What was the purpose of TomoSense?

The TomoSense experiment was conceived to provide the scientific community with unprecedented data to study the features of radar scattering from temperate forests, comprising tomographic and fully polarimetric SAR surveys at P-, L-, and C-band, acquired in mono- and bistatic mode by simultaneously flying two aircraft. The TomoSense dataset is complemented by a detailed forest census, Terrestrial Laser Scanning (TLS), and Airborne Lidar Scanning (ALS) products.

Specific objectives of TomoSense were to:

• assess the potential and quantify the information content of C-band single-pass interferometry and tomography for forest mapping and parameter retrieval
• document complementarities and synergies of P-, L- and C-band SAR
• derive initial recommendations with respect to P-, L- and C-band missions and mission concepts
• provide a unique database for further analyses in the context of future studies on Earth Observation missions

A significant part of data processing activities was dedicated to interferometric and tomographic calibration of SAR data, necessary to finely estimate platform motion and achieve accurate tomographic focusing. Calibration activities resulted in the generation of finely coregisted, phase calibrated, and ground steered complex SAR image stacks at all frequency bands, which were included in the final data delivery to ESA. Calibrated image stacks were afterwards processed to generate multi-frequency mono- and bi-statictomographic cubes representing forest scattering in three dimensions, intended to serve as the basis for all subsequent scientific analyses and also included in the final data delivery.

Scientific analyses were conducted by investigating the connection of tomographic cubes to biophysical parameters. Essential to these activities was the availability of a large amount of biophysical information from independent measurements, including field-works, TLS, and ALS. In particular, analysis of TLS data allowed for the generation of new allometric relations, which resulted in an improved AGB (Above Ground Access) map of the whole area.

What was the outcome of TomoSense?

C-band: Tomographic analysis of C-band data indicates that the residual coherence in repeat-pass interferograms is mostly determined by scattering from the ground level, whereas the signal from the forest canopy is nearly impossible to detect because of temporal decorrelation. Analysis of interferograms formed by mono- and bi-static data collected in the same flight reveals that tall forests decorrelate almost immediately because of wind gusts, thus confirming the results from the BorealScat experiment. However, tomographic processing allowed for exploiting vestigial coherence from the vegetation, resulting in the possibility to detect forest canopies in parts of the image and observe a good match with regards to Lidar height.

Overall, results obtained at this site indicate that C-band waves care capable of penetrating down to the ground level. This finding provides an element in support of the feasibility of C-band tomography of temperate forests, clearly provided that acquisitions are taken at a temporal baselines at a few tens of milliseconds.

Considering the case of a two-satellite bistatic mission, fulfilling this requirement would entail to fly the formation within an along-track baseline systematically within a few tens of metres. Such a requirement was fulfilled by the TanDEM-X system. Still, it might turn out to be too demanding for small satellite or opportunity missions. In this case, zero-temporal baseline could only be achieved by using formations of three or more satellites.

Concerning airborne campaigns, the flight formation designed for the TomoSense experiment proved successful in delivering the correct range of across-track baselines, whereas flight safety imposed to fly an along-track baseline larger than planned. A valuable alternative design is described by Treuhaft et al in by flying three phase centers on the same aircraft, whose altitude was let vary across different overpasses. For C-band, however, the problem of such a design is that it requires to vary flight altitude from about 1000 m to over 8000 m. Accordingly, it could never be implemented using small aircrafts and low-power Radar sensors. Another alternative would be to use three aircraft uniformly spaced in the along-track direction. Such a formation would allow forming a zero-temporal baseline interferogram by using the mono-static image collected by the central sensor and the bi-static image collected by the sensors at the two ends of the formation. In this case, the fundamental requirement would be to ensure that the central sensor is located at the same along-track distance with regards to the ones at the two ends to within a few metres.

P- and L-band: both P and L band are observed to provide sensitivity to the whole vegetation layer, in that both frequencies allow for a clear detection of terrain and forest canopies. Moreover, both frequencies are robust with regards to temporal decorrelation over few hours, resulting in the possibility to produce high-quality tomographic imaging from repeat-pass campaign data. Accordingly, no fundamental difference was observed between P- and L-band at this forest site, but rather different sensitivities to different forest elements, L-band being more sensitive to canopy scattering and P-band to scattering from the terrain level (which includes double-bounce scattering). Concerning the sensitivity to forest AGB, observations for both frequencies showed the fundamental role played by tomographic layering, resulting in a much clearer connection to AGB than using intensity data only.

Interestingly, within this study the use of normalized tomographic indicators like fractional volume intensity was observed to provide sensitivity to AGB as well, the best result being assessed in slightly over 20% on the aggregated forest class using bistatic L-band data.

This preliminary result indicates the potential to explore a new direction to retrieve AGB without requiring absolute radiometric calibration. If confirmed, through further research and at different test sites, this result would indicate the possibility for accurate observation of forested areas from space using formation of small satellites, for which absolute radiometric accuracy would not be required, and fine synchronization could be achieved in post-processing. In this context L-band appears to have advantages over P-band, due to the smaller antenna size, larger available bandwidth, as well as the current international interest towards this frequency band. In addition to that, the concept could as well be implemented by adding a few passive companion satellites to existing or planned missions.

Download the TomoSense Final Report

Digital Object Identifier: European Space Agency, 2024, TomoSense campaign, https://doi.org/10.57780/esa-1206ab0

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