Articles | Volume 16, issue 6
https://doi.org/10.5194/tc-16-2225-2022
© Author(s) 2022. This work is distributed under
the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/tc-16-2225-2022
© Author(s) 2022. This work is distributed under
the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
A leading-edge-based method for correction of slope-induced errors in ice-sheet heights derived from radar altimetry
Weiran Li
CORRESPONDING AUTHOR
Department of Geoscience and Remote Sensing, Delft University of Technology, Delft, the Netherlands
Cornelis Slobbe
Department of Geoscience and Remote Sensing, Delft University of Technology, Delft, the Netherlands
Stef Lhermitte
Department of Geoscience and Remote Sensing, Delft University of Technology, Delft, the Netherlands
Related authors
Weiran Li, Sanne B. M. Veldhuijsen, and Stef Lhermitte
The Cryosphere, 19, 37–61, https://doi.org/10.5194/tc-19-37-2025, https://doi.org/10.5194/tc-19-37-2025, 2025
Short summary
Short summary
This study used a machine learning approach to estimate the densities over the Antarctic Ice Sheet, particularly in the areas where the snow is usually dry. The motivation is to establish a link between satellite parameters to snow densities, as measurements are difficult for people to take on site. It provides valuable insights into the complexities of the relationship between satellite parameters and firn density and provides potential for further studies.
Weiran Li, Stef Lhermitte, Bert Wouters, Cornelis Slobbe, Max Brils, and Xavier Fettweis
EGUsphere, https://doi.org/10.5194/egusphere-2024-3251, https://doi.org/10.5194/egusphere-2024-3251, 2024
Short summary
Short summary
Due to the melt events in recent decades, the snow condition over Greenland has been changed. To observe this, we use a parameter (leading edge width; LeW) derived from satellite altimetry, and analyse its spatial and temporal variations. By comparing the LeW variations with modelled firn parameters, we concluded that the 2012 melt event has a long-lasting impact on the volume scattering of Greenland firn. This impact cannot fully recover due to the recent and more frequent melt events.
Weiran Li, Stef Lhermitte, and Paco López-Dekker
The Cryosphere, 15, 5309–5322, https://doi.org/10.5194/tc-15-5309-2021, https://doi.org/10.5194/tc-15-5309-2021, 2021
Short summary
Short summary
Surface meltwater lakes have been observed on several Antarctic ice shelves in field studies and optical images. Meltwater lakes can drain and refreeze, increasing the fragility of the ice shelves. The combination of synthetic aperture radar (SAR) backscatter and interferometric information (InSAR) can provide the cryosphere community with the possibility to continuously assess the dynamics of the meltwater lakes, potentially helping to facilitate the study of ice shelves in a changing climate.
Sofie Van Winckel, Jonas Simons, Stef Lhermitte, and Bart Muys
Biogeosciences, 22, 4291–4307, https://doi.org/10.5194/bg-22-4291-2025, https://doi.org/10.5194/bg-22-4291-2025, 2025
Short summary
Short summary
Insights on management's impact on forest carbon stocks are crucial for sustainable forest management practices. However, accurately monitoring carbon stocks remains a technological challenge. This study estimates above-ground carbon stock in managed and unmanaged forests using passive optical, synthetic aperture radar (SAR), and light detection and ranging (lidar) remote sensing data. Results show promising potential in using multiple remote sensing predictors and publicly available high-resolution data for mapping forest carbon stocks.
Ann-Sofie P. Zinck, Bert Wouters, Franka Jesse, and Stef Lhermitte
EGUsphere, https://doi.org/10.5194/egusphere-2025-573, https://doi.org/10.5194/egusphere-2025-573, 2025
Short summary
Short summary
Ocean-driven basal melting of ice shelves can carve channels into the ice shelf base. These channels represent potential weak areas of the ice shelf. On George VI Ice shelf we discover a new channel which onset coincides with the 2015 El-Nino Southern Oscillation event. Since the channel has developed rapidly and is located within a highly channelized area close to the ice shelf front it poses a potential thread of ice shelf retreat.
Weiran Li, Sanne B. M. Veldhuijsen, and Stef Lhermitte
The Cryosphere, 19, 37–61, https://doi.org/10.5194/tc-19-37-2025, https://doi.org/10.5194/tc-19-37-2025, 2025
Short summary
Short summary
This study used a machine learning approach to estimate the densities over the Antarctic Ice Sheet, particularly in the areas where the snow is usually dry. The motivation is to establish a link between satellite parameters to snow densities, as measurements are difficult for people to take on site. It provides valuable insights into the complexities of the relationship between satellite parameters and firn density and provides potential for further studies.
Weiran Li, Stef Lhermitte, Bert Wouters, Cornelis Slobbe, Max Brils, and Xavier Fettweis
EGUsphere, https://doi.org/10.5194/egusphere-2024-3251, https://doi.org/10.5194/egusphere-2024-3251, 2024
Short summary
Short summary
Due to the melt events in recent decades, the snow condition over Greenland has been changed. To observe this, we use a parameter (leading edge width; LeW) derived from satellite altimetry, and analyse its spatial and temporal variations. By comparing the LeW variations with modelled firn parameters, we concluded that the 2012 melt event has a long-lasting impact on the volume scattering of Greenland firn. This impact cannot fully recover due to the recent and more frequent melt events.
Julius Sommer, Maaike Izeboud, Sophie de Roda Husman, Bert Wouters, and Stef Lhermitte
EGUsphere, https://doi.org/10.5194/egusphere-2024-3105, https://doi.org/10.5194/egusphere-2024-3105, 2024
Short summary
Short summary
Ice shelves, the floating extensions of Antarctica’s ice sheet, play a crucial role in preventing mass ice loss, and understanding their stability is crucial. If surface meltwater lakes drain rapidly through fractures, the ice shelf can destabilize. We analyzed satellite images of three years from the Shackleton Ice Shelf and found that lake drainages occurred in areas where damage is present and developing, and coincided with rising tides, offering insights into the drivers of this process.
Filippo Emilio Scarsi, Alessandro Battaglia, Maximilian Maahn, and Stef Lhermitte
EGUsphere, https://doi.org/10.5194/egusphere-2024-1917, https://doi.org/10.5194/egusphere-2024-1917, 2024
Short summary
Short summary
Snowfall measurements at high latitudes are crucial for estimating ice sheet mass balance. Spaceborne radar and radiometer missions help estimate snowfall but face uncertainties. This work evaluates uncertainties in snowfall estimates from a fixed near-nadir radar (CloudSat-like) and a conically scanning radar (WIVERN-like), determining that WIVERN will provide much better estimates than CloudSat, and at much smaller spatial and temporal scales.
Thore Kausch, Stef Lhermitte, Marie G. P. Cavitte, Eric Keenan, and Shashwat Shukla
EGUsphere, https://doi.org/10.5194/egusphere-2024-2077, https://doi.org/10.5194/egusphere-2024-2077, 2024
Short summary
Short summary
Determining the net balance of snow accumulation on the surface of Antarctica is challenging. Sentinel-1 satellite sensors, which can see through snow, offer a promising method. However, linking their signals to snow amounts is complex due to snow's internal structure and limited on-the-ground data. This study found a connection between satellite signals and snow levels at three locations in Dronning Maud Land. Using models and field data, the method shows potential for wider use in Antarctica.
Lena G. Buth, Valeria Di Biase, Peter Kuipers Munneke, Stef Lhermitte, Sanne B. M. Veldhuijsen, Sophie de Roda Husman, Michiel R. van den Broeke, and Bert Wouters
EGUsphere, https://doi.org/10.5194/egusphere-2023-2000, https://doi.org/10.5194/egusphere-2023-2000, 2023
Preprint archived
Short summary
Short summary
Liquid meltwater which is stored in air bubbles in the compacted snow near the surface of Antarctica can affect ice shelf stability. In order to detect the presence of such firn aquifers over large scales, satellite remote sensing is needed. In this paper, we present our new detection method using radar satellite data as well as the results for the whole Antarctic Peninsula. Firn aquifers are found in the north and northwest of the peninsula, in agreement with locations predicted by models.
Ignace Pelckmans, Jean-Philippe Belliard, Luis E. Dominguez-Granda, Cornelis Slobbe, Stijn Temmerman, and Olivier Gourgue
Nat. Hazards Earth Syst. Sci., 23, 3169–3183, https://doi.org/10.5194/nhess-23-3169-2023, https://doi.org/10.5194/nhess-23-3169-2023, 2023
Short summary
Short summary
Mangroves are increasingly recognized as a coastal protection against extreme sea levels. Their effectiveness in doing so, however, is still poorly understood, as mangroves are typically located in tropical countries where data on mangrove vegetation and topography properties are often scarce. Through a modelling study, we identified the degree of channelization and the mangrove forest floor topography as the key properties for regulating high water levels in a tropical delta.
Ann-Sofie Priergaard Zinck, Bert Wouters, Erwin Lambert, and Stef Lhermitte
The Cryosphere, 17, 3785–3801, https://doi.org/10.5194/tc-17-3785-2023, https://doi.org/10.5194/tc-17-3785-2023, 2023
Short summary
Short summary
The ice shelves in Antarctica are melting from below, which puts their stability at risk. Therefore, it is important to observe how much and where they are melting. In this study we use high-resolution satellite imagery to derive 50 m resolution basal melt rates of the Dotson Ice Shelf. With the high resolution of our product we are able to uncover small-scale features which may in the future help us to understand the state and fate of the Antarctic ice shelves and their (in)stability.
Diana Francis, Ricardo Fonseca, Kyle S. Mattingly, Stef Lhermitte, and Catherine Walker
The Cryosphere, 17, 3041–3062, https://doi.org/10.5194/tc-17-3041-2023, https://doi.org/10.5194/tc-17-3041-2023, 2023
Short summary
Short summary
Role of Foehn Winds in ice and snow conditions at the Pine Island Glacier, West Antarctica.
Lena G. Buth, Bert Wouters, Sanne B. M. Veldhuijsen, Stef Lhermitte, Peter Kuipers Munneke, and Michiel R. van den Broeke
The Cryosphere Discuss., https://doi.org/10.5194/tc-2022-127, https://doi.org/10.5194/tc-2022-127, 2022
Manuscript not accepted for further review
Short summary
Short summary
Liquid meltwater which is stored in air bubbles in the compacted snow near the surface of Antarctica can affect ice shelf stability. In order to detect the presence of such firn aquifers over large scales, satellite remote sensing is needed. In this paper, we present our new detection method using radar satellite data as well as the results for the whole Antarctic Peninsula. Firn aquifers are found in the north and northwest of the peninsula, in agreement with locations predicted by models.
Zhongyang Hu, Peter Kuipers Munneke, Stef Lhermitte, Maaike Izeboud, and Michiel van den Broeke
The Cryosphere, 15, 5639–5658, https://doi.org/10.5194/tc-15-5639-2021, https://doi.org/10.5194/tc-15-5639-2021, 2021
Short summary
Short summary
Antarctica is shrinking, and part of the mass loss is caused by higher temperatures leading to more snowmelt. We use computer models to estimate the amount of melt, but this can be inaccurate – specifically in the areas with the most melt. This is because the model cannot account for small, darker areas like rocks or darker ice. Thus, we trained a computer using artificial intelligence and satellite images that showed these darker areas. The model computed an improved estimate of melt.
Weiran Li, Stef Lhermitte, and Paco López-Dekker
The Cryosphere, 15, 5309–5322, https://doi.org/10.5194/tc-15-5309-2021, https://doi.org/10.5194/tc-15-5309-2021, 2021
Short summary
Short summary
Surface meltwater lakes have been observed on several Antarctic ice shelves in field studies and optical images. Meltwater lakes can drain and refreeze, increasing the fragility of the ice shelves. The combination of synthetic aperture radar (SAR) backscatter and interferometric information (InSAR) can provide the cryosphere community with the possibility to continuously assess the dynamics of the meltwater lakes, potentially helping to facilitate the study of ice shelves in a changing climate.
Annelies Voordendag, Marion Réveillet, Shelley MacDonell, and Stef Lhermitte
The Cryosphere, 15, 4241–4259, https://doi.org/10.5194/tc-15-4241-2021, https://doi.org/10.5194/tc-15-4241-2021, 2021
Short summary
Short summary
The sensitivity of two snow models (SNOWPACK and SnowModel) to various parameterizations and atmospheric forcing biases is assessed in the semi-arid Andes of Chile in winter 2017. Models show that sublimation is a main driver of ablation and that its relative contribution to total ablation is highly sensitive to the selected albedo parameterization and snow roughness length. The forcing and parameterizations are more important than the model choice, despite differences in physical complexity.
Diana Francis, Kyle S. Mattingly, Stef Lhermitte, Marouane Temimi, and Petra Heil
The Cryosphere, 15, 2147–2165, https://doi.org/10.5194/tc-15-2147-2021, https://doi.org/10.5194/tc-15-2147-2021, 2021
Short summary
Short summary
The unexpected September 2019 calving event from the Amery Ice Shelf, the largest since 1963 and which occurred almost a decade earlier than expected, was triggered by atmospheric extremes. Explosive twin polar cyclones provided a deterministic role in this event by creating oceanward sea surface slope triggering the calving. The observed record-anomalous atmospheric conditions were promoted by blocking ridges and Antarctic-wide anomalous poleward transport of heat and moisture.
Christiaan T. van Dalum, Willem Jan van de Berg, Stef Lhermitte, and Michiel R. van den Broeke
The Cryosphere, 14, 3645–3662, https://doi.org/10.5194/tc-14-3645-2020, https://doi.org/10.5194/tc-14-3645-2020, 2020
Short summary
Short summary
The reflectivity of sunlight, which is also known as albedo, is often inadequately modeled in regional climate models. Therefore, we have implemented a new snow and ice albedo scheme in the regional climate model RACMO2. In this study, we evaluate a new RACMO2 version for the Greenland ice sheet by using observations and the previous model version. RACMO2 output compares well with observations, and by including new processes we improve the ability of RACMO2 to make future climate projections.
Thore Kausch, Stef Lhermitte, Jan T. M. Lenaerts, Nander Wever, Mana Inoue, Frank Pattyn, Sainan Sun, Sarah Wauthy, Jean-Louis Tison, and Willem Jan van de Berg
The Cryosphere, 14, 3367–3380, https://doi.org/10.5194/tc-14-3367-2020, https://doi.org/10.5194/tc-14-3367-2020, 2020
Short summary
Short summary
Ice rises are elevated parts of the otherwise flat ice shelf. Here we study the impact of an Antarctic ice rise on the surrounding snow accumulation by combining field data and modeling. Our results show a clear difference in average yearly snow accumulation between the windward side, the leeward side and the peak of the ice rise due to differences in snowfall and wind erosion. This is relevant for the interpretation of ice core records, which are often drilled on the peak of an ice rise.
Cited articles
Abdalati, W., Zwally, H. J., Bindschadler, R., Csatho, B., Farrell,
S. L., Fricker, H. A., Harding, D., Kwok, R., Lefsky,
M., Markus, T., Marshak, A., Neumann, T., Palm,
S., Schutz, B., Smith, B., Spinhirne, J., and Webb, C.: The ICESat-2 laser altimetry mission, P. IEEE, 98, 735–751, https://doi.org/10.1109/jproc.2009.2034765, 2010. a
Adodo, F. I., Remy, F., and Picard, G.: Seasonal variations of the backscattering coefficient measured by radar altimeters over the Antarctic Ice Sheet, The Cryosphere, 12, 1767–1778, https://doi.org/10.5194/tc-12-1767-2018, 2018. a
Aublanc, J., Moreau, T., Thibaut, P., Boy, F., Rémy, F., and Picot, N.: Evaluation of SAR altimetry over the antarctic ice sheet from CryoSat-2 acquisitions, Adv. Space Res., 62, 1307–1323, https://doi.org/10.1016/j.asr.2018.06.043, 2018. a, b, c
Blarel, F. and Legresy, B.: Investigations on the Envisat RA2 Doppler slope correction for ice sheets, in: European Space Agency-CNES Symp., 24–29 September 2012, Venice, Italy, edited by: Ouwehand, L., vol. 710, p.103, ISBN 978-92-9221-274-2, 2012. a
Bobach, T. A.: Natural Neighbor Interpolation – Critical Assessment and New Contributions, PhD thesis, Technische Universität Kaiserslautern, https://kluedo.ub.unikl.de/frontdoor/deliver/index/docId/2104/file/diss.bobach.natural.neighbor.20090615.pdf (last access: 9 June 2022), 2009. a
Brenner, A. C., Bindschadler, R. A., Thomas, R. H., and Zwally, H. J.: Slope-induced errors in radar altimetry over continental ice sheets, J. Geophys. Res., 88, 1617, https://doi.org/10.1029/jc088ic03p01617, 1983. a, b
Cooper, A.: Slope Correction By Relocation For Satellite Radar Altimetry, in: 12th Canadian Symposium on Remote Sensing Geoscience and Remote Sensing Symposium, IEEE, 10–14 July 1989, Vancouver, BC, Canada, https://doi.org/10.1109/igarss.1989.577978, 1989. a
European Space Agency: L1b LRM Precise Orbit. Baseline D, https://doi.org/10.5270/CR2-cbow23i, 2019a. a
European Space Agency: L2 LRM Precise Orbit. Baseline D, https://doi.org/10.5270/CR2-k1o4pyh, 2019b. a
European Space Agency: New Ice Baseline E and Near Real Time Processors, https://earth.esa.int/eogateway/news/new-ice-baseline-e-and-near-real-time-processors (last access: 18 April 2022), 2021. a
Hai, G., Xie, H., Du, W., Xia, M., Tong, X., and Li, R.: Characterizing slope correction methods applied to satellite radar altimetry data: A case study around Dome Argus in East Antarctica, Adv. Space Res., 67, 2120–2139, https://doi.org/10.1016/j.asr.2021.01.016, 2021. a, b, c, d
Helm, V., Humbert, A., and Miller, H.: Elevation Model of Greenland derived from CryoSat-2 in the period 2011 to 2013, links to DEM and uncertainty map as GeoTIFF, Pangaea [data set], https://doi.org/10.1594/PANGAEA.831393, 2014b. a, b, c, d
Howat, I. M., Porter, C., Smith, B. E., Noh, M.-J., and Morin, P.: The Reference Elevation Model of Antarctica, The Cryosphere, 13, 665–674, https://doi.org/10.5194/tc-13-665-2019, 2019. a
Hurkmans, R. T. W. L., Bamber, J. L., and Griggs, J. A.: Brief communication “Importance of slope-induced error correction in volume change estimates from radar altimetry”, The Cryosphere, 6, 447–451, https://doi.org/10.5194/tc-6-447-2012, 2012. a
Lacroix, P., Dechambre, M., Legrésy, B., Blarel, F., and Rémy, F.: On the use of the dual-frequency ENVISAT altimeter to determine snowpack properties of the Antarctic ice sheet, Remote Sens. Environ., 112, 1712–1729, https://doi.org/10.1016/j.rse.2007.08.022, 2008. a
Meloni, M., Bouffard, J., Parrinello, T., Dawson, G., Garnier, F., Helm, V., Di Bella, A., Hendricks, S., Ricker, R., Webb, E., Wright, B., Nielsen, K., Lee, S., Passaro, M., Scagliola, M., Simonsen, S. B., Sandberg Sørensen, L., Brockley, D., Baker, S., Fleury, S., Bamber, J., Maestri, L., Skourup, H., Forsberg, R., and Mizzi, L.: CryoSat Ice Baseline-D validation and evolutions, The Cryosphere, 14, 1889–1907, https://doi.org/10.5194/tc-14-1889-2020, 2020. a
National Snow and Ice Data Center (NSIDC): ATL06 release 005 known issues, https://nsidc.org/sites/nsidc.org/files/technical-references/ICESat2_ATL06_Known_Issues_v005.pdf (last access: 18 April 2022), 2021. a
Noh, M.-J. and Howat, I. M.: Automated stereo-photogrammetric DEM generation at high latitudes: Surface Extraction with TIN-based Search-space Minimization (SETSM) validation and demonstration over glaciated regions, GISci. Remote Sens., 52, 198–217, https://doi.org/10.1080/15481603.2015.1008621, 2015. a
Noh, M.-J. and Howat, I. M.: The Surface Extraction from TIN based Search-space Minimization (SETSM) algorithm, ISPRS J. Photogramm., 129, 55–76, https://doi.org/10.1016/j.isprsjprs.2017.04.019, 2017. a
Porter, C., Morin, P., Howat, I., Noh, M.-J., Bates, B., Peterman, K., Keesey, S., Schlenk, M., Gardiner, J., Tomko, K., Willis, M., Kelleher, C., Cloutier, M., Husby, E., Foga, S., Nakamura, H., Platson, M., Wethington Jr., M., Williamson, C., Bauer, G., Enos, J., Arnold, G., Kramer, W., Becker, P., Doshi, A., D'Souza, C., Cummens, P., Laurier, F., and Bojesen, M.: ArcticDEM, Harvard Dataverse, https://doi.org/10.7910/DVN/OHHUKH, 2018. a
Remy, F., Mazzega, P., Houry, S., Brossier, C., and Minster, J.: Mapping of the Topography of Continental Ice by Inversion of Satellite-altimeter Data, J. Glaciol., 35, 98–107, https://doi.org/10.3189/002214389793701419, 1989. a
Schröder, L., Horwath, M., Dietrich, R., Helm, V., van den Broeke, M. R., and Ligtenberg, S. R. M.: Four decades of Antarctic surface elevation changes from multi-mission satellite altimetry, The Cryosphere, 13, 427–449, https://doi.org/10.5194/tc-13-427-2019, 2019. a
Slater, T., Shepherd, A., McMillan, M., Muir, A., Gilbert, L., Hogg, A. E., Konrad, H., and Parrinello, T.: A new digital elevation model of Antarctica derived from CryoSat-2 altimetry, The Cryosphere, 12, 1551–1562, https://doi.org/10.5194/tc-12-1551-2018, 2018. a
Smith, B., Fricker, H. A., Holschuh, N., Gardner, A. S., Adusumilli, S., Brunt, K. M., Csatho, B., Harbeck, K., Huth, A., Neumann, T., Nilsson, J., and Siegfried, M. R.: Land ice height-retrieval algorithm for NASA's ICESat-2 photon-counting laser altimeter, Remote Sens. Environ., 233, 111352, https://doi.org/10.1016/j.rse.2019.111352, 2019.
a
Smith, B., Fricker, H. A., Gardner, A., Siegfried, M. R., Adusumilli, S., Csathó, B. M., Holschuh, N., Nilsson, J., Paolo, F. S., and the ICESat-2 Science Team: ATLAS/ICESat-2 L3A Land Ice Height, Version 4, National Snow and Ice Data Center (NSIDC), https://doi.org/10.5067/ATLAS/ATL06.004, 2020a. a
Smith, B., Hancock, D., Harbeck, K., Roberts, L., Neumann, T., Brunt, K., Fricker, H., Gardner, A., Siegfried, M., Adusumilli, S., Csathó, B., Holschuh, N., Nilsson, J., and Paolo, F.: Algorithm Theoretical Basis Document (ATBD) for Land Ice Along-Track Height Product (ATL06), Tech. rep., ICESat-2 Project Science Office, https://nsidc.org/sites/nsidc.org/files/technical-references/ICESat2_ATL06_ATBD_r004.pdf (last access: 9 June 2022), 2020b. a
Smith, B., Hancock, D., Harbeck, K., Roberts, L., Neumann, T., Brunt, K., Fricker, H., Gardner, A., Siegfried, M., Adusumilli, S., Csathó, B., Holschuh, N., Nilsson, J., and Paolo, F.: ICESat-2 Algorithm Theoretical Basis Document for Land Ice Height (ATL06) Release 005, https://nsidc.org/sites/nsidc.org/files/technical-references/ICESat2_ATL06_ATBD_r005.pdf (last access: 18 April 2022), 2021. a
Wingham, D. J., Rapley, C. G., and Griffiths, H.: New Techniques in Satellite Altimeter Tracking Systems, in: Digest – International Geoscience and Remote Sensing Symposium (IGARSS), 8–11 September 1986, vol. ESA SP-254, Zurich, 1339–1344, https://www.researchgate.net/publication/269518510_New_Techniques_in_Satellite_Altimeter_Tracking_Systems (last access: 9 June 2022), 1986. a
Short summary
This study proposes a new method for correcting the slope-induced errors in satellite radar altimetry. The slope-induced errors can significantly affect the height estimations of ice sheets if left uncorrected. This study applies the method to radar altimetry data (CryoSat-2) and compares the performance with two existing methods. The performance is assessed by comparison with independent height measurements from ICESat-2. The assessment shows that the method performs promisingly.
This study proposes a new method for correcting the slope-induced errors in satellite radar...