Low Earth Observing instruments that are used to monitor the incoming solar and outgoing long wave radiation have been a crucial part of studying the Earths radiation budget for the past three decades. These instruments go through several robust design phases followed by vigorous ground calibration campaigns to set their baseline characterization spectrally, spatially, temporally and radiometrically. The knowledge from building and calibrating these instruments has aided in technology advancements and the need for developing more accurate instruments has increased. In order to understand the on-ground instrument performance, NASA Langley Research Center has partnered with the Thermal Radiation Group of Virginia Tech to develop a first-principle, dynamic, electrothermal, numerical model of a scanning radiometer that can be used to enhance the interpretation of an Earth radiation budget-like instrument on orbit. This paper will summarize the current efforts of developing this high-fidelity end-to-end model and also highlight how it can be applied to an Earth radiation budget instrument

Ashraf, Anum Rauf Bark

Mahan, James Robert

Priestley, Kory James

English

NASA Technical Reports Server

FIRST-PRINCIPLE DYNAMIC ELECTRO-THERMAL NUMERICAL 
MODEL OF A SCANNING RADIOMETER FOR EARTH RADIATION 
BUDGET APPLICATIONS  
Anum Rauf Barki Ashraf  
Climate Science Branch 
NASA Langley Research Center 
Hampton, VA. USA 
anum.r.barki@nasa.gov 
Dr. Kory James Priestley 
Climate Science Branch 
NASA Langley Research Center 
Hampton, VA. USA 
kory.j.priestley@nasa.gov 
Dr. James Robert Mahan 
Department of Mechanical Engineering 
Virginia Polytechnic Institute and State University 
Blacksburg, VA USA 
jrmahan@vt.edu 
Abstract—Low Earth Observing instruments that are 
used to monitor the incoming solar and outgoing longwave 
radiation have been a crucial part of studying the Earth’s 
radiation budget for the past three decades.  These 
instruments go through several robust design phases 
followed by vigorous ground calibration campaigns to set 
their baseline characterization spectrally, spatially, 
temporally and radiometrically.  The knowledge from 
building and calibrating these instruments has aided in 
technology advancements and the need for developing 
more accurate instruments has increased.  In order to 
understand the on-ground instrument performance, NASA 
Langley Research Center has partnered with the Thermal 
Radiation Group of Virginia Tech to develop a first-
principle, dynamic, electro-thermal, numerical model of a 
scanning radiometer that can be used to enhance the 
interpretation of an Earth radiation budget-like 
instrument on orbit.  This paper will summarize the 
current efforts of developing this high-fidelity end-to-end 
model and also highlight how it can be applied to an Earth 
radiation budget instrument.   
Keywords— Earth radiation budget, MCRT, modeling, 
RBI, CERES 
I. INTRODUCTION 
The radiation budget for the Earth is a balance 
between the energy the Earth receives from the Sun and 
the energy that is radiated back into space.  The four 
radiation components that contribute to this balance are 
solar incident, solar reflected, earth absorbed, and earth 
emitted energy.  In order to maintain an energy balance, 
the earth must emit, over the course of time, the same 
amount of energy as it absorbs from the sun.  Such a 
balance ensures that the temperature of the Earth will 
remain constant thereby resulting in a stabilized global 
climate over a long period of time.  
For the past three decades, the science and the 
engineering community at NASA Langley Research 
Center (LaRC) has been involved with designing, 
building, calibrating, flying, collecting and processing the 
data for instruments involved in making the observations 
for our earth radiation budget.  These instruments, which 
are often manifested as Low Earth Observing (LEO) 
payloads, go through extensive design phases followed 
by vigorous calibration protocols to set the baseline 
requirments spectrally, spacitaly, temporally and 
radiometrically.  Over the years, the science community 
has made considerable advancements in making these 
measurements more accurate with the help of modeling 
tools that allow for on-ground parametric analysis of 
these instruments.  In order to better understand the 
performance on ground, NASA LaRC has partnered with 
the Thermal Radiation Group (TRG) of Virginia Tech to 
develop a high-fidelity, first-principle, dynamic, electro-
thermal, numerical model of a scanning radiometer that 
can be used to enhance the interpretation of an Earth 
radiation budget-like instrument in orbit.  An effort such 
as this helps the science community in supporting and 
validating the engineering design and fabrication phase 
for the instrument, and also helps with quantifying 
various anomalous effects and uncertainties in 
knowledge of system parameters.   
The objective of this effort is to model the complete 
end-to-end science signal chain which starts from 
photons arriving at the entrance apperture of the 
https://ntrs.nasa.gov/search.jsp?R=20200002547 2020-05-24T04:50:10+00:00Z
instrument to reading the data out as digital counts.  Fig. 
1 shows a block diagram for a sample instrument signal 
chain.  This end-to-end model will help understand the 
science data stream output for an Earth radiation 
instrument with any viewing scenes such as calibration 
targets, earth scene or any other user-defined radiance.    
 
Fig. 1.  Flow of signal chain for an instrument.  Photons converted to 
digital counts. 
II. GENERAL INSTRUMENT MODEL FLOW FOR AN EARTH 
RADIATION BUDGET INSTRUMENT 
The end-to-end radiometric model consists of several 
subcomponents including: external sources, optical train, 
detector module, and electronics.  The framework of this 
model begins with radiance from an external source, 
which is typically a calibration target or geo-scene. The 
input radiance is carried through the optical train, where 
a radiative and optical analysis is performed using a 
Monte-Carlo Ray Trace (MCRT) technique.  The results 
from the ray trace are then implemented into a finite-
element thermal diffusion or a finite-difference electro-
thermal diffusion detector module.  Lastly, the output is 
carried through the electronics signal chain to produce 
the digital counts for the radiative input.  
Several numerical modeling platforms or techniques 
can be used to complete an end-to-end system 
characterization of an instrument.  These include, but are 
not limited to, MCRT techniques, finite-difference 
electro-thermal model, electric circuit model, and other 
off-the-shelf software tools such as Zemax Optical 
Design Software and Thermal Desktop.  For a complete 
description of MCRT and finite-difference techniques, 
the reader can refer to [1, 2].  The remainder of this paper 
will talk about applying the tools and techniques of the 
afformentioned end-to-end model characterization to 
NASA LaRC’s Radiation Budet Instrument.  
III. OVERVIEW OF THE RADIATION BUDGET 
INSTRUMENT  
The next generation Radiation Budget Instrument 
(RBI), successor to the Clouds and the Earth’s Radiant 
Energy System (CERES) instruments [3,4], will measure 
the Earth’s reflected sunlight and emitted thermal 
radiation at the top of atmosphere (TOA) and ensure 
continuity with long-standing data records maintained by 
the Earth Radiation Budget (ERB) programs.  RBI is 
scheduled to fly on the Joint Polar Satellite System 2 
(JPSS-2) spacecraft, a polar-orbiting, sun-synchronous 
satellite in low earth orbit. The goal of this mission is to 
continue and extend the unique global climate 
measurements of the Earth’s radiation budget provided 
by CERES since 1998. RBI will contain three scanning 
radiometers that measure three spectral bands at TOA: a 
total band from 0.3 to > 50+um, a shortwave band from 
0.3 to 5.0 um, and a longwave band from 5.0 to 50+um.  
Radiometric calibration is essential to the RBI mission, 
therefore, the instrument itself will contain three on-board 
calibration targets to provide and hold the calibration 
stable over the full RBI spectral range and throughout the 
mission lifetime. RBI along with its internal modular 
view highlighting three on-board calibration targets and 
the sensor and focal plane module can be seen in Fig. 2.  
 
Fig. 2. Modular view of the RBI highlighting the three on-board 
calibration targets, and the sensor and focal plane module. 
 
In order to assess the accurate performance of RBI 
during its design and build phase and to evaluate the 
impact of instrument performance on science data, an 
end-to-end characterization of the instrument is 
necessary.  The end-to-end radiometric system model 
consists of the RBI components that are directly involved 
with radiometric performance and calibration, and a 
realistic earth model that is based on real scene 
observations.  The model is broken into subcomponents 
that consists of scenes and targets, the optical sensor 
module, the focal plane module that houses the on-board 
thermopile detectors, and finally the signal conditioning 
electronics that output science digital counts.  This effort 
will allow us to simulate the entire science data stream 
output, photons in to bits out, and give us the flexibility 
to input various scenes that might be comprised of 
calibration targets or Earth scenes.  The framework of 
this effort is shown in Fig. 3.  
 
IV. RBI END-TO-END MODELING SUBCOMPONENTS 
The following sections will highlight the different 
subcomponents that are currently being developed to 
complete the end-to-end modeling effort for RBI. These 
sections are: A. on-board calibration targets, B. the sensor 
module, C. the focal plane module, D. signal conditioning 
electronics, and E. the Earth model. 
Digital
Counts 
Photons Optical 
Module
Focal Plane 
Module
Electronics
Power
Distribution on 
Detector
Detector
Output
Sensor and Focal 
Plane Module
Optical Module 
located under 
bench shroud
Attaches to 
Cross-track 
Scan Module
Visible Calibration 
Target
Solar Calibration 
Target
Infrared Calibration 
Target
 Fig. 3.  Modeling framework of the RBI instrument model 
A. On-board Calibration Targets 
RBI consists of three on-board calibration targets:  
the Infrared Calibration Target (ICT) used to calibrate 
the total and longwave channel, the Visible Calibration 
Target (VCT) used to calibrate the total and shortwave 
channel, and the Solar Calibration Target (SCT) used to 
calibrate the shortwave and total channel.  The geometry 
and surface properties for all of the targets were used and 
modeled using the MCRT technique mentioned 
previously.  The thermal analysis in conjunction with 
MCRT analysis provides the complete description of 
power leaving the calibration targets and arriving at the 
entrance aperture of the sensor module. Preliminary 
results from all three of the calibration targets have been 
completed. 
B. Sensor Module 
The sensor module of this model consists of the fore 
baffles, the telescope primary and secondary baffles, the 
spectral filters for the longwave and shortwave channel 
and finally the focal plane, where the energy is absorbed.  
Similar to the on-board calibration targets, MCRT 
techniques were used to model the geometry and surface 
properties of this module.  The output of this module is 
a time-series of radiation arriving at the focal plane.  A 
transient thermal analysis of this module has also been 
performed in parallel to assess background infrared (IR) 
signal noise. 
C. Focal Plane Module 
This module consists of two thermopile detectors 
housed on a Molybdenum baseplate.  A finite difference 
technique was used to model the electro-thermal 
diffusion behavior of this module.  With given 
knowledge of the geometry and detector properties, this 
module takes the time-series of radiation from the sensor 
module and converts it into a voltage time series signal.  
The voltage is produced due to the temperature 
difference across the thermopile detectors, a change that 
is detected due to the heat sink of the detector and the 
arriving radiation.  The time-series of voltages from this 
module is then carried through to the next phase.  
D. Signal Conditioning Electronics 
The signal conditioning electronics module for RBI 
is composed of three sub-modules: A Focal plane 
module, which consists of the thermopile detectors and 
the preamplifier, a detector electronics board, which 
consists of the programmable gain and the delta sigma 
modulator, and finally a Field Programmable Gate Array 
(FPGA) which also includes the Bessel filter.  Data 
received from the thermal diffusion model is processed 
and amplified on the focal plane module.  This analog 
signal is then passed to and processed by the detector 
electronics board and converted to a digital signal using 
an A/D converter.  Finally, the digital signal is then 
processed through a Bessel filter to provide the science 
output of digital counts.  The model accepts the (time 
series) signal voltage and provides 20-bit science data at 
one hundred samples per second.  
E. Earth Model 
In addition to developing a complete end-to-end 
signal chain model of RBI, the science team at LaRC is 
also developing Earth scenes to be used in conjunction 
with the aforementioned end-to-end modeling tool.  
These realistic Earth scenes, based on real observations 
from previous Earth radiation budget programs and 
imager instruments, will include spatial, spectral, and 
temporal variations in different scene types to provide 
the most realistic concept of operations for an instrument 
on-orbit.  This effort will require multiple auxiliary 
datasets and various algorithms working together to 
define a synthetic orbit in terms of geo type and 
atmospheric state, scene viewing geometry, and sun 
illumination geometry.  This combined information will 
provide a complete description of a radiation field that 
the instrument will capture as it is scanning in a sample 
orbit.      
 
V. CURRENT STATUS AND FUTURE WORK 
      The efforts described in this paper are currently 
underway.  A complete high-fidelity independent model 
of the ICT, VCT, sensor module, and the electronics 
signal chain is complete.  A first-order model of the SCT 
and the focal plane module is also complete.  The team 
is currently working on the interfaces between the 
different subcomponents to simulate the complete end-
to-end signal chain.  Short-term studies, such as stray 
light behavior, filter heating and re-emission, 
temperature variations in the sensor module and the 
instrument point spread function have been completed, 
and on-going sensitivity and parametric analyses are also 
underway. 
 
Ground 
Calibration Sources
On-board Calibration 
Sources
Sensor Module
(Optical + Thermal)
Focal Plane Module
(Thermal+Electrical)
Signal Conditioning 
Electronics
Science Data 
Packets
Infrared 
Calibration Target
Visible Calibration 
Target
Solar Calibration 
Target
Scene Library
Earth Targets
External Sources
“Photons in”
Optical Train
Transfers the photons 
to the detector
Detector
Converts the photons 
into voltage
Electronics
Converts voltage to 
counts  
Science team takes the 
bits and converts it to 
Radiances using count 
conversion algorithms. 
REFERENCES 
 
[1] Priestley, K. J. (1997). Use of first-principle numerical models to 
enhance the understanding of the operational analysis of space-
based Earth radiation budget instruments (Doctoral 
dissertation). 
[2] Sorenson, I., 2002, “Design and analysis of radiometric 
instruments using high-level numerical models and genetic 
algorithms,” Doctor of Philosophy Dissertation, Virginia Tech.  
[3] Wielicki, B. A., Barkstorm, B. R., Harrison, E. F., Lee III, R. B., 
Louis Smith, G., & Cooper, J. E. (1996). Clouds and the Earths 
Radiant Energy System (CERES): An earth observing system 
experiment.  Bulletin of the American Meterological Socieity, 
77(5), 853-868. 
[4] Smith, G. L., Priestley, K. J., & Loeb, N.G. (2014). Clouds and 
earth radiant energy system:  From design to data. IEEE 
Transactions on Geoscience and Remote Sensing, 52(3), 1729-
1738.
 


First-Principle Dynamic Electro-Thermal Numerical Model of a Scanning Radiometer for Earth Radiation Budget Applications

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