The ASAR (Advanced SAR) instrument derives from the AMI istrument of ERS-1 and ERS-2. It measures the radar backscatter of the Earth's surface at C-band with a choice of five polarization modes: VV, HH, VV/HH, HV/HH, or VH/VV.
Compared to ERS AMI, ASAR is a significantly advanced instrument employing a number of new technological developments which allow extended performance. The replacement of the centralized high-power amplifier combined with the passive waveguide slot array antenna of the AMI by an active phased array antenna system using distributed elements is the most challenging development. The resulting improvements in image and wave mode beam elevation steerage allow the selection of different swaths, providing a swath coverage of over 400-km wide using ScanSAR techniques.
The ASAR instrument is a phased array radar with 320 T/R-modules arranged across the antenna, such that by adjusting individual module phase and gain, the transmit and receive beams may be steered and configured.
The instrument comprises two major functional groups, the antenna subassembly (ASA) and the central electronics subassembly (CESA) with subsystems as shown in the functional block diagram. The active antenna contains 20 tiles with 16 T/R-modules each. The ASAR instrument is controlled by its instrument control equipment (ICE), which provides the command and control interface to the satellite. Macrocommands are transferred from the payload management computer to the ICE where they are expanded and queued. The ICE maintains and manages a database of operation parameters such as transmit pulse and beam characteristics for each swath of each mode and timing characteristics such as pulse repetition frequencies and window timings. The ICE downloads parameters from the database during transition to the operation mode. The ICE provides the operational control of the ASAR equipment, including the control of power and telemetry monitoring.
The transmit pulse characteristics are set within the data subsystem by coefficients in a digital chirp generator which supplies in-phase (I) and quadrature (Q) components. The output of the data subsystem is a composite up-chirp centred at the IF carrier.
The signal is then passed to the RF subsystem where it is mixed
with the local oscillator frequency to generate the RF signal
centred on 5.331 GHz. The upconverted signal is routed via the
calibration/switch equipment to the antenna signal feed waveguide.
At the antenna, the signal is distributed by the RF panel feed
waveguide network to the tile subsystems. The T/R modules apply
phase and gain changes to the signal in
Echo signals are received through the same antenna array, passing to the T/R modules for low noise amplification and phase and gain changes which determine the receive beam shape. The outputs from each module are routed at RF via the corporate feed and antenna RF distribution system which acts as a combiner, effectively adding signal inputs coherently and noise inputs incoherently.
Coherent RF/IF conversion of the RF echo signals is performed in the downconverter. I/Q detection of the IF echo signal is accomplished in the demodulator of the data subsystem. The resulting baseband I/Q signals are further processed in the data equipment, which performs filtering, digitalization, and compression of this data. After buffering and packetizing, the echo data is transmitted to the measurement data I/F.
The power conditioning unit (PCU) provides a regulated supply to the data subsystem, the RF subsystem and auxilary power to the antenna power switching and monitoring unit.