Collocation (remote sensing)

<h2 id="satellites">Satellites</h2>

The <a href="/facts/Satellite/PTX8c68t">satellites</a> that most concern us are those with a <a href="/facts/Low_Earth_orbit/rq1TwayJ">low-Earth</a>, <a href="/facts/Polar_orbit/JLUlIPlq">polar orbit</a> since <a href="/facts/Geostationary/VBq1v5Kr">geostationary</a> satellites view the same point throughout their lifetime.
The diagram shows measurements from <a href="/facts/Advanced_Microwave_Sounding_Unit/hQhmaDRJ">AMSU-B</a>
instruments mounted on three satellites over a period of 12 hours.
This illustrates both the orbit path and the scan pattern which runs crosswise.
Since the <a href="/facts/Orbit/nD2TPySG">orbit</a> of a satellite is <a href="/facts/Deterministic_system/oT1NvyHD">deterministic</a>, 
barring <a href="/facts/Orbital_maneuver/HAlEFRSJ">orbit maneuvers</a>, we can predict the location of the
satellite at a given time and, by extension, the location of
the measurement pixels.
In theory, collocations can be performed by inverting the
determining equations starting from the desired time period.
In practice, partially processed data (usually referred to as
level 1b, 1c or level 2) contain the coordinates of each of
the measurement pixels and 
it is common to simply feed these coordinates to a nearest neighbor search.
As mentioned previously, the satellite data is always <a href="/facts/Data_binning/542EIHcu">binned</a>
in some manner. 
At minimum, the data will be arranged in 
swaths extending from pole to pole.
The swaths will be labelled by time period and the
approximate location known.

<h2 id="radiosondes">Radiosondes</h2>

<a href="/facts/Radiosonde/NRNr47JN">Radiosondes</a> are particularly important for collocation studies
because they measure atmospheric variables more accurately and more
directly than satellite or other remote-sensing instruments.
In addition, radiosonde samples are effectively instantaneous point measurements.
One issue with radiosondes carried aloft by <a href="/facts/Weather_balloon/yYenKMYA">weather balloons</a> is
balloon drift. In,<a class="footnote-ref" id="fnref:1" href="#fn:1">1</a>
this is handled by averaging all the satellite pixels within a 50 km radius
of the balloon launch.

If high-resolution sonde data, which normally has a constant
sampling rate or includes the measurement time, is used,
then the lateral motion can be traced from the wind data.
Even with low-resolution data, the motion can still
be approximated by assuming a constant ascent rate.
Excepting a short bit towards the end, 
the linear ascent can be clearly seen in the figure above.
We can show that the ascent rate of a balloon is given
by the following equation

<a class="footnote-ref" id="fnref:2" href="#fn:2">2</a>

v
        =
        
          
            
              
                g
                k
                h
                (
                1
                −
                
                  R
                  
                    a
                  
                
                
                  /
                
                
                  R
                  
                    s
                  
                
                )
              
              
                c
                
                  D
                
              
            
          
        
      
    
    {\displaystyle v={\sqrt {\frac {gkh(1-R_{a}/R_{s})}{c_{D}}}}}

where g is gravitational acceleration,
k relates the height, h, and surface area, A,
of the balloon to its volume: V = khA; 
Rs is the equivalent "gas constant" of the balloon, 
Ra is the gas constant of the air
and cD is the drag coefficient of the balloon.
Substituting some sensible values for each of the constants,
k=1. (the balloon is a perfect cylinder), h=2. m, cD = 1.
and Ra is the gas constant of helium,
returns an ascent rate of 4.1 m/s. Compare this with the
values shown in the histogram which compiles all of the
radiosonde launches from the <a href="/facts/Polarstern/PmfleNgp">Polarstern</a> research vessel
over a period of eleven years between 1992 and 2003.

<h2 id="interpolation">Interpolation</h2>
For gridded data such as <a href="/facts/Data_assimilation/8E2Mb5of">assimilation</a> or <a href="/facts/Meteorological_reanalysis/eR0mmflj">reanalysis</a> data,
<a href="/facts/Interpolation/8PyetX4f">interpolation</a> is likely the most appropriate method for performing any type of comparison.
A specific point in both physical position and time is easy to locate
within the grid and interpolation performed between the nearest neighbors.
<a href="/facts/Linear_interpolation/galkvRGy">Linear interpolation</a> (<a href="/facts/Bilinear_interpolation/yoke6SzF">bilinear</a>, <a href="/facts/Trilinear_interpolation/neuRVbGU">trilinear</a> etc.) is the most common, 
though cubic is used as well but is probably not worth the extra computational overhead.
If the variable of interest has a relatively smooth rate of change (temperature is a good example of this because it
has a diffusion mechanism, <a href="/facts/Radiative_transfer/nYE4KojG">radiative transfer</a>, not available to other atmospheric variables), 
then interpolation can eliminate much of the error associated with collocation.
Interpolation may also be appropriate for many types of satellite instruments,
for instance a cross-track scanning instrument like <a href="/facts/Landsat/4DI1PFaq">Landsat</a>.
In <a class="footnote-ref" id="fnref:3" href="#fn:3">3</a> data derived from the <a href="/facts/Advanced_Microwave_Sounding_Unit/hQhmaDRJ">Advanced Microwave Sounding Unit</a> (AMSU) are
interpolated (although not for the purposes of collocation) using a slight variation
of trilinear interpolation.
Since measurements within a single scan track are laid out in an approximately rectangular
grid, bilinear interpolation can be performed.
By searching for the nearest overlapping scan track both forwards and backwards in time,
the spatial interpolates can then be interpolated in time. 
This technique works better with derived quantities rather than raw brightness temperatures since
the scan angle will already have been accounted for.
For instruments with a more irregular sampling pattern, such as the <a href="/facts/Aqua_(satellite)/QXaLRTAv">Advanced Microwave
Scanning Radiometer-EOS (AMSR-E)</a> instrument which has a circular scanning pattern,
we need a more general form of interpolation such as <a href="/facts/Kernel_estimation/otWLlBbA">kernel estimation</a>.
A method commonly used for this particular instrument, as well as <a href="/facts/SSM%2fI/WVPM1nUA">SSM/I</a>,
is a simple daily average within regularly gridded, spatial bins
.<a class="footnote-ref" id="fnref:4" href="#fn:4">4</a>

<h2 id="trajectories">Trajectories</h2>
To collocate measurements of a medium- to long-lived <a href="/facts/Flow_tracer/ssA1CASQ">atmospheric tracer</a> with a second instrument, running <a href="/facts/Trajectory_(meteorology)/JB9fu7FK">trajectories</a> can considerably improve the accuracy. It also simplifies the analysis somewhat: a trajectory is run both forwards and backwards from the measurement location and between the desired time window. Note that the acceptable time window has now become longer because the error from transport induced changes in the tracer is removed: the tracer lifetime would be a good window to use. Since the trajectories provide a location for every point in time within the time window, there is no need to check multiple measurements from the second instrument. Every time within the trajectory is checked for the distance criterion but within a very narrow window. Alternatively, the exact times of the measurements for the second instrument are interpolated within the trajectory. Only the smallest distance error below the threshold is used and the distance criterion can be made smaller as a consequence.

<h2 id="example-pol-ice-campaign">Example: Pol-Ice Campaign</h2>

Collocations of sea ice thickness and <a href="/facts/Brightness_temperature/6kwjCTEn">brightness temperatures</a> taken during the
Pol-Ice Campaign are an excellent example since they illustrate many of the most important principles as well as demonstrating the necessity of taking into account the individual case. The Pol-Ice campaign was conducted in the N. Baltic in March 2007 as part of the SMOS-Ice project in preparation for the launch of the <a href="/facts/Soil_Moisture_and_Ocean_Salinity_satellite/lBc3U9Rs">Soil Moisture and Ocean Salinity satellite</a>. 
Because of the low frequency of the SMOS instrument, it is hoped that it will render
information on sea ice thickness, therefore the campaign comprised measurements
of both sea ice thickness and emitted brightness temperature.
Brightness temperatures were measured with the EMIRAD L-band microwave radiometer
<a class="footnote-ref" id="fnref:5" href="#fn:5">5</a>
carried on board an airplane. Ice thickness was measured with the E-M Bird ice thickness meter which was carried by a helicopter. The E-M Bird measures ice thickness with a combination of inductance measurements to determine the location of the ice-water interface and a <a href="/facts/LIDAR/b9SPUmLM">laser altimeter</a> to measure the height of the ice surface.<a class="footnote-ref" id="fnref:6" href="#fn:6">6</a>
The map above shows the flight tracks of both instruments which were approximately coincident but obviously subject to pilot error.

Since the flight paths of both aircraft were approximately linear, the first step in the collocation process was to convert all the coincident flights to Cartesian coordinates with the x-axis being lateral distance and the y-axis transverse distance. In this way, collocations can be performed in two ways: crudely, by matching only the x distances, and more precisely by matching both coordinates.
More importantly, the footprint size of the radiometer is many times larger
than that of the E-M Bird meter. The figure to the left shows
the <a href="/facts/Radiation_pattern/5t0QU2mv">antenna response function</a> for the radiometer.
The <a href="/facts/Full_width_at_half_maximum/oXSLXf7S">full width at half maximum</a> is 31 degrees.<a class="footnote-ref" id="fnref:7" href="#fn:7">7</a>
Since the aircraft was flying at approximately 500 m, this translates
to a footprint size of 200 m or more.
Meanwhile, the footprint size of the E-M Bird was roughly 40 m
with a sample spacing of only 2 to 4 m.<a class="footnote-ref" id="fnref:8" href="#fn:8">8</a>
Rather than looking to nearest neighbors, which would have produced
poor results, a weighted average of the thickness measurements was
performed for each radiometer measurement.
Weights were calculated based on the radiometer response function which is almost
a perfect Gaussian up to about 45 degrees.
Points could be excluded based on distance along the flight
path.
For validation of sea ice emissivity forward model calculations, 
this was further refined by performing an emissivity calculation
for each thickness measurement and averaging over the radiometer
footprint.
<a class="footnote-ref" id="fnref:9" href="#fn:9">9</a>
The figure below illustrates relative measurement locations
from each of the instruments used in the Pol-Ice campaign.
Two overpasses are shown: one from the airplane carrying the
EMIRAD radiometer and one from the helicopter carrying the
E-M Bird instrument.
The x-axis is along the line of the flight path.
EMIRAD footprints are drawn with lines, E-M Bird
inductance measurements are represented by circles
and LIDAR measurements with dots.

<h2 id="references">References</h2>

<ol>
<li id="fn:1">S. A. Buehler; M. Kuvatov; V. O. John; U. Leiterer; H. Dier (2004). "Comparison of Microwave Satellite Humidity Data and Radiosonde Profiles: A Case Study". Journal of Geophysical Research. 109 (D13103): D13103. Bibcode:2004JGRD..10913103B. doi:10.1029/2004JD004605. <a href="http://urn.kb.se/resolve?urn=urn:nbn:se:ltu:diva-10542" target="_blank">http://urn.kb.se/resolve?urn=urn:nbn:se:ltu:diva-10542</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></li>
<li id="fn:2">Peter Mills (2004). Following the Vapour Trail: a Study of Chaotic Mixing of Water Vapour in the Upper Troposphere (PDF) (Thesis). University of Bremen. Archived from the original (PDF) on 2011-07-21. Retrieved 2010-12-16. <a href="https://web.archive.org/web/20110721214241/http://www.sat.ltu.se/members/pmils/Masters_Thesis_Peter_reduced.pdf" target="_blank">https://web.archive.org/web/20110721214241/http://www.sat.ltu.se/members/pmils/Masters_Thesis_Peter_reduced.pdf</a> <a href="#fnref:2" class="footnote-back-ref">↩</a></li>
<li id="fn:3">Peter Mills (2009). "Isoline retrieval: An optimal method for validation of advected contours" (PDF). Computers & Geosciences. 35 (11): 2020–2031. arXiv:1202.5659. Bibcode:2009CG.....35.2020M. doi:10.1016/j.cageo.2008.12.015. S2CID 1637949. <a href="http://peteysoft.users.sourceforge.net/Mills2009.pdf" target="_blank">http://peteysoft.users.sourceforge.net/Mills2009.pdf</a> <a href="#fnref:3" class="footnote-back-ref">↩</a></li>
<li id="fn:4">G. Spreen; L. Kaleschke; G. Heygster (2008). "Sea ice remote sensing using AMSR-E 89 GHz channels". Journal of Geophysical Research. 113 (C02S03): C02S03. Bibcode:2008JGRC..113.2S03S. doi:10.1029/2005JC003384. <a href="https://doi.org/10.1029%2F2005JC003384" target="_blank">https://doi.org/10.1029%2F2005JC003384</a> <a href="#fnref:4" class="footnote-back-ref">↩</a></li>
<li id="fn:5">N. Skou; S. S. Sobjaerg; J. Balling (2007). EMIRAD-2 and its use in the CoSMOS Campaigns (Technical report). Electromagnetic Systems Section Danish National Space Center, Technical University of Denmark. ESTEC Contract No. 18924/05/NL/FF. <a href="#fnref:5" class="footnote-back-ref">↩</a></li>
<li id="fn:6">G. Heygster; S. Hendricks; L. Kaleschke; N. Maass; P. Mills; D. Stammer; R. T. Tonboe; C. Haas (2009). L-Band Radiometry for Sea-Ice Applications (Technical report). Institute of Environmental Physics, University of Bremen. ESA/ESTEC Contract N. 21130/08/NL/EL. <a href="#fnref:6" class="footnote-back-ref">↩</a></li>
<li id="fn:7">N. Skou; S. S. Sobjaerg; J. Balling (2007). EMIRAD-2 and its use in the CoSMOS Campaigns (Technical report). Electromagnetic Systems Section Danish National Space Center, Technical University of Denmark. ESTEC Contract No. 18924/05/NL/FF. <a href="#fnref:7" class="footnote-back-ref">↩</a></li>
<li id="fn:8">G. Heygster; S. Hendricks; L. Kaleschke; N. Maass; P. Mills; D. Stammer; R. T. Tonboe; C. Haas (2009). L-Band Radiometry for Sea-Ice Applications (Technical report). Institute of Environmental Physics, University of Bremen. ESA/ESTEC Contract N. 21130/08/NL/EL. <a href="#fnref:8" class="footnote-back-ref">↩</a></li>
<li id="fn:9">Peter Mills; Georg Heygster (2010). "Sea ice emissivity modelling at L-band and application to Pol-Ice campaign field data" (PDF). IEEE Transactions on Geoscience and Remote Sensing. 49 (2): 612–627. Bibcode:2011ITGRS..49..612M. doi:10.1109/TGRS.2010.2060729. S2CID 20981849. <a href="http://peteysoft.users.sourceforge.net/smos_ieee.pdf" target="_blank">http://peteysoft.users.sourceforge.net/smos_ieee.pdf</a> <a href="#fnref:9" class="footnote-back-ref">↩</a></li>
</ol>

Collocation (remote sensing) open-in-new

Collocation (remote sensing)