Conference Agenda

Advanced spaceborne 3D Tomographic SAR (Tomo-SAR) [1-5] missions based on companion/tandem (i.e. formation-flying) configurations are emerging for remote sensing of forests [2-4,6], key in the CO2 cycle matters. In fact, this implementation of Tomo-SAR, obtaining the height profile from a sequence of track-pair only coherence data, bypasses typical long-term decorrelation issues of repeat pass monostatic SAR configurations [2,3]. However, because of safety/orbit control issues, the formation satellites do not pass exactly simultaneously [6,7]. Therefore, wind-induced short-term (fractions of second scale) decorrelation phenomena can still affect tandem Tomo-SAR processing [7] in forest scenarios. Also, it is noted that the Tomographic blurring sources are height-localized [2,3].

In our work, a methodology is thus reported for new analyses of 4D (3D + Time [5]) Differential Tomography type of short-term forest temporal decorrelation phenomena, exploiting a ground-based X-band miniradar array (ack. IDS Italy), placed vertically, with very quick acquisition capabilities (up to 1000 array firings per second) [7]. In particular, multiple vertical beamformings are performed, one for each array snapshot, followed by height-by-height statistical analyses along the quickly sampled temporal domain.

Moreover, corresponding real data results have been obtained, developing characterizations of both height- and time-varying behaviours of short-term decorrelation in a representative experiment for a stand of elms and poplars trees in Italy (UniPi PisaScat experiment [7]). The data were acquired in two different wind conditions and seasons.

For example, advanced height-varying Doppler spectra estimates have been obtained; increasing Doppler bandwidths with height and for stronger wind velocity resulted, indicating corresponding increasing velocity of the random motions of windblown branches and leaves.

Through the well-known signal bandwidth-coherence time relation, short-term coherence times (for coherence to decay to a steady-state) have been also measured; these can be useful to state if the short-term decorrelation issue influences a Tomo-SAR tandem satellite system, according to its formation-flying time-lag [6], or if this issue can be neglected. Noteworthy, a variability of coherence time along the trees layers (decreasing with higher heights), and a decorrelation timescale of small fractions of second, have resulted to be apparent. As a comparison, ensemble (height-integrated) Doppler measures have also been carried out, resulting mostly sensitive just to the lower (less critical) heights.

Other obtained findings include height-varying short-term steady-state coherence levels (which can tell about influence degree for tandem satellite Tomo-SAR); time-varying height scattering profiles and (sliding window-based) mean Doppler shifts under wind gusts, for the different heights; and direct autocorrelation function measures of the scattering at very short sampling scale (representing a physical view of the height-varying short term decorrelation process and a confirmation of the Doppler-based coherence time results). These will be presented at the conference, together with physical comments, and other acquisition system, scenario and methodology details.

Such investigations of height-varying characteristics of short-term decorrelation processes complement ESA radar tower [3] measures, which are instead at some seconds scale and basically of ensemble kind.

Our advanced characterization methodology and the obtained findings (possibly carrier frequency-scaled) may be useful for development, or definition refinement of the operative envelope, of the emerging tandem satellite programs and missions; examples are LuTan-1, already operative (as TanDEM-X), the NASA DART, and SAOCOM-CS-like system [6] concepts, currently being developed.

Ack.: This work has been partially supported by the Italian Ministry of Education and Research (MUR) in the framework of the FoReLab project (Departments of Excellence).

[1] T.G. Yitayew, L. Ferro-Famil, T. Eltoft, “High Resolution Three dimensional Imaging of Sea Ice using Ground-based Tomographic SAR Data,” Proc. 10th EUSAR, Berlin, Germany, 2014, pp.1325-1328.

[2] M. Lavalle, M. Simard, S. Hensley, “A Temporal Decorrelation Model for Polarimetric Radar Interferometers,” IEEE TGRS, 50(7), pp.2880-2888, 2012.

[3] T. Dinh Ho Minh, et al., “Vertical Structure of P-Band Temporal Decorrelation at the Paracou Forest: Results from TropiScat,” IEEE GRSL, 11(8), pp.1438-1442, 2014.

[4] A. Reigber, A. Moreira, “First Demonstration of Airborne SAR Tomography using Multibaseline L-band Data,” IEEE TGRS, 38(5), pp.2142-2152, 2000.

[5] D. Reale, G. Fornaro, A. Pauciullo, X. Zhu, R. Bamler, “Tomographic Imaging and Monitoring of Buildings with Very High Resolution SAR Data,” IEEE GRSL, 8(4), pp.661-665, 2011.

[6] P. López-Dekker, H. Rott, P. Prats-Iraola, B. Chapron, K. Scipal and E. D. Witte, “Harmony: an Earth Explorer 10 Mission Candidate to Observe Land, Ice, and Ocean Surface Dynamics,” Proc. 2019 IEEE IGARSS, Yokohama, Japan, 2019, pp.8381-8384.

[7] F. Lombardini, G. Buttitta, “On the Issue of Short-term Decorrelation in Forest Tomography with Companion SARs,” Proc. 2024 IEEE IGARSS, Athens, Greece, 2024, pp.3104-3107.