A portable remote sensing system for precision column measurements of methane has been developed, built and tested at NASA GSFC. The sensor covers the spectral range from 1.636 micrometers to 1.646 micrometers, employs an air-gapped Fabry-Perot filter and a CCD camera and has a potential to operate from a variety of platforms. The detector is an XS-1.7-320 camera unit from Xenics Infrared solutions which combines an uncooled InGaAs detector array working up to 1.7 micrometers. Custom software was developed in addition to the graphical user basic interface X-Control provided by the company to help save and process the data. The technique and setup can be used to measure other trace gases in the atmosphere with minimal changes of the etalon and the prefilter. In this paper we describe the calibration of the system using several different approaches

Georgieva, E. M.

Heaps, W. S.

Huang, W.

NASA Technical Reports Server

A new remote sensing filter radiometer employing a Fabry-Perot etalon and a CCD 
camera for column measurements of methane in the Earth atmosphere 
Georgieva, E.M1, Huang W.3, Heaps, W.S.2 
1Joint Center for Earth Systems Technology (JCET), UMBC, Baltimore, MD 21228, USA 
e-mail: Elena.M.Georgieva@nasa.gov 
2 NASA, Goddard Space flight Center, Greenbelt, MD 20771, USA 
3 Science Systems and Applications, Inc., Lanham, MD, USAA 
 
Abstract. A portable remote sensing system for 
precision column measurements of methane has been 
developed, built and tested at NASA GSFC. The sensor 
covers the spectral range from 1.636 µm to 1.646 µm, 
employs an air-gapped Fabry-Perot filter and a CCD 
camera and has a potential to operate from a variety of 
platforms. The detector is an XS-1.7-320 camera unit 
from Xenics Infrared solutions 1which combines an 
uncooled InGaAs detector array working up to 1.7 μm.  
Custom software was developed in addition to the 
graphical user basic interface X-Control provided by the 
company to help save and process the data. The 
technique and setup can be used to measure other trace 
gases in the atmosphere with minimal changes of the 
etalon and the prefilter. In this paper we describe the 
calibration of the system using several different 
approaches. 
Instrumentation; measurement; metrology; remote 
sensing; atmospheric composition; optical instruments; 
absorption; interferometry; Fabry-Perot. 
I.  INTRODUCTION 
Methane measurements are becoming increasingly 
important as a component of NASA’s programs to 
understand the global carbon cycle and quantify the threat of 
global warming. Methane is the third most important 
greenhouse gas in the Earth’s radiation budget (after water 
vapor and carbon dioxide) and the second most important 
anthropogenic contributor to global warming. Individual 
molecules of methane are ~25 more effective than carbon 
dioxide in trapping heat in the earth’s atmosphere. In its 
2007 report the International Panel on Climate Change 
identified methane as a key uncertainty in our understanding 
saying that “the causes of recent changes in the growth rate 
of atmospheric CH4 are not well understood”2. In US global 
change research program strategic plan 2012–202 is stated 
that “The primary cause of climate change is increasing 
greenhouse gases in the atmosphere, mainly carbon dioxide 
and methane.” We do know that methane arises from a 
number of natural sources including wet lands and the 
oceans plus man made sources from agriculture, as well as 
coal and petroleum production and distribution. It has 
recently been pointed out that large amounts of methane are 
frozen in the permafrost of Canada and Siberia. There is a 
fear that melting of this permafrost driven by global 
warming may release methane very suddenly further 
exacerbating climate change3-6. 
II. DESCRIPTION OF INSTRUMENT AND BASIC THEORY 
 
In the current setup (Fig.1) sunlight enters the 
instrument from the right, passes through a bandpass filter 
and is then split. One half goes directly to a detector and the 
other half pass through a Fabry-Perot interferometer7,8 tuned 
to transmit at wavelengths where methane absorbs light. The 
detector for this half then responds strongly to changes in 
the atmospheric methane. The difference with the previous 
setup reported10-14 is that instead of two InGaAs detectors 
for the Reference and FP channel we use a CCD array to 
sample both the reference and the Fabry-Perot signal 
simultaneously on different areas of the array. It increases 
the signal to noise ratio9 (SNR) while permitting a reduction 
in the overall size of the instrument.  
 
III.  EXPERIMENTAL  
 
As with any instrument, calibration and validation are 
crucial to the success of the measurement. To effectively 
address questions regarding sources and sinks of methane, 
precision at the level of 1.0% or better is required. 
Figure 1 shows the sesor with the Xenics CCD camera.
https://ntrs.nasa.gov/search.jsp?R=20120013487 2019-08-30T21:53:44+00:00Z
The methane filter radiometer instrument uses a differential 
optical absorption measurement technique. In this 
technique, it makes measurements of light at an atmospheric 
absorption wavelength and in a nearby atmospheric 
transmission wavelength region. The process for converting 
these measurements to a mixing ratio determination for an 
optical  
 
 
species involves a ratio of these two measurements, which 
were obtained using very nearly the same instrumental 
components. Because of this, a change in the reflectivity of 
a mirror or the transmission of some component in the 
common optical path “divides out” so such changes do not 
affect the derived mixing ratio.  
The methane instrument calibration/validation covers three 
areas—spectral, radiometric, and pointing. The spectral 
calibration proceeds from ground testing.  
A. Wavelength calibration 
 
The radiometer has been calibrated by different methods – 
using sunlight, an Agilent tunable laser and using calibrated 
lamps and integrating spheres in the Radiometric facility at 
Goddard. For the first method because of the difficulty in 
testing the camera from the roof directly pointing it at the 
sun we implemented a fiber optically coupled system in 
which a roof mounted suntracker produces an image of the 
sun on a fiber bundle. The fiber bundle is led down into the 
third floor 
laboratory and the 
light is introduced 
into the Xenics 
camera. Two Lab 
View software 
programs were 
developed to 
assist in 
wavelength 
camera calibration 
and the analysis of 
the collected data. 
Tunable laser light 
from a source locked to a spectral reference line is 
introduced to the sensor via multimode fiber coupled to a 
fiber bundle. Tuning the laser permits a wavelength map to 
be made of the ‘bulls-eye” pattern in the focal plane (Fig.3) 
We first use a tunable laser (Agilent 81649) and a single 
mode fiber and look at the displacement of FP and Reference 
channel images on the CCD array. In this way the images 
become minimal in size. We adjust the vertical and 
horizontal positions of the images so they look similar for 
both the FP and Reference channels. The displacement can 
be obtained from the image data with an error < 0.5  pixel. 
Than we use a fiber bundle to input a uniformly illuminated 
Agilent laser beam into the sensor. The laser wavelength was 
changed in steps of 0.1 nm from 1630nm until 1641nm, 
which is the maximum wavelength for our laser. 
We record images for several wavelengths to derive the free 
spectrum range of the FP and extrapolate the fringe position 
for wavelengths from 1641nm to 1650nm. The Agilent laser 
wavelength can be calibrated using CH4 lines below 
1641nm. 
Figure 4 shows a screenshot taken November 28, 2011 - 
Each wavelength is a line in the curves on the right. The two 
images on the two-dimensional IR detector array vary in 
wavelength along the array diagonal. The left image is our 
Reference channel and the right image is the Fabry-Perot 
channel. The comb filter that we use is an air-gapped etalon 
with diameter 50.8 mm, 80% reflection at 1565-1670 nm 
and free spectral range (FSR) of 2.56 nm. The etalon is 
tuned by tilting at 10 deg. Intensity variation is a result of 
passing clouds. Each point in the curve is 1 ms integration 
of CCD. Shown are the focal plane images of a hexagonal 
 fiber optic array feeding the methane sensor from a roof 
mounted suntracker (Fig5). The right hexagon is the 
reference channel and the left hexagon has been encoded in 
wavelength by passing through a Fabry-Perot solid etalon 
before imaging. The (almost) horizontal band of pink across 
the left hexagon is caused by methane absorption at the 
wavelengths represented by those pixels. The color change 
at the right edge of both hexagons is caused by cloud partly 
obscuring the solar image.  
Figure 4 A screenshot with sunimages and panels for 
Reference, FP and Ratio channels 
Figure 2, Left-FP transmission bands and prefilter shape 
Right: shows the transmitted wavelengths on the CCD array 
Figure 3, Images of Ref and FP 
channel during the wavelength 
calibration 
  
 
Figure 5. The telescope attached to the suntracker focuses an 
image of the sun on the end of a 52 fiber bundle used to conduct 
light down to the instrument in the lab. 
 
After normalizing the left image by the right one the 
intensity of each dot can be plotted against the wavelength 
that it represents to produce a short absorption spectrum for 
methane. 
For the last calibration method we used bulbs in various 
combinations and an integrating sphere, so a range of output 
illuminations can be achieved.  
 
B. Intensity calibration 
 
The camera had to be calibrated against a known 
radiation source. It converts the arbitrary units of the camera 
into physical units of energy. Here we discuss the radiometry 
theory required to calibrate the camera and apply this 
calibration to field images. Radiometric calibration is 
accomplished using a NIST-calibrated white light source; a 
pixel map is generated for the focal plane array. This 
includes sensitivity, dark current, and read noise. A full 
instrument calibration again generates a pixel map that is 
wavelength-specific the array. 
Integrating sphere systems are highly Lambertian and show 
very uniform radiance distribution. We measured the 
linearity of the camera by irradiating the array with varying 
light levels and measuring the signal produced.  
C. Preliminary Data collection using a 
fibercoupled suntracker 
 
Light for the measurement is gathered by a sun tracker on 
the roof that focuses an image of the sun onto the end of a 
fiber bundle. Each fiber within the bundle collects light 
from a different place on the sun and the instrument forms 
an image of each fiber in the bundle. Images of the fiber 
bundle FP and REF channel have been shown in Figure 4. 
Figure 7 shows results from roughly three days of 
continuous observation by the suntracker. The traces labeled 
"C" correspond to different rings in the Fabry-Perot's 
mapping the pixels. Zero time begins on the afternoon of 
March 8, 2012. Rings C15 through C18 show an initial 
decrease. This is because as the sun sets the atmospheric 
path length grows longer and the effect of methane 
absorption grows stronger.  
Rings C14 and C19 are not mapped onto a wavelength that 
methane absorbs so they do not change. Seconds 1000 
through 5500 represent sunset, night when the FP and REF 
signals are near zero, and then sunrise. During sunrise and 
sunset the pathlength becomes very long and the probability 
of clouds in the path becomes very high. This is why the 
ratio becomes extremely noisy during these transitions. 
 
Figure 7. This shows the FP/REF ratio plotted for almost four days 
of continuous operation. Only data from the day time periods has 
value. 
 
During the first full day (March 9) rings C15 - C 18 show in 
increase during the morning as the atmospheric path length 
grows shorter reaching a minimum at local noon and then 
increasing again in the afternoon as on March 8. Rings C14 
and C19 show little variation because they are off the 
methane absorption. Two additional nights and days are 
Figure 6 Grande 1 m diameter sphere with the sensor 
in front of it, Goddard Calibration Facility 
shown exhibiting the same pattern. March 11 is very noisy 
because the day was mostly cloudy.  
Figure 8 shows data taken at two times in March 9 – early 
morning and noon. Rows 11, 12, 13, and 14 corresponding 
to rings C15, C16, C17, and C18 in Figure 7 show a 
variation in ratio caused by changes in the airmass between 
these two times of day. Other rings less sensitive to methane 
show no such variation. We can calculate the pathlength 
from the instrument to the sun very precisely.  
 
 
Figure 8. A plot of the ring ratios (basically a spectrum) for two 
times of day. Row 12 (ring C17) shows a large change between the 
deep trough for the early morning data and the shallower trough 
for noon time data. Rows that do not lie on the methane absorption 
line do not show any variation.  
 
Knowing the wavelength assignments of the pixels and the 
methane absorption spectrum will permit an accurate 
determination of the column methane concentration. The 
final wavelength calibration of the instrument has not been 
done yet because the instrument is still undergoing changes 
to improve optical efficiency and stability. Over the next 
months these improvements will be completed and the final 
calibration will be performed.  Additional data taken with 
this instrument will be available at the AMIGO website: 
http://acdb-ext.gsfc.nasa.gov/People/Weaver/AMIGO.html. 
 
IV.  CONCLUSIONS 
 
We are reporting a new improved filter radiometer 
which uses a CCD array to sample both the reference and 
the Fabry-Perot signal simultaneously on different areas of 
the array. It increases the signal to noise ratio (SNR) while 
permitting a reduction in the overall size of the instrument. 
It can evolve to become an extremely flexible instrument 
exploiting the superior throughput and resolution of the 
Fabry-Perot interferometer. With changes of the prefilter 
and the etalon the system can measure different greenhouse 
gases in the atmosphere.  
 
 
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A New Remote Sensing Filter Radiometer Employing a Fabry-Perot Etalon and a CCD Camera for Column Measurements of Methane in the Earth Atmosphere

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