Key Facts - ERS

Basic principles of radar imaging

• Microwave sensors register images of the earth in the microwave region of the electromagnetic spectrum.
• Two modes of Microwave sensors exist, the active and the passive modes.
• Active sensors carry on board an instrument that sends a microwave pulse to the surface of the earth and register the reflections from the surface of the earth.
• The passive sensors just register the natural occurring microwave signature of the earth. Some satellites carry more than one system combining active and passive instruments on board.

A system exists for each mode:

• SAR (Synthetic Aperture Radar) is an active sensor
• SMMR (Scanning Multi-channel Microwave Radiometer) a passive sensor
• Both were flown on several satellites together or apart. Seasat had on board a SAR and an SMMR sensor alongside some other instruments. Different sensors collect data utilising different bands, or 'windows', in the microwave region. The figure below shows the division of the Microwave region of the electromagnetic spectrum with their respective names.

Frequency and respective wavelength of bands in the microwave region.

Active sensors send their pulses and then register the reflected pulse. Different surfaces reflect microwave energy in different ways.

• Rough surfaces appear bright as the microwave energy is reflected to the satellite sensor.
• Smooth surfaces appear dark as the microwave energy is reflected away from the satellite sensor.
• Microwave energy is sensitive to texture.
• Microwave energy is sensitive to the physical characteristic of the bodies it is bouncing off.
• Water changes the physical characteristics of the bodies it penetrates or covers.

Advantages and disadvantages of microwave-sensing

Advantages of microwave sensors are:

• Operational under all weather conditions with capabilities for sensing the Earth day and night.
• Provide description of the surface texture.
• In the case of the active sensors provide own source of illumination.
• Cloud and fog cover are not a problem.
• Vegetation and subsurface penetration capabilities.
• Information enhancements through SAR and Optical imagery combination.

The disadvantages are:

• Image distortions.
• Extensive shadowing of areas characterised with relief.
• Coarse resolution, especially for passive applications.
• Radar images are rather difficult to deal with. The few commercial software packages that exist to deal with radar imagery offer a limited amount of functions.

SAR systems

A good technical description, What is Imaging Radar?, is provided by Tony Freeman, of the Jet Propulsion Laboratory.

SAR systems evolved from Side Looking Airborne Radar (SLAR) systems. SLAR systems beam microwave signals and collect the returning signals from the ground. The ground resolution, range & azimuth resolution, is controlled by two independent parameters, pulse length and antenna beam width. The pulse length determines the range resolution which in turn is determined by the duration of the pulse transmission. For wavelength and Antenna Length AL, the azimuth resolution (Ra) is determined by the beam width (S) and the range of the sensor (Rs):

Ra = Rs * b

where b =  l / AL

Thus the resolution of SLAR for a predetermined wavelength is dependent on the length of the antenna. To avoid having a large antenna on board the sensor its length can be synthesized by processing the signal received back from a point over a period of time, hence the name synthetic. This is done in SAR systems through modified data recording and processing techniques of the returning signal.

Where would I find SAR instruments?

SAR systems were flown on the following platforms:

• Seasat SAR (L-band).
• Shuttle Imaging Radar SIR (L-band), SIR-A, SIR-B.
• Spacelab Microwave Remote-Sensing Experiment (MRSE) (X-band). A malfunction prevented the production of images.

The above systems were experimental and short lived. Longer lived are:

• SIR-C (L- & C-bands by USA and X-band by Germany/Italy) & SIR-D.
• ERS-1 and ERS-2 SAR (C-band). European Space Agency (ESA) project. 30m resolution.
• RADARSAT SAR (C-band). Canadian Project. 25m resolution.
• JERS-1 SAR (L-band) by Japan. 25m resolution.

SMMR

SMMR systems have been flown on Seasat and Nimbus 7, both launched in 1978. SMMR operate five wavelengths 4.45, 2.8, 1.66, 1.36, & 0.81 cm. The sensor is dual polarised thus providing 10 channels. SMMR is a passive sensor registering the emitted microwave signatures by earth surface bodies. These emitted waves occur either naturally or are reflected radiations from other sources such as the sun. The resolution of SMMR varies from 30 to 159 km, the resolution differs for the various channels, rendering the sensor as only useful for global applications. SMMR applications in water related studies has proven of good quality. It provided information on different aspect of water studies, such as water vapour profiles, sea surface temperature, cloud water content, snow cover mapping, snow cover water equivalent, sea ice and surface wind.