Abstract
Abstract. Magnetic interference source identification is a critical preparation step for magnetometer-mounted unmanned aircraft systems (UAS) used for high-sensitivity geomagnetic surveying. A magnetic field scanner was built for mapping the low-frequency interference that is produced by a UAS. It was used to compare four types of electric-powered UAS capable of carrying an alkali-vapour magnetometer: (1) a single-motor fixed-wing, (2) a single-rotor helicopter, (3) a quad-rotor helicopter, and (4) a hexa-rotor helicopter. The scanner's error was estimated by calculating the root-mean-square deviation of the background total magnetic intensity over the mapping duration; averaged values ranged between 3.1 and 7.4 nT. Each mapping was performed above the UAS with the motor(s) engaged and with the UAS facing in two orthogonal directions; peak interference intensities ranged between 21.4 and 574.2 nT. For each system, the interference is a combination of both ferromagnetic and electrical current sources. Major sources of interference were identified such as servo(s) and the cables carrying direct current between the motor battery and the electronic speed controller. Magnetic intensity profiles were measured at various motor current draws for each UAS, and a change in intensity was observed for currents as low as 1 A.
Highlights
The carriage was instrumented with the magnetic survey system intended for installation; a potassium-vapour total field (TF) magnetometer system (GSMP-35UAV, GEM Systems) powered by a 4 Ah lithium polymer (LiPo) battery and a triaxial fluxgate magnetometer (Mag649, Bartington Instruments) which recorded to a data acquisition system (DAS; Raspberry Pi 3) powered with a 1.8 Ah LiPo battery
The measurements of the background magnetic intensity were made along the track length without the Unmanned aircraft systems (UAS) present in order to a. measure the spatial distribution of the background and provide a correction for isolating the anomalous field associated with the UAS
Using the relationship between aliasing and the height-to-line-spacing ratio calculated for aeromagnetic surveys (Reid, 1980) and considering the limitation imposed on collection time by the UAS battery, a line spacing of 30 cm was chosen for the FW because of its larger dimensions, whereas a line spacing of 10 cm was chosen for the other UAS (Table 3)
Summary
Unmanned aircraft systems (UAS) are being used as an alternative method for traditional ground geomagnetic surveys on the scale of < 1 km (Eck and Imbach, 2012; Macharet et al, 2016; Parshin et al, 2018; Parvar et al, 2018; Versteeg et al, 2010), < 10 km (Kaneko et al, 2011; Koyama et al, 2013; Malehmir et al, 2017; Wood et al, 2016), and larger (Anderson and Pita, 2005; Cherkasov and Kapshtan, 2018; Funaki et al, 2014; Pei et al, 2017; Wenjie, 2014). One method has been to tow the magnetometer below the UAS at a distance where the interference becomes negligible, often reported as > 3 m (Cherkasov and Kapshtan, 2018; Koyama et al, 2013; Malehmir et al, 2017; Parvar et al, 2018; Walter et al, 2018) This can introduce new issues such as location and heading error (Walter et al, 2020), reduced flight stability, increased drag, and increased risk of impact damage to the magnetometer upon landing (Kaneko et al, 2011). As a complement to each mapping, interference profiles were collected at different motor current draws to illustrate the impact of amperage on the magnetic signature of the UAS
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