend leads to extremely challenging requirements for achieving better image resolution, finer feature discrimination, and higher penetration depth beyond ground and water surfaces; thus, more powerful instruments are required for appropriately equipping the constellation satellites [11]. This, in turn, translates into larger transmitting power, wider bandwidth, higher sensitivity, and wider steering capability of active electronic steerable antennas (AESAs) in synthetic aperture radar (SAR) systems [12, 13, 14, 15]. A key element of this type of antenna system is the transmit/receive module (TRM) [16, 17, 18, 19] that provides the appropriate amplitude and phase to the specific radiating element of the antenna array while it is transmitting and detects and amplifies the received signal while working in receiving mode [20, 21]. The TRM developed and proposed in this paper has the capability to cover the whole X-band (8-12 GHz) associated with a very high transmitted power (>20 W): this enables very-high-resolution SAR. Its quad multilayer integrationand very compact design enables high steering angles, whereas the inclusion of delay lines allows for the avoidance of squinting when scanning the whole bandwidth. Several technological challenges are embedded into the design of the proposed TRM in terms of electrical performance, due to the very wide operational bandwidth radiated power, reduced size, and thermal requirements, in order to ensure proper module functionalities while working in different thermal (high temperature variability while the satellite is facing the sun) conditions and while being affected by harsh environments characterized by mechanical stress (vibrations) [22; 23]. Although the promising compact solution proposed in [24; 25; 26; 27] based on a challenging 3D packaging technology is able to successfully fulfill the electrical requirements, the thermal handing capabilities requires a more advanced and effective design in order to extract and dissipate the thermal energy from the large TRM array when they are actively driving the entire AESA. The requirements for space technologies are very strict compared to commercial products in terms of mechanical stress, drift in the long-term performances of active devices due to the harsh environment, and lack of possibility for maintenance and electrical tuning of the module; thus, eventually, effective electrical and mechanical reliability are needed. Some of such space-compliant requirements are usually based on the European Space Agency (ESA) and military standard (MIL-STD) guidelines. Therefore, the materials and the manufacturing technologies used for building up the mechanical parts and electrical circuits can be selected on a quite limited set and they should be space-qualified in order to be able to deal with wide temperature variations, vibrations and mechanical shocks, to ensure immunity to space radiation and hermeticity in order to avoid any multipactor and corona effects. To meet these requirements, and to overcome the bandwidth and steering capabilities of the currently flying SAR solutions [28; 29; 30], a complete network is proposed for the TRM. The TRM design is based on a custom integrated core-chip based on SiGe technology and a custom integrated single-chip front-end (SCFE) in GaN technology [31] that were developed in the framework of the project ATOS [32].