The aircraft was re-engineered in 1999 to host the Mobile Flux Platform (MFP), which consists of a set of sensors for atmospheric measurements. The installation was certified to operate under both FAA (Federal Aviation Administration, USA) and JAR (Joint Aviation Regulations, EU) aeronautical regulations. Atmospheric turbulence measurements are made with the “Best Aircraft Turbulence” (BAT) probe, developed by NOAA-ATDD and ARA Australia (see figure below). In brief, the BAT probe measures the velocity of air with respect to aircraft using a hemispheric 9-hole pressure sphere that records static and dynamic pressures by means of four differential pressure transducers (Crawford and Dobosy, 1992).

The Sky Arrow engine is mounted in a pusher configuration, allowing the BAT probe to be installed directly on the aircraft’s nose, thus reducing most of airflow contamination due to upwash and sidewash generated by the wing (Crawford et al. 1996). The actual wind components (horizontal U, V and vertical W) relative to the ground are calculated introducing corrections for three-dimensional velocity, pitch, roll and heading of the aircraft. Those corrections are made using a combination of GPS velocity measurements and data from two sets of three orthogonal accelerometers mounted at the centre of gravity of the aircraft and in the centre of the hemisphere. Aircraft velocity relative to the ground is measured by means of a conventional differential GPS (RT20, Novatel USA) at 10 Hz.

An additional 4-antenna GPS system (AT4, Javad, USA) is used to measure aircraft attitude angles at frequencies up to 20 Hz (figure VI.1.2-c). Finally, the GPS and accelerometer signals are blended to obtain attitude and velocity data at frequencies up to 50 Hz. Accordingly, atmospheric turbulence is actually measured at a frequency of 50 Hz and since the aircraft can fly at relatively slow speed (35 m s-1), a horizontal spacing of 0.7 m between 50 Hz measurements in no-wind conditions can be achieved. In this way, eddies of wavelengths larger than 1.4 m can be detected. The probe is equipped with a fast thermocouple to measure air temperature with a response time of 0.02 s. A platinum resistance thermometer is used for a mean air temperature reference.

A net radiometer (Q*7, REBS USA) and upward and downward looking PAR radiometers (200s, LiCor USA) are mounted on the aircraft’s horizontal stabilizer. Low frequency air moisture measurements are made using a chilled mirror dew point sensor (EdgeTech, USA). Surface temperature is measured using an infrared thermometer (4000.4GH, Everest USA). Atmospheric densities of carbon dioxide and water vapour are sampled and recorded at 50 Hz by a LiCor 7500 (LiCor, Lincoln, Nebraska) open path infrared gas analyser installed on the aircraft nose (see figure above). All the digitally converted signals from the BAT Probe and the sensors are stored on a PC located on-board. Additional details of underlying theory and the technical implementation of flux aircraft can be found in Crawford and Dobosy (1992) and Crawford and Hacker (2001), Dumas et al. (2001).

table VI.1.2-a : Equipment installed on board the Skyrrow ERA