Instrument Description - Sentinel-1 SAR Technical Guide - Sentinel Online
SAR Instrument Description
SENTINEL-1 satellites carry a single payload consisting of a C-band Synthetic Aperture Radar (SAR) instrument. The instrument is composed of two major subsystems:
- the SAR Electronics Subsystem (SES)
- the SAR Antenna Subsystem (SAS).
The radar signal is generated at base band by the chirp generator and up-converted to C-band within the SES. This signal is distributed to the high power amplifiers inside the Electronic Front End (EFE) Transmit/Receive Modules (TRMs) via the beam-forming network of the SAS. Signal radiation and echo reception are performed with the same antenna using slotted waveguide radiators. When receiving, the echo signal is amplified by the low noise amplifiers inside the EFE TRMs and summed up using the same network as for transmit signal distribution. After filtering and down-conversion to base band inside the SES, the echo signal is digitised and formatted for recording.
SAR Electronics Subsystem (SES)
The SES forms the core of the radar instrument connecting the SAS for transmission of Tx pulses and receiving of backscattered pulses from ground targets. The SES provides all radar control, IF/RF signal generation and receive data handling functions comprising:
- radar command and control, timing control, and redundancy control
- transmit chirp generation, frequency generation, up-conversion/down-conversion, modulation/demodulation and filtering
- digitisation, data compression and formatting.
A digital chirp generator and selectable receive filter bandwidths allow an efficient use of on board storage capacity considering the ground range resolution dependence on the incidence angle. The radar signal is generated at base band by the chirp generator and up-converted to C-band within the SES. This signal is distributed to the high-power amplifiers inside the EFE TRMs via the beam-forming network of the SAS. Signal radiation and echo reception are performed with the same antenna using slotted waveguide radiators. When receiving, the echo signal is amplified by the low noise amplifiers inside the EFE TRMs and summed up using the same network as for transmit signal distribution. After filtering and down-conversion to base band inside the SES, the echo signal is digitised and formatted for recording. Flexible dynamic block adaptive quantisation allows the efficient use of on-board storage and to minimise downlink times with negligible impacts on image noise. The SES hardware comprises the following units:
- Integrated Central Electronics (ICE) unit
- Mission Dependent Filter Equipment (MDFE)
- Transmit Gain Unit (TGU).
The ICE unit is the principal module of the SES providing the radar with its core functionality, control and monitoring. The subsystem uses a fully digital design approach for both the derivation of the C-band chirped radar signal and the digital receiver which samples the echo signal at an IF close to base band. The ICE maintains and manages a database of operational parameters such as transmit pulse and beam characteristics for each swath of each mode, and timing characteristics like pulse repetition frequencies and window timings. The MDFE is passive, providing a set of RF filters for the Tx path (to control out-of-band transmissions) and for the Rx path (to limit out-of-band interference). The TGU provides the final RF amplification of the Tx pulse signal before sending it to the SAS. The TGU receives its own dedicated primary power supply from the platform. Switching the TGU on and off is performed by the ICE.
SAR Antenna Subsystem (SAS)The SAS is responsible for:
- signal radiation and reception
- distributed transmit signal high power amplification
- distributed receive signal low noise amplification with LNA protection
- signal and power distribution (corporate feed, power converter)
- phase and amplitude control, including temperature compensation
- internal calibration loop
- deployment mechanisms, including hold down and release
- antenna mechanical structure.
The instrument is based on a deployable planar phased array antenna carrying TRMs allowing for horizontal and vertical polarisations. The dual polarised antenna allows either transmission in one single but selectable polarisation (H or V) or, simultaneous reception of both H and V polarisation, at any time. The C-SAR antenna comprises two wings, stowed on the platform's lateral panels during launch, which are deployed once in orbit. Each antenna wing consists of two antenna panels. An antenna panel consists in principle, of a panel frame and a number of antenna tiles. The SAS central panel comprises two antenna tiles, whereas the wing panels comprise three antenna tiles each. The complete antenna is symmetrical around the middle of the central panel.
The active phased array antenna is capable of performing rapid electronic beam steering, beam shaping and polarisation selection, providing fast scanning in elevation and azimuth to cover the large range of incidence angles and to meet the image quality requirements for the TOPSAR mode. TRMs are arranged across the antenna such that, by adjusting the gain and phase of individual modules, the transmit and receive beams may be steered and shaped.
The SAS consists of 14 tiles with 20 dual-polarised sub-arrays on each tile. Each sub-array is a dual-polarised unit with two parallel slotted resonant waveguides. The vertical polarisation is excited by offset longitudinal slots in a ridge waveguide, while the horizontal polarisation is generated by transverse narrow wall slots excited by inserted tilted wires.
A SAS tile is composed of 10 'Waveguide 4' assemblies (two vertically and two horizontally polarised waveguides), which form the smallest building block in the tile manufacturing.
The instrument comprises three RF networks: Tx network, RxH (H polarisation) network and RxV (V polarisation) network. During radar operation the Tx network carries the transmit RF signal and the RxH/RxV networks carry the echo signals. The dual polarised antenna allows transmission in one single but selectable polarisation (H or V) and simultaneous reception of both H and V polarisation.