MTP Instrument Descriptions
An MTP is a passive microwave radiometer that measures the
thermal emission and absorption from oxygen molecules in the atmosphere
along the instrument's line of sight, and then uses this information to
retrieve
a vertical profile of temperature by assuming horizontal temperature
stratification and by using an atmospheric microwave
opacity model . [In this link, the three altitudes of 0 km, 10 km,
and 20 km are representative of ground based, DC8 and ER2/WB57
projects. This absorption model is from P.W. Rosenkranz, Ch. 2 and
Appendix, in Atmospheric Remote Sensing By Microwave
Radiometry, M.A. Janssen, Editor, New York : Wiley, 1993. ].
Because any oxygen emission is absorbed in proportion to its distance
from the radiometer, the integrated emission can be represented by a "weighting function" which
characterizes the weighted mean distance of the emission. In the
special case where the temperature lapses at a constant rate with
distance, it is easy to show that the brightness temperature measured
by the radiometer is exactly equal to the physical temperature at the
e-folding distance for the absorption. More complicated situations
require more detailed treatment.
An MTP generally consists of two assemblies: a sensor unit (SU),
which receives and detects the signal, and a data unit (DU), which
controls the SU and records the data. In addition, on some platforms
there may be a third element, a real-time analysis computer (RAC),
which analyzes the data to produce temperature profiles and other data products in real time. The
SU is connected to the DU with power, control, and
data cables. In addition the DU has interfaces to the aircraft
navigation
data bus and the RAC, if one is present.
Navigation data is needed so that information such as altitude,
pitch
and roll are available. Aircraft altitude is needed to perform
retrievals
(which are altitude dependent), while pitch and roll are needed for
controlling
the position of a stepper motor which must drive a scanning mirror to
predetermined elevation angles. Generally, the feed horn is nearly normal to the flight
direction and the scanning mirror is oriented at 45-degrees with
respect to receiving feed horn to allow viewing from near nadir to near
zenith. At each viewing position a local oscillator (LO) is sequenced
through two or more frequencies. Since a double sideband receiver is
used, the LO is generally located near the "valley" between two
spectral lines, so that the upper and lower sidebands are located near
the spectral line peaks to ensure the maximum absorption. This is
especially important at high altitudes where "transparency" corrections
become important if the lines are too "thin."
Because each frequency has
a different effective viewing distance , the MTP is able to "see"
to different distances by changing frequency. In addition, because the
viewing direction is also varied and because the atmospheric opacity is
temperature and pressure dependent, different effective viewing
distances are also achieved through scanning in elevation . If the
scanning is done so that the applicable altitudes (that is, the
effective viewing distance times the sine of the elevation angle) at
different frequencies and elevation angles are the same, then
inter-frequency calibration can also be done, which improves the
quality of the retrieved profiles. For a two-frequency radiometer with
10 elevation angles, each 15-second observing cycle produces a set of
20 brightness temperatures, which are converted
by a linear retrieval algorithm to a profile of air temperature versus
altitude, T(z).
Finally, radiometric calibration is performed using the
outside
air temperature (OAT) and a heated reference target to determine the
instrument gain. However, complete calibration of the system to include
"window corrections" and other effects, requires tedious analysis and
comparison with radiosondes near the aircraft flight path. This is
probably the most important single factor contributing to reliable
calibration. For stable MTPs, like that
on the DC8, such calibrations appear to be reliable for many years.
Such
analysis is always performed before MTP data are placed on mission
archive
computers.
The JPL DC8 Microwave Temperature Profiler (MTP/DC8) is a passive
microwave radiometer that measures the natural thermal emission from
oxygen molecules at three frequencies (55.51, 56.66 and 58.79 GHz). The
instrument views ten elevation angles between -80 and +80 degrees by
using a scanning mirror, located behind a microwave window on the
sensor unit,
to change the viewing direction. The sensor unit is located in a window just aft
of the forward starboard exit door. This window has always been
considered inaccessible for science instruments or inlets because it is
immediately behind the Mission Manager's console. The sensor fairing is
far
enough ahead of other useable windows that it should not interfere with
any
future sampler inlets. The control
and analysis electronics for the MTP are located in the Mission
Manager's rack. The MTP retrieves profiles of air temperature versus
altitude, which it displays on a dedicated color LCD display beside the
Mission Manager's console; a video signal is also available for
distribution to DADS monitors throughout the aircraft.
Comments on Molecular Oxygen
Rotational Spectrum
Some FAQs on DADS-related
Parameters
ER2
Introduction
The airborne MTP/ER2 instrument is a passive microwave radiometer that
measures thermal emission from oxygen molecules along the viewing
direction. A stepper motor rotates a 45-degree shaped reflector so that
radiation entering a receiving horn is sequenced through a set of 10
elevation angles, ranging from -58.2 to +60.0 degrees (within a
vertical plane that is offset 20 degrees in azimuth from the direction
of flight). At each viewing position a local oscillator is sequenced
through two frequencies: 56.66 and 58.80 GHz. Each 14-second observing
cycle produces a set of 20 brightness temperatures,
which are converted by a linear retrieval algorithm to a profile of air
temperature versus altitude. Altitude coverage is 15 to 25 km while
flying
at 19 km. T(z) profiles are obtained every 2.9 km along the flight
path.
Science
T(z) data can be used to derive a "curtain cross-section" of the
temperature field along the flight path . Regions cold enough to
condense water vapor, or nitric acid, can be identified (with mixing
ratio assumptions). The
temperature field is also used to derive cross-sections of isentrope
surface
altitudes, which can be used to locate mountain waves. "Waviness" of
the
isentrope surface altitudes can also be used to quantify the mesoscale
vertical
motions of air parcels and their associated temperature fluctuations.
Temperature fluctuations are needed to determine an effective
temperature for those chemical
reactions and solubilities which vary in a strongly non-linear manner
with
temperature. The tropopause altitude can be located along the flight
path
when the aircraft is within about 3 km of the tropopause, which often
occurs
during brief descent/ascents midway in a flight.
History
The MTP/ER2 was used during the 1987 Airborne Antarctic Ozone
Experiment, AAOE, the 1989 Airborne Arctic Science Experiment, AASE,
the 1992 Airborne Arctic Science Experiment, AASE II, and the 1994
Airborne Southern Hemisphere Ozone Experiment. Measurements for
Assessing the Effects of Stratospheric Aircraft, ASHOE/MAESA, based in
New Zealand and Hawaii. AAOE measurements led to the discovery and
quantitative characterization of mountain waves in the lower
stratosphere over the Antarctic Peninsula. AASE measurements led to the
discovery and quantification of an ever-present component of
mesoscale vertical motions in the lower stratosphere, which has
implications
for polar stratospheric cloud formation and drop size evolution. AASE
II
measurements were used to validate a model for mountain wave production
(and CAT breakdown), which has implications for momentum flux transfer
to
the stratosphere. ASHOE/MAESA measurements led to the discovery that
filaments
sometimes produce a distortion to the temperature field, and this may
provide
a means for studying filament entrainment and transport.
Performance
Profile Accuracy (for flight at
19
km)
- < I K from 18 to 20.5 km
- < 2 K from16.2 to 23.2 km
- < 3 K from 15 to 25 km
|
Tropopause altitude accuracy
- 0.2 km when within 1 km of the tropopause
- 0.3 to 0.5 km when 2 to 3 km away
|
| Temporal Response
: new profile every 14 seconds |
Location:
Right engine "cheek" |
| Mass: 11 kg |
Power: 100 W |
WB57
MTP will fly for the first time on the NCAR WB57 in March 1998 on the
WB57 Aerosol Mission (WAM). The sensor unit and data unit are the ER2
instrument. An old superpod fairing has been modified to allow scanning
from +70 to
-70 degrees. In addition, a real time analysis computer (WAC) has been
added and DC8 realtime analysis code modified so that the back seat
scientist can have realtime tropopause altitude guidance.
