basic measurements in progressive science
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6/21/06 #1
Basic Measurements
in
Progressive Science
Al Cooper
21 June 2006
6/21/06 #2
Measurements to Discuss
● Temperature
● Pressure
● Wind (Friesen)
– horizontal
– vertical
● Humidity
● Derived Variables
● GPS and D-values
– (Jensen)
● Dropsondes
6/21/06 #3
Temperature
● Five sensors available: TT_A, TTH{L,R}{1,2}
● TT_A comes from Gulfstream avionics (with
their processing)
● TTH are heated sensors, hence slow-
responding (>second)
● TTX is selected for use in derived quantities
like potential temperature. For PS, ==TT_A
6/21/06 #4
Processing Uncertainties
● Calibration! Based on bath calibration, but
there were inconsistencies still not
understood and problems with TTH
measurements
● Mach Number dependence: NOT used, but
expected. See variables TTH{L,R}{1,2}C
● Recovery factor: Used 1.0, but still fine-
tuning
6/21/06 #5
6/21/06 #6
6/21/06 #7
6/21/06 #8
6/21/06 #9
Suggestions, Temperature
(for now):● Use TT_A and associated variables. (Ambient T is
AT_A, and this is also ATX). No evidence of delay
or filtering in AT_A; seems as good as ATH for
response (unlike avionics pressure)
● Avoid ATHL1: many bad flights.
● Use caution for values <-60C for now. There is an
apparent calibration difference at T<-60., with AT_A
about 5C above others. There is also uncertainty in
the recovery factor, not yet resolved.
6/21/06 #10
Time response of heated sensors(here TTHR3)is obviouslyslow vs this
unheated (TTRL)one from TREX.
6/21/06 #11
Pressure:
● As for T, there is an avionics-supplied static
pressure: PS_A. The aircraft is “RVSM”
certified, which needs altitude accuracy of ca.
80 ft or, at 40Kft, pressure accuracy of ca.
0.8 mb.
● Pressure is measured independently as
PSFC (after corrections are applied). This is
the reference pressure used for Prog. Sci.
6/21/06 #12
6/21/06 #13
Other aspects of pressure:
● The avionics pressure PS_A is inherently
slow and has an evident lag, so it is best
avoided except as a reference for calibration.
● Beware of regular several-minute oscillations
created by the auto-throttles, which produce
oscillations in P, Z, TAS, and sometimes
other variables. (These are filtered from
PS_A.)
6/21/06 #14
Recommendation, Static P
● Avoid PS_A except for calibration: It has slow
time response and an inherent lag.
● Use PSFC (or PSX) for moderate accuracy.
● See the study to be presented by Jorgen
Jensen for higher accuracy, as needed for
combining pressure with GPS-altitude to
study pressure fields.
6/21/06 #15
Wind Measurements
● Dick Friesen will discuss in some detail
● Measurements look reasonable as first
estimates, but need (and are undergoing)
fine-tuning for such things as radome
calibration, removal of airspeed offset, and
an apparent but variable bias in vertical wind.
● Important qualification: Sensors are slower
than expected, don't provide >~5Hz
6/21/06 #16
Humidity
● Three measurements:
– DPLC and DPRC: dew point from chilled-mirror
instruments. 1/s samples used (serial); also
sampled at higher rate as analog output. “C”
refers to correction from measured frost point to
calculated dewpoint.
– TDL, provided in two channels with different
sensitivity. (archived as ppmv)
6/21/06 #17
DP sensors:* slow response * searching leads
to errors * reasonable match* lower DP than
prev.* P correction? * still, RMS<1.5C
6/21/06 #18
Beware of MR fluctuations:
● frost-pt values both
have floors at -74C,
which is d.p. of about -
87C
● As T changes, MR
changes, giving false
signal not representing
humidity changes.
6/21/06 #19
TDL:
● Does not have this lower-limit problem
● Calibration is independent of DPs
● Much more sensitive at low humidity
● Archived value is ppmv. Convert to MR in
g/kg by multiplying by 0.622E-3.
● vs DPs, values appear to be biased high?
6/21/06 #20
Comparison, MR from TDL and from DP
6/21/06 #21
Derived Variables: Some traps
● Be wary of MR fluctuations; they may not be
real for low values. Similar: q, RH, etc.
● Potential temperature is calculated from
AT_A and PSFC as in data files, and these
are best values. ATHxx and PS_A are less
suited to this calculation. Be aware of a
remaining calibration issue with T, though, at
low values: This still needs to be resolved.
6/21/06 #22
GPS position and altitude● Basic aircraft system provides high accuracy
● For best accuracy, differential GPS data are available,
but recorded separately and not yet part of the
processed files.
● There is high potential to use these measurements to
study pressure fields with good precision and spatial
resolution
● Errors are now such that the error in pressure is the
dominant source of uncertainty. See Jorgen Jensen's
talk for ongoing work on this source of error.
6/21/06 #23
Example● Geostrophic wind Wg=-(g/f) (dz/dn):
● measured D-value slope indicates about 80 m/s wind at
left, decreasing to right
6/21/06 #24
Dropsondes
● New installation encountered some problems
with sondes sticking in the exit tube and with
some striking the aircraft on release.
● System was modified, and after modification
mostly worked satisfactorily.
6/21/06 #25see ftp://ftp.eol.ucar.edu/pub/temp/users/kbeierle/progressive_sci/
6/21/06 #26
Expected Revisions:
● Wind: refinements underway (Friesen)
● Pressure: refinements underway (Jensen)
● Temperature: still need to resolve calibration, but
may not revise unless ATX=AT_A needs revision
● Higher-rate processing being implemented
● Differential GPS handling: may be merged to
primary dataset?
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