A conventional Sun sensor determines the orientation of the Sun in two axes. If mounted on Earth in a known coordinate system, it can determine the azimuth and elevation of the Sun. When mounted on a spacecraft, it can determine the pointing direction toward the Sun. Traditionally, Sun sensors have been based on optical/detector configurations that would change the magnitude of an electrical signal based on the angular incidence of the Sun (analog Sun sensor). Other types of Sun sensors have relied on optics and a geometric pattern placed over the detectors that generated an on/off (digital) signal depending on the solar angle (digital Sun sensor).
Existing Sun sensors only determine the pointing direction towards the Sun. They do not determine clocking orientation around the Sun. In other words, they only provide two angles, while three angles are required for complete spacecraft attitude. The proposed three-axis Sun sensor can determine all three angles of attitude information simply by observing the Sun.
The proposed Sun sensor determines the orientation to the Sun similar to a conventional two-axis Sun sensor (imaging the Sun and calculating the centroid). However, it also measures the solar rotation axis, which provides the third axis of attitude information. Since the Sun is bright, the proposed Sun sensor has the ability to operate at large slew rates (short exposure times) compared to a conventional star scanner/tracker that images faint stars.
The Sun is rotating with a rotation period of approximately 25 days. This causes the solar photosphere close to the Sun’s equator to move at a speed of ≈2 km/second. At one limb it will move toward the observer, and at the other limb it will move away from the observer. This causes a Doppler shift of the solar spectrum and the absorption lines in the solar spectrum (of ≈5 pm at 770 nm). The proposed Sun sensor operates by imaging the Sun in a narrow band centered on a potassium absorption line at ≈770 nm (it is also possible to utilize other solar lines like the sodium absorption line at 589 nm). By utilizing the Zeeman effect, it is possible to measure very small shifts in the wavelength.
The filter section of the Sun sensor is comprised of two crossed polarizers (P1 & P2) surrounding a glass cell (Cell 1) containing potassium vapor. A longitudinal magnetic field is imposed on the cell by magnets placed around it. The magnetic field results in the Zeeman splitting of the absorption lines of the potassium vapor. In the wings of these Zeeman split lines, the linearly polarized light entering the cell has its plane of polarization rotated by magneto-optical effects (principally circular birefringence), allowing light to pass through the second polarizer. Two transmission bands are produced, one on either side of the potassium resonance line, at 770 nm. The wing selector is composed of a second vapor cell (Cell 2), also in a strong longitudinal magnetic field, followed by a quarter wave plate, a narrow band filter (Φ1) that rejects out-of-band light that leaks through the polarizers, and a Wollaston prism (W1).
The magnetic field strength in the second cell is selected to produce Zeemansplit absorption lines that coincide with the transmission passbands from the filter section. Partial absorption via the inverse Zeeman effect causes the two transmission bands to become circularly polarized in opposing senses, as the longitudinal magnetic field orientation only allows circularly polarized light to be absorbed. The quarter wave plate switches the circularly polarized light back to linearly polarized light, with each passband now having orthogonal linear polarization. Finally, the Wollaston prism separates the two linearly polarized passbands into two separate beams, which are imaged on the CCD (charge coupled device).