ALTERNATIVE METHODOLOGIES FOR BORESIGHT CALIBRATION OF GNSS/INS-ASSISTED PUSH-BROOM HYPERSPECTRAL SCANNERS ON UAV PLATFORMS

Abstract

Low-cost unmanned aerial\nvehicles (UAVs) utilizing push-broom hyperspectral scanners are poised to\nbecome a popular alternative to conventional remote sensing platforms such as\nmanned aircraft and satellites. In order to employ this emerging technology in\nfields such as high-throughput phenotyping and precision agriculture, direct\ngeoreferencing of hyperspectral data using onboard integrated global navigation\nsatellite systems (GNSS) and inertial navigation systems (INS) is required.\nDirectly deriving the scanner position and orientation requires the spatial and\nrotational relationship between the coordinate systems of the GNSS/INS unit and\nhyperspectral scanner to be evaluated. The spatial offset (lever arm) between\nthe scanner and GNSS/INS unit can be measured manually. However, the angular\nrelationship (boresight angles) between the scanner and GNSS/INS coordinate\nsystems, which is more critical for accurate generation of georeferenced\nproducts, is difficult to establish. This research presents three alternative calibration\napproaches to estimate the boresight angles relating hyperspectral push-broom\nscanner and GNSS/INS coordinate systems. For reliable/practical estimation of\nthe boresight angles, the thesis starts with establishing the optimal/minimal\nflight and control/tie point configuration through a bias impact analysis\nstarting from the point positioning equation. Then, an approximate calibration\nprocedure utilizing tie points in overlapping scenes is presented after making\nsome assumptions about the flight trajectory and topography of covered terrain.\nNext, two rigorous approaches are introduced – one using Ground Control Points\n(GCPs) and one using tie points. The approximate/rigorous approaches are based\non enforcing the collinearity and coplanarity of the light rays connecting the\nperspective centers of the imaging scanner, object point, and the respective\nimage points. To evaluate the accuracy of the proposed approaches, estimated\nboresight angles are used for ortho-rectification of six hyperspectral UAV\ndatasets acquired over an agricultural field. Qualitative and quantitative\nevaluations of the results have shown significant improvement in the derived\northophotos to a level equivalent to the Ground Sampling Distance (GSD) of the\nused scanner (namely, 3-5 cm when flying at 60 m).