3.3.3. Synthetic Aperture Radar (SAR) Altimeter

SAR altimeter differs from real aperture radar altimeter (conventional) in that it exploits coherent processing of groups of transmitted pulses, while conventional altimeters is exploited to make the most efficient use of the power reflected from the surface. The SAR altimeter offers many potential improvements over conventional altimetry for measurements, since it increases the resolution and offers multilook processing.

Currently, three mini-satellites are dedicated to altimetry with SAR processing, such as SARAL, Sentinel-3A, and Sentinel-3B. The planned missions are Sentinel 6 (Jason-CS). Table A12 summarizes the main characteristics of radar altimeters with SAR processing. Typical requirements are similar to the conventional altimeters for mini-platform: 100 W average power consumption, 1.2 m antenna diameter, 63 kg payload mass.

The geophysical variables of interest to analyze with SAR altimeter are ocean surface currents, significant wave height, dominant wave direction, sea ice cover, sea ice type, sea ice thickness, and horizontal wind speed over the sea surface.

## 3.3.4. Synthetic Aperture Radar (SAR) Imager

Spaceborne SAR imager sensors have been widely used for ocean monitoring (e.g., sea-ice cover, oil spills monitoring, sea-ice type, wave direction, dominant wave period, sea level, etc.), and land applications (e.g., soil moisture indices, vegetation monitoring, classification, fire fractional cover, fraction of vegetation over land, landslides and motion risk assessment, permafrost, and others) to support the environment management, with resolutions comparable to those of optical systems. The manufacturing and implementation related to a small SAR satellite mission have opened a market for a new technology which has recently been developed: the constellations of small SAR satellites, being the principle of Fractionated and Federated Satellites (FSS) [71], and/or bistatic SARs as companion satellites (e.g., SAOCOM [72]).

The use of SARs imager in small satellites poses some major challenges, such as the antenna dimensions and power requirements of the system. Another challenge is how to generate the power required by this sensor, reducing the transmitted power, resulting in a narrow swath and therefore increasing the revisit time. In this line, SARs are now feasible in small platforms—for example, NovaSAR-S [73] and ICEYE's Synthetic Aperture Radar [74]. NovaSAR-S is a novel platform for small synthetic aperture Radar (S-band) development by Surrey Satellite Technology Ltd. (Guildford, United Kingdom), with a mass of 500 kg and peak power of 1.8 kW. The antenna is a microstrip patch phased array with size of 3 × 1 m. ICEYE's Synthetic Aperture Radar is a microsatellite developed by ICEYE, with a satellite mass of 100 kg, and phase array antenna at X-band. According to the frequency band of the SAR, beyond 2028, there will be no X-band SAR mission in orbit, but there will be L- and C-band SARs mission (Figure 2). On this subject, the frequency band selected for SAR instrument is X-band, in order to obtain a smaller instrument and cover the frequency gap.

The geophysical variables of interest to analyze with SAR imager are iceberg tracking, sea ice cover, sea ice type, sea ice thickness, sea ice drift, sea ice extent, wind speed, ocean surface currents, dominant wave direction, dominant wave period, wind speed, and significant wave height. Nevertheless, single, large SAR satellites are not compatible with the requirements of 3 h of revisit time. Constellations of small SAR Satellites are under development or implementation stages [74]. In contrast, large SAR Satellites have been in orbit for years. Small SAR satellites can replace large SAR, for some specific applications requiring medium resolution imagery and smaller areas covered (due to power limitations). If the frequency band is higher (X-band), the spatial resolution and swath wide can be adjusted, therefore reducing the size and mass of the system. Table A13 presents a survey of the representative SAR image missions and classifies each instrument into mini or large according to capabilities of commercial platforms surveyed in the previous chapter.

**Figure 2.** Frequency bands of future (2020–2030) European Union (EU) mission carrying Synthetic Aperture Radar (SAR) imager instruments.
