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1. Find Collocations

A set of observations from a pair of instruments within a common period (e.g. 1 day) is required as input to the algorithm. The first step is to obtain these data from both instruments, select the relevant comparable portions and identify the pixels that are spatially collocated, temporally concurrent, geometrically aligned and spectrally compatible and calculate the mean and variance of these radiances.

Figure2: Step 1 of Generic Data Flow, showing inputs and outputs

1.1. Select Orbit

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1.1.1. Purpose

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We first perform a rough cut to reduce the data volume and only include relevant portions of the dataset (channels, area, time, viewing geometry). The purpose is to select portions of data collected by the two instruments that are likely to produce collocations. This is desirable because typically less then 0.1% of measurements are collocated. The processing time is reduced substantially by excluding measurements unlikely to produce collocations.

Data is selected on a per-orbit or per-image basis. To do this, we need to know how often to do inter-calibration – which is based on the observed rate of change and must be defined iteratively with the results of the inter-calibration process (see 4.f).

1.1.2. General Options

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The simplest, but inefficient approach is “trial-and-error”, i.e., compare the time and location of all pairs of files within a given time window.

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Class IReeGL

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For inter-calibrations between geostationary and sun-synchronous satellites, the orbits provide collocations near the GEO Sub-Satellite Point (SSP) within fixed time windows every day and night. In this case, we adopt the simple approach outlined in general option v0.1.

We define the GEO Field of Regard (FoR) as an area close to the GEO Sub-Satellite Point (SSP), which is viewed by the GEO sensor with a zenith angle less than a threshold. Wu [2009] defined a threshold angular distance from nadir of less than 60^\circ based on geometric considerations, which is the maximum incidence angle of most LEO sounders. This corresponds to \approx \pm 52^\circ in latitude and longitude from the GEO SSP. The GEO and LEO data is then subset to only include observations within this FoR within each inter-calibration period.

Mathematically, the GEO FoR is the collection of locations whose arc angle (angular distance) to nadir is less than a threshold or, equivalently, the cosine of this angle is larger than min_cos_arc. We chose the threshold min_cos_arc = 0.5, i.e., angular distance less than 60 degree.

Computationally, with known Earth coordinates of GEO nadir G (0, geo_nad_lon) and granule centre P (gra_ctr_lat, gra_ctr_lon) and approximating the Earth as being spherical, the arc angle between a LEO pixel and LEO nadir can be computed with cosine theorem for a right angle on a sphere (see Figure 3):

Equation 1: \cos (GP) = \cos (gra\_ctr\_lat) \cos (geo\_nad\_lon - gra\_ctr\_lon)

If the LEO pixel is outside of GEO FoR, no collocation is considered possible. Note the arc angle GP on the left panel of Figure 3, which is the same as the angle \angle GOP on the right panel, is smaller than the angle \angle SPZ (right panel), the zenith angle of GEO from the pixel. This means that the instrument zenith angle is always less than 60 degrees for all collocations.

Figure 3: Computing arc angle to satellite nadir and zenith angle of satellite from Earth location

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Class IReeGL  Instruments SeviriIasi

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