# SAR Image Mode

In image mode the SAR provides high resolution two-dimensional images with a spatial resolution of 26 m in range (across track) and between 6 and 30 m in azimuth (along track). Image data is acquired for a maximum duration of approximately ten minutes per orbit As the data rate is too high for on-board storage it is only acquired within the reception zone of a suitably equipped ground receiving station (see Chapter Ground Segment Facilities). The main characteristics of the AMI in image mode are:

Spatial resolution: along track <=30 m;
across-track <=26.3 m
Peak sidelobe ratio: along track >=20 dB;
across-track >=18 dB
Spurious sidelobe ratio: along track >=25 dB;
across-track >=25 dB
Integrated sidelobe ratio: >=8 dB
Ambiguity ratio: along track >=20 dB;
across-track >=31 dB
Radiometric resolution: <=2.5 dB at sigma-zero = -18 dB
Dynamic range: >=21 dB
Radiometric stability: <=0.95 dB
Crss polarisation (one way): >=15 dB
Maximum operation time: 10 minutes per orbit
Swath width: 102.5 km (telemetered)
80.4 km (full performance)
Swath standoff: 250 km to the right
of the satellite track
Localisation accuracy: along track <=1 km;
across-track <=0.9 km
Incidence angle: near swath 20.1deg.,
mid swath 23deg.,
far swath 25.9deg.
Incidence angle tolerance: <=0.5 deg.

The rectangular antenna of the SAR is aligned along the satellite's line of flight to direct a narrow beam sideways and downwards onto the Earth's surface (see the figure) to obtain strips of high resolution imagery of about 100 km in width. Imagery is built up from the time delay and strength of the return signals, which depend primarily on the roughness and dielectric properties of the surface and its range from the satellite.

The SAR's high resolution in the range direction is achieved by phase coding the transmit pulse with a linear chirp and compressing the echo by matched filtering; range resolution being determined by means of the pulse travel time; and the azimuth esolution is achieved by recording the phase as well as the amplitude of the echoes along the flight path.

During operation, the command to generate a radar pulse is initiated by the SAR processor . A `short pulse' signal is generated and is used as the input to a dispersive delay line (Surface Acoustic Wave (SAW) device) which produces the linear Frequency Modulated (FM), or 'chirped' pulse . This chirp signal is generated at a programmable Pulse Repetition Frequency (PRF) in the range 1640-1720 Hz and is passed to the transmitter and up-converter , where it is mixed with the local oscillator signal (5176 to 7442 MHz), produced within the Frequency Generator . The level of the RF output pulse (peak power) is controlled by the Automatic Gain Control (AGC) loop. This output is amplified by approximately 45 dB by the High-Power Amplifier sub-system before it is routed via the waveguide through the Circulator Assembly to the SAR antenna.

The echo signal received by the SAR antenna is routed via the waveguide through the Circulator Assembly to the receiver. The received radar echo is then amplified in a Low-Noise Amplifier and mixed with the local oscillator signal to provide a signal at the intermediate frequency as input to the IF Radar . The echo signal is sent to the SAR processor where it is resolved into in-phase and quadrature components. The nominal operation mode is an on-ground range compression (OGRC) of the received pulse, providing complex samples with 5 bits in-phase (I), 5 bits quadrature (Q), while an on-board range compression (OBRC) mode provides complex samples of 6 bits I, 6 bits Q. The ground processing requires auxiliary data (chirp replica, noise measurement, calibration pulse), in addition to the radar echo, in order to produce the required image.

## Active Microwave Instrument

  AMI Technical Details
  SAR Image Mode
  SAR Data Products

ESA Earth Observation