Transcript No Slide Title

Attitude determination for limb-scanning satellites: The "KNEE" at 305 nm
Christopher E.
1
Sioris ,
1
Savigny ,
Christian von
2
Gattinger ,
1
McConnell ,
Richard L.
Jack C.
2
3
Edward J. Llewellyn , and the Odin team
Ian C.
1
McDade ,
Erik
1
Griffioen ,
1CRESS,
York University, 4700 Keele St., Toronto, M3J 1P3, Canada
2Department of Physics, University of Saskatchewan, Saskatoon, Canada
3Odin web page, http://www.snsb.se/Odin/Odin.html
Abstract
Methodology
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Knee sensitivity (km)
TH (km)
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O3 profile: tropical vs. subarctic winter (subarc-w)
Pressure: tropical vs. subarc-w
O3 & pressure (tropical vs. subarc-w)
SZA: 90 vs 57º
O3, pressure & SZA (tropical vs. subarc-w, 87.7 vs 57º)
Optically thick stratospheric cloud#
Stratospheric aerosol (moderate volcanic vs. backgr. profile)*0
Ground albedo (A=0 or 1)
Temperature (subarc-s vs. tropical vs. subarc-w)
Azimuthal angle (d= 60 to 120º)
+1
+3
+3
+2
+4
0
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15
0.0
0
0
0
10
0
100
200
300
400
500
600
700
800
900
0
0.00E+00
Intensity (DN/s/pixel)
5.00E-10
1.00E-09
1.50E-09
2.00E-09
2
2.50E-09
3.00E-09
-1
Radiance (W/cm /sr/cm )
Observed limb radiance profile (lat: 74-77 N, lon:
10-20W, July 30th 14:53-14:54 UTC, SZA=63.8º)
by OSIRIS on Odin. The knee is at ~45 km after
TH correction. To correct for internal scattering,  =
2.5e-6 was used (Evans and Alfred, Can. J. Phys.,
in press).
Modelled limb radiance profile at 305 nm
(MODTRAN4, 16 stream DISORT) for subarctic
summer, SZA=63.8º, d=90º, A=0.2, background
Mie scattering
345 nm
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40
40
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20
5000
Acknowledgements
This work has been supported by the Canadian Space Agency (CSA) and the National
Sciences and Engineering Research Council of Canada (NSERC). Odin is a Swedish-led
satellite project funded jointly by Sweden (SNSB), Canada (CSA), Finland (Tekes) and
France (CNES). We are indebted to G. Koenig-Langlo for providing the O3 sonde data.
References
Cho, J. Y. N. and J. Röttger, An updated review of polar mesosphere summer echoes: Observation,
theory, and their relationship to noctilucent clouds and subvisible aerosols, J. Geophys. Res. 102,
2001-2020, 1997.
Evans, W. F. J. and J. M. Alfred, Algorithm for correction of internal scattering and spectral crosstalk in the UV/vis band of the OSIRIS Instrument Flight Model, Can. J. Phys., in press.
Flittner, D. E., P. K. Bhartia, and B. M. Herman, O3 profiles retrieved from limb scatter
measurements: Theory, Geophys. Res. Lett., 27, 2061-2064, 2000.
30
20
10
Gerber, H., Y. Takano, T. J. Garrett and P. V. Hobbs, Nephelometer measurements of the
asymmetry parameter, volume extinction coefficient, and backscatter ratio in Arctic clouds, J.
Atmos. Sci. 57, 3021-3034, 1999.
90
4000
80
3000
2000
70
Koenig-Langlo, G. and B. Marx, The Meteorological Information System at the Alfred Wegener
Institute, in Climate and Environmental Database Systems, edited by M. Lautenschlager and M.
Reinke, Kluwer Academic Publisher, Boston, 1997, p. 117-125.
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10
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0
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10000
20000
30000
40000
Intensity (DN/s/pixel)
50000
60000
70000
0
0.0E+00
2.0E-08
4.0E-08
6.0E-08
8.0E-08
1.0E-07
1.2E-07
-1
Radiance (W/cm2/sr/cm )
1.4E-07
1.6E-07
Number density (cm-3)
• The success of the 305 nm knee suggests that a vertically imaging limb
sensor in the UV-visible (like the IR imager onboard ODIN) could remotely
sense stratospheric trace gases without the need for dedicated
instrumentation for pointing determination
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TH (km)
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TH (km)
Tangent height (km)
6000
3.0e+9
• Profiles of PMCs, O3, and NO2 altitude-calibrated using the 305 nm knee
show very good agreement in height with independent measurements
using various established techniques
Using a rotation matrix (approporiate to SMR) and the achieved
quaternions provided by the attitude control system, an offset can be
calculated between OSIRIS and the Odin control frame which is slightly
variable and slightly smaller (ΔTH=6.35±0.25 km) than the offset
provided by 305 nm knee.
Because the two instruments onboard Odin were co-aligned prior to
launch, the TH data provided by the ACS for the SMR could have been
used for OSIRIS as well. However, the extent of their post-launch coalignment was somewhat uncertain. Their co-alignment was partially
verified by the simultaneous viewing of Jupiter by OSIRIS and the SMR.
However, even this technique of determining pointing has some inherent
error because of the angular size of the Jovian disk viewed from Earth
(46 arc sec). The difference between the Odin control frame and the
OSIRIS viewing direction obtained from the Jupiter data translates to a
TH difference of 7.41±0.64 km at TH=45 km.
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2.5e+9
• The 305 nm knee occurs at a TH where radiance is very predictable
(ozone is fairly invariable, clouds are not directly observed and aerosols
contribute weakly). None of this is true for the 345 nm knee.
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60
2.0e+9
Conclusions
Validation of attitude determination
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1.5e+9
resolution of OSIRIS.
optically thick (=8 at 305 nm) cloud was inserted into the lower
stratosphere (11-30 km) with an asymmetry parameter of g=0.737,
appropriate for visible wavelengths and glaciated cloud, containing
mostly large bullet rosette ice crystals [Gerber et al., 1999].The 305 nm
phase function should be very similar due to the large crystal size.
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1.0e+9
The ozonesonde data has been smoothed with
a 2-km boxcar. This is the approximate vertical
#An
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5.0e+8
OSIRIS
HALOE
*In both cases, radiative properties were those of background aerosols.
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A sample limb scan
Variable(s)
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NO2, Aug 15, 31º N
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305 nm
Tangent height (km)
OSIRIS, the Optical Spectrograph and Infrared Imager System [Llewellyn
et al., 2001; Warshaw et al., 1998] is one of two instruments onboard the
Swedish/Finnish/French/Canadian Odin satellite. The other instrument is
a co-aligned limb-viewing sub-mm radiometer (SMR) and observes
molecular emissions. Odin was launched on February 20, 2001 from
Svobodny in eastern Russia into a circular, sun-synchronous nearterminator orbit with an inclination of 97.8° and an ascending node at
1800 LST. The nodding satellite enables OSIRIS to scan the Earth's limb
within a tangent height (TH) range between about 10 km and 100 km,
with ~2 km vertical sampling in the stratosphere. The optical component
of OSIRIS consists of a grating spectrograph covering the spectral range
from 280 nm to 800 nm with a resolution of about 1 nm. The Infrared
Imager has three channels: 1.53 m (featuring the OH Meinel (4-2) and
(3-1) bands), 1.27 m and 1.26 m (O2 IR atmospheric band). The
accuracy of the attitude control system (ACS) in limb-scanning mode is
about 1.2 minutes of arc, translating to about 1 km in terms of TH.
However, this accuracy value is appropriate for the look direction of the
SMR and the Odin control frame and does not correspond to specifically
to viewing direction of OSIRIS.
Summary of Sensitivity Study
The method consists of taking the difference in TH between the observed
305 nm knee (appropriate to the Odin control frame) and that modelled
by MODTRAN4 for the same conditions. The differences between the
observed and modelled knee THs for each scan are averaged over
several orbits of data to eliminate vertical undersampling errors in
individual scans. The resulting mean offset was 7.07±0.69 km.
Interpolation (e.g. cubic spline) is not used to find the true knee TH in the
vertically undersampled scan, the observed knee TH is simply the height
of the image with the maximal 305 nm radiance.
The atmosphere is modelled with three latitude bands (high, mid and
low) and two seasons (summer or winter). The SZA is calculable from the
measured time, latitude and longitude (of the tangent point). For OSIRIS,
the solar zenith angle (SZA) is always >57° and the azimuthal angle (d)
between the sun to tangent point line and the line of sight is in the range
57< d <123 ° due to the specifics of Odin’s orbit.
The important predictor variables for the knee TH are the ozone column
above 45 km, atmospheric pressure at 45 km, and SZA. The main cause
of the knee at 305 nm is increasing ozone absorption through the
Huggins bands with decreasing altitude, not Rayleigh scattering. Luckily,
the ozone column density is fairly latitude-independent above the altitude
of the knee (~45 km). Using model atmospheres, ozone spatial and
temporal variability can be taken into account adequately.
Instrumental
Retrievals of stratospheric trace gas profiles
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Altitude (km)
The limb scatter technique has, up until now, not been used to its full
potential for the retrieval of 3-D fields of stratospheric trace gases. The
technique combines good vertical resolution with tremendous spatial
coverage. OSIRIS (Optical Spectrograph and Infrared Imager System)
on the Odin satellite has begun to realize the enormous potential of this
technique. A number of upcoming missions (e.g. SCIAMACHY on
Envisat and OMPS on NPOESS) will also rely on this technique.
Pointing errors are the major limitation in remote sensing of stratospheric
ozone from the limb scatter technique (Flittner et al., 2000). Errors in
pointing of 1 km in tangent height (TH) can lead to O3 retrieval errors of
30% at z=20 km (Flittner et al., 2000). Altitude inaccuracies lead to
significant retrieval errors for limb scanning techniques in general (e.g.
occultation, longwave emission). In this poster, the issue of pointing
determination is addressed. A reliable technique using the tangent height
of the 305 nm limb radiance maximum, dubbed “the knee”, is discovered
which gives a pointing accuracy of 0.5 km. The main cause of the knee
at 305 nm is increasing ozone number density with decreasing altitude.
Intensity (arbitrary units)
Introduction
A technique based on observation and modelling of ultraviolet radiative transfer is developed to determine pointing to <1 km accuracy for satellite instruments viewing
Earth’s limb. The method consists of locating the ‘knee’, defined as the maximum in the limb radiance profile, at a wavelength of 305 nm. The tangent altitude of this
observational point (pixel or image, depending on whether or not the instrument is an imager) is known from radiative transfer modelling and is insensitive to tropospheric
clouds, stratospheric aerosol, temperature, surface albedo and change in azimuth angle. Observations over several orbits with OSIRIS (optical spectrograph and infrared
imager system) onboard the Odin satellite and model calculations show the knee at 305 nm is at ~44 km. The insensitivity of the knee to these spatially variable
geophysical parameters implies that the instrument measuring the limb radiance profile need not be an imager for this technique to be applicable. Although there is some
sensitivity to SZA, ozone number density and pressure, it is predictable using appropriate model atmospheres and scattering geometry. Previously [e.g. McPeters et al.,
2000], the knee at 345-355 nm was suggested for attitude sensing but our calculations and OSIRIS observations have shown this wavelength range to be sufficiently
45
sensitive to clouds and SZA that there is no knee for certain scenarios. Internal scattering limits this technique with OSIRIS to wavelengths longer than ~300 nm.
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1.8E-07
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Wavelength (nm)
Succesive images of Jupiter: two methane polyads
are readily apparent at longer wavelengths.
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Llewellyn, E. J., et al., The OSIRIS instrument on the ODIN satellite, Can. J. Phys., in press.
Intensity (DN/s/pixel)
PMC layer observed at ~84 km. It is interesting to
note that the PMC does not affect the knee
height (= 45 km) either.Because polar
mesospheric clouds (PMC) occur in a narrow
height range 82-84 km (Cho and Röttger, 1997),
the tangent altitude determined from the knee
can also be validated to some extent by the
height of these ice layers.
McPeters, R. D., S. J. Janz, E. Hilsenrath, T. L. Brown, The retrieval of O3 profiles from limb scatter
measurements: Results from the Shuttle Ozone Limb Sounding Experiment, Geophys. Res. Lett.
27, 2597-2600, 2000.
Warshaw, G. D., D.-L. Desaulniers, and D. Degenstein, Optical design and performance of the
Odin UV/visible spectrograph and infrared imager instrument, Proc. 10th Annual AIAA/USU
Conference on Small Satellites, 1996.