Experimental Assessment of Microwave Penetration Depth in Layered Soil by Wideband (0.3–6 GHz) Transmissometry and Reflectometry

The depth to which a Global Navigation Satellite System (GNSS) signal senses soil moisture is set by the electromagnetic power penetration depth (δp), the depth at which the power transmitted into the soil falls to 1/e, seldom measured directly and instead inferred from dielectric mixing models whose parameters are fitted to permittivity rather than to propagation through real, stratified soil. This work reports a controlled, wideband (0.3–6 GHz) Vector Network Analyzer (VNA) experiment in which two identical Left-Handed Circular Polarization (LHCP) spiral antennas measure the reflection (S11, S22) and transmission (S21) of a four-layer silt-loam soil column, a laboratory analogue of GNSS Reflectometry (GNSS-R) and GNSS Transmissometry (GNSS-T) geometries. Soil texture and per-layer moisture were measured, not assumed, with a layered-medium transfer-matrix forward model as the reference from which the depths are derived. In this monostatic, normal-incidence, same-hand circular arrangement the reflection channel was insensitive to the moisture profile due to helicity reversal on specular reflection suppressing the same-hand return by more than 15 dB, below the antenna’s own port reflection, whereas transmission preserved it. This is a configuration effect, not a general limitation of GNSS-R or of an opposite-hand dual-polarized reflectometer. Neither the Peplinski–Dobson nor the Mironov MBSDM model outperformed the other across all bands: Peplinski–Dobson fits best in 27 of the 48 moisture–band cells, whereas MBSDM performs best in the driest and two wettest states and in a few additional bins. Using the best-fitting model per band, and separating the parametric uncertainty of that model from the structural difference between the two, the model-derived δp decreases with moisture and frequency: under the driest evaluated state (volumetric moisture content mv=10.3%) it spans 26.4−1.5+1.8 cm at 868 MHz, a value higher than the 20 cm of soil actually sampled and therefore an extrapolation of the selected model, to 9.5−0.6+0.7 cm at 2.2 GHz, falling at mv=20.8% to 15.9−0.6+0.6 cm and 9.8−1.4+2.1 cm, respectively. At L1, L2, and L5 carriers, δp≈15–20 cm at mv=10.3% and δp≈5–7 cm at mv=41.3%. A second estimate, in which no dielectric model intervenes, is obtained from the wet-slab thickness sweep, the excess attenuation following from the slope of the differential transmissivity on propagation distance: at mv=22.9% it bounds δp from above at 20.9−1.8+2.2 cm at 868 MHz, 12.6−0.1+0.1 cm at L1, and 9.8−0.1+0.1 cm at 2.2 GHz, agreeing to within about one centimeter with the model-derived depths interpolated to the same moisture at the GNSS carriers and at S-band. Throughout, δp is a property of the soil alone. It is not the effective sensing depth of a GNSS observation, which also depends on the observation geometry, the polarization, the surface roughness, and the forward model used in the inversion.