Low-Cost GNSS-R Altimetry on a UAV for Water-Level Measurements at Arbitrary Times and Locations.

Kaoru Ichikawa,Kei Yufu,Masanori Konda,Takuji Ebinuma

doi:10.3390/s19050998

Abstract

Together with direct Global Navigation Satellite System (GNSS) signals, the signals reflected at the water surface can be received by an unmanned aerial vehicle (UAV). From the range difference between two GNSS signal paths, the height of the UAV above the water level can be geometrically estimated using the weighted least squares method, called GNSS reflectometry (GNSS-R) altimetry. Experimental low-cost GNSS-R altimetry flights with a UAV were conducted at the coast of Lake Biwa, Japan. Although the height estimated by the GNSS-R altimeter included large short-term noises up to 8 m amplitude, it agreed well with the UAV altitude measured by the post-processed kinematic positioning. By selecting better weight functions in the least square method and using sufficient temporal averaging, the GNSS-R altimetry achieved accuracy in the order of 0.01 m if a sufficient number of GNSS satellites with high elevation angles were available. The dependency of the results on the weight functions is also discussed.

Highlights

Sea surface height is a fundamental variable in physical oceanography that is used in various studies, such as studies of waves, tides, tsunamis, geostrophic currents, and climate change
For the use of oceanography, a wide spatial coverage is required, and Global Navigation Satellite System (GNSS) receivers can be mounted on drifters [8], ships [9], unmanned aerial vehicles (UAVs), aircraft [10] and satellites [11]
With the slight tilt of the flat water surface in the radial direction, θ, the slant range from the actual reflection point to the antenna, h/ sin( Ei + 2θ ), differs from the assumed ideal range, h/ sin( Ei ), by 2hθ/ sin( Ei ) tan( Ei ). This means that the effect of tilting slopes would decrease as sin( Ei ) tan( Ei ) increases, so that we introduce another weight function as wst = sin( E) tan( E) in order to further account for noises caused by tilting slopes of the water surface

Summary

Introduction

Sea surface height is a fundamental variable in physical oceanography that is used in various studies, such as studies of waves, tides, tsunamis, geostrophic currents, and climate change. Precise and Innovative Radar Altimeter) [6], are planned to provide better spatial resolution and coverage, additional frequent height observations are necessary to satisfy the requirements of the temporal resolution. For the use of oceanography, a wide spatial coverage is required, and GNSS receivers can be mounted on drifters [8], ships [9], unmanned aerial vehicles (UAVs), aircraft [10] and satellites [11]. For those receivers on a platform, the vertical distance

Methods

Results

Discussion

Conclusion