AIRBORNE RADIOMETRIC MEASURING SYSTEM (RAMS)
Broad Spectral Bandpass Hemispherical Field-of-View Solar Radiometer (BBHSR):
These systems use an electrically calibrated pyroelectric detector, optical chopping and null-
balanced operation. Prototypes were tested during NASA and NOAA field experiments
>see Valero et al. 1982, 1983, 1984; Ackerman and Valero 1984, Valero et al., 1989).
Narrow Spectral Bandpass, Narrow Field-of-View Multiple Spectral Channels Infrared
Radiance Radiometer (NFOVR): A liquid nitrogen cooled reference black body together
with the electrically calibrated pyroelectric detection system provides outstanding accuracy.
See Liou et al., 1990.
Broadband Hemispherical Field-of-View Multiple Spectral Channels Infrared Flux
Radiometer (HFOVR): Similar to the BBHSR except that this instrument incorporates a
long pass interference filter and uses the field of view inversion method. Used to measure
infrared net fluxes.
Total-Direct-Diffuse Multiple Spectral Channels Radiometer (TDDR): Measures total,
direct, and scattered radiation fields in several spectral channels. A unique optical radiation
collecting system is used to collimate and spectrally separate hemispherical radiation. A
prototype was tested during a NOAA field experiment, (Valero et al., 1989).
Broad Spectral Bandpass Hemispherical Field-of-View Solar Radiometer (BBHSR)
Instrument Concept: The electrically calibrated pyroelectric radiometers differ from other
radiometers in the use of optical chopping and null-balanced operation. Optical chopping is
made possible by the fast response of the lithium tantalate pyroelectric detector and is
effective in eliminating drift. A gold-black coating on the detector gives it a spectral range
from the ultraviolet to the far infrared. Null-balanced operation, in turn, renders the
resultant measurement insensitive to detector responsivity and amplifier gain and hence to
ambient temperature. A laboratory model of the instrument is described by Geist and
Blevin, 1973. An airborne model is described by Valero et al., 1982 and 1984.
Instrument Description: The most important design objectives are to:
a) maximize the signal-to-noise ratio (SNR) by the use of synchronous AC detection
which is made possible by the fast response of the detectors.
Objective a) is met by using a lithium tantalate pyroelectric detector. This is a very fast
detector which easily permits signal modulation and therefore synchronous amplification
and detection. Furthermore, as in the case of the broad spectral bandpass radiometers,
synchronous amplification is conducive to a large SNR and the rapid acquisition of data
which is important in the case of fast moving clouds and/or aircraft.
b) eliminate the error introduced by infrared emission from the optical components (filters,
windows, domes, etc.).
c) use a detection system insensitive to changes in the responsivity of the detector and
amplifier gain and to variations in ambient temperature.
Objective b) is met by using both optical chopping and the proper location of spectral
filters. Radiation emitted by any component of the system located between the chopper and
the detector produces a DC signal and, hence, does not contribute to the output signal of the
synchronous amplifier. Provision is therefore made for the placement of filters between the
chopper and the detector.
The key to the operation of the radiometer is the generation of an electrical signal
synchronized 180 degrees out of phase with the optical chopper. This signal is fed into the
gold-black coating on the detector surface which also serves as a resistive heating element;
this gives a thermal signal in opposition to that resulting from the chopped optical radiation.
The amplitude of the electrical drive is then varied until a null is detected at the output of the
synchronous amplifier. At this point, the optical power absorbed by the detector is
nominally equal to the electrical drive power. The latter is measured and digitally recorded
on computer memory. Objective c) is fulfilled by the use of this detection method since the
null detection is independent of circuitry changes and temperature.
The optical system is built around a double elliptico-logarithmic light collection cavity that
directs the radiation through the chopper toward the detector assembly. A spectral filter
introduced between the chopper and the detector eliminates any unwanted radiation
(including infrared radiation) from the optical and other components ahead of the filter.
The whole assembly, including preamplifiers, is encapsulated in a vacuum tight container
to eliminate the systemıs sensitivity to microphonics and environmental moisture.
Calibration: All our instruments are carefully calibrated in our laboratory and, depending
on the particular instrument, energy response calibrations are made at the NOAA Mauna
Loa Observatory and from high altitude aircraft like the NASA ER-2. In general,
calibration of the instruments are performed in three areas: energy response, zenith angular
response, and azimuth angular response. Each of these are discussed below and apply to
all the instruments. Special requirements of particular instruments are discussed in the
corresponding sections. Spectral response is verified using the facilities of our
Energy Response: The only optical calibration required is characterization of the detector to
determine the degree of equivalence of electrical and optical heating. The instrumental
response will be calibrated in the laboratory using National Institute of Standards and
Technology (NIST) traceable standards and standard detectors. NIST developed
laboratory versions of electrically calibrated pyroelectric detectors are used. Accuracies of
1% or better are achieved. The detectors are linear within 0.1%.
Zenith Angular Response: The zenith angular or ³cosine² response of the instrument can
be made quite accurately. Small deviations are taken care of by angular calibration.
Selection of proper diffuser materials eliminates most of the cosine response imperfections.
Our experience shows that we can achieve a cosine response accuracy of 0.5%. The
calibration of the angular response is done in the laboratory with a well collimated beam of
light where the response to radiation entering at various angles is measured.
Azimuthal Angular Response: The azimuthal angular response of the instrument can also
be made quite accurately.
Narrow Spectral Bandpass, Narrow Field-of-View Multiple Spectral Channels Infrared
Radiance Radiometer (NFOVR):
Instrument Concept and Description: The instrument design is directed towards obtaining
accurate infrared radiance measurements by careful choice of materials, components,
layout, and the best available technology, together with a superior approach to the physical
problems of IR measurements.
The most important design objectives include the objectives discussed above (a-c) for the
solar radiometers plus an additional key requirement for accurate detection of infrared
d) The accuracy of the measurements depends on the ability of the instrument to remain
calibrated under all foreseeable conditions and for long periods of time. We achieve this by
using the electrically calibrated pyroelectric technology (as described for the broad band
solar radiometer above) plus, in the case of infrared measurements, a liquid nitrogen cooled
blackbody reference that provides the zero radiation signal level. Identical optical elements
in the scene and reference optical paths provide a symmetric, self-compensating, balanced
infrared detection arrangement.
Before discussing how the design objectives are met, a brief description of the prototype
airborne system is in order. The radiometer uses three germanium lenses, an electrically
calibrated pyroelectric detector, optical chopping, a null-balanced method of measurement,
and a liquid nitrogen cooled blackbody surface as a zero-radiation source. The two
channels are identical except that one is a mirror image of the other, and they may differ in
the spectral filtering used. The radiation passes through a 25 mm diameter objective lens,
reflects off the gold-plated surface of the optical chopper, and is then focused on a 25 mm
diameter field lens which images the objective on the detector. During the half-cycle when
the optical chopper has rotated out of the path of the upward radiation, a third lens identical
to the objective lens permits the detector field to be filled by the cold blackbody surface.
This lens is also imaged on the detector.
In the hemispherical solar radiometer the chopper blade blocks the atmospheric radiation
from the detector. For the narrow field-of-view infrared radiometer, the chopper blade is
not used to block but to reflect the atmospheric radiation into the detector field. The
electrical heating of the detector surface occurs during the half-cycle in which the liquid-
nitrogen-cooled surface is viewed. In other respects the detection schemes are the same.
Design objective d) is met by using identical objective lenses mounted in the same heat sink
for viewing both the field radiation and the liquid-nitrogen-cooled blackbody surface. This
insures that the optical path characteristics are the same for the two fields viewed and that
radiation emitted by each lens and viewed by the detector is the same. The primary
difference between the two optical paths is the presence of the optical chopper when
viewing the radiation field to be measured. Radiation emitted by the chopper is then
received by the detector along with the radiation from the atmosphere. This chopper
emission is relatively small since the gold-plated chopper has an emissivity less than 0.01 at
the operating temperature. This small contribution is taken care of as discussed in the
following calibration section.
The detector is an absolute device and calibration in the usual sense is not required. The
fraction of radiation attenuated by the lenses and the filter can be measured and a resultant
loss factor applied to the measured signal. Alternatively, the complete radiometer may be
calibrated using either a standard source or a standard detector. The latter approach has
proven superior in our experience with the 2-channel aircraft prototype.
Broadband Hemispherical Field-of-View Multiple Spectral Channels Infrared Flux
Radiometer (HFOVR): For aircraft operations we have used an infrared version of the
broad band solar hemispherical system described above to measure IR Net Fluxes with
very high accuracy. We were able to achieve this by reversing the field of view of the
radiometer to determine upwelling and downwelling fluxes. To obtain the net flux, one is
subtracted from the other eliminating the systematic error introduced by the temperature
dependent IR emission from the optical components (Valero et al., 1982).
The calibration procedures for the BBHIR will be the same used for the NFOVR. Our
calibration laboratory is equipped with infrared radiation sources suitable for energy
response calibration. Angular calibrations for zenith and azimuth will be performed
following our standard procedure for hemispherical radiometers. We confirm the spectral
response of the system using Fourier transform spectrometers that cover the wavelength
range from the ultraviolet to beyond 100 microns.
Total-Direct-Diffuse Multiple Spectral Channels Radiometer (TDDR)
We describe here an instrument that we have developed and successfully used for research
in the Arctic from high altitude aircraft. We have adapted this basic prototype to produce a
radiometric system easily compatible with a ground-based operation.
Normally, optical depths in the Earthıs atmosphere are measured using hand-held or
suntracking sunphotometers. However, automatic sun tracking systems are complex and
require attention and considerable sophistication to maintain tracking accuracy within the
very narrow field of view of the instruments. An alternative approach, that we have
adopted, is to render the suntracking requirement unnecessary by using an optical system
that is capable of separating the contribution of the direct (parallel) solar beam from the total
hemispheric radiative flux. We have achieved this by incorporating an oscillating shadow
ring in front of the optical aperture of the hemispherical field-of-view radiometer. In this
fashion, at some point during the oscillation cycle, the ring will project a shadow that will
exclude the direct solar beam from the field of view of the radiometer; only the scattered or
diffuse component, Fd, of the total radiation field will reach the aperture of the optical
system. On the other hand, when the oscillating ring is out of the field of view of the
radiometer, the total hemispherical radiation field, Ft, is detected. From the values of Fd
and Ft the direct solar beam, Fs, is obtained. The values of Fs at different altitudes in the
atmosphere are then used to determine optical depths. By incorporating several spectral
channels, optical depths as a function of wavelength can be measured. Additionally, the
total hemispherical flux and the diffuse radiation field obtained using this technique are
fundamental parameters that, through radiative transfer calculations, can be used to
determine important quantities as discussed above.
An important consequence of the TDDR method of operation is that the oscillating shadow
arm provides a semi-independent set of measurements comprised of flux values determined
when different sectors of the hemispherical diffuse radiation field are shadowed. These
measurements can be inverted to derive the angular dependence of the scattered field. The
shadow arm operation also provides a way to infer the forward scattered flux directly from
measurements which, in turn, permits the determination of ³true² optical depths (free of
forward scattering error).
Optics: We employed very simple optical arrangement for the radiometer. The diffuser-
light trap arrangement provides a hemispherical field of view with incident radiation being
collimated by the high reflectance walls of the exponential-logarithmic cavity. Enough
collimation of the radiation is achieved with this design that narrow spectral bandpass
interference filters can be used to select desired wavelength regions.
Electronics: The instrument electronics includes five major functional blocks. They are the
detectors signal conditioning block, the data processing block, the system controller block,
the shadow ring drive and control block, and the data storage block.
The signal detectors are silicon photodiodes operating in the photovoltaic mode and
covering the spectral range from about 0.3 to l.l µm. Their signals are converted into
electrical voltages by low noise FET input operational amplifiers. Programmable gain
amplifiers allow adjustments for dynamic range, and filter circuits condition the signals for
analog to digital processing. Data processing units consist of an analog multiplexer circuit,
a sample-and-hold circuit, and an analog to digital converter providing a 12-bit resolution
output. The shadow ring is driven by a DC motor rotating at a constant speed. A motor
controller is used to maintain motor speed. The system controller provides the timing
necessary to perform all the systemıs tasks. It sets the shadow ring in motion and steps
through the detectorıs outputs, maintaining the proper dynamic range for the analog to
digital converter by selecting the proper amplifier gain. It also controls the analog to digital
conversion and selectively stores data.
Calibration of the instrument is to be performed for its function in four areas: energy
response, zenith angular response, azimuthal angular response (as described in a previous
section), and exoatmospheric constants. Here we will only discuss those calibration aspects
that are particular to this instrument.
Azimuthal Response: In the TDDR instrument used for Earth atmosphere studies, the
azimuthal angular response varies by ~10% in flux due to the configuration of the detector
in the instrument (a linear array of silicon photodiodes are used). This is removed in the
data analysis by calibrating the azimuthal angular response in the laboratory as described
above. With very accurate information on the orientation of the TDDR with respect to the
sun, the variation in azimuthal response can be removed from the data.
Exoatmospheric Constants: The measured values of direct solar flux, Fs, outside the
atmosphere need to be determined prior to evaluating optical depths at levels within the
Earthıs atmosphere. We derive these values from a Langley method calibration at the
Mauna Loa Observatory. Preliminary tests from an urban site, where aerosol mass loading
was somewhat variable during data acquisition, resulted in extrapolated constants at three
radiometer channels having a precision of better than 2%. The conditions at the Mauna Loa
Observatory, which lies well above the convective layer, enable us to improve that
precision to a few tenths of 1%.