Project #2000-828 AESOP/TEXAQS-2000

Texas Air Quality Study

Principal Investigator(s): Fred Fehsenfeld, et al.

NSF/NCAR L-188 Electra (N308D)


Data Quality Report

by Allen Schanot


This summary has been written to outline basic instrumentation problems affecting the data set and is not intended to point out every bit of questionable data. It is hoped that this information will facilitate use of the data as the research concentrates on specific flights and times.

The following report covers only the RAF-supplied instrumentation and is organized into two sections. Section I lists recurring problems, general limitations and systematic biases in the standard RAF measurements. Section II lists isolated problems occurring on a flight-by-flight basis.

Section I: General Discussion

  1. Aircraft position, ground speed, and attitude are provided by a Honeywell Inertial Laser Reference System (IRS). The values of PITCH, ROLL, and THDG are recorded at 50 sps and are highly accurate. However, the position and ground speed data are susceptible to long-term drift known as the Schuler oscillation, typically reducing their accuracy over the duration of a flight. A Trimble Global Positioning System (GPS) was used as a more accurate position reference during the program. When the GPS is working properly, it can provide data with an accuracy of better than 100 meters in position and 1 meter/sec in ground speed. The GPS provides a good absolute measurement over the entire duration of a flight while the IRS provides a good relative measurement and is good for short term variations.

    Both systems generally performed well, and the GPS position values are normally recommended for all research efforts. However, sharp turns do occasionally interrupt the reception from some satellites. These cases are characterized by a small, instantaneous shift in position and thus ground speed. Once the aircraft has come out of the turn, the track will typically shift back, but the wind calculations can be adversely affected by the resulting discontinuities in the key input variables. Rather than use the GPS data directly, the RAF uses the GPS measurements to correct the position and ground speed errors that are inherent in the IRS measurements. The simplest way to take advantage of the relative strengths of these two measurements when both are present is to apply a low-pass filter to the GPS measurements and the complementary high-pass filter to the IRS measurements and then add the two. The RAF algorithm accomplishes this with the addition of some tests and corrections for when the GPS signal is not present. For the most accurate data with smooth transitions under all conditions, RAF recommends that the LATC/LONC position data be used.

  2. The wind data for this project were derived from measurements taken with the radome wind gust package. As is the case with all wind gust systems, the ambient wind calculations can be adversely affected by either sharp changes in the aircraft's flight attitude or excessive drift in the onboard Inertial Reference System (IRS). Turns, and more importantly climbing turns, are particularly disruptive to this type of measurement technique. Wind data reported for these conditions should be used with caution.

    As an additional enhancement to this data set, a second-pass correction was applied to the wind data using position and ground speed updates provided by the GPS . Both the GPS-corrected and uncorrected values are included in the final data set.

  3. RAF flies redundant sensors to assure data quality. Performance characteristics differ from sensor to sensor with certain units being more susceptible to various thermal and dynamic effects than others. Good comparisons were typically obtained between the three standard temperatures (ATRL, ATRR, ATFH), three dynamic pressures (QCRC, QCFC, QCWC), two liquid water sensors (PLWCC, PLWCC1), the two dew pointers (DPT, DPB) and the three static pressures (PSFDC, PSFC, PSWC). The reference pressure used in all of the derived measurements (PSFDC) was obtained with a Model 1501 unit. The two remote-surface temperature sensors (RSTB, RSTB1) generally functioned well, but there were short intervals when the two measurements differ significantly. Various derived measurements require input from one or more of these standard sensors. Reference sensors are selected for each project, depending on a review of their performance. Reference sensor designations can be found in the project's Aircraft Variable List.

  4. Temperature measurements were made using the standard heated (ATFH) and two unheated (ATRL) Rosemount temperature sensors. All of the standard temperature sensors performed reasonably well. A comparison of the data sets yielded good correlation in instrument signatures and only small differences in absolute value (±0.2 °C) throughout the program. A comparison of pre- and post-program calibrations indicated that all units maintained stable and independent calibrations. ATRL was selected as the reference value used in calculating the derived measurements.

  5. Humidity measurements were made using two collocated thermoelectric dew point sensors and one Lyman-alpha fast-response hygrometer. Generally speaking, the humidity sensors performed well. As is typically the case, the two dew point sensors (DPBC, DPTC) were set up differently to provide the best coverage under the widest range of ambient conditions. DPTC was set up for fast response, but its dynamic range was more limited and had a tendency to break into oscillation. DPBC was a little slower but had the capability of measuring greater dew point depressions. A comparison of the data sets yielded good correlation in instrument signatures during the largest portions of the flights when both instruments were functioning normally. DPBC was selected as the reference value used in calculating the derived measurements for most of the flights. On four flights (RF02, RF04, RF10, RF14) DPTC performed better and was substituted into the calculations as the reference.

    All Lyman-alpha hygrometers are susceptible to in-flight drift in the instrument's bias voltage. Due to this problem, RAF uses a special data-processing technique to remove the bias drift by referencing the long-term humidity values to one of the more stable thermoelectric dew point sensors. In this manner absolute humidity (RHOLA1) and mixing ratio (MRLA1) data from the lyman-alpha can provide improved response and fine-scale structure while maintaining the overall accuracy of the dew point sensors. During flights RF01-RF03, the unit was unusually susceptible to thermal drift. After the sensing head was replaced following flight RF03, performance was greatly improved, and the output was well correlated with the output of the reference dew point sensor.

  6. A set of upward- and downward-looking radiometers was deployed on the Electra for the measurement of downwelling and upwelling ultraviolet (UV), shortwave (SW) and infrared (IR) radiation. RAF recently implemented a processing algorithm that removes the effects of aircraft attitude from the upward-looking shortwave radiometer data. Users should note that the uncorrected upward-looking shortwave radiometer's variable name is SWT, while the correct, upward-looking shortwave variable name is SWTC. A description of this correction process is provided in RAF Bulletin No. 25.

    The algorithm used to remove the effects of aircraft attitude from the upward-looking shortwave radiometer data requires estimates of the shortwave direct and diffuse radiation fractions. For the AESOP/TEXAQS-2000 data set, a simple radiative transfer model was used to estimate these direct and diffuse fractions, and it was determined during this analysis that the same average fraction values could be used for processing each of the 14 research flights. The direct and diffuse adiation fractions used during the processing of the data were 0.66 and 0.34, respectively.

    All three up-looking systems (SWT, UVT, IRTC) and the down-looking infrared (IRBC) worked well throughout the project. A variety of problems with the down-looking shortwave (SWB) and ultraviolet (UVB) systems plagued the project for the entire deployment. Both sensors were replaced during the project, and water accumulated in the bottom radiometer pod following a rain storm during a static ground display, apparently damaged some of the interface wiring. Data from both systems were unusable for many of the research flights.

  7. Thermal temperature chamber experiments have indicated that the Heimann sensors used to remotely measure the surface temperature (RSTB, RSTB1) are sensitive to some thermal drift, primarily at the colder temperatures. In an attempt to deal with these problems, the units were equipped with a temperature-controlling heater system. Some drift is still apparent in the data set. RSTB1 seemed to be the more stable of the two units and exhibited better accuracy.

    An additional Heimann was placed looking upward (RSTT) to monitor the presence of cloud decks above the altitude of the aircraft. When no upper-layer decks are present, RSTT will be pegged around -50 °C. During in-cloud passes or with upper-level cloud decks present, the output of the sensor will reflect either the near-field ambient temperature or the temperature of the cloud base, respectively.

  8. The altitude of the aircraft was measured in several ways. The primary measurement (PALT, PALTF) is derived from the static pressure using the hydrostatic equation and the U.S. Standard Atmosphere, which assumes a constant surface pressure of 1013 mbar and a base surface temperature of 288°K. Whenever the aircraft is operating in areas that significantly differ from this basic profile, adjustments are typically made to the input constants to improve the calculation. In this case the input surface temperature was changed to 305°K for all flights. The value of this change is apparent in comparisons made with the alternate altitude measurements made over the Gulf of Mexico.

    The Inertial Reference System (IRS) outputs a similar measurement of altitude (ALT) by combining static pressure measurements with vertical accelerations. This output is well correlated, but no adjustment can be made to this value for operations in conditions that vary significantly from the standard atmosphere. RAF therefore recommends PALT be used as the reference value.

    Two radar altimeters were on board the aircraft for the project. The primary unit (HGME) functions at all altitudes but can be affected by surface signal reflections at altitudes below 50 meters AGL. The secondary unit (HGM) provides high precision data from the surface to a maximum altitude of 800 meters AGL. Once the maximum limit on HGM has been exceeded, the signal becomes very noisy and will no longer be of use. HGME or HGM should be used as the low-altitude reference.

    The Trimble Global Positioning System (GPS) also provides an altitude readout (GALT). In the past this output was 'de-tuned' by the military and was not particularly useful due to a superimposed oscillation of ±150 M. However, the signal has now been cleaned up and seems to be as accurate as the PALT measurements.

  9. Two hot-wire liquid-water sensors were used on the Electra during the program. The PMS King Probes (PLWCC, PLWCC1) worked reasonable well, and a comparison of the two units yielded a good correlation in most cases. Special note should be made that both these instruments are calibrated for a specific range of aircraft speeds. Small changes in the baseline are apparent with speed changes, as are small zero offsets. Each cloud penetration will require a baseline adjustment with the relative change providing the sampled liquid water content.

  10. The calculation of CN-sized aerosol particle concentrations (CONCN) is dependent upon total particle counts (CNTS) and the measurement of sample flow (FCN,FCNC). Sample flows are routinely corrected for altitude changes, but obstructions are possible. During this project, the CN sample flow was drawn from a special, moisture-separating inlet. This inlet avoids the false, order-of-magnitude jumps in in-cloud measurements resulting from droplet shattering. During a routine servicing of the unit following flight RF03, a leak developed in the intake for this instrument, and the unit was partially sampling cabin air for several flights. The data (CONCN) are bad for flights RF04 through RF10. After a repair the unit functioned well for the remaining flights.

  11. Three PMS particle probes (FSSP-100, PCASP, FSSP-300) were used on the project. It should be noted that the three probes have overlapping ranges. Routine comparisons of the total particle size distributions show good agreement between the different data streams. Some specific details on each of the probes are summarized below:

  12. Data recording typically begins well in advance of the actual aircraft takeoff time. During flights RF01 and RF08, the data system was even left running during short refueling stops. Virtually all measurements made on the aircraft require some sort of airspeed correction, or the systems are simply not active while the aircraft remains on the ground. None of the data collected while the aircraft is on the ground should be considered valid.

  13. Problems with data streams that have been identified in this quality assurance process have been noted in the following section. To avoid any confusion or misinterpretation of the data provided by RAF, the output from systems deemed to be bad for an entire flight have been replaced with the missing data flag: -32767.

 

Section II: Flight-by-Flight Summary

Note: All times listed below are Coordinated Universal Time (UTC).

 

RF01

 

RF02

 

RF03

 

RF04

 

RF05

 

RF06

 

RF07

 

RF08

 

RF09

 

RF10

 

RF11

 

RF12

 

RF13

 

RF14
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Last update: Thu Oct 19 17:42:13 MDT 2000