Project #2002-302 CRYSTAL-FACE
Cirrus Regional Study of Tropical Anvils and Cirrus Layers - Florida Area Cirrus Experiment
Hans Verlinde
U.S. Naval Research Laboratory P-3 Orion
Data Quality Report
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.
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 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 parameters 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.
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, or 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 positioning system. Both the GPS-corrected and uncorrected values are included in the final data set.
The aircraft true airspeed (TASX) is a critical measurement that factors into most of the in-situ data calculations. Redundant dynamic pressure sensors (QCR, QCF) are included in the instrumentation package to prevent any loss of data in this critical area. The two systems functioned well throughout the project and exhibited excellent correlation under most all conditions. QCF was selected as the reference sensor for all of the CRYSTAL-FACE flights. Note that the RAF uses a 'wet' airspeed in the calculation of true airspeed. In moist conditions this correction can be as great as 0.5 m/s. Due to an intermittent problem with the dew point sensor, several flights had to be processed using a 'dry' true airspeed. Specific flights where this occurred are identified in the flight-by-flight section of this QA summary. Note that there will be a slight degradation in the accuracy of the 3-D wind data for those flights.
Temperature measurements were made using a standard, unheated (ATRR) Rosemount temperature sensor. While fast and accurate, this sensor is susceptible to wetting during cloud penetrations. While the system appeared to function well, there was no redundant temperature measurement on the P-3 for a QA comparison. An evaluation of the pre & post project calibrations showed a stable response over the duration of the field deployment.
Humidity measurements were made using a thermoelectric dew point sensor. An experimental TDL laser hygrometer was supposed to have been added to the instrumentation complement, but the mounting location on the P-3 proved to be unsuitable. The TDL hygrometer was, therefore, not included in any of the CRYSTAL-FACE missions, and no data from this instrument will appear in the data set. In general terms, the dew point sensor functioned fairly well. There were short intervals on several flights where the unit lost its optical signal lock. In most instances the system was successfully reset in flight through a re-balancing of the sensor optics. There was no redundant dew point sensor available either as a QA comparison or as a fill-in data source.
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 288K. 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 305K for all flights.
The inertial reference system outputs a similar measurement of altitude (ALT) by combining static pressure measurements with vertical accelerations. These outputs are well correlated, but no adjustment can be made to this value for operations in conditions that vary significantly from the standard atmosphere.
The GPS positioning system also provides an MSL altitude (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 is unaffected by changes in the surface pressure or temperature. The RAF uses GALT as the primary input for the calculation of the reference MSL altitude (ALTX). Small discontinuities in GALT, resulting from switches in the satellite 'lock' configuration are smoothed out using the IRU altitude data.
Data from the pilot's radar altimeter were recorded during the project. The unit (RALTM) provides high precision data from the surface to a maximum altitude of roughly 1200 meters AGL. Once the maximum limit on RALTM has been exceeded the signal becomes very noisy and will eventually peg out at a maximum limit.
Data recording typically begins well in advance of the actual aircraft takeoff time. 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 as valid.
Problems with data streams that have been identified in this quality-assurance process have been noted in the following section. In order 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.
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