sensors Article Accuracy of Flight Altitude Measured with Low-Cost GNSS, Radar and Barometer Sensors: Implications for Airborne Radiometric Surveys Matteo Albéri 1,2,* ID , Marica Baldoncini 1,2, Carlo Bottardi 1,2 ID , Enrico Chiarelli 3, Giovanni Fiorentini 1,2, Kassandra Giulia Cristina Raptis 3, Eugenio Realini 4, Mirko Reguzzoni 5, Lorenzo Rossi 5, Daniele Sampietro 4, Virginia Strati 3 and Fabio Mantovani 1,2 ID 1 Department of Physics and Earth Sciences, University of Ferrara, Via Saragat, 1, 44122 Ferrara, Italy; [email protected] (M.B.); [email protected] (C.B.); fi[email protected] (G.F.); [email protected] (F.M.) 2 Ferrara Section of the National Institute of Nuclear Physics, Via Saragat, 1, 44122 Ferrara, Italy 3 Legnaro National Laboratory, National Institute of Nuclear Physics, Via dell’Università 2, 35020 Legnaro (Padova), Italy; [email protected] (E.C.); [email protected] (K.G.C.R.); [email protected] (V.S.) 4 Geomatics Research & Development (GReD) srl, Via Cavour 2, 22074 Lomazzo (Como), Italy; [email protected] (E.R.); [email protected] (D.S.) 5 Department of Civil and Environmental Engineering (DICA), Polytechnic of Milan, Piazza Leonardo da Vinci 32, 20133 Milano, Italy; [email protected] (M.R.); [email protected] (L.R.) * Correspondence: [email protected]; Tel.: +39-329-0715328 Received: 7 July 2017; Accepted: 13 August 2017; Published: 16 August 2017 Abstract: Flight height is a fundamental parameter for correcting the gamma signal produced by terrestrial radionuclides measured during airborne surveys. The frontiers of radiometric measurements with UAV require light and accurate altimeters flying at some 10 m from the ground. We equipped an aircraft with seven altimetric sensors (three low-cost GNSS receivers, one inertial measurement unit, one radar altimeter and two barometers) and analyzed ~3 h of data collected over the sea in the (35–2194) m altitude range. At low altitudes (H < 70 m) radar and barometric altimeters provide the best performances, while GNSS data are used only for barometer calibration as they are affected by a large noise due to the multipath from the sea. The ~1 m median standard deviation at 50 m altitude affects the estimation of the ground radioisotope abundances with an uncertainty less than 1.3%. The GNSS double-difference post-processing enhanced significantly the data quality for H > 80 m in terms of both altitude median standard deviation and agreement between the reconstructed and measured GPS antennas distances. Flying at 100 m the estimated uncertainty on the ground total activity due to the uncertainty on the flight height is of the order of 2%. Keywords: airborne gamma-ray spectrometry; low-cost GNSS; barometric sensors; radar altimeter; IMU 1. Introduction Airborne Gamma-Ray Spectroscopy (AGRS) is a proximal remote sensing method that allows quantifying the abundances of natural (40K, 238U, 232Th) and artificial (e.g., 137Cs) radionuclides present in the topsoil (~30 cm depth) over relatively large scales. Studying the spatial distribution of these radionuclides is strategic for monitoring environmental radioactivity [1], producing thematic maps of geochemical interest [2–4], identifying radioactive orphan sources [5] or investigating areas potentially contaminated by nuclear fallout [6]. Sodium iodide scintillation detectors (NaI(Tl)) are widely employed in AGRS measurements thanks to the high portability and high detection efficiency which allow performing surveys over extended areas in reasonable times and minimizing costs. Sensors 2017, 17, 1889; doi:10.3390/s17081889 www.mdpi.com/journal/sensors Sensors 2017, 17, 1889 2 of 21 Sensors 2017, 17, 1889 2 of 21 in AGRS measurements thanks to the high portability and high detection efficiency which allow performing surveys over extended areas in reasonable times and minimizing costs. InIn the last last decade decade there there has has been been a strong a strong effort effort in improving in improving spectral spectral analysis analysis techniques techniques which whichled not led only not to only high-accuracy to high-accuracy identification identification of radionuclides of radionuclides present presentin the environment in the environment [7], but also [7], butto the also possibility to the possibility of performing of performing real time real surveys, timesurveys, especially especially in the framework in the framework of homeland of homeland security securityapplications applications [8]. The [ 8].spread The spread of Unmanned of Unmanned Aerial Aerial Vehicles Vehicles (UAVs) (UAVs) is isboosting boosting research research andand developmentdevelopment inin the the field field of AGRSof AGRS applied applied to innovative to innovative sectors, sectors, such as precisionsuch as farmingprecision or farming emergency or responseemergency in response case of nuclear in case accidents, of nuclear both accidents, in terms bo ofth thein terms technologies of the technologies employed (e.g., employed prototypes (e.g., equippedprototypes with equipped CdZnTe with detectors CdZnTe [9]) detectors and of spectral [9]) and analysis of spectral methods analysis [10]. UAVs methods equipped [10]. withUAVs a lightweightequipped with gamma a lightweight spectrometer gamma typically spectrometer fly at altitudes typically and fly speeds at altitudes lower than and aspeeds helicopter, lower with than the a objectivehelicopter, of with enhancing the objective the terrestrial of enhancing gamma the signal terrestrial intensity gamma [11]. signal intensity [11]. InIn thesethese newnew technologicaltechnological scenariosscenarios aa preciseprecise evaluationevaluation ofof flightflight altitudealtitude isis mandatorymandatory forfor avoidingavoiding systematicsystematic effects effects in thein gammathe gamma signal signal corrections. corrections. In the lastIn decadethe last sophisticated decade sophisticated analytical techniquesanalytical techniques based on inverse based problemon inverse methods problem [12 ]methods as well as [12] Monte as well Carlo as simulations Monte Carlo [13 simulations] have been proposed[13] have forbeen studying proposed Digital for Elevationstudying ModelDigital (DEM) Elevation effect Model corrections (DEM) together effect corrections with corresponding together uncertaintieswith corresponding in airborne uncertainties gamma-ray in airborne spectrometry. gamma-ray This spectrometry. work addresses This these work topics addresses improving these thetopics analysis improving of data the collected analysis from of data seven collected altimeters from and seven reducing altimeters a source and of uncertaintyreducing a likesource DEM. of Inuncertainty particular like altitude DEM. measurements In particular altitude have been meas performedurements have with fourbeen low-costperformed GNSS with receivers, four low-cost one radarGNSS altimeter receivers, and one two radar low-cost altimeter barometric and two sensors low-cost in a barometric series of flights sensors over in the a series sea exploring of flights a wideover rangethe sea of exploring altitudes (froma wide 35 range to 2194 of altitudes m; Table1 (from and Figure 35 to 21941). The m; goalTable of 1 this and paper Figure was 1). toThe estimate goal of the this accuraciespaper was ofto flightestimate altitude, the accuracies investigating of flight statistical altitude, and investigating systematic effects statistical due to and calibration systematic methods, effects post-processingdue to calibration analysis methods, and post-processing sensor performances. analysis and sensor performances. Figure 1. The left panel shows a map of the paths flown during the four surveys over the sea between Figure 1. The left panel shows a map of the paths flown during the four surveys over the sea between ForteForte deidei MarmiMarmi (LU)(LU) andand MarinaMarina didi PisaPisa (PI)(PI) inin Italy.Italy. TheThe fourfour panelspanels onon thethe rightright showshow thethe meanmean altitudealtitude profilesprofiles measuredmeasured byby GPSABCGPSABC ofof eacheach flight.flight. Sensors 2017,, 17,, 18891889 3 of 21 Table 1. Main parameters of the four flights. Hmin and Hmax (minimum and maximum height) refer to Tablethe flight 1. Main height parameters above sea of level the four calculated flights. by Hmin averagingand Hmax the(minimum measurements and maximum of the different height) sensors. refer to Averagethe flight horizontal height above and seavertical level speeds calculated are calculated by averaging using the the measurements data from GPSABC. of the different sensors. Average horizontal and vertical speeds are calculated using the data from GPSABC. Flight Hmin Hmax Acquisition Average Horizontal Average Vertical Date Time (CEST) ID (m) (m) Time (s) Speed (m/s) Speed (m/s) Acquisition Average Horizontal Average Vertical Flight ID Date Time (CEST) H (m) H (m) F11 30/03/16 17:42:11–19:29:38 79 min 2018 max 6447 Time (s) Speed18.9 (m/s) Speed0.8 (m/s) F12 31/03/16 18:13:55–18:33:12 129 237 1158 15.5 0.5 F11 30/03/16 17:42:11–19:29:38 79 2018 6447 18.9 0.8 F14 F1205/04/16 31/03/16 16:37:15–17:33:0418:13:55–18:33:12 464 1292194 2373350 115821.1 15.5 0.50.8 F15 F1405/04/16 05/04/16 19:15:19–19:27:3916:37:15–17:33:04 35 46466 2194740 335034.4 21.1 0.80.6 F15 05/04/16 19:15:19–19:27:39 35 66 740 34.4 0.6 2. Instruments and Methods 2. Instruments and Methods The aircraft used for the surveys is the Radgyro (Figure 2–4), an experimental autogyro devoted to airborneThe aircraft multiparametric