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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Collocation (remote sensing) | 2/2 | https://en.wikipedia.org/wiki/Collocation_(remote_sensing) | reference | science, encyclopedia | 2026-05-05T09:53:37.734675+00:00 | kb-cron |
== Interpolation == For gridded data such as assimilation or reanalysis data, interpolation 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. Linear interpolation (bilinear, trilinear 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, radiative transfer, 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 Landsat. In data derived from the Advanced Microwave Sounding Unit (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 Advanced Microwave Scanning Radiometer-EOS (AMSR-E) instrument which has a circular scanning pattern, we need a more general form of interpolation such as kernel estimation. A method commonly used for this particular instrument, as well as SSM/I, is a simple daily average within regularly gridded, spatial bins .
== Trajectories == To collocate measurements of a medium- to long-lived atmospheric tracer with a second instrument, running trajectories 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.
== Example: Pol-Ice Campaign ==
Collocations of sea ice thickness and brightness temperatures 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 Soil Moisture and Ocean Salinity satellite.
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
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 laser altimeter to measure the height of the ice surface. 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 antenna response function for the radiometer. The full width at half maximum is 31 degrees. 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. 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.
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.
== References ==