# US 8,772,690 B2 · Multi-functional star tracker with precision focal plane position control for small cubesat-class satellites

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- Number: US 8,772,690 B2
- Document ID: US8772690B2
- Status: Granted
- Kind: B2
- Application number: 13/364,261
- Filed: 2012-02-01
- Published: 2013-08-01
- Granted: 2014-07-08
- Earliest claimed: 2012-02-01
- Assignees: The Charles Stark Draper Laboratory, Inc.; Massachusetts Institute of Technology
- Applicants: Matthew W. Smith; Christopher M. Pong; Sungyung Lim; David W. Miller; Sara Seager; Shawn Murphy
- Inventors: Matthew W. Smith; Christopher M. Pong; Sungyung Lim; David W. Miller; Sara Seager; Shawn Murphy
- Web page: https://patentgrep.com/patent/US8772690B2

## CPC

- G01J 1/0271: {Housings; Attachments or accessories for photometers}
- B64G 1/242: {Orbits and trajectories}
- B64G 1/244: {Spacecraft control systems}
- B64G 1/361: {using star sensors}
- G01C 21/025: {with the use of startrackers}
- G01C 21/24: specially adapted for cosmonautical navigation
- G01J 1/0266: {Field-of-view determination; Aiming or pointing of a photometer; Adjusting alignment; Encoding angular position; Size of the measurement area; Position tracking; Photodetection involving different fields of view for a single detector}
- G01J 1/0403: {Mechanical elements; Supports for optical elements; Scanning arrangements}
- G01J 1/0411: {using focussing or collimating elements, i.e. lenses or mirrors; Aberration correction}
- G01J: MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY (light sources F21, H01J, H01K, H05B; investigating properties of materials by optical means G01N)
- B64G: COSMONAUTICS; VEHICLES OR EQUIPMENT THEREFOR
- G01C: MEASURING DISTANCES, LEVELS OR BEARINGS; SURVEYING; NAVIGATION; GYROSCOPIC INSTRUMENTS; PHOTOGRAMMETRY OR VIDEOGRAMMETRY (measuring liquid level G01F; radio navigation, determining distance or velocity by use of propagation effects, e.g. Doppler effects, propagation time, of radio waves, analogous arrangements using other waves G01S)

## Abstract

An optical system for use in an Earth-orbiting satellite includes a plurality of image sensors disposed on a focal plane having a reference axis orthogonal thereto, optics configured to focus incident light onto the image sensors, a piezoelectric actuator coupled to the image sensors and configured to translate the image sensors in at least two axes each orthogonal to the reference axis, and at least one controller operably coupled to the plurality of image sensors and the piezoelectric actuator. The image sensors are configured to generate at least one image frame from light detected by the plurality of image sensors, the image frame including a target star and at least one guide star. The controller is configured to stabilize the position of the target star by adjusting the position of the piezoelectric actuator based on the changes in the position of the guide star.

## Claims

### Claim 1

An optical system for use in an Earth-orbiting satellite, the optical system comprising:
a plurality of image sensors disposed on a focal plane having a reference axis orthogonal thereto, the plurality of image sensors configured to generate at least one image frame from light detected by the plurality of image sensors, the at least one image frame including a target star and at least one guide star;
optics configured to focus incident light onto the plurality of image sensors;
a piezoelectric actuator coupled to the plurality of image sensors and configured to translate the plurality of image sensors in at least two axes each orthogonal to the reference axis, the piezoelectric actuator having a position with respect to the reference axis; and
at least one controller operably coupled to the plurality of image sensors and the piezoelectric actuator and configured to:
determine a position of the target star within the at least one image frame;
track changes in a position of the at least one guide star within the at least one image frame;
stabilize the position of the target star by adjusting the position of the piezoelectric actuator based on the changes in the position of the at least one guide star;
generate a pointing error of the satellite based on the changes in the position of the at least one guide star; and
stabilize an attitude of the satellite based at least on the pointing error.

### Claim 2 (depends on 1)

The optical system of claim 1, wherein at least one of the plurality of image sensors has a smaller pixel size than another one of the plurality of image sensors.

### Claim 3 (depends on 2)

The optical system of claim 2, wherein the plurality of image sensors includes a science optical sensor and at least one tracking sensor disposed adjacent to the science optical sensor, wherein the science optical sensor is configured to operate at a science optical sensor rate, and wherein the at least one tracking sensor is configured to operate at a tracking sensor rate that is not less than the science optical sensor rate.

### Claim 4 (depends on 3)

The optical system of claim 3, wherein the at least one tracking sensor is configured to generate the at least one image frame including the at least one guide star.

### Claim 5 (depends on 4)

The optical system of claim 4, wherein the science optical sensor is configured to generate the at least one image frame including the at least one target star.

### Claim 6 (depends on 5)

The optical system of claim 5, wherein the science optical sensor includes one of a charge coupled device (CCD) optical sensor and a complementary metal-oxide-semiconductor (CMOS) detector.

### Claim 7 (depends on 6)

The optical system of claim 6, wherein the at least one tracking sensor includes a CMOS detector.

### Claim 8 (depends on 1)

The optical system of claim 1, wherein the optics includes a single-lens reflex (SLR) camera lens.

### Claim 9 (depends on 1)

The optical system of claim 1, wherein the optics includes folded optics.

### Claim 10 (depends on 1)

The optical system of claim 1, wherein the optics includes a Cassegrain reflector.

### Claim 11 (depends on 1)

The optical system of claim 1, wherein the focal plane array, the optics, the piezoelectric actuator and the controller are configured to be mounted in a satellite having dimensions of approximately 10×10×34 cm3.

### Claim 12 (depends on 11)

The optical system of claim 11, wherein the mass of the satellite is approximately 5 kilograms or less.

### Claim 13

A method of operating an optical system in an Earth-orbiting satellite, the optical system including a plurality of optical sensors arranged on a focal plane and configured to generate an image, optics having a reference axis therethrough and configured and arranged to focus incident light onto the plurality of optical sensors, and a piezoelectric stage actuator coupled to the plurality of optical sensors and configured to translate the plurality of optical sensors in at least two axes each orthogonal to the reference axis, the method comprising:
detecting at least three guide stars and a target star within the image;
estimating an attitude of the satellite from positions of the at least three guide stars on the basis of a guide star database;
stabilizing, within a coarse threshold, the attitude of the satellite with respect to a desired attitude by commanding changes to the attitude of the satellite based on a difference between the estimated attitude and the desired attitude;
estimating an angular rate of the focal plane from rates of the positions of the at least three guide stars;
estimating a position error of the target star based on the angular rate of the focal plane; and
stabilizing, within a fine threshold that is smaller than the coarse threshold, a position of the target star with respect to one of the plurality of optical sensors by adjusting a position of the plurality of optical sensors with respect to the reference axis based on the estimated position error of the target star.

### Claim 14 (depends on 13)

The method of claim 13, wherein estimating the attitude of the satellite includes estimating an attitude quaternion of the satellite using a quaternion-based attitude estimation algorithm.

### Claim 15 (depends on 14)

The method of claim 14, further comprising refining the estimated attitude using an extended Kalman filter.

### Claim 16 (depends on 13)

The method of claim 13, further comprising calculating positions of the at least three guide stars based on the image and at least one of a position of the piezoelectric stage actuator and a position of the optics.

### Claim 17 (depends on 16)

The method of claim 16, further comprising continuously estimating an angular velocity of the focal plane based on a rate of change of the estimated positions of the at least three guide stars.

### Claim 18 (depends on 16)

The method of claim 16, further comprising continuously estimating a rate of change of the position of the target star based on the angular velocity of the focal plane.

### Claim 19 (depends on 18)

The method of claim 18, further comprising intermittently calculating the position of the target star based on the image and at least one of a position of the piezoelectric stage actuator and a position of the optics.

### Claim 20 (depends on 19)

The method of claim 19, wherein estimating the position of the target star includes continuously estimating the position of the target star based on the intermittently calculated position of the target star and a rate of change of the position of the target star.

### Claim 21 (depends on 20)

The method of claim 20, further comprising combining two intermittently calculated positions of the target star and the rate of change of the position of the target star using a Kalman filter.

### Claim 22 (depends on 21)

The method of claim 21, further comprising calculating the position error between the estimated position of the target star and a desired position of the target star.

### Claim 23 (depends on 13)

The method of claim 13, wherein the coarse threshold is approximately 120 arcseconds (3σ) or less, and wherein the fine threshold is approximately 5 arcseconds (3σ) or less.

### Claim 24

An optical system for use in an Earth-orbiting satellite, comprising:
a plurality of optical sensors;
optics having a reference axis therethrough and configured to focus incident light onto the plurality of optical sensors;
an actuator coupled to the plurality of optical sensors and configured to translate the plurality of optical sensors in at least two axes each orthogonal to the reference axis; and
means coupled to the plurality of optical sensors and the actuator for independently stabilizing, using images generated by the plurality of optical sensors, an attitude of the satellite and a position of the plurality of optical sensors with respect to a target star.