ATDD has played an important role in the development of the instrumentation system for the Sky Arrow 650 ERA (Environmental Research Aircraft), manufactured by Iniziative Industriali Italiane (3I) in Monterotondo, Italy. There are currently seven Sky Arrow aircraft being operated around the world with instrumentation from NOAA/ATDD.
The Sky Arrow features a strong, lightweight all carbon-fiber airframe and is powered by a 100 HP pusher engine. It is equipped with a fixed pitch propeller, a rectangular wing supported by two struts, and a fixed landing gear. Its maximum flight speed is 105 knots, with a low stall speed of 35 knots. The aircraft is certified (FAR 23/AC 2311) in the normal category, with research instruments installed!
The 650 ERA variant has a gross weight of 650 kg. This model has hard points, portholes and power capabilities to accommodate the ATDD BAT probe and MFP system mounting and power requirements. The aircraft is especially suited for high-fidelity flux measurements at 70 knots flight speed and altitudes up to 12,000 ft MSL. For more information, visit Sky Arrow.
The Sky Arrow uses a variety of instruments to measure velocity vectors needed to compute winds with respect to Earth and scientific parameters such as dry air temperature, dew point temperature, incoming and reflected solar radiation, Earth surface temperature, and trace gas concentration. Additional remote sensing capabilities have been added both as part of the MFP system and as independent instrument packages.
Aircraft motion and attitude are measured using a combination of GPS and accelerometer measurements. Several different GPS systems have been used as low frequency references to measure the position, velocity, and attitude angles of the aircraft in the different Sky Arrow systems. The NovAtel OEM 4 series GPS system is a 12-channel GPS system capable of recording raw satellite pseudo-range data that allows differential correction of aircraft position and velocity measurements and is common to all Sky Arrow 650 ERA aircraft. The NovAtel GPS was chosen because of its unparalleled ability to measure the vertical velocity of the aircraft to extreme accuracy. This is the most important measurement made by the MFP system. For more information, visit NovAtel.
Aircraft attitude measurement has always been a challenge, but has improved in reliability and accuracy as instruments change and improve over time. The first attitude measurements were made with a Trimble TANS (Trimble Advanced Navigation System) Vector Attitude GPS system, followed by the Javad AT-4 attitude GPS. Most recently, a Systron-Donner CMIGITS GPS/INS system has been used to measure aircraft attitude angles on some Sky Arrow systems. For more information, visit Javad.
The attitude GPS systems work by measuring differences between carrier phase signals using 4 antennas arranged in a fixed array, the position of each antenna relative to a master antenna can be very accurately measured and the attitude of the aircraft computed. The resulting attitude data (pitch, roll, and heading) is accurate to 0.1 degree and is provided at either 10 or 20 Hz, depending on the particular system. Attitude data are output in a package format, with each measurement accompanied by a GPS time tag. Advantages of this system are guaranteed absolute accuracy with every measurement, while disadvantages are high cost and sometimes unreliable attitude angle output depending on the aircraft's orientation and rotation rate about the axes (e.g. the GPS tends to drop out during steep turns).
The GPS/INS (Global Positioning System/Inertial Navigation System) uses MEMS (Micro-machined Electro-Mechanical Sensor) technology to measure angular rates of rotation about each axis which are then integrated to produce pitch, roll, and heading measurements. The GPS is integrated using Kalman filters to blend velocity and position data from the GPS with rate data from the INS to provide stable measurements. Like other INS systems, this system requires initialization prior to use. The chief benefits of this system are more reliable attitude output and reduced cost (< 2x) from the GPS attitude systems described above. The biggest disadvantage is attitude angle drift over time, especially in heading, if the aircraft is flown at constant attitude and headings for extended periods of time. Also, this case is exacerbated when heading does not match ground track (e.g. crabbing flight in a crosswind). For more information, visit Systron.
High frequency measurements of the aircraft's attitude, position, and velocity are made with two groups of three orthogonal accelerometers. One set is mounted in the Best Aircraft Turbulence (BAT) probe at the front of the aircraft (Ax, Ay, Az), while the other set is mounted near the aircrafts center of gravity (Axb, Ayb, Azb). By using differences between the front and rear accelerometers, acceleration, velocity, position, and angles in all three axes can be calculated. These measurements are blended with lower frequency attitude, position, and velocity measurements to create aircraft motion measurements at 50 Hz.
Wind velocity relative to the aircraft is measured using two Honeywell solid-state differential pressure transducers and one Honeywell solid-state absolute pressure transducer. The two differential sensors measure pressure differences in the horizontal (Py) and vertical (Pz) directions on the hemisphere. Dynamic pressure (Px) is measured using the absolute pressure sensor. All pressure measurements are recorded at 50 Hz. From these pressure measurements, the magnitude and direction of the incident wind vector relative to the aircraft can be computed. For more information, visit Honeywell.
Aircraft altitude is measured using three independent measurements. First, a combination of GPS and accelerometers are used to measure the aircraft altitude above mean sea level (MSL), to an accuracy of +5 meters. Pressure altitude is also measured by a Honeywell absolute pressure sensor, capable of measuring static atmospheric pressure to resolutions of 0.1 mb. To measure absolute altitude AGL, a Riegl LD90-3 laser distance sensor is used, which provides altitude measurements accurate to 2 cm up to a range of 1000 meters. Laser altitude data is interrogated and stored at 50 Hz. For more information, visit Riegl.
The aircraft are equipped with a downward pointing Everest Interscience Model 4000.4GL infrared temperature sensor. This device has a 4 degree field of view, allowing skin temperature at the surface to be measured over a small spot. From an altitude of 50 m, for example, the spot area would be approximately 8.6 sq cm. Surface temperature measurements are typically recorded at 50 Hz. For more information, visit Everest Interscience.
Solar radiation measurements are also made from the Sky Arrow. The most recent Sky Arrow aircraft use a Kipp & Zonen NR Lite net radiometer and two Apogee PAR (Photosynthetically Active Radiation) sensors, one upward-looking and the other downward-looking, mounted on the horizontal stabilizer of the aircraft. Older aircraft use a Radiation Energy Balance Systems (REBS) Q*7.1 Frission-type net radiometer and two LiCor PAR sensors. For more information, visit Kipp & Zonen and Apogee.
Dry air temperature is measured using two sensors. A platinum resistance temperature device measures air temperature at 1 Hz, and is used as a low frequency reference. In addition, two fast response micro-bead thermistors are used to measure temperature fluctuations in the 1-20 Hz frequency range. More recent additions include the FUST (Fast Ultra-Sensitive Temperature) device that uses a fine-wire thermocouple exposed to the fast slipstream just below the BAT probe hemisphere to measure temperature fluctuations.
Capabilities for measuring CO2/H2O gas concentrations are also provided. In early installations, a Licor 6262 CO2/H2O analyzer was used to measure gas concentrations at frequencies below 1 Hz, while an ATDD IRGA (InfraRed Gas Analyzer) instrument is used to measure concentrations in the 1 Hz-50 Hz frequency range. In more recent installations, the LiCor-7500 IRGA is used to measure CO2/H2O gas concentrations. Dew-point temperature is measured at 1 Hz using an EdgeTech Model 200 DewTrak humidity transmitter and sensor. As is the case with other instruments, the water vapor concentration data from the IRGA and the dew point humidity transmitter are blended to produce 50 Hz water vapor fluctuations. For more information, visit LiCor and EdgeTech.
The NOAA/ATDD Mobile Flux Platform (MFP) system uses an IBM-PC with a Pentium class processor running the Linux operating system to store data collected from the various instruments. Data are ingested into the computer using RS-232/RS-422 serial ports, and/or the Universal Serial Bus (USB). The system has the capability of logging up to 32 (optional 48) analog channels at user-selectable frequencies and logging up to 8 serial data streams at baud rates up to 460 Kbit/sec.
Analog signals are acquired by Best Aircraft Turbulence-REMote (BAT-REM) data acquisition modules, built by Airborne Research Australia. These modules acquire and digitize 16 differential analog channels with 16-bit resolution. All channels are filtered using 5-pole Butterworth anti-alias filters, then digitized at 200 Hz. An additional digital filter is applied to the 200 Hz signal, reducing its frequency to 50 Hz. With a full-scale range of +5 volts DC, each channel has a voltage resolution of 0.152592 mv/count. These data are then transmitted via 460 Kbit/sec RS-422 serial interface to the host computer. For more information, visit ARA.
Time synchronization of the data from the BAT-REM modules is performed using a 1 pulse-per-second (PPS) timing signal from the NovAtel GPS that drives a 50 Hz TTL pulse that triggers the BAT-REM modules to begin their data acquisition process. The process, including transmission of data to the host computer, is complete approximately 18 ms following the trigger. The GPS time message is used as a common time-base for all data storage.
The current version of the BATSTORE data acquisition program was written using gcc for Linux. The program reads serial data from each serial device by separate processes and sends the data through the message bus back to the parent process, which writes each package to a data file. All data packages are sent to the parent process and data file in the order in which they are received, thus ensuring data are time aligned. For more information, visit the BATSTORE FTP site
Three data files are written by the MFP system. First, the primary binary data file, designated ORG file, contains all raw binary data for both platform motion and scientific instruments. Second, an ASCII file describing the significant events of the data file, with comments as to ancillary information (weather conditions, observations, etc) is used to analyze useful portions of the ORG file. Finally, a differential correction file, designated BIN file, contains data to allow differential corrections of the aircraft position and velocity.
After flight, the BIN file is used to generate a differential correction file, which is then combined with the ORG file. The resulting RAW file is then written in a network Common Data Format (netCDF) file. This file type is self-describing and allows easy access to channel data with a variety of software packages. For more information, visit the NetCDF site
For quick viewing of the data, as well as some data reduction, we use MATLAB. ATDD has developed tools to allow netCDF files to be read using MATLAB and the toolbox available from Charles Denham at USGS. For more information, visit MATLAB NetCDF.
ATDD is a strong supporter of the Network of Airborne Environmental Research Scientists, started by the late Dr. Timothy Crawford and the airborne research team at NOAAs Field Research Division in Idaho Falls, ID and Dr. Jorg Hacker, Airborne Research Australia. Each of the Sky Arrow programs mentioned above are also active supporters of the group. For more information, visit NAERS.
The following is a list of groups operating Sky Arrow 650 ERA aircraft and their respective web pages:
San Diego State University Global Change Research Group, San Diego, California USA.
URL - http://gcrg.sdsu.edu/
Regional Assessment and Monitoring of Carbon Balanace (RECAB), Lund University, Sweden
URL - http://www.biosphere.ibimet.cnr.it/File_activities/02_aircraft_research.htm
IBIMet Institute of Biometeorology, Florence, Italy
URL - http://www.ibimet.cnr.it/biosphere/
ISAFoM Institute for Mediterranean Agriculture and Forest Systems, Naples, Italy
URL - http://www.cnr.it/istituti/FocusByN_eng.html?cds=084&nfocus=3
The University of Alabama Atmospheric and Environmental Research Operations Laboratory (AERO), Tuscaloosa, Alabama, USA
URL - http://uanews.ua.edu/anews2004/oct04/resairplane100104.htm
WUR-Alterra, The Netherlands
URL - http://www.me2.alterra.nl
Instituto de Clima y Agua - Centro Nacional de Investigaciones Agropecuarias - Instituto Nacional de Tecnología Agropecuaria (INTA) - Argentina
URL - http://www.ifeva.edu.ar/