Determination of sunglint location and its characteristics on observation from a METEOSAT 9 satellite
R.H. Gardashova* and M.Sh. Eminovb aDepartment of Caspian Sea Problems, Institute of Geography, ANAS, Baku, Azerbaijan; bFaculty of Computer Science and Electrical Engineering, Kiel University of Applied Sciences, Kiel,
Germany (Received 6 May 2013; accepted 7 March 2015)
On the basis of observations from a geostationary satellite, a method for the determination of sunlight location on the ocean surface, based on two parameters – (1) the
Greenwich time and (2) the longitude of the satellite – was developed. The problem is solved in three stages: first, the position of the Earth in its orbit for any given point of time is determined; second, for this point of time, the relative position of Sun–Earth– satellite is defined; and, third, the latitude and longitude of the principal point of sunglint (PPS) was found. The outputs of the program based on this method are: (1) the geographical coordinates of PPS; (2) the boundary of a simultaneously illuminated and observed region of the Earth; (3) the contour of the sunglint (disk) on the smooth ocean surface; and (4) the distribution of the sunglint brightness on the rough (waved) ocean surface. This method is applied to detect sunglint characteristics in the images gathered from the METEOSAT 9 satellite. 1. Introduction
A geostationary orbit is a circular orbit in the equatorial plane of the Earth with a radius (r) of 42,164.2 km. A satellite placed in geostationary orbit has an angular velocity which is equal to the angular velocity of the Earth’s rotation and, therefore, it is fixed relative to the Earth. Approximately half of the Earth is observed from a geostationary satellite.
Because of these properties, geostationary satellites are especially suitable for communications, meteorology, and military aims. As there is only one geostationary orbit, the numbers of satellites in it are constantly growing and, at present, there are more than 400.
With the construction of devices with higher resolution, the range of tasks undertaken by geostationary satellites is also expanding.
In problems of remote sensing of oceans and seas, issues caused by sunglint always occur. In some instances, sunglint is a ‘nuisance’ (e.g. a study of the properties of the sea waters), whereas in others, it is a ‘useful signal’ (e.g. a study of the state of the surface).
Consequently, knowledge of the location of sunglint and its characteristics are required for developing methods of remote sensing of oceans and seas.
Under the known relative positions of the Sun, the Earth, and the geostationary satellite; a method for determining regions covered by sunglints and their brightness distribution within this region was developed by Gardashov and Barla (2001). A similar problem was considered by Prakash, Varma, and Bhandari (1994), where the geographical coordinates of sunglint were determined by solving a system of two non-linear equations *Corresponding author. Email: email@example.com
International Journal of Remote Sensing, 2015
Vol. 36, No. 10, 2584–2598, http://dx.doi.org/10.1080/01431161.2015.1042119 © 2015 Taylor & Francis by the Newton–Raphson method. The authors of this work note that the problem of convergence of the iterations arises and, therefore, special care is required. However, in the method offered by Gardashov and Barla (2001), due to appropriate selection of the reference plane, one non-linear equation with a unique solution is obtained and no problem of convergence arises. Emecen et al. (2006) had extended the method developed by Gardashov and Barla (2001) to the case: the definition of the characteristics of sunglint for any given time and coordinate (longitude) of geostationary satellite.
In the case of a perfectly smooth ocean surface, the sunglint seems to have an oval shape for an observer. Therefore, for accuracy it must be noted that under the term ‘sunglint coordinates’, we mean the coordinates of a ‘principal point of sunglint’ (PPS), which is the point of the smooth surface that reflects the beam coming from the sun disk centre to the observation point (sensor). When a wave appears, the whole sunglint (disk) is divided into small glitters that cover some parts of the ocean’s surface, which we call ‘the glint-covered area’. The stronger the wave, the greater is this area.
When the waves grow, facets with large slopes appear. Accordingly, the rays coming from the Sun are reflected from the distant (relative to the centre of the glint-covered area) facets to the point of observation. The size and shape of the glint-covered area, as well as the distribution of sunglint brightness in this area depend on the degree of waves, and the geometry of incidence and observation.
When the Earth is observed from a geostationary orbit, depending on the relative positions of the Sun, the Earth, and the satellite, some parts of the Earth’s surface are illuminated by direct sunbeams and are observed from the satellite. This part of the Earth’s surface is called the ‘simultaneously illuminated and observed region’. Obviously, the glint-covered area is located within this region.
In this paper, we present a method for determination of the geographical coordinates of sunglint and its characteristics. This method is applied to images taken by the
METEOSAT 9 geostationary satellite. For this, an algorithm and a FORTRAN-based program have been developed. The inputs of this program are: (1) the time t [in
Greenwich Mean Time (GMT)] and (2) the longitude geostationary satellite, φsat. The outputs of this program are: (1) the geographical coordinates of sunglint; (2) the boundary of the simultaneously illuminated and observed region; (3) the contour of the sunglint (disk) image on the smooth ocean surface; and (4) the distribution of the brightness of sunglints on the rough (wavy) ocean surface. 2. Geometry and basic formulae