The AVIRIS instrument uses an onboard calibration system to provide auxiliary calibration data. The system consist of a tungsten halogen cycle lamp imaged onto a fiber bundle through an eight position filter wheel. The fiber bundle illuminates the back side of the foreoptics shutter during a pre-run and post-run calibration sequence. The filter wheel contains two neutral density filters, five spectral filters and one blocked position. This paper reviews the general workings of the onboard calibrator system and discusses recent modifications

Chovit, Chris

Chrien, Thomas G.

Eastwood, Mike

Green, Robert O.

Hajek, Pavel

Johnson, Howell

Sarture, Charles

English

NASA Technical Reports Server

N95- 33739
Airborne Visible/Infrared Imaging Spectrometer (AVIRIS) Onboard Calibration System
Thomas G. Chrien, Mike Eastwood, Robert O. Green, Charles Sarture, Howell Johnson, Chris Chovit, and Pavel Hajek
Jet Propulsion Laboratory, California Institute of Technology
4800 Oak Grove Drive, Pasadena, California USA 91109
The AVIRIS instrument uses an onboard calibration system to
provide auxiliary calibration data. The system consists of a
tungsten halogen cycle lamp imaged onto a fiber bundle through
an eight position filter wheel. The fiber bundle illuminates the back
side of the foreoptics shutter during a pre-run and post-run
calibration sequence. The filter wheel contains two neutral density
filters, five spectral filters and one Hocked position. This paper
reviews the general workings of the onboard calibrator system and
discusses recent modifications.
INTRODUCTION
AVIRIS is an airborne sensor which measures high spatial
resolution image data of the earth in 224 spectral channels in four
spectrometers (A, B, C, and D) coveting the range from 380 to
2500 nm. These data are spatially, spectrally and radiometrically
calibrated (Vane, 1987, Chrien 1990, Chrien 1993b).
Modifications to AVIRIS have resulted in substantial
improvements in signal-to-noise ratio, calibration accuracy and
operability of the sensor (Cbrien, 1991, Cbrien 1993a). Validation
experiments are conducted to verify instrument performance and
calibration in flight (Green, 1993a). The onboard calibration data
can be used to correct for variations in the AVIRIS radiometric
response (Green 1993b). Recent modifications to the onboard
calibration system are discussed.
CALIBRATION REQUIREMENT
Imaging spectrometers are used to measure the contiguous spectral
signature of the upweLling radiance reflected and emitted from the
suxface of the earth and its atmosphere. The information contained
in this data is in general a non-linear mixture of atmospheric
molecular absorptions and particle scattering signatures, surface
reflected molecular absorption signatures and bidirectional
reflectance properties and source spectral characteristics such as
solar spectral radiance and surface temperatures. The scientific
study of any one of these effects requires a separation of these
complex interactions.
The instrument calibration of an imaging spectrometer removes an
additional (and unnecessary) complication to the non-linear
unmixing problem discussed above. In general, the requirement
for calibration accuracy is determined by the tolerance of unmixm'g
algorithms to resid_ calibration errors. This tolerance decreases
as the signal-to-noise ratio of an imaging spectrometer increases in
order to avoid unmixing that is instrument calibration error limited.
Recent improvements in both the signal-to-noise ratio of the
AVIRIS instrument and the unmixing algorithms applied to
AVIRIS data have prompted a re-evaluation of its calibration
requirements. Of primary interest are improvements to the current
instrument radiometric accuracy of 5% absolute and spectral
accuracy of 1 to 2 nanometers.
ONBOARD CALIBRATION SYSTEM
In 1991 the onboard calibrator under went a major modification
(Chrien, 1991) that routed light into the four AVIRIS
spectrometers by illuminating the back of the closed forcoptics
shutter. A color balance filter (2 mm Schott KG-2) was installed to
provide good detector response across the 224 spectral channels of
the sensor. An improved current stabilized lamp drive circuit was
also installed. These changes greatly improved the utility of the
onboard calibrator data to correct for residual drifts in the
radiometric response of the sensor. Since that time a number of
addition improvements have been made.
End of Run Dark Images
The AVlRIS instrument now records 100 lines of dark imagery at
the end of each run of data. The fore-optics shutter is dosed during
this time as it is during end-of-line dark measurements. It is
expected that this dark image will provide a data set on which to
base coherent noise level measurements and to enable the
construction of coherent noise filters for the datg A high resolution
counter (HRC) has been added to the engineering telemetry to
measure the number of instrument clock cycles between
successive lines (which varies due to roll correction). The high
resolution counter provides a way to time code detector reads from
line to line and may prove useful in filtering time dependent noise
sourS.
Onboard Calibration Lamp
Prior to the 1994 flight season the OBC lamp was replaced with a
5000 hour rated lifetime bulb. Unfortunately, during the 1994
flight season the onboard calibrator experienced an excessive
number of lamp failures. The first ocurred just before shipping
AVIRIS to Ames. Two other failures occurred during the flight
season. These failings oecured at a fraction of the 5000 hour life of
the bulb. Upon visual inspection, no fracture or burn-through of
the filament was visible, and indeed, the second lamp that failed in
AVIRIS lit right up in the lab.
The failed bulbs were returned to the manufacturer for autopsy,
and the cause of failure was found to be degradation of the pigtail-
wire-to-lamp-pin crimp-weld connections owing to corrosion. The
corrosion arose from chemical byproducts from the heat
degradation of shrink tubing placed over the crimp-welds before
the lamp was potted in its metal base. This corrosion caused the
wire-to-pin connections to rise in resistance beyond what the OBC
constant-current power supply could handle (owing to an upper
limit on the voltage it was able to supply to the bulb and wires
combination), and hence the bulbs failed to light. The bulb
intensity was probably unstable for some time before the
resisitance due to the crimp-weld rose so far as to preclude the
hrnp from lighting at all.
The corrosion explanation fit well with the observed lamp behavior
before failme, and with lamp "filament" resistance measurements
after failure. The manufacturer uses teflon tubing for all its higber-
wattage lamps, but for this 10 watt lamp typical eleoronio-cabling-
variety shrink tubing was used. The manufacturer did not forsee
the lamp base getting very hot, but we had taken steps to make
sure that the lamp did get to at least 250 degrees C in order to keep
the lamp's internal halogen cycle effective and hence maximize the
life and the light output stability of the bulb. The manufacturer has
fabricated new lamps for the AVIRIS OBC application which
incorporate the high temperatme tolerant teflon tubing around the
crimp-welds. We expect these to survive our nominal operating
temperature.
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https://ntrs.nasa.gov/search.jsp?R=19950027318 2020-06-16T06:59:30+00:00Z
Also included in this year's changes is a conversion from a
constant-current powcr supply to a constant-light-ou_ut power
supply. We have mounted a _licon photodiodeJamplifier hybrid
(Model #UDT-455) on the top of the OBC lamp housing. The
lamp filament is imaged onto the photodiode active area through a
543nm narrow-pass filler. The is fed back to the
lump power supply to maintain constant light output from the
bulb.
Currently an investigation of lamp stability over operating
temperatme is underway. Historical data show that the temperature
internal to the OBC remains between 20 ° C and 30 ° C over the
course of a fright season. Narrow-banding the light hitting the
detector minimizes the effect of the silicon photodiode's
_y-dependent tesponsivity drift with temperature.
Spectral Calibration Filters
Four new spectral filters have been designed for the onhoard
calibration source filer wheel The Éllter wheel has been modified
to accommodate the additional filters. The new filers consist of a
bi_fringent material sandwiched between linear polarizers to make
a single stage Lyot filler. The transmission of a Lyot filler varies as
the cosine squared of one over the wavelength depending on the
filler retardance. The four filters are designed to have a period of
approximately 20 nanometers at the short wavelength end of each
AVIRIS spectrometer. This insures that there will be sharply
sloping spectral features across the entize 0.4 to 2.5 _ _.
The spectral transmittance of the four filter set is shown in Figure
1. Measured data is shown for the first three filters which cover the
0.4 to 1.9 pin range, while the predicted performance is shown for
the fourth 1.7 to 2.5 _rn filter. A temperature sensor placed on the
filter wheel housing updates the engineering telemetry once per
second with a 0.3°C precision.
60
%T 2
400 450 500 550 (dX) 650 700
30
700 800 900 1000 II00 1200 1300
30
0l 1-" .... t - - "e I Y I I
II00 1300 1500 1700 1900
4O
1°.t0
1700 1900 2100 2300 2500
Wavelength [rim]
Figme I. Lyot spectral calibration falter set
The data from the onboard calibrator is converted to spectral
transmittance by dividing the Lyot minus Dark filter whee_ spectra
by the High minus Dark spectra. The sharply sloping
tJansmittance features provide high sensitivity to spectral shifting
infight. Preliminaryanalysis shows that a 0.I nm shift in spectral
response is detectable using these filters,
REFERENCES
Chrien, T. G.,R. O. Green,and M. L. Eastwcod, "Accuracyof
the Spectral and Radiometric Laboratory Calibration of the
Airborne Visible/I_ Imaging Speet_ometer", Proc. SPIE
Conference on Aerospace Sensing, Imaging Spectroscopy of
the Terrestrial Environment, Orlando, Florida, 16-20 April,
1990.
Chnen, Thomas G., Michael L. Eastwood, Charles M. Sarture,
Robert O. Green, and Wallace M. Porter, "Current Instrument
Status of the Airborne Visible/Infrared Imaging Spectrometer
(AVIRIS)", Proc. Third AVIRIS Workshop, JPL Publication
91-28, Jet Propulsion Laboratory, Pasadena, CA, pp. 302-313,
1991.
Chrien, Thomas G., R. O. Green, "Instantaneous Field of View
and Spatial Sampling of the Airborne Visible/Infrared Imaging
Spectrometer (AVIRIS)," Summaries of the Fourth Annual JPL
Airborne Geoscience Workshop, JPL Publication 93-26, Vol. 1,
Jet Propulsion Laboratory, Pasadena, CA, 1993a.
Chrien, Thomas G., R. O. Green, C. M. Sarture, C. Chovit, NL L.
Eastwood, and B. T. Eng, "Airborne Visible/Infrared Imaging
Spectrometer (AVIRIS): Recent Improvements to the Sensor,"
Summaries of the Fourth Annual JPL Airborne Geoscienoe
Workshop, JPL Publication 93-26, Vol. 1, Jet Propulsion
Laboratory, Pasadena, CA, 1993b.
Green, Rohe_ O., James E. Conel, Mark Helndinger, Jeannette
van den Bosch, Chris Chovit and Tom Chrien, "lnflight
Calibration of AVIRIS in 1992 and 1993," Summaries of the
Fourth Annual JPL Airhome Geoscience Workshop, JPL Pub.
93-26, Vol. 1, Jet Propulsion Laboratory, Pasadena, CA,
1993a.
Green, Robert O., "Use of Data from the AVIRIS Onboard
Calibrator," Summaries of the Fourth Annual JPL Airborne
Geoscience Workshop, JPL Publication 93-26, Vol. 1, Jet
Propulsion Laboratory, Pasadena, CA, 1993b.
Vane, Gregg, T. G. Chrien, E. A. Miller, and J. H. Reimer,
"Spectral and radiometric calibration of the Airborne
Visible/Infrared Imaging Spectrometer (AVIRIS)," Proc.
SPlE, 834, (1987).
ACKNOWLEDGMENTS
The research described in this paper was carried out by the Jet
Propulsion Laboratory, California Institute of Technology, under a
contract with the National Aeronautics and Space Administration.
32


Airborne Visible/Infrared Imaging Spectrometer (AVIRIS) onboard calibration system

Airborne Visible/Infrared Imaging Spectrometer (AVIRIS) onboard calibration system

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