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SAR Image Mode 

image 

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 are:

Geometric specifications

Spatial resolution:  along track <=30 m 
across-track <=26.3 m 
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. 

Radiometric specifications:

Frequency:  5.3 GHz (C-band) 
Wave length:  5.6 cm 
Bandwidth:  15.55+-0.1 MHz 
Polarization:  VV 
Peak sidelobe ratio:  along track >20 dB 
across-track >18 dB 
Spurious sidelobe ratio:  across-track >25 dB 
along 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 
Cross polarisation (one way):  >15 dB 

Operational specifications:

Require ground station visibility   
Maximum operation time:  <12 minutes per orbit in total 
<10 minutes per orbit on descending passes 
<4 minutes per orbit in eclipse 
Maximum on/off switch:  6 per orbit 

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.

SAR Wave Mode

image 

The SAR wave mode provides two-dimensional spectra of ocean surface waves ( see the figure ). For this function the SAR records regularly spaced samples within the image swath. The images are transformed into directional spectra providing information about wavelength and direction of wave systems. Automatic measurements of dominant wavelengths and directions will improve sea forecast models, but the imagettes can also show the effects of other phenomena, such as internal waves, slicks, small scale variations in wind and modulations due to surface currents and the presence of sea ice. The idea of wave mode is that much useful information can be obtained from the power spectra of ocean waves. In particular, the wavelengths and directions of swell wave systems can be measured readily from spectra. Series of spectra can be used to determine the evolution of such systems. While operating in wave mode the AMI measures the change in radar reflectivity of the sea surface due to the ocean surface waves. In this mode the system operates as a SAR (see image mode, however the wave mode differs in that the RF power and hence imaging capability are reduced.

The AMI characteristics in the wave mode are the same as those of the AMI SAR in image mode, except for the following characteristics:

  • the swath width corresponds to between 9.6 km and 12 km in OBRC mode depending on position, or 5 km in OGRC , every 200 km along track
  • the data rate permits global operation
  • the swath position is programmable anywhere within the SAR imaging swath
  • the A/D quantisation is 4 bits I, 4 bits Q, in OBRC mode, and 2 bits I, 2 bits Q, in OGRC mode
  • the data can be generated in a stand-alone mode or interleaved with wind mode data.

The main technical characteristics of the AMI in wave mode are listed below:

Wave direction / length: 0-180deg. (180deg. ambiguity) / 100-1000 m
Accuracy direction / length: +-20deg. / +-25%
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 rati (point target): along track >=25 dB; across-track >=31 dB
Radiometric resolution: <=2.0 dB
Dynamic range: -12 to 3 dB
Radiometric stability: <=0.95 dB
Cross polarisation (one way):>=15 dB
Swath length: >=5 km
Swath width (with OBRC): >=9.6 km (far swath); 12 km (near swath)
Swath width (with OGRC): >=5 km
Swath position step size: <=2.5 km
Localisation accuracy: along track <=2 km; across-track <=1.8 km
Incidence angle tolerance: <=0.5 deg.

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