1. Backscatter bursts at Ka band
01/01/2018
Backscattering bursts at Ka-band over land using SARAL data
Frappart F.a,b; Blarel F.*a, Blumstein D.a,c, Niño F. a
(a)- Laboratoire d’Etudes en Géophysique et Océanographie Spatiales (LEGOS-GRGS), Université de Toulouse, CNES, CNRS, IRD, UPS. Toulouse, France
(b)- Géosciences Environnement Toulouse (GET-GRGS), Université de Toulouse, CNES, CNRS, IRD, UPS. Toulouse, France
(c)- Centre National d’Etudes Spatiales (CNES), 18 Avenue Edouard Belin, Toulouse, France,
Several studies showed that the backscattering values measured at Ka-band by AltiKa on-board SARAL over land can reach larger values than previpously oberved at lower frequency [e.g. 1]. In this study, we present global maps of along-track average backscattering coefficients (from ice-1 [2] and ice-2 [3] retracker) over the lands and associated standard deviation at Ka band using SARAL data [4].
The resulting maps have been obtained from a original method developed to compute any statistics along the pass. This preliminary results using the first of SARAL measurements at Ka-band showed the potential of Ka-band for detecting surface soil moisture (SSM) changes and water even under forest canopy. SARAL data exhibit very high values (>50 dB) of backscattering coefficients (i.e., bursts) at Ka-band over land. In this study, we propose an identification of these bursts and an analysis of their cause.
Introduction
Method
Along-track altimetry data were first sorted using a normalized pass [5, 6] in order to merge the data measurements into cells regularly organized along the pathway. The mean pass is then defined by four parameters: the mean longitude and latitude of each cell composing the mean pass, and its size given by its dimensions along (δAlong-track) and cross track (δCross-track). If δCross-track is chosen arbitrarily (equals two kilometers in this study as this distance corresponds to the maximum cross-track variations along the orbit of ERS-2, ENVISAT, SARAL and Jason-2 missions), δAlong-track is given by : δAlong-track = Vsat δt
where Vsat is the velocity of the satellite along the orbit equals to 7.45 km.s-1 and δt is chosen equals to 1 second, that mean that every cell contains, each cycle of SARAL a maximum of 40 measurements.
Note that these parameters defining and/or identifying the reference ground track and altimeter points have been integrated into the GDR products distributed by the CTOH.
Mean pass
δCross-track
δAzimuth=Vsat*δt
Cell along average track.
(λcell, φcell)
Altimetry data in one cell for several cycles
Mean altimetry pass (black arrow)obtained as the average of all altimetry data acquired during the life time of an altimetry
mission. Altimetry measurements are
shown using dots, each color
representing a cycle. Rectangles
represent the cells along the
altimetry pass defined by
δAlong-track and δCross-track for a
given location (λcell, φcell)
(yellow and
black star) 1
Datasets
In this study, we used the following parameters, from cycle 1 (March 2013) to 33 (Juin 2016) present in the GDR E of the SARAL mission:
Ice-1 and Ice-2 based backscattering coefficients, Ice-1 peakiness and Ice-2 leading edge width and amplitude See [2, 3] for the definition of these parameters. These data are made available by the Centre de Topographie des Océans et de l'Hydrosphère (CTOH) at Laboratoire d'Etudes en Géophysique et Hydrologie Spatiales (LEGOS):
Backscatter at Ka band
Mean of Bs (dB)
StD of Bs (dB)
Mean of Peakiness (count)
Mean of Bs amplitude (dB)
Mean of Leading Edge Width (m)
Mean of Leading Edge Amplitude (dB)
Fig. 1
Fig. 2
Fig. 3
Fig. 4
Fig. 5
Fig. 6
The altimeter backscattered responses at Ka-bands exhibit a wide range of spatial and temporal variations over these different areas (Fig.1). The backscattered energy by each surface can be related to its soil roughness and soil dielectric constant and their variations against time. Largest values, that can reach 50 dB at Ka-band are found over flat areas such as large river basins with extensive wetlands, as well as in large irrigated zone where specular returns occur. For example, this is clearly visible along the Ganges, and especially in the Ganges-Brahmaputra-Meghna delta, the Plata or the Ob’, a large peri-arctic river. Along the Ganges and in the Mekong basins, the large backscattering values are also due to the inundated rice paddies. Over rivers with extensive floodplains covered with forest, as in the Amazon Basin, the mean backscattering coefficient is low due to the presence of the forest once averaged on 7 km along the track.
The other parameters also exhibit spatial patterns which can be related to the presence of bright targets (especially open water) over land surfaces. Annual amplitudes of backscattering (Fig. 4) are likely to reach 50 dB in some large watersheds. The peakiness value (Fig. 3) is also a good indicator of the presence of bright targets such as land water bodies (the larger the peakiness value is, the more specular the waveform).
A particularly striking feature is the high values of peakiness values over land waters (Fig. 3). It clearly appears in the Nelson, Northern Dvina, Ob', Lena, Mississippi, Tocantins, La Plata, Ganges-Brahmaputra, Mekong, ... where extensive floodplains are present. The presence dense vegetation coverage can account for the lower peakiness values (Fig. 3) in some basins such as Amazon, Orinoco and Congo. Some extensive floodplains also presents high values of peakiness such as the Beni-Mamoré at the Bolivian-Brazilian borders. Similar patterns are also present one the Ice-2 retracking parameters (Fig. 5 and 6): leading edge width and leading edge amplitude.
Backscatter bursts at Ka band
Results & Conclusion
Very strong responses observed at Ka-band (> 40 dB) in some occasions over most land surfaces compared with other frequencies used in radar altimetry.
Larger backscattering values observed over inland water bodies.
Results consistent with high peakiness values, i.e., very peaky, quasi-Dirac waveforms
> 40.0 dB
> 45.0 dB
References
From the three maps, it is shown the strong backscatter over 40.0 dB above, over 45.0 dB above on the right hand side and over 50.0 dB on the right hand side.
This three maps show the areas of strong backscatter values. All land surfaces, except deserts and montainous areas exhibit, large backscattering values (>40 dB) during the SARAL observation period (March 2013 – June 2016). As the threshold value on the backscattering at Ka-band increases, the areas presenting backscattering bursts are decreasing and appears to be mostly located over inland water bodies such as small lakes, river watersheds and wetlands.
[1] Frappart F., Fatras C., Mougin E., Marieu V., Diepkile A.T., Blarel F., Borderies P. (2015). Radar altimetry backscattering signatures at Ka, Ku, C and S bands over West Africa. Physics and Chemistry of the Earth, Parts A/B/C, 83-84, , doi: /j.pce
[2]- Wingham et al, Sepember 1986, Proceedings of the IGARSS Symposium: New techniques in satellite altimeter tracking systems, Zurich, Switzerland
[3]- Legresy et al, 2005, Remote Sensing of Environment 95, n 2, ᵒdoi: /j.rse
[4]- CNES
[5]- Blarel et al CTOH Technical report: ”Normalized Altimetric Pass”.
[6]: Blarel, F., Frappart, F., Legrésy, B., Blumstein, D., Rémy, F., Fatras, C., Mougin E., Papa F., Prigent C., Nino F., Borderies, P., Biancamaria S, Calmant S., “Altimetry backscattering signatures at Ku and S bands over land and ice sheets.” Proceedings of SPIE Remote Sensing - Remote Sensing for Agriculture, Ecosystems, and Hydrology, 17, , doi: /
> 50.0 dB
Ocean Surface Topography Science Team, November 2016, La Rochelle, France