Linear depolarization ratio

The linear depolarization ratio ( English linear depolarization ratio , LDR) is the ratio of cross to co- polarized backscattered energy, and is in dB given. The cross-polarized part is created by depolarization . The LDR and other polarimetric measured variables are used to characterize the backscattering objects, such as hydrometeors in polarimetric radar or LIDAR or vegetation in SAR . For the measurement of the depolarization ratio, the radar has the so-called STAR mode ( English S imultaneous t ransmit A nd R eceive ) off: so the radar can not simultaneously horizontally and vertically polarized send (and would thus be in the far field than + 45 ° or -45 ° polarized), but may only transmit either horizontally or vertically polarized and must receive horizontally and vertically polarized at the same time.

${\displaystyle L_{dr}={\frac {\langle \left|\sigma _{\bot }\right|^{2}\rangle }{\langle \left|\sigma _{\parallel }\right|^{2}\rangle }}}$

${\displaystyle \mathrm {LDR} =10\log _{10}(L_{dr})}$  

The σ therein are the proportions of the scattering cross-section related to the polarization direction of the transmitted pulse for the case of backscattering . Cross-polarized ( ) means in the case of horizontal linearly polarized transmitting precipitation radars vertically linearly polarized, while co-polarized ( ) in this case means the horizontal polarization direction. The angle brackets mean here an averaging over the function of the size distribution and the orientations of the hydrometeors. ${\displaystyle \bot }$ ${\displaystyle \parallel }$ ${\displaystyle \langle \,\rangle }$

For spherical particles (e.g. very small raindrops) LDR = 0. Values ​​down to about −40 dB are practically measurable. This value is limited by the practically incomplete isolation between the two receiving channels, essentially between the vertically and horizontally polarized antenna elements. Complete depolarization occurs in chaff clouds . Since these metal-coated glass fibers float down almost exclusively in a horizontal position at resonance length, the reflected radar signal is completely polarized horizontally. In this special military case, the vertical linear depolarization ratio is at its maximum (LDR is transmitted with vertically polarized transmission); the horizontal depolarization ratio (LDR when sent horizontally polarized) is minimal.

literature

• VN Bringi, V. Chandrasekar: Polarimetric Doppler weather radar , (esp. Chapter 7, The polarimetric basis for characterizing precipitation ).
• Michael I. Mishchenko, Joop W. Hovenier, Larry D. Travis: Light scattering by nonspherical particles . [1]

Individual proof

1. ↑ Sergey Y. Matrosov, Depolarization Estimates from Linear H and V Measurements with Weather Radars Operating in Simultaneous Transmission – Simultaneous Receiving Mode in Journal of Atmospheric and Oceanic Technology, Volume 21, p. 576 (Formula 6d) ( online )