Frequency dependence

The usefulness of SAR data for the extraction of biophysical parameters is most dependent on frequency. Microwaves at lower frequencies such as L-band (2.0-1.0 GHz) and P-band (1.0-0.3 GHz) are better able to penetrate the canopy and interact more extensively with its structural components (leaves, branches, trunks). Microwaves at higher frequencies (C-band, 3.8 - 7.5GHz) tend to interact primarily with the upper portion of the canopy. The sensors at higher frequency therefore have a more limited potential use in the discrimination of variations in dry biomass whereas it is expected that L-band data and P-Band data can be used to discriminate increasing biomass variations.

9.3.2 Polarization Dependence

Le Toan et al., 1992 Le Toan et al 1992 showed that the dynamic range of backscatter over uniform pine stands had a strong variation with polarization e.g., the dynamic range for such data obtained at P-band for the HV polarization was approximately 15 dB, at HH approximately 11 dB, and at VV approximately 4.8 dB. At L-band, the dynamic range was reduced , with a dynamic range of approximately 8.6 dB at HV, approximately 5.3 dB at HH and approximately 4.6 dB at VV. At C-band the dynamic range was significantly lower, with data for the HV polarization showing the greatest range (approximately 4 dB) and those in HH and VV polarizations showing a dynamic range of only 2dB.

It is believed that the backscatter for the HV polarization at longer wavelengths can be more effectively used to characterize forest biomass as shown in the work of Dobson et al.Dobson et al.,1992(e.g. Figure 9-27.)

Figure 9-27a
C-Band

Figure 9-27b
L-Band

Figure 9-27. Calibrated C-band and L-band backscatter as a function of total above ground biomass (tons/ha) of maritime pine and loblolly pine plotted on a logarithmic scale (from Dobson et al 1992).