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Seen from Space 2005

Observing Forests with Synthetic Aperture Radar

Fig. 1 Forest spreading over the northwest part of Tomakomai, Hokkaido observed by Airborne L-band SAR radar

Figure 1 depicts forests in northwest Tomakomai, Hokkaido, observed by Airborne L-band SAR radar (PiSAR*1) at a wavelength of 23.6 cm. The image includes the National Forest and the Hokkaido University Forest, which are mainly larch, two kinds of spruce (Japanese Aka-ezomatu, Ezo-matsu), one fir (Japanese Todo-matsu), and some broadleaf trees. The rectangular area seen in Fig. 1 and expanded in Fig. 3 represents one forest stand. The black areas seen in the upper left and upper right are Lake Shikotsu and New Chitose Airport. The downtown area of Tomakomai is to the lower left.

Fig. 2 SAR observations of forest and open areas

Satellite radio sensors use synthetic aperture radar (SAR) to transmit radar signals to the earth from their antennas. We use the backscattered signals from the ground to assemble an image. Because the radar signals can penetrate clouds, SAR makes observations even on cloudy or rainy days. The radar signal cannot return in the direction of the antenna over open areas or water surfaces, and those areas of the image are dark (Fig. 2). When radar illuminates forested areas, the signal interacts with the trees, reflecting some of the signal back in the direction of the antenna. Color variation in the forest areas is due to the variations in backscattering intensity, depending on the number and species of trees (see "Explanation of the Image" below). These characteristics of the L-band SAR data can be used to detect deforestation and to estimate forest biomass.


Fig. 3 L-band SAR image taken before typhoon	Fig. 4 L-band SAR image taken after typhoon. Almost all trees in this area were felled by the wind	Fig. 5 Photograph of open area where the fallen trees were removed

An intense typhoon, 200418 (SONGDA), struck Hokkaido on September 7, 2004,and many trees were felled by the strong winds(*2). Almost all the trees in the area seen in Fig. 4 were felled, and those areas of the image are dark. The red square in Fig. 4 shows the area where the fallen trees were removed, creating an open area (Fig. 5). (Pink highlights in the open area are caused by the calibration device.) L-band SAR (PALSAR) will also be carried aboard the Advanced Land Observing Satellite (ALOS), which JAXA has scheduled for launch this year (2005). Using the L-band SAR, JAXA and other users will examine the status of the world's forests.

The study done in Tomakomai has been cooperated with the National Institute for Environmental Study.

(*1)Pi-SAR is an airborne synthetic aperture radar. It consists of X-band SAR from the National Institute of Information and Communications Technology (NICT) and L-band SAR from the Japan Aerospace and Exploration Agency (JAXA). Details and the image library acquired by SAR are available at "Pi-SAR (L-band) - About Pi-SAR (Japanese only)".

(*2)Typhoon 195415 struck this area from September 25 to 27, 1954, and leveled trees at this that time.

Explanation of the Image
(Fig.1,Fig.3,Fig.4)

Sensor:	Polarimetric and Interferometric Airborne Synthetic Aperture Radar L-band (Pi-SAR), Wavelength: 23.6cm
Time and Date:	February 8, 2004 (Figs. 1 and 3) February 10, 2005 (Fig. 4)

Three of the four Pi-SAR polarization modes are used together to compose a color image (HH polarization, red; HV polarization, green; and VV polarization, blue). The polarization represents a radio-signal component oscillating in a specific plane. In H polarization, the radio-signal component oscillates parallel to the ground, and in V, perpendicular to the ground. HV indicates that an H polarized signal emitted from the antenna returns as a V polarized signal due to ground scattering. The spatial resolution of the original image is about 3m.

Black:	Water surface, runway, road, golf course, or no-data area.
Purple:	Wetland or low vegetation area (e.g., near mountains).
Orange or gree:	Forest

Related sites:

Mangrove forest in Iriomote Island observed by synthetic aperture radar

Pi-SAR L-band Data Archive (Japanese only)