Tools for DIY site-testing

Tools for DIY site-testing Federico Floresa , Roberto Rondanellia , Accel Abarcab , Marcos Diazb and Richard Querelb a Dept.

Geophysics; Electrical Engineering; Universidad de Chile, Santiago, Chile. b Dept.

ABSTRACT Our group has designed, sourced and constructed a radiosonde/ground-station pair using inexpensive opensource hardware. Based on the Arduino platform, the easy to build radiosonde allows the atmospheric science community to test and deploy instrumentation packages that can be fully customized to their individual sensing requirements. This sensing/transmitter package has been successfully deployed on a tethered-balloon, a weather balloon, a UAV airplane, and is currently being integrated into a UAV quadcopter and a student-built rocket. In this paper, the system, field measurements and potential applications will be described. As will the science drivers of having full control and open access to a measurement system in an age when commercial solutions have become popular but are restrictive in terms of proprietary sensor specifications, “black-box” calibration operations or data handling routines, etc. The ability to modify and experiment with both the hardware and software tools is an essential part of the scientific process. Without an understanding of the intrinsic biases or limitations in your instruments and system, it becomes difficult to improve them or advance the knowledge in any given field. Keywords: Site-testing, Atmosphere, Radiosonde, Remote sensing, Arduino

1. INTRODUCTION All ground-based astronomical observatories are subject to the environmental conditions of their geographical locations. For this reason, prior to positioning an observatory, a systematic characterization of the relevant atmospheric and environmental parameters should be performed. This could include: temperature, pressure, humidity, wind, dust, seeing, turbulence, aerosols, extinction, cloud coverage, light pollution, etc. This type of study and analysis is collectively referred to as site-testing. The process can be an expensive undertaking since these candidates sites are often remote and lacking of any infrastructure. Dedicated site-testing campaigns are mounted, sometimes over several years in order to gain the statistical confidence required to make sound recommendations to the site selection committees. Each of these atmospheric or environmental parameters requires a dedicated piece of hardware to make the measurement. In this paper we describe a sensing platform developed by engineering students as part of a 1-semester undergraduate project course.

2. RADIOSONDES Radiosondes are the recognized standard for atmospheric measurements and are often used to ground-truth measurements made with complementary instruments during a site-testing campaign. A radiosonde is a set of sensors typically flown on a balloon whose data are transmitted down to a ground-station. Their simplicity, cost and size are unparalleled when compared to the sophisticated and prohibitively expensive alternatives such as sounders, radiometers and space-based systems. The radiosonde allows in a matter of minutes and at relatively low cost, measurement and transmission of data from regions mostly inaccessible to humans. Although satellites have filled much of the gaps in our knowledge of the upper atmosphere, there is still the need for radiosondes to provide high quality and high resolution data not only for calibration of other observing systems, but also for basic meteorological research, for feeding weather forecasting models and for detecting climate trends. With this relevance to the atmospheric science community in mind, our group has developed a radiosonde Further author information: (Send correspondence to R.R.Q.) R.R.Q.: E-mail: [email protected]

transmitter/receiver pair using widely available and inexpensive open source hardware and software.1 A summary of the hardware platform and some of our results will be described in this paper. Current radiosondes are a well developed set of instruments that measure temperature, pressure and humidity. Wind can be inferred from a GPS-equipped sonde using its position assuming that the carrying balloon acts as a passive tracer with respect to horizontal winds and filtering the pendular motion of the radiosonde package. Many of the technological improvements of the radiosonde have been led by the Finnish company Vaisala (especially since the 1980s) which produced about 70% of the radiosondes used globally by 2002.2 The World Meteorological Organization has established requirements for the accuracy of the data gathered by a radiosonde.3 For instance, operational standards allow for errors of 1 K in the tropospheric temperature and less than 7.5% in the relative humidity. Only about 1000 synoptic radiosonde stations exist around the globe. This is explained by the relatively large operational cost of the ground stations, including the cost of the receiving station, the gas supply, the radiosondes and the personnel in charge.4 Receiving stations are usually up to 100K US$ (or at the lower end, 5K US$ for InterMet boundary layer systems that consists mostly of a radio receiver, a modem interface and the software). Each individual radiosonde costs about 200 US$. The cost of establishing a radiosonde ground station makes it difficult to increase the spatial extent of the radiosonde network, which is particularly deficient in vast regions of the Southern Hemisphere. This prohibitive set-up cost is exaggerated in the case of site-testing when the entire system must be remotely located. With the prohibitive set-up costs and an interest in developing an engineering project, we set about building an “open hardware”, recoverable, cost-effective alternative to the typical commercial radiosonde equipment. Open systems also allow for the possibility of benefiting from the development accelerated by a community of users. Eventually, a standard open platform might be used and easily adapted by the global sensing community. The possibility of adding and comparing different sensors can also lead to improvements in the accuracy, price, weight, and durability of sensors, batteries, actuators and other components.

3. SENSOR PLATFORM 3.1 Hardware The computational core of the radiosonde is an Arduino board, an open-source electronics prototyping platform (http://arduino.cc/). Arduino was selected because it provides a complete, flexible, easy-to-use hardware and software platform that is widely used not only by engineers, but also by artists, designers, and hobbyists.5 The sensors, GPS, transceiver, release-linear-motor, memory card and camera in the radiosonde are connected to the Arduino via a customized daughter board, or “shield” in Arduino nomenclature. The ground receiver consists of an Arduino and a transceiver module which are connected to a computer where data is stored. Pictures taken by the radiosonde camera are stored internally on the memory card and not transmitted due to bandwidth limitations. Figure 1 shows a view of the components as they were laid out in our final prototype. The radiosonde and receiver hardware used in our tests were based on the Arduino Duemilanove. Our source code has also been tested on newer versions of the Arduino board: the Uno and Mini Pro. The choice of original sensors was a combination of cost versus the ability to provide high quality meteorological data. Table 1 shows a description of the sensors used in building the radiosonde. More detailed specifications of the sensors, parts and their connections to the Arduino board, as well as circuit diagrams are available at http://www.dgf.uchile.cl/radiosonde/.

3.2 Software The information flow from the radiosonde is as follows: raw sensor data (voltages) are measured by the sensor/transmitter unit and communicated to the receiver unit which is connected to a computer where the voltages are converted to actual meteorological values. The radiosonde software is divided into three distinct programs: Arduino instructions for the transmitter/sensor unit, Arduino instructions for the receiver, and a computer-based GUI interface to process and display the received data.

Figure 1. Photo of the radiosonde, detailing the different components and sensors.

4. CALIBRATION AND CROSS-COMPARISON 4.1 Preliminary experiments Several test experiments were performed in preparation for the field launch. A tethered balloon campaign was conducted where the radiosonde was first used to transmit meteorological data from an altitude of about 1000 m above ground level. We also tested the data-link from the transmitter to the receiver by communicating between the top of Cerro San Cristbal (850 m.a.s.l) and the top of the Geophysics Department building in Santiago (520 m.a.s.l) about 4 km line-of-sight with an unobstructed view of the hill. A successful transmission of meteorological data was completed including GPS data. A second communication test successfully transmitted data from a park near the Andes piedmont and the top of the Geophysics building, about 13 km line-of sight.

4.2 Methodology for release and recovery Besides being able to compare data measured with our sonde with those obtained with research-grade radiosondes, we tested the possibility of recovering the equipment. This would further decrease the overall launch costs and if successful allow for more expensive and sophisticated instruments to be flown without fear of loss. In order to recover the radiosonde, we had technological advantages not available to the early balloonists who also recovered their balloon-borne recording devices. First, our release system was able to detach a balloon from the sonde at any given pressure level, upon reaching a certain temperature, or elapsed flight time, allowing the sonde to travel a pre-determined distance in the vertical. Second, the GPS signals from our sonde and one of

the commercial sondes, allowed us to accurately know their positions. Third, we made use of a WRF (Weather Research Forecasting Model,6 ) forecast simulation. The model forecast winds were used to determine the sonde trajectory, allowing a recovery team to be ready near the projected landing zone. We used the “multiple balloon technique”:7, 8 two balloons would produce the lift necessary for an ascent of ∼ 5 m/s of the payload (three sondes weighting in total ∼ 1250 g) and in our case, at a given pressure level the linear actuator would release one of the balloons so that the remaining balloon would carry the payload back to the ground at ∼ 2 m/s of vertical descent velocity. In our case, evidence from comparing WRF-forecast and real sonde trajectories showed that the trajectories were sufficiently close to the real trajectories (especially for the region below 10 km height and within the bounds of the city) so as to allow a recovery team to be within the range of the radiosonde signals at ground level (much smaller than the range of the reception of the sonde in flight), and perhaps, even observe the descent of the sonde. The multiple balloon technique has an additional advantage over the most common approach/technique of using a parachute; if the cord-length of the sonde is sufficiently long, the attached balloon serves as a marker for the position of the radiosonde package.

4.3 Field tests Together with our sonde (which we call FCFM, from the Spanish acronym for the Faculty of Physical and Mathematical Sciences) we launched two commercially available research-grade radiosondes: a Vaisala RS-92 and an InterMet iMet-1 radiosonde. The three sondes were attached to a styrofoam structure. Our first campaign in November 2011 did not proceed as originally planned because several of the sensors malfunctioned (from the commercial sondes as well as from our own sonde). The radiosondes and structure were successfully recovered later that day and the details of the launch and flight are fully described elsewhere.1 A second campaign was carried out in May 2012 to test the radiosonde at higher altitude, i.e colder and larger communication range. The sonde was released at Santo Domingo (33.63°S, 71.65°W, 75 m.a.s.l), one of the four official sounding stations located in Chile, and the closest to Santiago. Figures 2 and 3 show the comparison between a simultaneous iMet, Vaisala and FCFM sounding. Temperature and Relative Humidity are plotted versus Pressure. Also shown is the difference between the sensor measurements relative to the data from the Vaisala sonde. Despite of the eventual loss of the payload in this second launch, the radiosonde transmitted good data even during the descent. We observed a well-mixed boundary layer between surface and 970 hPa (almost constant virtual potential temperature in the layer Figure 2). Stratocumulus was present at the top of the mixed layer. There is a relatively deep inversion layer from 970 hPa to about 900 hPa. The FCFM-Sensirion temperature sensor has a long time lag which is enough to smooth out the rapid change at the top of the mixed boundary layer. Figure 2 shows more clearly the lag in temperature by the FCFM-Sensirion, nevertheless, the height of the mixed boundary late could be clearly identified from the sounding. The performance of the analog temperature sensor was surprisingly good, following very closely the behavior of the Vaisala sounding. The mean absolute error of the temperature was about 0.85 K for the FCFM-Sensirion and only 0.4 K for the FCMF-Analog. The FCFM-Sensirion exceeded its operating range and transmitted a fixed temperature after reaching 229 K. The iMet temperature sensor data was corrupted for unknown reasons, having a step discontinuity part way through the sounding of ¿20 K. For this reason it was not included in the analysis. Both the IMet and RS-92 Vaisala show relative humidities near 100 % at the top of the mixed layer. Our sensor, however, shows a ∼ 15% dry bias within the mixed boundary layer (Figure 3). The inversion at the top of the well-mixed boundary layer coincides with a rapid drying from near saturation at the top of the mixed layer to about 20% RH at the top of the inversion layer (about 900 hPa). The performance of our sensor in RH improves for drier conditions compared to Vaisala, consistent with the performance during the first campaign. Near 400 hPa, there was an ice cloud which is apparently detected by the Vaisala sounding as a thin layer of ∼ 100% RH with respect to ice. In this case, both iMet and in particular our sonde have trouble responding to the presence of the cloud, and therefore show a much smoother curve (Vaisala sounding shows a rich structure in humidity) giving also a lower value for the pressure level of the cloud. The slow time response of both iMet and FCFM-Sensirion is clearly seen in the increase of RH above 500 hPa and also in the increase of RH from about 450 hPa to 400 hPa which is the level of the cloud (Figure 3).

Figure 2. The panels show ascent vertical profiles for the free troposphere intercomparison campaign of May 4th 2012. (Left) Temperature [K] as a function of Pressure. The boundary layer is shown in the expanded sub-plot. (Right) Temperature difference relative to the Vaisala values. The iMet temperature sensor data was not valid for this sounding.

Although we did not recover this first prototype of our radiosonde, the campaign was successful in that it provided us with a wider range of atmospheric conditions to compare and to test the performance of the sensors and the electronics. Also, we were able to receive the signal of the radiosonde over a much longer path than previously tested (about 30 km in a direct line of sight, as calculated from the balloon trajectory inferred from a WRF simulation).

5. UAV PLATFORM A second application of the sensing hardware described above is achieved by mounting the transmitter to a controllable vehicle, rather than simply a balloon that is carried away with the wind. In this case it was a radiocontrolled aircraft with a ∼ 3.8 m wingspan, shown in Figure 4. For this set-up a slightly different approach was taken. The aircraft was already equipped with a DAQ and telemetry system, so rather than transmitting a second signal, the sensor data was interfaced to the aircraft system and transmitted alongside it’s housekeeping system data. For this trial, a commercial iMet radiosonde unit was used. The serial interface of the iMet probe was read using an Arduino board. The output of the iMet is an ASCII string that was parsed by the Arduino into decimal data to be passed to the aircraft and subsequently relayed to the ground station. The Arduino also had a microSD card-reader so data could be stored onboard in case of signal loss during the flight. While the aircraft was capable of automated flight, for these tests is was controlled manually to accommodate the variable wind conditions and to better follow the elevation of a co-located tethered balloon.

Figure 3. The panels show ascent vertical profiles for the free troposphere intercomparison campaign of May 4th 2012. (Left) Relative Humidity [%] as a function of Pressure. (Right) Relative Humidity difference relative to the Vaisala values.

In order to validate the sensors and systems prior to the flight tests, a series of ground tests were performed in the lab and a wind tunnel. As was the case for our radiosonde, the iMet sonde was calibrated with respect to temperature, pressure and humidity sensors in the environmental and pressure chambers at the Chilean National Weather Service (DMC). The airspeed probe on the aircraft was validated in a wind tunnel located in U.Chile’s Department of Civil Engineering. Wind speed in the tunnel was varied from 0 − 27 m/s and back down to 0 in 5 m/s steps at time intervals sufficient to allow the sensor readings to stabilize.

5.1 Flying Tests The flight tests were performed at an aeromodelling park West of Santiago. The park is ∼ 470 m.a.s.l. We had two flights with maximum altitudes of 780.7 m and 821.8 m.a.s.l., respectively. The second flight path is shown in Figure 5. Radio transmission limits were not tested at this time. During the flights the calibrated iMet sonde was interfaced to the aircraft as described above. These measurements were compared to those recorded from a tethered-balloon carrying a radiosonde that we had launched near the runway. This gave the opportunity to determine if the measurements made at airspeed were different than those recorded from a fixed location (moving slightly in the wind). The data sets were found to be similar though not exact, given that they only roughly represented the same parcels of atmosphere, seeing as the aircraft was 100−200 m away from the balloon. During the higher altitude flight, the UAV circled near 770 m.a.s.l. for ∼ 5 minutes in an attempt the sample conditions

Figure 4. Photo of the UAV.

Figure 5. Flight path of the UAV.

at that fixed height. The difference between the velocity registered by the GPS and the airspeed recorded by the UAV allowed for an approximate determination of the windspeed at that height. The relative wind speed at 770 m.a.s.l. is plotted in Figure 6. GPS heading information was not recorded by the aircraft electronics, so a vector map of wind velocity was not possible.

Figure 6. Wind speed at 770 m.a.s.l. as determined by the relative speed between the GPS and the UAV.

6. FUTURE DEVELOPMENT In order to improve the measurement of humidity, we are currently developing a chilled-mirror hygrometer using Peltier cooling elements based in part on the one described by Vomel et al.9 Control routines to sequentially cool and warm the Peltier elements can be easily programmed to the Arduino. Also, we are attempting the integration of our system with an ozonesonde provided by CENMA (Chilean National Environment Center), which, being an expensive instrument to begin with (∼ 1000 US$ each ozonesonde), makes the possibility of the recovery of the ozonesonde payload much more attractive. Companies that produce electrochemical ozonesondes have designed their instruments to communicate with research-grade radiosonde. Ozonesondes of this type can be reused, provided that the reactants are replaced and the sonde is recalibrated. Equally important for the continuation of the project is to improve the quality of the data obtained from the commercially available sensors, attempting to get as close as possible to the research-grade accuracy and response-time in all meteorological variables. Although our comparisons are relative to RS92 Vaisala and iMet-1 radiosondes, these instruments also suffer from calibration biases and random errors, e.g. errors in the relative humidity measurements made by the RS92 Vaisala sonde.10, 11 These data can be improved with better sensing hardware and read-outs, but also from software corrections due to the time response of the sensors and possibly improvements in the layout of the instruments in the payload. Attempting to solve some of these challenges make for interesting ideas for future instances of the workshop class. Once the hardware and sensors used in our radiosondes are shown to have uncertainties equal or less that those of the operational standards, then the proposed idea of adaptive sounding4 becomes feasible using our system, given the relatively low cost of implementation of a ground receiving station (the limiting issue then becoming the availability of a gas supply). Although the cost of our radiosonde turned out to be of the order of the current commercial radiosondes (200 US$), the receiver (ground station) is several orders of magnitude cheaper than commercial solutions (100 US$). In addition to this, the cost of the radiosonde can be of less relevance if we notice two extra advantages of developing a reusable radiosonde, specially as an open software/hardware initiative: 1) open initiatives tend to be developed faster than closed ones due to the community cooperation and

2) specific needs (e.g., radio observatories needing very accurate humidity measurements in the upper troposphere to perform interferometry observations) can be addressed by adding new custom made sensors in a much easier and flexible manner. Other improvements to the system involve the visualization and trajectory software to allow for the real-time correction of the forecast trajectory of the radiosonde to speed up the recovery of the radiosonde package. The recovery of the payload requires not only the technology to be functional but also a strict set of protocols to be followed by the people participating in the campaign. In addition to the UAV aircraft mentioned above, the sensing hardware is being adapted into quadrocopters currently being designed and constructed by our group. These will accommodate a 1 kg payload and offer flight times of ∼ 10 minutes. This will allow for the interesting opportunity to create 3-dimensional maps of the liquid water structure of fog clouds, fly sunphotometers to measure the vertical profiles of aerosols, even potentially to bounce lasers off of under-mounted reflectors and determine molecular abundances through absorption along the beam-path. Several possibilities exist for atmospheric measurements from a controllable airborne platform. It is important to remember that these sensing platforms are complementary. Between the ground-based sensors and satellite systems, there is a large area where UAVs could fill measurement niches near the surface, while balloons would continue to sample the atmosphere above.

7. CONCLUSION The lessons learned from the campaigns have led to the development of many improvements to our radiosonde system. Our first two full campaigns were successful in accomplishing the main goals initially proposed to the students, namely the transmission of data during flight from most of the depth of the troposphere, the ability to recover the radiosonde, the functioning of the electronics subject to very cold temperatures and the ability to generate data that compared reasonably well with the operational standards of commercial radiosondes. We have shown that using off-the-shelf components it is possible to create sensing instrumentation whose abilities and performance come close to the operational levels set by the industry standards of the field, all the while being cost-effective and adhering to open standards.

ACKNOWLEDGMENTS We appreciate the help of the Chilean National Weather Service (Direccin Meteorolgica de Chile, DMC) for providing access to an environmental chamber to make the calibration of our sensors, and also for providing the ground station for retrieving the Vaisala measurements.

REFERENCES [1] Flores, F. et al., “The life cycle of a radiosonde.,” Bulletin of the American Meteorological Society (Submitted 2012). [2] Dabberdt, W., Cole, H., Paukkunen, A., Horhammer, J., Antikainen, V., and Shellhorn, R., [Radiosondes], vol. 6, 1900–1913, Academic Press, San Diego (2002). [3] Nash, J., Oakley, T., V¨ omel, H., and Wei, L., “WMO intercomparison of high quality radiosonde systems, Yangjian China, 12th July to 3rd August.,” tech. rep., World Meteorological Organization (2011). [4] Douglas, M., “Adaptive sounding arrays for tropical regions,” in [29th Conference on Hurricanes and Tropical Meteorology], American Meteorological Society (2010). [5] Sarik, J. and Kymissis, I., [Lab kits using the Arduino prototyping platform], T3C–1, 40th ASEE/IEEE Frontiers in Education Conference, IEEE (2010). [6] Skamarock, W., Klemp, J., Dudhia, J., Gill, D., Barker, D., Wang, W., and Powers, J., “A Description of the Advanced Research WRF Version 3,” tech. rep., NCAR (2008). NCAR Tech Notes-475+ STR. [7] Hergesell, H., “ Ballon-Aufstiege u ¨ber dem freien Meere zur Erfassung der Temperatur- und Feuchtigkeitsverh¨ altnisse sowie der Luftstr¨omungen bis zu sehr grossen H¨ ohen der Atmosph¨are,” Beitr. z. Phys. d. fr. Atm 1, 200–204 (1906).

[8] DuBois, J., Multhauf, R., and Ziegler, C., [The Invention and Development of the Radiosonde with a Catalog of Upper-Atmospheric Telemetering Probes in the National Museum of American History, Smithsonian Institution ], no. 53 in Smithsonian studies in history and technology, Smithsonian Institution Press, Washington, D.C. (2002). [9] V¨ omel, H., Fujiwara, M., Shiotani, M., Hasebe, F., Oltmans, S., and Barnes, J., “The behavior of the snow white chilled-mirror hygrometer in extremely dry conditions,” Journal of Atmospheric and Oceanic Technology 20(11), 1560–1567 (2003). [10] Miloshevich, L., V¨ omel, H., Whiteman, D., and Leblanc, T., “Accuracy assessment and correction of vaisala rs92 radiosonde water vapor measurements,” J. Geophys. Res 114, D11305 (2009). [11] Otarola, A. C., Querel, R., and Kerber, F., “Precipitable Water Vapor: Considerations on the water vapor scale height, dry bias of the radiosonde humidity sensors, and spatial and temporal variability of the humidity field,” ArXiv e-prints (Mar. 2011).

Responsetime/ Sampling frequency

US$ 1.95

Value

Error

1.2 s

US$1.47

/

Variable Function

Measuring / Operating Range

TTC 3 s (@ 5 m/s)

LM235Z

HIH-4010001

Vendor / Manufacturer

-40 to 125 C

1C

Num-

Temperature

-40 to 125 C

± 3.5 %

Part ber

Temperature

0 to 100 % RH

5 s (slow moving air) 0.1 ms

2322 640

Humidity

0 to 1000 hPa

US$ 20.24

Pressure

Olimex / Vishay BCcomponents Digikey / National Semiconductor Corporation Digikey / Honeywell Digikey / Motorola

US$ 16.09

MPX5100D

Table 1. The radiosonde components: sensors, characteristics and providers

10 k Thermistor

Description

Analogue Temperature Sensor

Name

Analogue Temperature Sensor

Analogue Humidity Sensor Analogue Pressure Sensor

0-100 % RH 40 to 124 C

US$ 15.00 Humidity Temperature

8s

Sensirion

± 4.5 % ± 0.5 C

SHT10

US$ 50.25

US$ 6.95

US$ 14.95

US$ 29.95

US$ 13.95

US$ 39.95

US$ 50.00