Variable list: A list of the parameters measured by the GV core instrumentation and available in the production release of data.

Instrumentation

Pressure:

Static pressure is available using two different systems: Research and Avionics.

Research static pressure is measured with a Paroscientific (MODEL 1000) with a stated accuracy of 0.01% of full scale. This measurement is output in the netCDF files as:

• PSF (measured): static pressure as measured using the fuselage holes
• PSX (reference): same as PSF. Used to choose reference variable if more than one instrument provides measurement of the same parameter.
• PSFC (measured): static pressure corrected for airflow effects (pcor)
• PSXC (reference): same as PSFC. Used to choose reference variable if more than one instrument provides measurement of the same parameter.

Use PSXC for the normal measure of pressure (e.g., in equation of state or hydrostatic equation).

Avionics static pressure is recorded from the GV avionics. 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.

Temperature:

Temperature was measured using four different sensors on the GV:

An unheated Rosemount sensor was used for fast-response measurements.  This sensor can be affected by icing and several occurrences of this were observed in HIPPO-1 (see detailed flight reports below).  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 serial channels and are affected by a variable recovery factor.  As a consequence, RAF recommends using the reference temperature, ATX (see below.) for all uses of the data set:

RAF recommends using ATX for the temperature in thermodynamic equations, etc.

For HIPPO-1 processing of the temperature data was accomplished using A/D temperature calibration corrections.

Dewpoint temperature and vapor density:

Humidity was measured using two Buck Research 1011C cooled-mirror hygrometers that are normally used for measuring tropospheric humidity.  They have a sandwich of three Peltier elements to cool the mirror, and in comparison to earlier generations of cooled-mirror hygrometers, they have a much-improved capability to measure at low temperatures.  These sensors are assumed to measure dewpoint above 0°C and frostpoint below 0°C.  The instrument has a quoted accuracy of 0.1 °C over the -75 to +50 °C; however, based on examination of the measurements RAF is not comfortable with accuracies better 0.5 °C for dewpoint and 1 °C for frostpoint.  The cooled-mirror sensors are slow, in particular at lower temperatures, and this may give considerable differences between the measurements from the two units or when comparing with faster instruments.  Their cooling rates depend in part on the airflow through the sensor, and this may depend on the angle of the external stub relative to the airflow.  The angle may differ between the two sensors, and this may contribute to response-time differences between the sensors.  At very low temperatures the sensors may jump ("rail") to even lower temperatures.  The cooled-mirror temperatures are included even when they are outside the sensor operating range; this is caused by the need to use values of water vapor in other calculations (e.g., true airspeed). However, the impact of these out of bounds conditions on derived calculations that depend on humidity correction is very small since they occur at extremely low dew points.

The chilled mirror sensors are sensitive to flooding on rapid descents of the aircraft into the humid boundary layer. This results in temporary loss of the instruments ability to measure the dewpoint, which may last from 3 to 15 minutes, depending on conditions. This problem can also be seen in the form of "ringing", or a decaying sinusoidal oscillation of the signal, that appears after altitude changes, especially those following a period of cold soaking at high altitudes. During these periods it is advised to compare the data from both chilled mirrors and choose the one that recovers faster.

VCSEL Hygrometer was deployed for measuring atmospheric moisture content throughout the troposphere and lower stratosphere using high sensitivity optical absorption methods, using a new, near-infrared, vertical cavity surface emitting laser (VCSEL) at 1854 nm. In conjunction with a compact, multipass, open air cell and digital signal processor (DSP) electronics, this sensor consumes very low power (< 5 W), is lightweight (< 2 kg excluding the inlet housing), and occupies only the space within an aperture plate. The use of the 1854 nm VCSEL allows for a limit of detection of <1 ppmv, a precision of 3% or 0.05 ppmv max, and a minimum sampling frequency of up to 25 Hz.

RAF recommends using DPXC as a slow 'tropospheric' variable, and MRTDL as a fast-response 'tropospheric' variable.  MRTDL is also recommended for all 'stratospheric' use.

Attack and Sideslip:

Measurements of attack and sideslip were done using the 5-hole nose cone pressure sensors, ADIFR and BDIFR.  Although sampled at 50 sps, internal filtering in the Mensor pressure sensors (model 6100) limits usefulness of high-rate analysis to about 5 Hz.

Both AKRD and SSLIP were calibrated using in-flight maneuvers.

True airspeed:

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.

RAF recommends using TASX as the aircraft true air speed.

Position and ground speed:

The measurement of aircraft position (latitude, longitude and geometric altitude) and aircraft velocities relative to the ground are done using several sensors onboard the GV.

Novatel Omnistar-enabled 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 real-time accuracy is needed.  These variables are subsequently used to constrain the INS drift for the calculations of the GV winds; more about this below.

A secondary Garmin GPS system provided redundant position measurements that should be used during periods of noisy GPS data. These variables have the same naming convention as the reference GPS above with a suffix _GMN.

Honeywell inertial reference system 1 and 2:  The GV is equipped with three inertial systems.  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.

The choice of variables for position analysis depends on the type of analysis; in general the Novatel Omnistart GPS is sufficiently accurate.

Not all INS variables are output in the final data set, including IRS2. If you require more detailed INS data please contact RAF.

• Attitude angles:
  
  Aircraft attitude angles are measured by the two Honeywell IRS units.

The values of pitch angle (PITCH) have been corrected using in-flight measurements to give approximately the same values as the aircraft attack angle (AKRD) for long parts of each flight; this correction is performed 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.

• Wind speeds:
  
  Wind speeds are derived from the 5-hole radome gust probe, other pressure measurements, temperature and inertial measurements supported by GPS data. The use of the Mensor 6100 pressure sensors for ADIFR, BDIFR and QCF results in the following limitations on the wind data: 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 following lists the most commonly used wind variables:

RAF recommends using the GPS corrected wind components, i.e., the variables ending in "C".

Liquid water content:

A PMS-King type liquid water content sensor was installed on the GV. The corrected liquid water content obtained from relating the power consumption (required to maintain a constant temperature) to liquid water content, taking into account the effect of convective heat losses.

Icing rate indicator

Rosemount Model 871FA Icing Rate Detector - Right Wing pylon Access Plate (RICE). The instrument was operational but reported little data due to infrequent penetrations of supercooled liquid water clouds.

The Microwave Temperature Profiler (MTP) was deployed. The MTP is a passive microwave radiometer, which measures the natural thermal emission from oxygen molecules in the earth-s atmosphere for a selection of elevation angles between zenith and nadir by scanning through an arc in the flight direction. The MTP observing frequencies are located on oxygen absorption lines at 56.363, 57.612 and 58.363 GHz. The measured brightness temperatures versus elevation angle are converted to air temperature versus altitude using a quasi-Bayesian statistical retrieval procedure. An altitude temperature profile (ATP) is produced in this manner every 18 seconds along the flight track. Temperature accuracy is approximately 1 K within 3 km of flight altitude, and < 2K within 6 km of flight altitude. Vertical resolution is approximately 150 meters at flight level, and approximately half the distance from the aircraft away from flight level. Data are provided in the form of altitude temperature profiles and color-coded temperature "curtains", and may be converted to isentropic cross sections.

CDP (Cloud Droplet Probe)

The CDP is a commercial instrument from Droplet Measuring Technologies (DMT). It measures the intensity of forward light scattering (4 - 12°) to determine the sizes of individual cloud droplets. An internal multi-channel analyzer assigns to individual particles to bins, and the data interface outputs a histogram of particle size and concentration. On the NSF/NCAR C­‑130 and HIAPER, it is mounted in a PMS canister. Variables output by the CDP instrument have _LWI suffix.

more information: http://www.dropletmeasurement.com/products/CDP.htm

Ultra-High Sensitivity Aerosol Spectrometer - UHSAS

The UHSAS is a single-particle light scattering instrument. It uses a CW high energy laser diode, wide angle collection optics centered at 90°, and four stages of amplification to size aerosol particles according to their scattered light. It assigns bins to individual particles and outputs a histogram of particle size and concentration. The RAF version of this instrument has been highly modified from the commercially-available lab bench version.  It uses volume flow controllers to keep the flow constant over a wide range of operating pressures and temperatures. It mounts in a PMS canister. The variables output by UHSAS have the suffix _RWO. The adaptation of the instrument to DSM logging causes an overlap of its four gain stages, which results in three ranges of size distributions that can not be finely resolved; these are visible on size distribution histograms as flat spots. Total concentrations are correct for all UHSAS ranges; only size distributions are affected.

more information: http://www.dropletmeasurement.com/products/UHSAS.htm

Data logging and averaging:

Analog data were logged at 500 sps and averaged to 1 sps. Serial data (e.g., RS-232), ARINC data (IRS units), etc. were recorded at the instrument-specific output rate.

The recordings listed for a given second contains measurements logged at e.g., 12:00:00.000 and until 12:00:01.  The value of "Time" corresponding to this interval is given a 12:00:00 in the released data set.

All measurements are "time-tagged" at the time of logging.  Subsequently these measurements are interpolated onto a regular grid and averaged.

RAF staff have reviewed the data set for instrumentation problems.  When an instrument has been found to be malfunctioning, specific time intervals are noted.  In those instances the bad data intervals have been filled in the netCDF data files with the missing data code of -32767.  In some cases a system may be out for an entire flight.

Calibrations

The following table identifies the sensors serial numbers and calibrations that were used on the GV for HIPPO Global Phase 1:

Data Quality Control

General:

• WIC data are reliable only during straight and level flight. Expect deviations whenever the aircraft is not in straight and level flight. For HIPPO there was very little straight and level flight and effort was made to tune the WIC processing to obtain good vertical wind in climbs and descents but deviations can still be seen.
• DPRS, DPLS, DPRC, DPLC: dewpointers tend to overshoot and oscillate after rapid temperature increase on aircraft descents. Using best judgment, these overshoots are removed from DPXC, which is the recommended reference dewpoint variable, but are left in DPLC, which is the source variable for DPXC, for comparison. The operating range for both DPLC and DPRC is down to approximately -70C and values below that should not be used for quantitative analyses. DP_VXL (dewpoint measurement produced by VCSEL hygrometer) is not affected by rapid  temperature changes and should be used whenever the DPXC is missing, and for any kind of high resolution analyses.
• Icing was a frequent occurrence during HIPPO-1. To detect icing conditions look for the onset of a difference between QCR and QCF in addition to PLWCC and RICE. The radome tip hope (QCR) ices over very quickly in icing conditions, resulting in the lag in response or constant pressure reading. These data are not representative of flight conditions (refer to QCF instead) but are useful for identifying periods of icing early, so they were left in the dataset. Variables dependent on the QCR (for example TASR) will also be unusable during radome icing.
• PLWCC processing: the background output of the King hot wire probe is loosely coupled to zero using a sliding window average. Once a cloud is encountered the coupling unlatches and the liquid water content is calculated using the last background value as a reference. This sometimes results in artifacts if the baseline continues to drift as a function of airspeed. This is an algorithm flaw that will be addressed in the future through refinements in the PLWCC airspeed parameterization.

Flight specific notes:

RF01: DPLS lost balance at high altitude under cold conditions at 16:38. The unit.did not recover until right before landing.

RF02: DPRS lost balance at high altitude under cold conditions at 21:02. The unit recovered at 21:53. Right wing DSM failed at 02:23 causing loss of UHSAS and RICE data after that time. Aircraft encountered light icing conditions throughout the flight, increasing quickly after 01:33 and reached noticeable level at 01:51. This effect can be seen as a deviation between QCF and QCR, with QCR being affected by radome icing. Digital imagery also will show loss of data whenever icing was encountered. Garmin and Novatel GPS altitudes diverge by up to 10 m and show noise off and on from 02:03 to 02:36.

RF03: Data gap encountered in Novatel GPS from 19:17:46 to 19:22:20. Use Garmin GPS data for that period. Noise in Garmin GPS altitude data and glitches in Novatel GPS data encountered around 01:06. DSM logging the INS data failed at 00:44 and did not recover until the end of the flight, resulting in the loss of 3D wind and combined position data (LATC and LONC) data. For position, use GPS variables instead of combination position.

RF04: Strong icing was encountered on the climb out at 19:32. Several instruments were impacted: MTP, VCSEL, DPLS and QCR. Each instrument recovered at a different time: VCSEL at 20:19, DPLS at 21:45, QCR at 21:34. MTP profiles may be affected by radome icing; see MTP data and documentation. Second period of icing started at 00:22 with radome iced over at 00:29, VCSEL following at 00:31 and recovering at 00:50. QCR recovered at 00:59. Third period of icing affected QCR from 01:10 to 01:37 and VCSEL from 01:14 to 01:29. Last icing period was encountered at 03:02 and affected only QCR, which recovered at 03:40. Period of noisy GPS data was encountered on both Garmin and Novatel receivers from 20:02 to 20:54. For some of the periods where DPXC was affected DPRC was performing well, so please use one or another dewpointer as necessary.

RF05: DPXC was removed for a number of intervals where it overshot. DPR* were also removed for a number of intervals where the probe did not work properly. VCSEL iced over from 23:54 to 00:01 and 00:07 to 00:12. QCR showed signs of icing from 23:57 to 00:12. Icing was encountered at 23:57 that affected QCR and lasted till 00:11. Temperature measurements were affected by the icing from 23:57 to 00:07; ATFR should not be trusted during this period.

RF06: DSM logging the INS data locked up from 21:38 to 22:43 resulting in the loss of the 3D winds data. Navigation data are available from the GPS. On this flight the brief loss of PSF data during the period when DSM was restarted to resume INS data acquisition caused loss of corrected temperature data from 22:48:08 to 22:43:27. This interval is very short and no change in processing was done for this flight (RF07 and RF08 have different temperature processing applied to correct for a similar issue, see below).

RF07: A period of icing was encountered from 22:19 and at 23:05-23:07. QCR and ADIFR (horizontal radome differential sensor) and dependent parameters appear to be affected. The icing of ADIFR caused the loss of 3D wind measurements from 00:03 to 00:22. Loss of the attack measurement affects the calculation of ambient temperatures, so a more crude corrections were applied to the entire flight for ATFR, ATHR1 and ATHR2: QCFC was set to QCF+2.26, where 2.26 is the mean offset between QCF and QCFC. The maximum differences in the ambient temperature caused by this substitution are during rapid descents and at high altitude leg and are on the order of ±0.5C, the mean difference is 0.1C. If this presents a problem, contact RAF for a data file that uses the original processing; it will have a gap in several variables from 22:19 and at 23:05-23:07.

RF08: Significant icing of the radome was encountered at 02:45 that lasted until 04:02. During the first part of this interval both QCR and ADIFR were affected, resulting in the loss of 3D wind data. 3D winds recovered at 03:13; QCR remained covered to 04:02. TASF was significantly different from the avionics airspeed from 02:46 to 03:13. Loss of the attack measurement affects the calculation of ambient temperatures, so a more crude corrections were applied to the entire flight for ATFR, ATHR1 and ATHR2: QCFC was set to QCF+2.26, where 2.26 is the mean offset between QCF and QCFC. For this flight the mean offset was different for the part with dips and the high altitude leg, so 2.26 was used for consistency with RF07. The maximum differences in the ambient temperature caused by this substitution are during rapid descents and are on the order of 0.3C, the mean difference is zero. If this presents a problem, contact RAF for a data file that uses the original processing; it will have a gap in several variables from 02:45 to 03:13.

RF09: INS data were lost from 05:54:40 to 06:01:05, resulting in the loss of 3D wind data. PSX was lost from 06:00 to 06:01:05 during the time the DSM recording INS data was being reset. At 08:34 on landing approach the power to the onboard network hub was interrupted, resulting in the loss of all data recoded on the ADS-3 from that point on.

RF10: No issues noted other than the dewpointer overshooting typical for all flights.

RF11: PLWC data had several periods where the sensor did not report: 17:54-18:01; 20:08-20:31; 21:06-21:20; 21:32-21:56.

July 22, 2010 Update

Changes to the HIPPO-1 production data, v. 2

A number of repetitive variables have been removed.

ATX, TTX: Reference ambient temperature and reference total temperature have been changed to be consistent with HIPPO-2 and HIPPO-3. Reference temperature used in this data release is AT_A, the temperature reported by the GV avionics. The research temperature sensors of the GV are undergoing quantification of the variable recovery factor at this time, and comparisons with the radiosondes indicated that the avionics temperature is the closest to that reported by radiosondes.

This change makes HIPPO-1 ATX consistent with the v.1 release of HIPPO-2 and HIPPO-3 data.

The reference temperature may change in the future when the research sensors are fully parameterized.

TCAB: Converted from the voltage output to degrees C.

RF03, RF06 and RF09: In addition to any v.1 comments: avionics measurements have been lost for a part of the flight. Therefore, ATX was set to an artificial temperature for this flight. The reference value has been generated by re-calibrating TTFR to closely match TT_A, and then setting ATX to ATFR. Since TTFR was measured for the entire flight, this allowed to recover temperature data for the part where the avionics data were lost. The difference between the avionics and calculated ATFR is from zero to 1C, with the maximum difference observed during high altitude, high speed flight. Note that the value of TTFR and ATFR in release v.2 for this flight is not the actual measurement acquired by the fast response sensor. Please compare ATX with AT_A for these sections if higher accuracy is needed for the temperature measurements.

April 7, 2011 Update

Changes to the HIPPO-1 production data, v. 3

An error was introduced in the previous release into the UHSAS size distribution data, removing the correction where bins were averaged across gain stage overlaps. The correction is re-introduced, size distributions are now corrected.

WP3 is obsolete and removed from the data file.

TVIR, virtual temperature, is added to the data file.

Calculations for the dew point from the chilled mirror sensors (DPRC, DPLC, DPXC); THETA, THETAV, PALT are updated based on commonly accepted published algorithms. This change is insignificant and is mainly for data traceability purposes.

Specific details are below:

-----------------------------------------

WP3 retirement: The IRS provides VSPD which is better (but needs some investigation to understand how it is processed and filtered). WP3 last appeared in the VOCALS and PLOWS datasets but was not used for the calculation of either WI or WIC in VOCALS and, while it was used to calculate WI in PLOWS, investigators were advised to use WIC instead (which depends on VSPD).

TVIR: The virtual temperature is the temperature of a dry-air parcel that would have the same density as the actual moist-air parcel.

DPxx: switch from Goff-Gratch formula to the Murphy-Koop algorithm.

THETA: Calculations based on Bolton (1980) are replaced with more accurate ones from Davies-Jones (2009); variable definition is changed to a more specific " pseudo-adiabatic equivalent potential temperature".

THETAE: abrupt deviations from other THETA variables are caused by the incomplete removal of the problematic dewpointer data from DPXC. Ignore these parts of the data.

PALT: use the International Standard Atmosphere or the equivalent U.S. Standard Atmosphere. The effects of the change are small but not entirely negligible, and this change makes the measurement consistent with the adopted standards for aviation. The differences are as follows:

1. The change in the constant for the low-altitude branch, to 44330.77 m, causes about a 5 m maximum change in results, with the maximum at 11000 m.
2. The change in the exponents for the low-altitude branch, to 0.1902632, causes at most a change of about 1 m in the results.
3. The change in the high-altitude branch increases with altitude, but at 120 mb (about 15 km) the change is about +5 m.

Feb 20, 2013 Update

December 18, 2012

HIPPO Global Data Change Notification

Dates affected: all phases, HIPPO-1 through HIPPO-5

Contact: P. Romashkin

Data affected: WI, WIC

Impact: Minimal

Description: An inconsistency in the processing of WI and WIC was discovered in which the two variables were calculated using a different vertical velocity for the different HIPPO deployments, leading essentially to a naming inconsistency. Serendipitously, another the inconsistency in the ASCII data tables for H-1 and 2 vs. H-3, H-4 and H-5 resulted in the correctly calculated data being available to the investigators: in H-1 and H-2, WI was included in the data, which was calculated using the more accurate vertical velocity; in H-3, H-4 and H-5 WIC was included, which was also calculated using the more accurate vertical velocity. At this time all five projects were re-processed to make the WI and WIC consistent across all five deployments. WIC is recommended for use and is calculated using the more accurate GPS-corrected vertical velocity VSPD_G.

This change is not expected to noticeably impact existing research because for the deployments 1 and 2 the WIC released at this time and recommended for future use is insignificantly different from the WI released previously.