Fundamental Technologies Voyager LECP Pages

Right Ascension and Declination Algorithm

The PI may wish to compute S/C centered right ascension and declination angles of his boresight or optic axis relative to the Earth Mean Equator and Equinox of 1950.0 (EME50). To obtain these angles the following two step algorithm is offered.

Step 1. Rotate ECL50 Unit Vector to EME50 Unit Vector.

The instrument boresight or optic axis unit vector is available from the SEDR relative to the Earth Mean Ecliptic and Equinox of 1950.0 (ECL50). This unit vector must be rotated through the mean obliquity of the ecliptic (angle between the ecliptic and equatorial planes) at 1950.0 to obtain the EME50 unit vector. The following transformation matrix will accomplish the required rotation.

Step 2. Compute the Right Ascension and Declination Angles.

Once the unit vector has been transformed to EME50 coordinates, the right ascension and declination angles can be computed by using the following equations:

a = Tan^-1 (y_EME50/x_EME50)

d = Sin^-1 (z_EME50)

Where a is the right ascension angle, d is the declination angle and x_EME50, y_EME50 and z_EME50 are the x, y and z components of the EME50 unit vector.

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Last modified 12/6/02, Tizby Hunt-Ward
tizby@ftecs.com