2820 JOURNAL OF ATMOSPHERIC AND OCEANIC TECHNOLOGY VOLUME 30 A Review and Practical Guide to In-Flight Calibration for Aircraft Turbulence Sensors CLEMENS DRU¨ E AND GU¨ NTHER HEINEMANN Department of Environmental Meteorology, University of Trier, Trier, Germany (Manuscript received 2 May 2012, in final form 5 June 2013) ABSTRACT A large number of quantities have to be measured and processed to determine the atmospheric-state variables, which are the actual measurands, from aircraft-based measurements. A great part of the de- pendencies between these quantities depends on the aerodynamic state of the aircraft. Aircraft-based me- teorological measurements, hence, require in-flight calibration. Most operators of research aircraft perform some kind of calibration, but the schemes used and the degree they are documented greatly vary. The flight maneuvers and calculation methods required, however, are published in a number of partly overlapping and partly contradictory publications. Some methods are only presented as a minor issue in publications mainly focused on atmospheric processes and are therefore hard to find. For an aircraft user, it is hence challenging to either perform or verify a calibration because of missing comprehensive guidance. This lack was stated on occasion of the in-flight calibration of the German research aircraft Polar5 carried out for the field experiment Investigation of Katabatic Winds and Polynyas during Summer (IKAPOS). In the present paper, a compre- hensive review of the existing literature on this field and a practical guide to the wind calibration of a research aircraft to be used for turbulent flux measurements are given. 1. Introduction the corrections needed due to flow distortion by the aircraft body require an in-flight calibration of each in- a. Motivation strumented aircraft (e.g., Lenschow 1976; Brown 1988; Aircraft-based in situ measurements are among the Tjernstrom€ and Friehe 1991; Bange and Roth 1999; most valuable tools for the investigation of physical Williams and Marcotte 2000). processes in the atmosphere. Although aircraft have Most aircraft available for atmospheric research are always been used as meteorological sensor platforms operated by instrumentation engineers and scientists (Moninger et al. 2003), and the equations to calculate who have great expertise in airborne measurements. wind vector and other quantities are well known (e.g., Hence, usually the operators ensure a proper calibration Tjernstrom€ and Friehe 1991; Lenschow 1986), obtaining (Lenschow et al. 2007). However, in some cases the re- good atmospheric measurements is still a complex and quired expertise is not present—for example, leased challenging task. Typically, 15 individual quantities have commercial aircraft or multipurpose aircraft that are to be measured to obtain the three-dimensional (3D) only occasionally instrumented—and a satisfying in- wind vector (e.g., Metzger et al. 2011). flight calibration may be left to the user. Planning and The design of the sensing probes is a science of its own performance of such a calibration, however, exceed the (for examples, see Spyers-Duran and Baumgardner expertise of many users. 1983; Crawford and Dobosy 1992; Haman et al. 2001; Lenschow (1986) gives a comprehensive recommen- Spiess et al. 2007; Wang and Geerts 2009), integrating dation for suitable calibration methods. But this book sensors requires engineering skills (e.g., AEEC 2001), was written before the public availability of the Navi- and is subject to strict regulations (FAA 1968, 1995). gation Satellite Timing and Ranging (NAVSTAR) sys- While the actual sensors can be calibrated in a laboratory, tem (Parkinson and Gilbert 1983)—often referred to as ‘‘the’’ global positioning system (GPS)—and fully digi- € € tal data acquisition. Hence, later advances (such as, e.g., Corresponding author address: Clemens Drue, Universitat Trier, € Umweltmeteorologie, FB VI, Behringstraße. 21, D-54286 Trier, Crawford et al. 1993; Tjernstrom and Samuelsson 1995; Germany. Matejka and Lewis 1997; Khelif et al. 1999; Kalogiros E-mail: [email protected] and Wang 2002) are not incorporated. DOI: 10.1175/JTECH-D-12-00103.1 Ó 2013 American Meteorological Society Unauthenticated | Downloaded 09/26/21 02:56 AM UTC € DECEMBER 2013 D R UE A N D H E I N E M A N N 2821 All sensing elements delivering analog output as well as the analog-to-digital converters (ADCs) had been calibrated prior to the experiment. For sensors with cal- ibrations expected to be stable, manufacturer-supplied calibration coefficients are used, such as for the inertial navigation system (INS), GPS, and the altimeters. Default data output of the Polar5 data acquisition system contains temperature values only with a resolu- tion of about 0.025 K. This turned out to be too coarse, for example, to investigate the sensor inertia (section 2f). Extracting analog-to-digital converter readings from a backup of the onboard database, however, allows calculation of calibrated sensor readings with almost five FIG. 1. Polar5 fitted with the nose boom carrying the turbulence significant digits. sensors. (inset) Detail of the nose boom: the tip to the left is the five- As known from Polar5’s predecessor, Polar2, the data hole probe, the housed sensors are (top to bottom) Lyman-a hy- acquisition system tends to record occasional spikes. grometer, inlet for dewpoint mirror through a deicable Rosemount It appears that they are caused by uncorrected data com- housing, slow temperature sensor (TE) in deicable Rosemount munication errors, since spikes even occur in fully digitally housing, and fast temperature sensor (TRvF) in non-deicable Rosemount housing (see section 2f for details). recorded quantities such as INS position (Th. Garbrecht 2002, personal communication). Although it was com- The intention of the present paper is, hence, to give monly believed that these were caused by very high aircraft data users a practical guide to a state-of-the art frequency (VHF) radio, the spikes persisted after the in-flight calibration. The recommendations given are use of VHF was discontinued. To identify spikes, a developed for the German research aircraft Polar5 on simple method is used that removes all values that deviate occasion of the experiment Investigation of Katabatic more than a threshold value from high-pass-filtered Winds and Polynyas during Summer (IKAPOS; time series. To determine the threshold, the normal- Heinemann et al. 2011), but they can be easily trans- ized probability density distribution of each high-pass- ferred to any other aircraft of similar weight, ranging filtered measurement time series is compared to a from single-engine light aircraft (e.g., Crawford and Gaussian error function (Drue€ 2001). For IKAPOS, a Dobosy 1992) over twin-engine aircraft (e.g., Kalogiros value of 3.5 standard deviations was determined as and Wang 2002) to quad-engine utility aircraft (e.g., a suitable threshold. Khelif et al. 1999) and twin-engine business jet Upon delivery, the manufacturer of the nose boom, (Tjernstrom€ and Friehe 1991). MessWERK GmbH, performed an extensive verifica- tion of wind measurements (Cremer 2008). This report, b. IKAPOS however, gives the empirical relationships determined IKAPOS was performed in June 2010, using the using formulations that differ from the equations German polar aircraft ‘‘Polar5’’ of the Alfred Wegener commonly used (e.g., by Lenschow 1986; Bogel€ and Institute (AWI), which was based at Qaanaaq (north- Baumann 1991; Khelif et al. 1999; Strunin and Hiyama west Greenland). The investigations comprised studies 2004) or by Vorsmann€ et al. (1989), which is a somewhat of the summertime katabatic wind system in the expanded version of Vorsmann€ (1990). Hence, the re- coastal area of north and northwest Greenland, and of sults are difficult to compare to these other calibration atmosphere/sea ice/ocean exchange processes over the methods. Furthermore, the local angle-of-attack cor- North Water (NOW) polynya (see Heinemann et al. rections are determined by quasi-static maneuvers that 2011). Polar5 is a Basler BT-67 (Fig. 1) that consists of a do not allow for time shifts between the time series modified Douglas DC-3 airframe retrofitted with tur- (Bogel€ and Baumann 1991), which might be not optimal boprop engines (Herber et al. 2008). The main modi- for turbulence measurements. fications comprise a slight stretching of the fuselage, reinforced structure, and redesigned outer wing lead- 2. In-flight calibration ing edge and wing tip. Polar5 has a permanent ‘‘basic’’ meteorological in- In general, ‘‘calibration’’ denotes setting up a conver- strumentation and was carrying its optional nose boom and sion between the sensor reading (measured quantity) various radiation sensors during IKAPOS. The sensors and the sought quantity (the measurand; JCGM 2012), involved in calculating energy fluxes are listed in Table 1. but most authors use in-flight calibrations to yield Unauthenticated | Downloaded 09/26/21 02:56 AM UTC 2822 JOURNAL OF ATMOSPHERIC AND OCEANIC TECHNOLOGY VOLUME 30 TABLE 1. Polar5 turbulence instrumentation. Quantity Sampling (Hz) Sensor Type Position 1 GPS Trimble 4000SSi Attitude and acceleration 50 INS Honeywell Laseref V family Height 100 Radar altimeter Honeywell KRA 405B Static pressure 20 Rosemount 1201F2A1B1A 3D wind 100 Five-hole probe Rosemount 858AJ Air temperature 100 Housed Pt100 Rosemount 102E4AL 100 Housed Pt100 Rosemount 102E Air humidity 100 Lyman-a Buck Research 100 Humicap Vaisala HMT333 100 Chilled mirror General Eastern 1011B Radiation