ASAR, ENVISAT-ASAR

European Space Agency

The Advanced Synthetic Aperture Radar (ASAR) sensor rides aboard the ENVISAT satellite and provides radar imagery of the earth for studying oceans, atmosphere, ice, and land. ENVISAT was launched in 2002 and is still operational.

Synthetic aperture radar sensors are a form of active remote sensing where, instead of recording the amount of sunlight reflected from a surface or radiation emitted from a surface (e.g., thermal imaging), radar pulses are emitted from the sensor and then the reflected responses are recorded. Radar sensors operate in the microwave region of the electromagnetic spectrum (EMS). Envisat's ASAR, in particular, operates in the C-band frequency range of the EMS - 2GHz to 4GHz (interestingly, this is the range of frequencies that many consumer wireless devices like cordless phones operate in).

Radar is a ranging sensor - meaning that it detects the distance to objects by timing how long it takes for emitted pulses to return to the sensor. This information is converted to elevation by processing the radar signal. By examining other properties of the radar responses (e.g., the amount of the signal that is returned), other properties of the surface (and in some cases even sub-surface properties like sub-canopy or sub-soil information) can be determined. For example, areas with high vegetative cover scatter much of the radar signal and will show up darker in a radar image than those with lots of bare ground that reflect more of the radar signal. The microwaves emitted by a radar sensor can be directed to oscillate in one particular direction. This is known as polarization. Some surfaces, especially vegetation, can cause changes in the polarization of microwaves, and measuring the amount of the signal returned in horizontal versus vertical polarization can also provide information on the type of surface. The ASAR sensor has the ability to emit or detect microwaves in any combination of horizontal or vertical polarizations.

Also, because radar sensors emit their own radiation, they can function day or night and can often image through clouds or smoke, making them very versatile.

Other synthetic aperture radar sensors include:

• Radarsat-1
• Radarsat-2
• ERS-1 and 2

The ASAR sensor has five modes that can image the earth's surface with different levels of polarization and detail and different image extents.

ASAR records reflected radiation in the C-band of the electromagnetic spectrum. ASAR also has five choices of polarization levels: VV, HH, VV/HH, HV/HH, or VH/VV, where the first letter refers to the polarization emitted by the antenna, and the second letter refers to the polarization detected by the receiving antenna.

Swath width varies by mode:

• The image, and alternating polarization modes have a swath width of up to 100km
• The wave mode has a 5km swath width
• The wide-swath and global monitoring modes have a swath width of 400km or more

Resolution of the images also varies by mode:

• The image, wave, and alternating polarization modes have a resolution of approximately 30m x 30m.
• The wide-swath mode has a resolution of approximately 150m x 150m
• The global monitoring mode has a resolution of approximately 1,000m x 1,000m

Begining and ending (if applicable) dates of availability Return interval

Information on cost of the imagery and process for acquiring imagery (i.e., download from archive, scheduled acquisition)

Format and delivery options for imagery from this sensor

Most applications of ENVISAT-ASAR data to date have been for estimating soil moisture. Other applications include monitoring desertification or estimating vegetative cover.

• Anh, H.V., Williams, M. and D. Manning. 2006. Remote-sensing monitoring of desertification using ASTER and ENVISAT ASAR: case study at semi-arid area of Vietnam. International Symposium on Geoinformatics for Spatial Infrastructure Development in Earth and Applied Sciences.
• Baup, F., Mougin, E., de Posnay, P, Tomouk, F. and I. Chenerle. 2007. Surface soil moisture estimation over the AMMA Sahelian site in Mali using ENVISAT/ASAR data. Remote Sensing of the Environment 109(4):473-481.
• Baup, F.; Mougin, E.; Hiernaux, P.; Lopes, A.; De Rosnay, P.; Chenerie, I. 2007. Radar Signatures of Sahelian Surfaces in Mali Using ENVISAT-ASAR Data. IEEE Transactions on Geoscience and Remote Sensing 45(7):2354 - 2363.
• Huanga, S., Pottera, C., Crabtree, R.L. Hagerc. S. and P. Grossd. 2009. Fusing optical and radar data to estimate sagebrush, herbaceous, and bare ground cover in Yellowstone. Remote Sensing of the Environment 114:251-264.
• Wagner W., Pathe, C., Sabel, D., Bartsch, A., Kunzer, C., and K. Scipal. 2007. Experimental 1km soil moisture products from ENVISAT ASAR for Southern Africa. Proceedings of ENVISAT Symposium. Montreux, Switzerland. SP-636.

Any special (or helpful) software or hardware requirements for using this kind of imagery (e.g., converters for reading NASA HDF format and saving data to a more user-friendly format)

• Wikipedia page on synthetic aperture radar: http://en.wikipedia.org/wiki/Synthetic_aperture_radar
• the European Space Agency's ASAR Frequently Asked Questions: http://envisat.esa.int/object/index.cfm?fobjectid=3772&contentid=3829

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