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. 2017 Aug 16;17(8):1889.
doi: 10.3390/s17081889.

Accuracy of Flight Altitude Measured with Low-Cost GNSS, Radar and Barometer Sensors: Implications for Airborne Radiometric Surveys

Matteo Albéri  1   2 , Marica Baldoncini  3   4 , Carlo Bottardi  5   6 , Enrico Chiarelli  7 , Giovanni Fiorentini  8   9 , Kassandra Giulia Cristina Raptis  10 , Eugenio Realini  11 , Mirko Reguzzoni  12 , Lorenzo Rossi  13 , Daniele Sampietro  14 , Virginia Strati  15 , Fabio Mantovani  16   17
Affiliations

Accuracy of Flight Altitude Measured with Low-Cost GNSS, Radar and Barometer Sensors: Implications for Airborne Radiometric Surveys

Matteo Albéri et al. Sensors (Basel). .

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: IMU; airborne gamma-ray spectrometry; barometric sensors; low-cost GNSS; radar altimeter.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Figure A1
Figure A1
Plot of the β(μ(E),z) factor as function of the survey altitude z: the red, green and blue line refer respectively to the 40K, 214Bi (238U) and 208Tl (232Th) gamma emission energies.
Figure 1
Figure 1
The left panel shows a map of the paths flown during the four surveys over the sea between Forte dei Marmi (LU) and Marina di Pisa (PI) in Italy. The four panels on the right show the mean altitude profiles measured by GPSABC of each flight.
Figure 2
Figure 2
Radgyro, the autogyro used for all the surveys described in Table 1.
Figure 3
Figure 3
Scheme of the placement of the different devices on the Radgyro: (A) GNSS antenna (GPSA), (B) GNSS antenna (GPSB), (C) GNSS antenna (GPSC), (D) GNSS antenna connected to IMU (GPSIMU), (E) pressure and temperature sensors of IMU (PTIMU), (F) pressure and temperature sensors (PT), (G) radar altimeter (ALT), (H) gamma spectrometer NaI(Tl). GPSA, GPSB and GPSC are placed at the following relative distances: dGPSAB = dGPSAC = (3.83 ± 0.01) m and dGPSBC = (1.96 ± 0.01) m.
Figure 4
Figure 4
A typical situation of flight over the sea with the Radgyro.
Figure 5
Figure 5
Percentage of outliers in the ALT dataset as a function of the orthometric height. The altitude of 340 m has been identified has a threshold above which the ALT dataset has been excluded from the global analysis.
Figure 6
Figure 6
(a) Reconstructed distance between GPSA and GPSB as a function of time during a portion of F15. The dashed red line represents the (3.83 ± 0.01) m reference distance and the brown line represents the average reconstructed dGPSAB during the flight. The large fluctuations observed in the reconstructed distance when flying over the sea are strongly reduced when flying over land, in particular when flying more than 3 km far from the coast (point A). (b) Mean height above the ground level z(m) (digital elevation model is subtracted) measured by GPSABC. (c) Flight path of F15.
Figure 7
Figure 7
Boxplot of the distribution of dGPSBC as a function of the orthometric height H for entire 0.2 Hz dataset. The blue line represents the (1.96 ± 0.01) m reference distance between GPSB and GPSC. Black points represent outlier data.
Figure 8
Figure 8
In the upper panel are shown the percentages of outliers identified in the dGPSAB, dGPSAC and dGPSBC datasets as function of the orthometric height H. In the bottom panel the acquisition statistics is as function of the orthometric height.
Figure 9
Figure 9
Temporal profile of the pressure measured by PT (in blue) and PTIMU (in red) not calibrated, and of the Radgyro horizontal velocity (in black) before the take-off. When the back screw is turned on, the PT sensor, which is significantly exposed to the air flux, measures a depression (point A). The pressure variation registered by both sensors in B is due to the rapid increase of velocity during the taxiing. The accelerating run along a runway starts in C and in D the aircraft takes off.
Figure 10
Figure 10
Linear regression between HGPSABC and HPT data for F11. In blue and red are reported the calibrated and not-calibrated barometric data respectively. The black straight lines are the linear fits to data: in both cases r2 = 0.999.
Figure 11
Figure 11
Distribution of σ(H) (standard deviations of heights) in the range (35–66) m (red solid line), (79–340) m (blue solid line) and (340–2194) m (green solid line) measured at 1 Hz.
Figure 12
Figure 12
Distribution of the residuals δHiJ in the (464–2194) m range of altitude: GPSIMU dataset in solid green line, PTIMU and PT dataset is reported in solid blue line, and GPSABC dataset in solid red line.
Figure 13
Figure 13
Distribution of σ(H) (standard deviations of heights) calculated for GPSABC built-in solution (red solid line) and with double-difference post-processing (blue solid line), in the altitude ranges (35–66) m (panel a) and (79–2194) m (panel b).
Figure 14
Figure 14
Distribution of σ(H) (standard deviations of heights) in the altitude ranges (35–66) m (red solid line), (79–340) m (blue solid line) and (340–2194) m (green solid line) measured at 0.2 Hz.
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