Active Technologies for Earth Observation
This collaborative project between the start-up Balamis and the Universitat Politècnica de Catalunya – BarcelonaTech, funded by Spanish Ministry of Science, Innovation and Universities and the Generalitat of Catalonia through its Industrial Doctorates programme, is framed in the field of Earth observation by means of active microwave imaging systems to improve our planet’s knowledge.
A better study of different ecosystems: forests, agricultural areas, urban areas, etc… using active microwave imaging systems can contribute to a better solution of the problems affecting our planet, such as climate change, biodiversity reduction, social changes, but above all, it can support the sustainable development of several economic activities. For example, in the case of forests, the information that can be provided by active microwave systems can help to better develop economic activities such as forest management, illegal logging control, biomass estimation or disasters management. In the case of agricultural areas, they can be very important for the development of precision agriculture. Active microwave systems have been also fundamental to monitor ground movements and subsidence phenomena, whether due to natural consequences (earthquakes, volcanoes, landslides, etc…) or anthropogenic (mining activities, extensive exploitation of aquifers, oil extraction, etc…). Additionally, these active microwave systems are being used, as far as possible, as early warning systems. Finally, this type of systems have also a scientific interest since they may support the development of future space borne missions or to increase our knowledge of both, man made and natural targets.
Currently, active microwave imaging systems focus almost exclusively on Synthetic Aperture Radars (SAR). These are an extremely useful tool for Earth observation as they are able to acquire data independently of weather or lighting conditions, with a high spatial resolution and at global scale when shipped in space platforms.
In a first period, covering from its conception in the 1950s to the early 1990s, SAR systems were characterized by having a single-channel nature and by being shipped on orbital platforms. But only from the early 90’s SAR technology has shown its full potential thanks to the appearance of multichannel or multidimensional SAR techniques. These include SAR interferometry (InSAR), SAR polarimetry (PolSAR), the combination of both in polarimetric SAR inteferometry (PolInSAR) and specially multitemporal SAR. As evidenced, multidimensional SAR systems allow the estimation of a greater number of geophysical and biophysical characteristics of the Earth’s surface. It is also from the 90s that SAR systems embarked on aircraft began to be developed, reducing operating costs and also allowing targeting particular areas of the Earth’s surface. Both, orbital and airborne SAR systems, despite their importance, have important limitations in those cases where a great flexibility is needed in terms of revisit time with a high temporal frequency or even through continuous observation. In the case of orbital systems, considering a single satellite, it is impossible to observe the same target in periods less than two or three times a month due to the limitations of the satellite’s orbit. This limitation could be partially resolved by airborne systems, but in many cases at a prohibitive cost.
To overcome the previous limitations, the design of the SAR system has recently begun to lead to the design and development of terrestrial SAR or GBSAR (Ground Based SAR) systems. The main advantage of GBSAR systems is that they allow greater flexibility when monitoring a target with high temporal flexibility, and also its ease of deployment and much lower operating cost. The high stability of the terrestrial platform and its flexibility in terms of revisit time make GBSAR systems an ideal option to monitor and detect changes in local areas of interest with high spatial resolution and with a temporal resolution of minutes. In this context, the first GBSAR systems that were developed were based on Vector Network Analyzers (VNA). This technology allows great versatility in signal generation terms, as well as the development of a GBSAR system without the need for complex development of electronic microwave systems. However, this solution has the fundamental drawback of requiring a long acquisition time for a single image. Nevertheless, the acquisition time of an image should be reduced, as much as possible, to avoid the appearance of distortions that impact negatively the quality of the final image.
To reduce the image acquisition time, GBSAR systems are currently designed as radar systems with an architecture based on the use of FMCW (Frequency Modulation Continuous Wave) continuous signals. These architectures allow to reduce the image acquisition time leading to high quality SAR images. Furthermore, the flexibility of this type of systems makes it possible to acquire images in InSAR and PolSAR modes. The main limitation of this solution is that the complete design and development of the radar hardware system is necessary, resulting into high costs and a long design time.
The current radar technology is undergoing a rapid transition from fully analog to digital systems, breaking also the chronic limitation of frequency band or bandwidth dependency and therefore allowing an increased versatility. In this context, the main objective of this R&D project is the design, implementation and validation of a commercial prototype of a reconfigurable, multi-frequency, GBSAR system.
Most current GBSAR systems focus almost exclusively on the development of systems at X-band systems (9 GHz) or higher frequencies. The novelty of this project that is the development of a dual RX channel, reconfigurable, multi-frequency, multi-mode GBSAR system capable to operate at various frequency bands, in both real and synthetic aperture modes, and in the later in Stop&Go and Fast Continuos modes to reduce acquisition times. In a first stage, the prototype operates from 0.5 GHz to 9 GHz, i.e., at P, L, C and X-bands, but with the possibility to operate a higher frequencies in the future. The main advantage to operate at different frequency bands, on the one hand, is that the frequency band can be adapted to the type of target observed, since the response of the target to the radar signal depends on the frequency. On the other hand, the targets of interest have different backscatter characteristics depending on the working frequency. Hence, a multifrequency system allow an improved characterization and study of the targets being observed. Its dual RX channel allows single-pass InSAR, as well as PolSAR acquisitions.
The final objective to develop the proposed GBSAR system in this R&D project is to be able to have a system whose reconfiguration capabilities makes it possible for the system to be adapted to the observation needs of a particular client.
Top image: First image acquisiton campaing in the area of Muntanya Rodona, Catalonia, Spain. Results at P, L, C and X-bands are presented below.