Download Kein Folientitel - IMK-ASF

Forschungszentrum Karlsruhe
Technik und Umwelt
C. E. Blom, T. Gulde, C. Keim, W. Kimmig, C. Piesch, C. Sartorius, H. Fischer
Institut für Meteorologie und Klimaforschung
Forschungszentrum Karlsruhe GmbH / Universität Karlsruhe
MIPAS-STR: a new instrument for stratospheric aircraft
MIPAS-STR (Michelson Interferometer for Passive Atmospheric
Sounding - STRatospheric aircraft) is a new instrument developed for
remote sensing of a large number of atmospheric trace compounds
(e.g. ClONO2, N2O5, NO, NO2 and HNO3) from high-altitude aircraft.
It will be operated from the Russian M-55 Geophysica in the
framework of the APE-GAIA (Airborne Polar Experiment Geophysica Aircraft In Antarctica) in September/October 1999.
We used modules of the Giessen diode laser system to test
Brault’s approach of time-equidistant sampling which was
implemented in the electronics of the interferometer of MIPAS-STR.
Components of the optic module

Scan unit: For accurate pointing and to correct for roll-angle
movements of the M55, the scan mirror is continuously
adjusted based on the data measured by the instrumentfixed AHRS (Attitude and Heading Reference System).


3-Mirror telescope for stray light suppression.
DPI: Interferometer based on the double pendulum principle
but with equidistant sampling in the time domain.

Cooling system: In order to reduce the background radiation
and improve NESR, the optic module is cooled to 200 K.

Detector unit: see Figure 3.
Line of sight stability
Optical path difference (2-sided)
Spectral resolution (unapodized)
Beam diameter inside the DPI
Etendue
Scan velocity
Detector type (area)
Field of view, FOV
Signal frequencies
1 arcmin (3)
 15 cm
0.034 cm-1
50 mm
2.6  10-3 cm2 sr
3 cm/s
Si:As (BIB) (1.6  1.6 mm2)
0.44 (full cone)
2.3 - 5.8 kHz
Noise equivalent spectra radiance (NESR):
- channel 1: 770 - 1000 cm-1 (13.0 - 10.0 m)
- channel 2: 1200 - 1370 cm-1 (8.3 - 7.3 m)
- channel 3: 1585 - 1645 cm-1 (6.3 - 6.1 m)
- channel 4: 1845 - 1940 cm-1 (5.2 - 5.4 m)
Temperature and emissivity of blackbodies:
BB1 in the optic module
BB2 in the upper isolation of the optics module
Accuracy of calibration
Sampling frequency (time-equidistant)
Sampling frequency (interpolated to equidistant
sampling positions)
Data rate (4 channels incl. HK data)
Refill intervals of cryogenics (lHe, lN2, dry-ice)
Electrical power (voltage)
Mass (optic module + electronics)
[nW/(cm2 sr cm-1]
25
11
3
2
Fig. 1: Artist’s view of the optic module. To reduce size of the instrument the optics is divided in
two levels: the upper level contains the scan unit and the telescope, the lower level the
interferometer.
Fig. 2: Linear representation of the optical path. The IR-radiation propagates from the scan
mirror via the telescope and the interferometer to the detector unit. The instrumental FOV is
defined by the lHe cooled apertures FS3 and AS3. The apertures AS, FS1 and shields reduce the
radiation from outside the FOV as well as scattered radiation reaching the front optics. The
radiation diffracted at the edges of the front optics is suppressed by the Lyot and aperture FS2.
Fig. 3: The detector system. The entire
focal plane with dichroic beam splitters,
optical filters, Si:As-detectors etc. is
cooled to 4 K. The division into 4 channels
is necessary for NESR improvement to
facilitate the detection of NO2 and NO
(channels 3 and 4) and allows an efficient
data reduction.
78 K / 200 K, 0.997 (cavity)
about 240 K (floating), 0.98 (plate)
1-3%
48.8 kHz (50 MHz / 1024)
47.4 kHz
50 kB/s
20 h
300 W (28 VDC)
200 kg (150 kg + 50 kg)
external PC's
Ethernet
main control unit
- expert system
- data storage
Table 1: Characteristic instrument data.
Onboard electronics and system
operation
transputer
interferometer
electronics
pilot control panel
aircraft avionics
link
housekeeping and
auxiliary electronics
line-of-sight
electronics
AHRS/GPS
angle encoder
interferometer drive
IR data acquisition
Main computer: rugged PC with standard interface
14

Unix operating system
12

Transputer network for communication with subsystems

Ethernet for connection to the ground station
Phase [ °]

relay, reset
shutter drive
housekeeping data
pointing mirror drive
Fig. 5: Results from blackbody
measurements with scan
velocities between 1.58 cm/s and
4.30 cm/s. The mean of the phase
spectra for forward and backward
scans shows the electrical
contributions to the phase.
10
8
6
Ground station
4
1000

has access to the main computer and all subsystems

transfers the measurement program before take-off

allows on-line visualization of all parameters and transfer of
data stored during the flight
Fig. 4: Scheme of the onboard
electronics. The electronics is
structured hierarchically. A transputer
network connects the central computer
with the independent subsystems. The
state of the systems is defined by
housekeeping and status data. Access
from the main computer enables full
control during operation of the
instrument.
1500
2000
2500
3000
3500
Frequency [Hz]
4000
4500
Fig. 6a: Spectrum of the diode
laser. Low current was applied
to the TDL to obtain almost
monochromatic radiation.
The new sampling technique
Problem:
Vibrations of the aircraft produce perturbations of the
scan velocity of the interferometer. If laser fringes
establish the sampling positions and a time delay
between the laser signal and the IR-interferogram
exists, the velocity variations lead to sampling errors
and the resulting spectra show phase ghosts or
sidebands.
Solution:
Brault’s approach of time-equidistant sampling was
implemented in the Interferometer electronics.
Test:
Use of Giessen TDL as monochromatic source.
Fig. 5: The aircraft M-55 Geophysica. Left: in Pratica di Mare (November 1996). Right: drawing of
the M-55 with the ‘dorsal bay’ for MIPAS on top.
Fig. 6b: Same as fig. 6b but with
expanded vertical scale. Since
no monochromator was used,
weak secondary lines (below
1%) can be observed.
Fig 6c: Spectrum obtained by
sine modulating the scan
velocity. The modulation
frequency was set to 150 Hz,
the amplitude (p-p) was 40%
of the nominal value of 3 cm/s.
A time delay of 0 s was used.
Fig 6d: Same as fig. 6c but
with time delay of 22 s
derived from the phase
spectra shown in fig. 5. Note
that the ghosts are reduced to
about 20%. The non-vanishing
part of the ghosts may be due
to simultaneous amplitude
modulation at the same
frequency.
August 1999