Measuring Biophysical Parameters of Wheat Canopy with MHz- and GHz-Frequency Range Impulses Employing Contactless GPR

• Published in: Remote sensing (Basel, Switzerland) Vol. 16; no. 19; p. 3547
• Main Authors: Muzalevskiy, Konstantin, Fomin, Sergey, Karavayskiy, Andrey, Leskova, Julia, Lipshin, Alexey, Romanov, Vasily
• Format: Journal Article
• Language: English
• Published: Basel MDPI AG 01.10.2024
• Subjects: above-ground biomass (AGB); Accuracy; Aircraft; Antennas (Electronics); Bandwidths; Bias; Biomass; Canopies; canopy height; Crops; Farming systems; Frequencies; Frequency ranges; Ground penetrating radar; ground-penetrating radar (GPR); Horn antennas; Impulses; Investigations; Measurement; Network analysers; Precision farming; Radar systems; Remote sensing; Sensors; Soil analysis; Soil investigations; Soil mapping; Soil moisture; soil moisture under vegetation; soil roughness; Soil surfaces; Spectral emittance; Timing; unmanned aerial vehicle (UAV); Unmanned aerial vehicles; Vegetation; Wheat
• DOI: 10.3390/rs16193547
• ISSN: 2072-4292

In this paper, the advantages of the joint use of MHz- and GHz-frequency band impulses when employing contactless ground penetration radar (GPR) for the remote sensing of biomass, the height of the wheat canopy, and underlying soil moisture were experimentally investigated. A MHz-frequency band nanosecond impulse with a duration of 1.2 ns (average frequency of 750 MHz and spectrum bandwidth of 580 MHz, at a level of –6 dB) was emitted and received by a GPR OKO-3 equipped with an AB-900 M3 antenna unit. A GHz-frequency band sub-nanosecond impulse with a duration of 0.5 ns (average frequency of 3.2 GHz and spectral bandwidth of 1.36 GHz, at a level of −6 dB) was generated using a horn antenna and a Keysight FieldFox N9917B 18 GHz vector network analyzer. It has been shown that changes in the relative amplitudes and time delays of nanosecond impulses, reflected from a soil surface covered with wheat at a height from 0 to 87 cm and fresh above-ground biomass (AGB) from 0 to 1.5 kg/m2, do not exceed 6% and 0.09 ns, respectively. GPR nanosecond impulses reflected/scattered by the wheat canopy have not been detected. In this research, sub-nanosecond impulses reflected/scattered by the wheat canopy have been confidently identified and make it possible to measure the wheat height (fresh AGB up to 2.3 kg/m2 and height up to 104 cm) with a determination coefficient (R2) of ~0.99 and a bias of ~−7 cm, as well as fresh AGB where R2 = 0.97, with a bias = −0.09 kg/m2, and a root-mean-square error of 0.1 kg/m2. The joint use of impulses in two different MHz- and GHz-frequency bands will, in the future, make it possible to create UAV-based reflectometers for simultaneously mapping the soil moisture, height, and biomass of vegetation for precision farming systems.