Lake Icepack and Dry Snowpack Thickness Measurement Using Coherent Multipath Interference of Wideband Planck Radiation

Abstract

The seasonal terrestrial snowpack is an important source of water for many parts of the globe. The global quantification of the amount of water in the snowpack reservoir has been a long term objective of most remote sensing applications. Thus far, the primary means of quantifying the amount of snow on the ground has been via the differential scatter-darkening mechanism, such as 19 and 37 GHz brightness difference. This technique is region specific and depends on the statistics of snow grain sizes. While a time series of more than 35 years of passive microwave data has been made, progress in understanding the scatter-darkening brightness signature of snow continues, especially for forested areas where vegetation scattering confounds the signature.

In addition, monitoring the ice thickness is important in analyzing the pressure exerted to off-shore structures such as wind farms. It is also an essential parameter for the safety of ice fishing and ice skating activities. The current and traditional method of ice thickness measurement is by drilling holes through the ice, which is not only cumbersome but also dangerous. Hence, an accurate remote sensing technique is needed to safely and non-destructively measure the ice and snow thickness.

In this work, a novel microwave radiometric technique, wideband autocorrelation radiometry (WiBAR), is introduced. The radiometer offers a direct method to remotely measure the microwave propagation time difference of multipath microwave emission from low-loss layered surfaces, such as a dry snowpack and a freshwater lake icepack. The microwave propagation time difference through the pack yields a measure of its vertical extent; thus, this technique provides a direct measurement of depth. It is also a low-power sensing method since there is no transmitter. A simple geophysical forward model for the multipath interference phenomenon is presented, and the system requirements needed to design a WiBAR instrument are derived. Three different versions of WiBAR instruments operating at L-, S-, and X-band are fabricated from commercial-off-the-shelf (COTS) components. To validate the WiBAR method, simulated laboratory measurements are first performed using a microwave scene simulator circuit. Finally, to prove the potential of this technique as an inversion algorithm, many field measurements were conducted in different winter seasons in the Upper Midwest region, Michigan and Minnesota. It is demonstrated that a WiBAR instrument operating in the frequency range of 7-10 GHz (X-band) can directly measure the icepack thicknesses from nadir to 59 degree of incidence angles. The WiBAR was able to measure the lake icepack thicknesses in the range of 22-59 cm with an accuracy of about 2 cm over this range of incidence angles.