T-REX Logo, 10983 byte jpg

Project # 2006-503 T-REX

Terrain-induced Rotor EXperiment

Principal Investigator(s):   Vanda Grubišic´, et al.

NSF/NCAR Gulfstream-V (N677F)

RAF Logo, 5,367 byte png

Data Quality Report (High-Rate)

By Jørgen Jensen, Project Manager

9 March 2007

Acknowledgments:  Allen Schanot produced the NSF/NCAR GV high-rate production data set for T-REX.  Much of the credit for generating the software and procedures should also go to Dick Friesen, Chris Webster and Al Cooper.  Ron Ruth did a careful review of this documentation.  Almost all of RAF's staff and many other EOL staff have been involved in preparing instruments for the T-REX deployment or they have participated in the GV operations, and they also deserve credit for the data.

Background:  T-REX was the first deployment of the NSF/NCAR GV research aircraft for which production data have been released.

Data with 1 sps data rate has previously been released, and a separate project manager's Quality Report has been issued.  Please refer to that for general questions about the T-REX NSF/NCAR GV data set.  The present report is intended as a supplement to describe the particular issues with the high-rate T-REX netCDF processing and files.

It should be noted that the two programs creating the low-rate and high-rate netCDF files are separate programs.  Due to a variety of reasons, the 1 sps data files are not simply averages of the 25 sps files.  However, the differences are mostly very small.

Contents of the T-REX GV high-rate data files:  The previously released low-rate GV T-REX data set (1 sample per second) contains both the variables recorded on the main aircraft data logger, and also a number of other measurements using separate data systems (Laser hygrometer, ozone, differential GPS, etc.).  The present high-rate data (maximum data rates of 25 sps) set contain only the data logged on the GV main aircraft data system.  The remainder of the data (Laser hygrometer, etc.) has either been released or will be released separately as high-rate files by the respective PIs.  Users are requested to contact the appropriate instrument PIs for information on these data.

Data were sampled at a variety of different rates depending on sensor characteristics:

The NetCDF file header contains all the information needed to examine the dependencies (i.e., what variables are used to calculate a given derived variable) as well as the sample rates of directly measured variables.  Several programs can be used to examine the header of the NetCDF files; the program ncplot (available from www.eol.ucar.edu/raf/Software/) can be used to both plot data and examine headers.  As an example for the data from research file RF01:  Enter the "view" button, then hit the "netCDF" button, and scroll down to

     float THETAE(time)
        THETAE:_FillValue = -32767.f ;
        THETAE:units = "K"
        THETAE:long_name = "Equivalent Potential Temperature"
        THETAE:Category = "Thermodynamic"
        THETAE:DataQuality = "Good"
        THETAE:Dependencies = "4 ATX PSXC EDPC MR"

The final line above shows that Thetae is calculated from the reference temperature measurements ATX (for T-REX this is a blended value of the slow avionics temperature AT_A and the fast-response Rosemount temperature ATRL); from the reference static pressure PSXC; from the reference water vapor pressure EDPC; and from the water vapor mixing ratio MR.  From other entries in the same netCDF file header, it can be seen that AT_A was sampled at 2 sps, ATRL is in part based on TTRL which was sampled at 500 sps, EDPC was calculated in part from DPXC which was sampled at 1 sps, etc.  The fact that each of the values above may have their own dependencies illustrates that many derived high-rate values are combinations of variables measured at different sample rates.  Investigators have all the material to examine the sample rates using the above measurements, and it is left to the investigator's decision to use the high-rate data file with caution.

The T-REX netCDF files contain output rates of 1 sps and 25 sps, depending on the sample rate of the sensors used.  For instance, although the avionics temperature, AT_A, was sampled at 2 sps, it is given as a 1 sps variable in the T-REX netCDF high-rate file.  A quick way to determine the data rate of a variable in the netCDF files is to use ncplot, then bring up a time series of the variable, followed by "view" and "spectrum".  The resulting plot for all the data on T-REX RF04 is shown in Fig 1.  It can be seen that the AT_A data is cut off at the Nyquist frequency of 0.5 Hz, implying that the variable is given a 1 sps variable in the T-REX netCDF file.  Similarly, the ATX variable has a Nyquist frequency of 12.5 Hz, implying that it is a 25 sps variable in the netCDF file.  The vertical drop-off at 12.5 Hz is a result of the digital filtering.

Power spectra of AT_A and high-rate temperature ATX
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Figure 1.  Power spectra of AT_A (red curve) and high-rate temperature ATX (blue curve).

The GV autopilot:  The flight management system on the GV allows the pilots to dial in a given flight altitude, air speed and heading.  The management systems then tries to keep the aircraft within a narrow range of the desired values but the system has small overshoots of the desired values.  In practice the aircraft engine power is increased and decreased with a period of about 3 s; this audible when flying in the GV cabin.  As a result, the GV will go through periodic pitch and attack changes, the result of which can best be seen from the power spectra shown in Fig. 2.  Both curves show relative peaks at about 0.4 Hz.

A properly characterized and calibrated aircraft wind system should not show any significant residuals of these oscillations in the spectra of vertical air velocity, WIC.  (See the green curve in Fig. 2.)  (As pointed out independently by Drs. Al Cooper and Rod Frehlich, this was an unresolved issue in the preliminary T-REX field data set).

Power spectra of the GV pitch angle PITCH, attack angle
   ATTACK and vertical air velocity WIC. The data are from part of T-REX RF04.
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Figure 2.  Power spectra of the GV pitch angle PITCH (red curve), attack angle ATTACK (blue curve) and vertical air velocity WIC (green curve).  The data are from part of T-REX RF04.

Section I: General Discussion of Sensors and Measurements

Section II:   Flight-by-Flight Summary

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

RF01 (03/02/2006) RF02 (03/05/2006) RF03 (03/09/2006) RF04 (03/14/2006) RF05 (03/25/2006) RF06 (04/02/2006) RF07A (04/06/2006) RF08 (04/08/2006) RF09 (04/15/2006) RF10 (04/16/2006) RF11 (04/21/2006) RF12 (04/26/2006)
Last update: Tue Mar 13 18:07:25 GMT 2007