Characteristics - ASAR

1. Envisat

1.1 Orbit Information

ENVISAT consists of ten radar and optical instruments providing continuous observation of the Earth since its launch in 2002 (Table 1). ENVISAT is the largest earth observation spacecraft ever built providing information about:

• Land cover
• Oceans
• Ice caps
• Atmosphere

Table 1. Information about ENVISAT launch and physical characteristics (ESA, 2006).

1.2 Instruments

Described below are the key features of each of the ENVISAT instruments (more in depth information can be found at ESA, 2006 http://envisat.esa.int/instruments/ ).

ASAR – Advanced Synthetic Aperture Radar

• Operates at C band
• Enhanced capabilities in terms of coverage, polarization, range of incidence angles and modes of operation
• ScanSAR mode of operation

MERIS – Medium Resolution Imaging Spectrometer

• Optical instrument, measures solar radiation reflected by the Earth
• Spatial resolution 300m, 15 spectral bands
• Global coverage of the Earth in 3 days
• Primarily used for measuring sea colour which can be converted into a measurement of chlorophyll pigment concentration, suspended sediment concentration and aerosol loads.

AATSR – Advanced Along Track Scanning Radiometer

• Infrared-visible radiometer
• Establish continuity of the ATSR-1 and ATSR-2 data sets of precise sea surface temperature (SST) providing a 10 year near-continuous data set.

RA-2 – Radar Altimeter 2

• Determine the two-way delay of the radar echo from the Earth’s surface
• Measures the power and shape of the reflected radar pulses
• Can be used to determine wind speed and significant wave height at sea

MWR – Microwave Radiometer

• Measurement of the integrated water vapour column and cloud liquid water content
• Useful for the determination of surface emissivity and soil moisture over land and surface energy budget investigations to support atmospheric studies and for ice characterization.

DORIS – Doppler Orbitography and Radiopositioning Integrated by Satellite

• Provided range-rate measurements of signals from a dense network of ground-based beacons.

GOMOS – Global Ozone Monitoring by Occultation of Stars

• New ESA instrument to monitor ozone which provides altitude-resolved global ozone mapping at a high accuracy
• Used to understand ozone chemistry and to validate models
• 600 profile measurements per day

MIPAS – Michelson Interferometer for Passive Atmospheric Sounding

• Fourier transform, spectrometer for measurement of high-resolution gaseous emission spectra at the Earth’s limb
• Operates in the near to mid infrared spectrum
• Monitors stratospheric chemistry and climatology

SCIAMACHY – SCanning Imaging Absorption SpectroMeter for Atmospheric CHartographY

• Primary objective is to perform global measurements of trace gases in the troposphere and stratosphere.

Figure 1. ENVISAT illustrating the location of instruments contained as part of the spacecraft (ESA, 2006)

2. ASAR Modes

ASAR works in a range of modes these are: -

• Global Monitoring
• Wave Mode
• Image Mode
• Alternating Polarisation (HH/HV, VV/VH, HH/VV)
• Wide Swath

The modes that can be used for interferometry are the Image Mode, Alternating Polarisation and Wide Swath Mode. Guarnieri et al., 2003 provide a good summary of each of the mode parameters in Table 2.The modes available (Figure 2) have provided an enhanced capability for interferometric application such as change detection, long term monitoring and classification (Guarnieri et al., 2003).

Table 2 ASAR interferometric modes (Guarnieri et al., 2003, p.194).

Figure 2. ENVISAT ASAR ‘interferometric’ modes (Guarnieri et al., 2003, p.194)