the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 18 Oct 2024
Characterization of surface clutter signal in presence of orography for a spaceborne conically scanning W-band Doppler radar
Abstract. The Earth's surface radar reflection is one of the most important signals received by spaceborne radar systems. It is used in several scientific applications including geolocation, terrain classification, and path-integrated attenuation estimation. A simulator based on the ray tracing approach has been developed to reproduce the clutter reflectivity and the Doppler velocity signal for a conically scanning spaceborne Doppler radar system. The simulator exploits topographic information through a raster digital elevation model, land types from a regional classification database, and a normalized radar surface cross‐section look-up table. The simulator is applied to the WInd VElocity Radar Nephoscop (WIVERN) mission, which proposes a conically scanning W-band Doppler radar to study in-cloud winds. Using an orbital model, detailed simulations for conical scans over the Piedmont region of Italy that offers a variety of landscape conditions are presented. The results highlight the strong departure of the reflectivity and Doppler velocity profiles in the presence of marked orography and the significant gradient in the surface radar backscattering properties. The simulations demonstrate the limitations and advantages of using the surface Doppler velocity over land as an antenna-pointing characterization technique. The simulations represent the full strength range of the surface radar clutter over land surfaces for the WIVERN radar. The surface clutter tool applies to other spaceborne radar missions such as the nadir pointing EarthCARE and CloudSat cloud profiling radars, or the cross-track scanning GPM precipitation radars.
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RC1: 'Comment on egusphere-2024-2779', Anonymous Referee #1, 15 Nov 2024
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This manuscript describes development of a clutter simulator that was applied to the WIVERN mission to generate simulated reflectivity and Doppler velocity profiles over a mountainous region in the northwest of Italy. The manuscript presents detailed analysis of a particular profile that demonstrates non-uniform beam filling (NUBF) and also statistical analysis of a collection of scans over the region. The statistical results begin to quantify the intuitive idea that regions with greater variability in terrain elevation and NRCS will generate clutter profiles with greater variability in reflectivity and Doppler.
The authors state the aim of the work is to “extend the simulations of the clutter signal to non-planer surfaces, … including a realistic variability of the surface backscatter” (lines 48-49). Their claimed novelty is “the application to a space-based configuration, the extension to the Doppler signal, and the inclusion of NUBF effects” (lines 51-54). The simulated data presented and analyzed appears to be high quality and analysis clearly illustrates NUBF and increased variability in reflectivity and Doppler profiles. The simulator considers only clutter (no atmosphere/hydrometeors simulated) and assumes no attenuation.
The work described in this manuscript is an incremental evolution of existing simulation techniques and the novelty is in the combination of high-resolution DEM with the WIVERN (W-band) mission. Other simulations with high-resolution DEMs have been previously described in the literature. The authors make no attempt to explain how the simulations would be useful for the WIVERN mission other than vague statements about how it may be difficult to use Doppler velocity as an antenna characterization technique over rough terrain (lines 10-11; 35-46; 223-225) . There is no attempt to quantitatively link the simulated clutter Doppler profiles to potential errors in mispointing corrections or any other aspects of the mission. For these reasons it is difficult to see how the manuscript meets the criteria of scientific significance required by this journal.
I suggest the authors make major revisions to this manuscript to address the issue of scientific significance before the manuscript is accepted for publication. In particular, the authors should make clear why it is a “substantial contribution” beyond existing simulation methods and make clear what benefits it will contribute to the WIVERN mission.
The following are other suggested corrections:
Lines 33-34: Introduction states “It is therefore timely to investigate and assess how beneficial such a scanning configuration could be in terms of reducing the signal-to-clutter ratio.” This topic is not addressed again in the manuscript and it is not clear how the simulator in its current state would contribute to such an investigation without simulating the atmosphere. Please clarify how the clutter simulator addresses this issue. Specifically how can the simulator be used in its current state when attenuation and scattering above the surface are neglected.
Line 60: NRCS model is an input that should be included in the description
Eqns 1, 4, 6 are stated as functions of r (LHS) but written as functions of t (RHS), please make them consistent.
Line 85: Indicate what limitations the statement “No attenuation effect has been included” places on the utility of the simulator.
Line 89: Justify choice of flat (plane) integration implicit in del x_ij del y_ij formulation of infinitesimal and why that is an appropriate choice even though spherical earth assumption is used otherwise in the model.
Line 139: The method proposed in Battaglia et al. (2024) is cited but that reference is currently unavailable (submitted) so this method cannot be evaluated in the context of this manuscript.
Line 157-159: The choice of these two cases is important in understanding the applicability of the work. The results would be strengthened by citation or further justification for the choice of the high correlation case. This is needed to give context to the conclusion “high correlation value … produces much better results and seems very promising” (line 231-232). rho on these lines should be changed to rho_HV
The case studies section should be reworked with emphasis given to readability.
Line 242: Please describe the renormalizing procedure.
Line 274-276: Indicate what value would be gained by improving the NRCS dataset, specifically what benefit would justify an additional field campaign. Also indicate what value the simulator would bring to the EarthCARE and CloudSat missions.
Citation: https://doi.org/10.5194/egusphere-2024-2779-RC1
Data sets
ASTER Global Digital Elevation Model Version 3 Jet Propulsion Laboratory https://asterweb.jpl.nasa.gov/GDEM.asp
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