Orbisat (Brazil)

Abstract. In a large number of scenarios and missions, the technical, operational and economical advantages of UAS-based photogrammetry and remote sensing over traditional airborne and satellite platforms are apparent. Airborne Synthetic Aperture Radar (SAR) or combined optical/SAR operation in remote areas might be a case of a typical "dull, dirty, dangerous" mission suitable for unmanned operation – in harsh environments such as for example rain forest areas in Brazil, topographic mapping of small to medium sparsely inhabited remote areas with UAS-based photogrammetry and remote sensing seems to be a reasonable paradigm. An example of such a system is the SARVANT platform, a fixed-wing aerial vehicle with a six-meter wingspan and a maximumtake- of-weight of 140 kilograms, able to carry a fifty-kilogram payload. SARVANT includes a multi-band (X and P) interferometric SAR payload, as the P-band enables the topographic mapping of densely tree-covered areas, providing terrain profile information. Moreover, the combination of X- and P-band measurements can be used to extract biomass estimations. Finally, long-term plan entails to incorporate surveying capabilities also at optical bands and deliver real-time imagery to a control station. This paper focuses on the remote-sensing concept in SARVANT, composed by the aforementioned SAR sensor and envisioning a double optical camera configuration to cover the visible and the near-infrared spectrum. The flexibility on the optical payload election, ranging from professional, medium-format cameras to mass-market, small-format cameras, is discussed as a driver in the SARVANT development. The paper also focuses on the navigation and orientation payloads, including the sensors (IMU and GNSS), the measurement acquisition system and the proposed navigation and orientation methods. The latter includes the Fast AT procedure, which performs close to traditional Integrated Sensor Orientation (ISO) and better than Direct Sensor Orientation (DiSO), and features the advantage of not requiring the massive image processing load for the generation of tie points, although it does require some Ground Control Points (GCPs). This technique is further supported by the availability of a high quality INS/GNSS trajectory, motivated by single-pass and repeat-pass SAR interferometry requirements.

In this work we focus on the Precise Topography- and Aperture-dependent algorithm, briefly PTA, proposed in the recent literature to remove the residual errors still affecting airborne SAR data focused with standard two-step Motion Compensation algorithms. More specifically, we first show that the azimuth-to-frequency mapping used by the PTA procedure is not fully appropriate, because it does not account for the presence of the residual motion errors themselves. Then, we show how to calculate the correct azimuth-to-frequency mapping able to fully account for the presence of the residual motion errors. Finally, we show how to modify the original PTA in such a way to properly exploit the correct azimuth-to-frequency mapping previously discussed.

Recently some experiments demonstrated that reliable DInSAR measurements can be achieved at any band and that further time-series analyses can be applied to airborne data. However, most of the airborne DInSAR results, including the time-series, published so far have focused on short-term analyses (baselines within hours or few days). This paper presents the first long-term airborne DInSAR survey at P- and X-band with temporal baselines of 1 year and 3 months. The survey was performed by the OrbiSAR system of OrbiSat under contract with Petrobras (PROTRAN), as part of a project to investigate the potential of DInSAR technique to identify, prior to structural damage, geohazards threats to the oil/gas pipelines in São Sebastião-SP, Brazil. After 1 year and 3 months, the P-band data have interferometric coherence equal or greater than 0.3 for approximately 80% of the imaged dense vegetated areas. At X-band, we achieved coherence equal or greater than 0.3 in the urban area. After removing all residual motion errors with proper up-to-date processing, we derived, for the P-band data, land movements with absolute accuracy in the order of centimeters. For X-band we derived land movement measurements with absolute accuracy in the order of millimeters. Through field work evidences, the paper analyses the causes of these centimeter to millimeter land movements, and how they are related to geodynamic processes and geohazard risks. A comparison between the DInSAR and in-loco inclinometer measurements is presented. The paper suggests some possible operational scenarios and discusses on the potential of the airborne DInSAR for geohazard risk monitoring.

Differential SAR Interferometry (DInSAR) is a remote sensing technique which allows monitoring ground deformation with accuracy of the order of the transmitted wavelength by exploiting the phase difference (interferogram) of two temporally separated SAR images relevant to the same area.

In this work we present the results of an airborne differential interferometry experiment carried out by using the ORBISAR X-band system. Use of a high track measurement accuracy allowed us to fulfill the requirements of Fornaro et al. (2003), where repeat pass phase accuracy has been quantified in terms of position measurement errors. The presented experiment can be considered a first step towards the development of operative airborne differential interferometry for accurate ground deformation monitoring.