`taneta = (sin @ToRadians( rho )*sin @ToRadians( lambda ))/(1-(sin @ToRadians( rho )*cos @ToRadians( lambda )))`

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Nadir Angle

is the direction pointing directly below a particular location;

that is, it is one of two vertical directions at a specified location, orthogonal to a horizontal flat surface there

where Lambda must be between 0 and 90 degree