All sensors systems designed to acquire quantitative data undergo radiometric calibration. The types and potential accuracies of calibration are discussed as well as the needs for calibration in the practical application of sensors. The recent and ongoing experience with the shuttle multispectral infrared radiometer is used as a reference

Goetz, A. F. H.

English

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RADIOMETRIC CONSIDERATIONS IN REMOTE SENSING SYSTEMS
Alexander F. H. Goetz
Jet Propulsion Laboratory
California Institute of Technology
Pasadena, CA 91109
Introduction
All sensor systems designed to acquire quantitative data undergo radio-
metric calibration. The following discussion describes the types and poten-
tial accuracies of calibration as well as the needs for calibration in the
practical application of sensors. The recent and ongoing experience with the
Shuttle Multispectral Infrared Radiometer (SMIRR) will be used as a reference.
Definition of Terms
Calibration is defined as the process by which the output of a sensor is
related to the input stimulus. In radiometers and radiometric imaging systems
using detectors that translate incoming photons into electrical signals, an
amplified output voltage is calibrated in terms of measured spectral radiance
in watts m-I sr -I Um-[. The uncertainty of this measurement is dependent on
the calibration source and the stability of the detector-electronlc sensor
system. The sources of spectral irradiance are lamps calibrated by the Na-
tional Bureau of Standards (NBS) to a standard of spectral radiance such as a
high temperature graphite blackbody. The lamps provide a specified spectral
irradiance at a given distance from the filament. The absolute accuracies in
irradiance under NBS conditions range from 1.4% at 350 nm, i.i % at 800 run and
1.2% at 1600 nm (Slater, 1980). However, international comparison of spec-
tral-irradiance measurements in the 300 - 800 run range reveal inconsistencies
as great as 6.5% (Gillham, 1977).
Although absolute calibration is necessary for establishing standards of
comparisons among systems, the present-day requirements for remote sensing of
the earth are best served by understanding a spectral sensing system in terms
of relative calibration.
Relative calibration means that the output of a sensor under a varying
input stimulus maintains the ratio between input and output in all spectral
bands. In other words, the spectral character of the reflectance or emittance
curves is maintained to the accuracy required for proper interpretation, but
the absolute values are not necessarily known. Relative calibration is more
readily achieved than absolute calibration and more important for the inter-
pretation of remote sensing data.
Detector response can be either linear or more often non-llnear wlthres-
pact to variation in irradiance. Within a sufficiently small range of irrad-
lance values, depending on the detector type, the output (O) can be described
in terms of the input (I) and two constants (a,b) by O = al + b. _¢er a lar-
ger range of values, a series of linear approximations must be used or a power
curve of the form O = aIc + b where c is usually greater than 0.5 and less
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https://ntrs.nasa.gov/search.jsp?R=19830020251 2020-03-21T02:02:17+00:00Z
than 2. Silicon detectors and photomultlplier tubes have inherent linear res-
ponses over a large dynamic range while detectors such as HgCdTe used in sen-
sors for wavelengths beyond i _m are notoriously non-linear.
Radlometric sensitivity defines the minimum amount of scene contrast that
can be detected by a sensor. Signal-to-nolse ratio is another term used to
describe the same sensor quality. Present-day practice is to define system
sensitivity in terms of the minimum detectable change in reflectance, nEAp or
nolse-equlvalent change in reflectance for the reflective portion of the spec-
trum, and nEAT or nolse-equlvalent change in temperature in the mid-IR.
Sensors
The analysis and interpretation of earth-looklng data today largely ig-
nores the question of calibration. Data are received from the appropriate
agency on computer tape in the form of digital numbers (DN) and they are
assumed to be relatively calibrated and linear with respect to frradiance.
Papers published on interpreted results do not reveal whether the authors were
aware of the nature of the calibration or the consequences of any possible
miscallbration. The reasons for this are two-fold.
I. Interpreters have a blind faith in the sensor builders.
2. The variability in the irradiances measured due to atmospheric scat-
tering and absorption, surface slope uncertainty, the inhomogeneities
in the surface reflectance or emittance, and the uncertainties in
interpretation are much greater than the inaccuracies introduced by
faulty calibration.
The blind faith in sensor builders is not misplaced. However, to an
interpreter who would llke to assure himself that his faith is well placed,
precious little data are available on calibration procedures and the results
of calibration for the spectral imaging systems in use today. An exception is
the Landsat MSS (NASA, 1976).
Aircraft scanners in general have reference blackbodles or calibration
lamps that provide data at the end of each mirror rotatlonor scan. These are
useful to set amplifier gains and for comparison among data taken at different
times. In most cases engineering measurements are ignored by the interpre-
ter. An exception is the use of thermal scanners to determine radiometric
temperatures of surfaces. Here high and low temperature blackbodles scanned
during each mirror revolution are set to bracket the temperature range to be
measured. A single ground temperature measurement is then sufficient as a
reference for low-flylng missions over a restricted area.
In all cases the relative response of the detector to an inputstlmulus
must be calibrated and the output llnearized either internally to the instru-
ment or the departures from linearlty available in an accessible calibration
report.
SMIRR
As an example of one calibration procedure, the Shuttle Multispectral
Infrared Radiometer (SMIRR) will be discussed (Goetz and Rowan, 1982). It was
calibrated with an irradiance standard traceable to an NBS standard (Goetz and
Brownell, 1982). The standard consists of a hemisphere containing two, I000 W
quartz-lodlde lamps. The hemisphere is coupled with a cone capped with an
etched fused-silica face plate. At the base of the cone, in front of the hem-
isphere exit port is an iris to control the face plate illumination and there-
fore the irradiance. During calibratlon runs the output of the hemisphere
calibrator is measured with a spot photometer containing a standard traceable
to NBS. The relative spectral output was measured with a calibrated spectrom-
eter over the range 0.4 - 2.5 _m. The estimated error in absolute irradiance
is 10% while the relative error is probably less than 5%. In comparison the
relative calibration using internal reference lamps •showed a 10% variation in
system response during the flight of SMIRR aboard Shuttle in November 1981.
During the three years of intermittent laboratory calibration measurement, the
system response decreased by 25% presumably due to detector degradation. The
above demonstrates that a one-time, absolute calibration of a sensor is not
sufficient and that a continuous relative calibration is necessary if high
precision measurements are desired.
Data Interpretation
The necessity for calibration or lack thereof in the interpretation of
natural surfaces can be drawn from the experience and the present-day under-
standing of the process of extracting information from airborne and spaceborne
spectral scanners.
The image interpreter is primarily interested in the quality of the image
and whether a combination of spectral images will yield the desired identifi-
cation or at least discrimination of the units of interest. Scanner images
should be free of "striping" which means that there should be no variations in
response from one image line to the next. With multiple detectors, such as
the 6 per band in the Landsat MSS, striping can be'a problem if the response
curves of the detectors are not matched. The matching can be obtained by
accurate relative calibration.
Accurate spectral reflectance curves from a radlometer or an imager are
only possible if relative calibration has been carried out. The effect of the
atmosphere can be modeled, but normally more parameters need to be known than
can be acquired. Therefore the interpreter, for instance in the case of
reflectance measurements, is forced to pick a calibration point on the ground
and normalize the surrounding data to this point. This procedure has been
used with success in the preliminary reduction of data from SMIRR in Egypt
(Goetz et al, 1982). Ground samples were collected and measured in the labor-
atory. The laboratory spectra were used to normalize the data and remove the
atmospheric effects. • By this means several hundred km of ground coverage
could be investigated and spectra obtained. The resulting spectra were inter-
preted on the basis of the relative inter-band spectral values giving charac-
teristic shape to the curves. The absolute reflectances were of little
•interest since the surface topography, the major source of overall brightness
variations, was unknown.
I0
_le use of ratio images (Rowan et al, 197_) immediately obviates the need
for absolute calibration but relies on a linear or known detector response.
Other techniques such as principal component analysis, in which the maximum
contrast is sought for display of an individual scene, does not require know-
ledge of sensor characteristics but makes the assumption thatthe detector
responses are linear (J. Conel, personal communication).
conclusion
The interpretation of spectroradiometric information from multispectral
sensors to present-day levels of accuracy requires thorough instrument cali-
bration, primarily to determine the response (non-linearity) of the detector,
spectral bandwidth and relative response among the spectral bands. Absolute
calibration is of value for intercomparison of instruments but does not con-
tribute significantly to data interpretation.
ACKNOWLEDGEMENTS
This research was conducted by the Jet Propulsion Laboratory, California
Institute of Technology, under contract with the National Aeronautics and
Space Administration.
REFERENCES
Gillham, E. J., Radiation standards, Applied Optics, 16, 300-301, 1977.
Goetz, A. F. H., and M. Brownell, SMIRR calibration, JPL Technica L Report, in
preparation, 1982.
Goetz, A. F. H., L. C. Rowan, and M. L. Kingston, Shuttle Multispectral Infra-
red Radiometer: Preliminary results from the second flight of Columbia,
Proceedings IEEE, Munich, Germany, 1982.
NASA, Landsat Data Users Handbook, Goddard Spaceflight Center Report No.
765D4258, 1976.
Rowan, L. C., P. H. Wetlaufer, A. F. H. Goetz, F. C. Billingsley, and J. H.
Stewart, Discrimination of rock types and detection of hydrothermally
altered areas in South-Central Nevada by the use of computer-enhanced ERTS
images, USGS Professional Paper 883, 1974.
Slater, P. N., Remote Sensing Optics and Optical Systems, Addison-Welsey Inc.,
Reading MA, 1980.
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