Lightweight mid-infrared methane sensor for unmanned aerial systems
The design and field performance of a compact diode laser-based instrument for measuring methane on unmanned aerial systems (UAS) is described. The system is based on open-path, wavelength modulation spectroscopy with a 3.27 µm GaSb laser. We design two versions of the sensor for a long-endurance fixed wing UAS and a rotary wing hexacopter, with instrument masses of 4.6 and 1.6 kg, respectively. The long-endurance platform was used to measure vertical profiles of methane up to 600 m in altitude and showed repeatability of 13 ppbv between multiple profiles. Additionally, the hexacopter system was used to evaluate the evolution of methane in the nocturnal boundary layer during the ScaleX field campaign in Germany, where measured data is consistent with supporting ground-based methane and meteorological measurements. Testing results on both platforms demonstrated our lightweight methane sensor had an in-flight precision of 5–10 ppbv Hz−1/2.
Methane is a strong greenhouse gas and plays a key role in determining the climate impact of sources such as wetlands, livestock, and oil and gas equipment [1, 2, 3]. There is a continuing need for technology to better understand and track the sources and sinks of methane at the local scale. Unmanned aerial systems (UAS) are attractive platforms for such measurements due to their flexibility and autonomous nature. In particular, small (<25 kg) UAS have low operation costs, relaxed regulatory requirements, and can operate near the surface, where most emissions originate. As an example, a near-infrared sensor (100 ppbv precision) equipped on a remote-controlled helicopter has been used to estimate the magnitude and temporal variability of methane emissions from a natural gas compressor station , and serves as an alternative to mobile laboratory and ground survey techniques which are more time-consuming or may miss elevated leak sources.
To leverage the potential of UAS, there is a need for high performance trace gas sensors meeting the payload constraints of typical fixed wing and copter platforms, including for methane . Due in part to this gap, air quality and trace gas monitoring with UAS is still a relatively new area of research. Methane instruments are available that have excellent performance, but are generally designed for aircraft-  or ground-based applications  where size, power consumption and mass are not significantly restricted. A sensor based on integrated cavity output spectroscopy (ICOS) technology from Los Gatos Research weighing 19.5 kg has been flown on the NASA SIERRA UAS platform , which like many long-endurance UAS has a large payload capacity. More compact methane sensors are available that typically either use standoff detection for path integrated measurements [9, 10], or have been targeted at high concentrations such as for direct sampling from natural gas leaks. In addition, several lightweight in situ methane sensors have previously been demonstrated for high-altitude balloon-borne or small UAS use [4, 11, 12]. These employed an open-path design, where air flows directly into the optical cell rather than using sampling equipment allowing for low weight and inherently high time response . However, each used near-infrared laser sources, so a further improvement in performance and weight is expected using a mid-infrared laser probing methane’s strong fundamental absorption band in the 3.3 μm region. A notable lightweight, mid-infrared system is the Tunable Laser Spectrometer, which has been used to constrain the possible levels of methane on Mars [13, 14].
We demonstrate for the first time a lightweight, mid-infrared, in situ methane sensor that was modified to operate on two different UAS platforms. The sensor employs open-path wavelength modulation spectroscopy (WMS) for sensitive detection of methane in the atmosphere. The first platform, a long-endurance fixed-wing UAS, had modest payload constraints and allowed us to test precision and stability on a multi-hour flight. The hexacopter system required additional reductions in mass and operates with much shorter flight times, but was well-suited for a nocturnal boundary layer application [15, 16], where methane accumulates in the stable, shallow layer near the surface. The sensor’s precision of 5–10 ppbv was adequate to resolve vertical gradients of atmospheric background methane (~2 ppmv) in both day- and night-time conditions, and meets previously estimated criteria of <40 ppbv needed to quantify emissions at a landfill , which may produce enhancements of 100–1000 s ppbv above the background level. The combination of both large, long-endurance, and small, flexible platforms allows for a variety of applications for probing the atmospheric boundary layer.
2 Design and methods
2.1 Description of instrument for both configurations
The common attributes of the sensor on both platforms are the core optical cell and most of the electronics components. The optical cell housing was fabricated using 3D printed ABS with spacing and rigidity maintained by three 8 mm hollow carbon fiber rods. A Herriott cell  was chosen due to its well-established stability in field environments and compact geometry, with an 11.2 cm base path and 2.7 m optical path length (N = 24 passes) confined by Ø (5.08 cm) gold-coated mirrors. As an open-path design, air moves directly through the optical cell based on atmospheric motions relative to the platform movement rather than using a pump. The laser and detector were attached to opposite sides of the cell and aligned using additional steering mirrors. We employed a GaSb distributed feedback (DFB) laser (Norcada), a LDTC0520 laser temperature and current controller (Wavelength Electronics), and an optically immersed HgCdTe photodetector with an AC coupled preamplifier (Vigo). A single board computer (Advantech) and PCIe-6251 data acquisition board (National Instruments) were dedicated to system control and data processing. Finally, a BMP180 sensor (Bosch) was situated at the edge of the electronics box to measure barometric pressure. While these aspects are the same for both configurations, details of the electronics box and mounting differed due to integration and flight operation needs.
2.1.1 Long-endurance system
2.1.2 Hexacopter system
Summary of sensor specifications in both configurations
5 ppbv Hz−1/2
10 ppbv Hz−1/2
0.68 kg (sensor head)
4.00 kg (control box)
1.45 kg (sensor head and control box)
0.14 kg (battery)
24 cm × 10 cm × 10 cm (sensor head )
20 cm × 26 cm × 11 cm (control box)
25 cm × 16 cm × 18 cm
For the hexacopter system, an Innovative Sensor Technology HYT 271 was added directly outside of the multipass cell to measure the ambient temperature and humidity. This is based on a proportional to absolute temperature (PTAT) measurement and capacitive polymer relative humidity measurement. Altitude and position were obtained from the hexacopter’s flight control, which incorporates both pressure and GPS information.
2.2 Spectroscopy and data processing
The harmonic signals were generated in real-time using a LabVIEW-based lock-in detection scheme  with methane and water vapor concentrations retrieved using a second harmonic (2f) peak-to-trough height calculation normalized by the first harmonic (1f) at the peak centers to account for intensity variations. The 2f/1f values were calibrated either by placing the whole sensor in an enclosed box and flowing calibration standards or with comparison to reference instruments, as described in Sect. 2.3. In both cases, a two-point calibration approach was used with a zero and linear slope.
In post processing, methane and supporting data were time synchronized to the GPS time and location information at 1 Hz. An internal GPS was used for the fixed-wing system, while for the hexacopter the copter GPS was used to avoid the need for an additional antenna. Data with low signal strength (defined as detector voltage <0.2 V) were then excluded to remove incidences where the beam was blocked. To account for pressure effects a correction function, κ(P) is applied , which was derived based on simulations using HITRAN 2012 data  to account for spectroscopic and density (ideal gas law) effects defined relative to standard conditions (296 K, 1013 hPa). This correction additionally accounts for the fact that a constant modulation amplitude was used, leading to changes in the modulation index and the shape of the 2f signal. For the fixed-wing configuration, temperature measurements were not available in-flight and a ground-based, fixed station was used. For the flight altitude envelope of 600 m during mid-day, the expected change in temperature is expected to be 3–6 K colder than the surface based upon environmental/adiabatic lapse rates. This temperature decrease results in an increase in the CH4 concentrations (i.e., absorption) by 1–1.5% from the ideal gas law and a decrease in absorption from the linestrength of 1–1.5%. In combination, these effects nearly cancel out and thus the overall uncertainty in the absolute concentration from ignoring temperature is negligible compared to instrument noise (5–10 ppbv).
An additional limitation was that active line locking (stabilization of the laser tuning) was not used, so the methane concentration was prone to drift if there were changes in the peak position. This was primarily an issue for the hexacopter flights, which were short in duration and had more rapidly varying environmental conditions than with the fixed-wing configuration. This was accounted for using a polynomial to correct for changes to the extent that drift in concentration was correlated with peak position. Individual hexacopter flights were excluded if peak position drifted outside of the preset window used by the single board computer for calculation of peak-to-trough heights.
A final aspect to the approach was inclusion of an in-line reference-cell containing the target gas (methane) at reduced pressure. This is a new variation on previous use of in-line reference cells which have targeted a separate gas that is spectrally non-overlapping or distinguishable using higher harmonics [19, 20]. Here the in-line reference cell was 2 cm long and contained 0.4% methane in N2 at 30 Torr, which can be effectively measured using direct absorption spectroscopy (DAS). The signal can be extracted using a time-multiplexed scheme where there are both scans with and without sinusoidal modulation every second . A duty cycle of 19:1 modulated to non-modulated scans was used, so the sensor was predominately aimed at WMS, while the hexacopter system ran in fully WMS mode since the reference cell had lost pressure. The parameters were optimized so that the direct absorption current offset was increased slightly, but not so much as to cause laser instability. Experimental spectra will be discussed in Sect. 3.1.
2.3 Field deployments
2.3.1 Long-endurance UAS
Flights with the hexacopter system were conducted in July 2016 as part of the ScaleX campaign (http://scalex.imk-ifu.kit.edu) , a “scale-crossing” experiment at the Fendt field site within the TERestrial ENvironmental Observatories (TERENO) network  in southern Germany. An innovative approach had been employed in ScaleX 2015 where a UAS was equipped with a Teflon drag-tube connected to a Picarro methane analyzer on the ground, where results have been described elsewhere . Here, we use our new sensor to obtain direct, in situ measurements with efforts focused during an intensive observational period from 21:00 to 06:00 (local time) on July 06–07.
Flight tracks are shown at the Fendt field site (Fig. 3b), along with the location of an instrumented 9 m tower containing air inlets at 1, 3, and 9 m. A gas switching system was used in connection with a Picarro G2508 that measured at these heights sequentially, sampling each height for 2.5 min. A container also located in Fendt (not shown) was equipped with a Los Gatos DLT-100, providing additional long-term information about methane patterns at the site .
In total, ten flights were conducted during the UAS intensive observational period, along with five additional daytime flights in Fendt and one at the IMK-IFU on July 18, 2016. One extended (2 h) intercomparison was conducted on July 11 with the sensor collocated at the Picarro tower’s 1 m inlet. A second intercomparison (2 h) collocated with the Los Gatos inlet on the roof of a measurement container was conducted on July 17 starting at sunset. Reference gas measurements (CH4 = 1893 ± 1% ppbv) were made several times during the campaign using a closed chamber, and zero gas measurements were done using nitrogen in a lab at the IMK-IFU campus. A constant linear calibration slope was used throughout campaign, but an offset was obtained by comparison with the Picarro each flight. This was a solution to help account for drifts by comparing to a highly stable ground-based instrument without having to do full re-calibrations in the field.
3.1 Fixed-wing based system
3.2 Hexacopter-based system
4 Summary and conclusions
We demonstrated a compact, open-path methane sensor with measurements conducted on both small and large unmanned aerial systems and which weighed 4.6 and 1.6 kg, respectively. In-flight precision of the fixed-wing system showed 5 ppbv at 1 s averaging time while the smaller system achieved 10 ppbv at 1 s averaging time. Vertical profiles were presented in the daytime over a coastal area up to 600 m a.g.l. and during night-time at a grassland site in southern Germany. The night-time vertical profiles were compared to available observations from two heights at a 9 m tower. Additionally, a same-gas reference cell was described providing additional evidence of stability during the flights.
The ability of the sensor to operate on small UAS opens up new opportunities for studies of trace gases in the atmospheric boundary layer, due to their low cost, ability to operate down to ground-level, highly flexible flight patterns, and reduced regulatory scrutiny. The combination of small UAS with a high-precision methane sensor will provide a more-detailed knowledge of vertical and horizontal heterogeneity of methane, as well as applications in localizing and quantifying emission sources. Further work will improve the long-term stability of the sensor to reduce the need for ground-based reference instruments and ruggedize the sensor for repeated field use.
We thank the staff of IMK-IFU for their support during ScaleX, and NSF IIP:1445031 for funding. The TERrestrial Environmental Observatory (TERENO, http://www.tereno.net) pre-Alpine infrastructure is funded by the Helmholtz Association and the Federal Ministry of Education and Research. We thank the Scientific Team of ScaleX Campaign 2016 (http://scalex.imk-ifu.kit.edu) for their contribution.
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