Project # 2006-503 T-REX
Terrain-induced Rotor EXperiment
Principal Investigator(s): Vanda Grubišic´, et al.
NSF/NCAR Gulfstream-V (N677F)
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:standard_name="equivalent_potential_temperature" 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.
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).
Static pressure is measured with a highly accurate Paroscientific (MODEL 1000) with a stated accuracy of 0.01% of full scale.
PSF/PSX Static pressure as measured using the fuselage holes PSFC/PSXC Static pressure corrected for airflow effects (pcorr)
Use PSXC for the normal measure of pressure (e.g. in equation of state or hydrostatic equation). PSF was sampled at a rate of 50 sps.
A second static pressure system is provided by the GV avionics system. This is slower than the Paroscientific measurement, but it has been corrected for airflow effects and it is certified for 'Reduced vertical separation minimum' (RVSM) through the calculation of pressure altitude. RAF has no documentation on how Gulfstream and Honeywell corrected this pressure measurement, but the measurement has passed very strict FAA certification requirements.
An unheated Rosemount sensor was used for fast-response measurements. This sensor can be affected by icing, but that did not appear to be a problem in T-REX. Two heated Harco sensors were used to give a slower response temperature that would also be adequate in icing conditions. A fourth measurement of temperature (slow and with some delay) was provided by the GV avionics instrumentation.
The Rosemount and Harco measurements were logged using analog channels that suffered from cross talk. As a consequence, RAF recommends using the blended temperature, ATX, (See below.) for all uses of the T-REX data set.
TTRL Total air temperature from fast Rosemount sensor TT_A Total air temperature from the avionics system ATRL Ambient air temperature from the Rosemount system AT_A Ambient temperature from the avionics system ATC/ATX Ambient temperature blended by low-pass filtering the avionics temperature and high-pass filtering the Rosemount measurements. Please note that this is the best available temperature measurement, but also that it is not perfect. The filter crossover point is at 30 s. In a sense we are using the absolute accuracy of the AT_A sensor and the high-frequency response of the ATRL to generate ATX.
RAF recommends using ATX for the temperature in thermodynamic equations, etc.
ADIFR Attack angle pressure sensor ATTACK Attack angle BDIFR Sideslip angle pressure sensor SSLIP Sideslip angle
Both ATTACK and SSLIP were corrected using in-flight maneuvers.
True airspeed was also measured primarily using a Mensor 6100 sensor, thus limiting the effective response to 5 Hz.
The radome pitot tube system uses the center hole of the 5-hole nose cone in conjunction with the research static pressure ports on the fuselage aft of the entrance door. A standard avionics pitot tube is also mounted on the fuselage aft of the radome, and this system is also referenced to the fuselage static ports aft of the main entrance door. It was found during empirical analysis that the fuselage pitot system gave more consistent results in reverse-heading maneuvers; it is suspected that this is due to random pressure changes at the radome center hole as has been suggested by modeling. The fuselage system is used for the calculation of the aircraft true airspeed, as well as for attack and sideslip angles. True airspeed is also provided from the aircraft avionics system, but this system is considered of slower response. Measurements using the radome and fuselage pitot systems were corrected using in-flight maneuvers.
TASR True airspeed using the radome system TASF/TASX True airspeed from the fuselage pitot system TASHC True airspeed using the fuselage pitot system and adding humidity corrections to the calculations; this is mainly of benefit in tropical low-altitude flight. TAS_A True airspeed from the avionics system
RAF recommends using TASX as the aircraft true airspeed.
The measurement of aircraft position (latitude, longitude and geometric altitude) and aircraft velocities relative to the ground are done using five different sensors onboard the GV:
Garmin GPS (Reference): These data are sampled at 10 sps and averaged to 1 sps. This is a simple GPS unit with a serial output, and the measurements are available in real-time. The values from this sensor start with a "G"; e.g.,:These are good values to use for cases where the highest accuracy is not needed. These variables are subsequently used to constrain the INS drift for the calculations of the GV winds; more about this below.
GGLAT Latitude GGLON Longitude GGALT Geometric altitude GGSPD Ground speed GGVNS Ground speed in north direction GGVEW Ground speed in east direction
GV GPS: The GV flight deck is equipped with another simple GPS unit, and the data from this unit has subscript "_G" at the end, i.e.,:Honeywell inertial reference systems 1 and 2: The GV has three inertial systems on the flight deck. Data from the first two of these are logged on the main aircraft data logger, with subscripts the latter having variable names with suffix "_IRS2". The advantage of the IRS values is that they typically have very high sample rates and very little noise from measurement to measurement. However, since they are based on accelerometers and gyroscopes, their values may drift with time. The drift is corrected for by filtering the INS positions towards the GPS positions with a long time constant filter; the filtered values have a "C" added to the end.
LAT_G Latitude LON_G Longitude ALT_G Geometric altitude GSF_G Ground speed VNS_G Ground speed in the north direction VEW_G Ground speed in the east direction VSPD_G Vertical speed of the aircraftThe choice of variables for position analysis depends on the type of analysis; in general the Garmin GPS is sufficiently accurate. However, for very precise analysis we recommend using the differential GPS data. For instance, in the T-REX area, the Garmin data have an accuracy of aircraft altitude of ±3 m, whereas the differential GPS is estimated to be accurate to better than ±0.3 m for 95% of the time. The avionics GPS may only have an altitude accuracy estimated to be ±15 m.
LAT latitude from IRS 1, no GPS filtering LATC latitude from IRS 1, filtered towards GPS values LAT_IRS2 latitude from IRS 2, no GPS filtering LON longitude from IRS 1, no GPS filtering LONC longitude from IRS 1, filtered towards GPS values LON_IRS2 longitude from IRS 2, no GPS filtering GSF ground speed from IRS 1, no GPS filtering GSF_IRS2 ground speed from IRS 2, no GPS filtering
PITCH pitch angle from IRS 1 (nose up is positive) PITCH_IRS2 pitch angle from IRS 2 ROLL roll angle from IRS 1 (right wing down is positive) ROLL_IRS2 roll angle from IRS 2 THDG true heading from IRS 1 THDG_IRS2 true heading from IRS 2
The values of pitch angle (PITCH) have been corrected using in-flight measurements to give approximately the same values as the aircraft attack angle (ATTACK) for long parts of each flight; this correction is done on a flight-by-flight basis to give a near-zero mean updraft over extended flight legs. The variation from flight to flight of this offset is caused by small differences in the pre-flight alignment of the inertial navigation system. No alignment correction has been applied to PITCH_IRS2.
The frequency response of PITCH is shown in Fig. 2.
These pressure measurements were sampled at 50 sps and thus resulting in power spectra to 25 Hz. Examination of power spectra and specifications from Mensor indicate that the sensors have internal filters with a -3dB (half-power) cutoff at 12 Hz, resulting in a noticeable roll-off in the spectra beginning approximately at 6 to 7 Hz. Users of wind data should be aware that contributions to covariances and dissipation calculations will be affected at and above these frequencies.
The flight-by-flight offset to PITCH has been implemented to give near-zero updrafts velocities over long time scales. Users doing analysis on shorter flight segments will have to decide if they feel that any remaining mean updrafts should be removed.
The following lists the most commonly used wind variables:RAF recommends using the GPS corrected wind components, i.e. the variables ending in "C".
UI Wind vector, east component UIC Wind vector, east component, GPS corrected for INS drift VI Wind vector, north component VIC Wind vector, north component, GPS corrected UX Wind vector, longitudinal component UXC Wind vector, longitudinal component, GPS corrected VY Wind vector, lateral component VYC Wind vector, lateral component, GPS corrected WI Wind vector, vertical gust component WIC Wind vector, vertical gust component, GPS corrected WS Wind speed, horizontal component WSC Wind speed, horizontal component, GPS corrected WD Horizontal wind direction WDC Horizontal wind direction, GPS corrected
PLWC Dissipated power (wet + dry terms) for the King probe PLWCC Cloud liquid water content (do not use at present)
Note: All times listed below are Coordinated Universal Time (UTC).