Full Polarimetric and Doppler Measurements with Poldirad: A Practical Application of the 3-PolD Method

Full Polarimetric and Doppler Measurements with Poldirad: A Practical Application of the 3-PolD Method.

V.Santalla del Rio1, J. Reimann2, M. Hagen2, J.G. Dios-Quiroga1, A. Martinez-Mera1 1 Department of Signal Theory and Communications, University of Vigo, SPAIN

2 Deutsches Zentrum fur¨ Luft- und Raumfahrt e.V. Institut fur¨ Physik der Atmosphare,¨ Oberpfaffenhofen-Wessling, GERMANY

The Poldirad radar of the DLR at Oberpfaffenhofen has been operated with research purposes since 1986. The main characteristics of the C-band system comprise the polarisation agility for transmitting, the dual-channel receiving, the Doppler capability, and the real time processing and display [1]. Its polarisation agility unique capabilities allow to explore and validate different polarimetric measurement techniques as it enables alternate transmission and reception of pulses with different polarizations, providing great flexibility in defining the sequence of pulse polarizations to be transmitted [2].

The 3-PolD method, proposed in [3], allows obtaining full polarimetric and Doppler information of the target if 3 different polarizations are alternately transmitted while simultaneously receiving the co- and cross-polar components. It has been theoretically demonstrated that this method possesses interesting properties. In particular, it provides minimum variance unbiased linear estimates of the polarimetric covariance matrix elements while allowing computation of the Doppler parameters. Interestingly, Doppler capabilities are not reduced as compared to a non-polarimetric radar with the same baseline PRF. Table I summarizes its main characteristics in comparison with the two well-known and established ATSR (Alternate Transmission Simultaneous Reception) and STSR (Simultaneous Transmission Simultaneous Reception) methods.

	3-PolD	ATSR	STSR
Horizontal reflectivity, Zh	Yes	Yes	Yes
Vertical reflectivity, Zv	Yes	Yes	Yes
Differential reflectivity, Zdr	Yes	Yes	Yes
Linear depolarization ratio, Ldr	Yes	Yes	No
HV co-polar correlation coefficient, hv	Yes	Yes1	Yes
Maximun unambiguous speed, vmax = PRF=4	Yes	No2	Yes
Assumes Shv = Svh = 0	No	No	Yes
Assumes Gaussian Doppler Spectrum	No	Yes	No
1 Requires low Doppler spectrum width (less than around 0.3vmax)
2 vmax is halved
TABLE I

Importantly, so long as the 3-PolD method does not make any assumption about the target properties and provides all elements of the covariance matrix, estimation of parameters defined for any other polarimetric basis, as for example the CDR or the ORTT, are straightforwardly obtained through a change of basis.

Besides, now that the possibility of using radars with a phased array antenna instead of a mechanically steered parabolic reflector to achieve faster and more flexible scanning capabilities is being investigated, it has been shown that, to cope with the loss of polarization purity and the increase of polarization channels coupling inherent to phased array antennas, the 3-PolD method requires some corrections only at reception.

The promising characteristics of the 3-PolD method have encouraged its testing and validation with real weather data. For this, the Poldirad radar has been operated to provide the required data.

The data was acquired on october the 1st, 2014. Four polarizations were alternately transmitted: vertical (V), horizontal (H), right-hand circular (R) and left-hand circular (L). At reception, the co- and cross-polar components of each transmitted pulse

were recorded. The baseline PRF was 1200 Hz, therefore, the PRF for each of the polarimetric channels was 300Hz. The pulse width was 1 s.

Fig. 1 shows the horizontal reflectivity RHI scan. The stratiform rain event data (the framed area in the RHI) was chosen to test the 3-PolD method. Data from 11 consecutive range gates was selected to test the 3-PolD method performance.

Fig. 1. Horizontal reflectivity RHI scan. The data corresponding to the framed area was used to test the 3-PolD method.

From each range gate 2816 pulses are available. Since the radar is scanning in elevation, 11 resolution cells have been defined at different elevation angles for each range gate. Therefore, a total of 121 resolution cells are considered with 256 received pulses available at each cell for polarimetric and Doppler parameter estimation.

Just as an example, Fig. 2 shows the results obtained for horizontal and vertical reflectivities that have been calculated using the classical sample estimator and the 3-PolD method. The differences are well explained by the statistical variability of the data.

Fig. 2. Vertical and horizontal reflectivities.

The preliminary results obtained show a good agreement of the 3-PolD method estimates with the sample estimates. Analysis of the data is still going on. More results will be presented at the conference.

ACKNOWLEDGMENT

This research is partially supported by the European Regional Development Fund (ERDF) and the Galician Regional Government under project CN 2012/260 "Consolidation of Research Units: AtlantTIC" and the Spanish State Secretary for Research under project TEC2011-28789-C02-02.

REFERENCES

[1]A. Schroth, M. Chandrai, and P. Meischner, "A c-band coherent polarimetric radar for propagation and cloud physics research," Journal of Atmospheric and Oceanic Technology, vol. 17, pp. 257--278, 2000.

[2]J. Reimann and M. Hagen, "Multipolarization measurements concerning various aspects of the star mode," ERAD2012. The Seventh European Conference on Radar in Meteorology and Hidrology., Toulouse, France. 24-29 June 2012.

[3]V. S. del Rio, "Least squares estimation of doppler and polarimetric parameters for weather targets." IEEE Trans. on Geosc. and Rem. Sens., vol. 45, pp. 3760--3772, 2007.