Star Tracker

• US 20160041265A1
• Filed: 08/06/2015
• Published: 02/11/2016
• Est. Priority Date: 08/06/2014
• Status: Active Grant

• Current Assignee: Charles Stark Draper Laboratory Incorporated
• Original Assignee: Charles Stark Draper Laboratory Incorporated
• Inventors: Waldron, Erik L., Laine, Juha-Pekka J., Blasche, Gregory P., Chaparala, Murali V., Dawson, Robin Mark Adrian, Lane, Benjamin F., Smith, Stephen P.
• Granted Patent: US 9,927,510 B2
• Time in Patent Office: Days
• US Class Current: 1/1
• CPC Class Codes: G01C 3/085 with electronic parallax measurement
• CPC Class Codes: G01S 3/7867 Star trackers navigation using star trackers G01C21/025

Abstract

A star tracker determines a location or orientation of an object, such as a space vehicle, by observing unpolarized light from one or more stars or other relatively bright navigational marks, without imaging optics, pixelated imaging sensors or associated pixel readout electronics. An angle of incidence of the light is determined by comparing signals from two or more differently polarized optical sensors. The star tracker may be fabricated on a thin substrate. Some embodiments have vertical profiles of essentially just their optical sensors. Some embodiments include micro-baffles to limit field of view of the optical sensors.

Claims

1. A method for determining a direction to a source of unpolarized electromagnetic radiation, the method comprising:

exposing a first sensor to the unpolarized electromagnetic radiation, the first sensor being primarily sensitive to electromagnetic radiation polarized along a first axis and configured to generate a first signal proportional to a magnitude of the unpolarized electromagnetic radiation detected by the first sensor;

exposing a second sensor to the unpolarized electromagnetic radiation, the second sensor being primarily sensitive to electromagnetic radiation polarized along a second axis, different than the first axis, and configured to generate a second signal proportional to a magnitude of the unpolarized electromagnetic radiation detected by the second sensor;

determining a ratio of the first signal to the second signal; and

using the ratio to calculate an angle of incidence of the unpolarized electromagnetic radiation.

5. A method according to claim 1, wherein:

exposing the first sensor to the unpolarized electromagnetic radiation comprises exposing a first array of nanoantennas to the unpolarized electromagnetic radiation, wherein all nanoantennas of the first array of nanoantennas are similarly oriented so as to be primarily sensitive to electromagnetic radiation polarized along the first axis, and all the nanoantennas of the first array of nanoantennas are electrically coupled together to generate the first signal; and

exposing the second sensor to the unpolarized electromagnetic radiation comprises exposing a second array of nanoantennas to the unpolarized electromagnetic radiation, wherein all nanoantennas of the second array of nanoantennas are similarly oriented so as to be primarily sensitive to electromagnetic radiation polarized along the second axis, and all the nanoantennas of the second array of nanoantennas are electrically coupled together to generate the second signal.

6. A method according to claim 1, further comprising:

storing a value representing the second signal;

tilting the second sensor;

exposing the tilted second sensor to the unpolarized electromagnetic radiation; and

generating a third signal proportional to a magnitude of the unpolarized electromagnetic radiation detected by the tilted second sensor;

wherein;

determining the ratio of the first signal to the second signal and using the ratio to calculate the angle of incidence of the unpolarized electromagnetic radiation comprises using the first signal, the stored value representing the second signal and the third signal to calculate the angle of incidence of the unpolarized electromagnetic radiation.

7. A method according to claim 1, further comprising compensating for a degree to which the unpolarized electromagnetic radiation is polarized.

8. A method according to claim 1, wherein:

exposing the first sensor comprises providing a sensor comprising a plurality of first elements, each element of the plurality of first elements being configured to generate a signal proportional to electromagnetic radiation incident upon the element;

exposing the second sensor comprises providing a sensor comprising a plurality of second elements, each element of the plurality of second elements being configured to generate a signal proportional to electromagnetic radiation incident upon the element;

the method further comprising;

automatically adjusting sensitivity of the plurality of first elements, such that each element of the plurality of first elements is preferentially sensitive to incident electromagnetic radiation having a first polarization; and

automatically adjusting sensitivity of the plurality of second elements, such that each element of the plurality of second elements is preferentially sensitive to incident electromagnetic radiation having a second polarization, different than the first polarization.

9. A method according to claim 8, wherein automatically adjusting the sensitivity of the plurality of first elements and the plurality of second elements comprises automatically adjusting the sensitivity of the plurality of first elements and automatically adjusting the sensitivity of the plurality of second elements, such that the first polarization is at least approximately perpendicular to the second polarization.

2. A method according to claim 2, wherein exposing the first and second sensors to the unpolarized electromagnetic radiation comprises exposing the first and second sensors to unpolarized electromagnetic radiation from a star.

3. A method according to claim 3, further comprising:

automatically accessing a database that stores a star catalog; and

automatically calculating a location in space based at least in part on the angle of incidence and information in the star catalog.

4. A method according to claim 3, further comprising:

automatically accessing a database that stores a star catalog; and

automatically calculating a direction to a star and identity of the star, based at least in part on the angle of incidence and information in the star catalog.

10. A star tracker for determining a direction to a source of unpolarized electromagnetic radiation, the star tracker comprising:

a plurality of electromagnetic radiation sensors, wherein each sensor of the plurality of electromagnetic radiation sensors is primarily sensitive to electromagnetic radiation polarized along a different axis and generates a proportional signal in proportion to a magnitude of unpolarized electromagnetic radiation detected by the sensor;

a ratio detector that receives the proportional signals generated by the plurality of electromagnetic radiation sensors and generates a ratio signal that represents a ratio of at least two of the proportional signals; and

an angle of incidence calculator that receives the ratio signal and generates therefrom a signal representing an angle of incidence of the unpolarized electromagnetic radiation.

11. A star tracker according to claim 10, further comprising:

a database that stores a star catalog; and

a navigation calculator that receives the angle of incidence signal, accesses the star catalog and calculates an orientation or location in space based at least in part on the angle of incidence signal and information in the star catalog.

12. A star tracker according to claim 11, wherein:

the star catalog includes information about a degree of polarization of electromagnetic radiation from at least one star; and

at least one of the ratio detector and the incidence angle calculator uses the information about the degree of polarization of the electromagnetic radiation from the at least one star to compensate at least one of the ratio signal and the signal representing an angle of incidence.

13. A star tracker according to claim 10, wherein each sensor of the plurality of electromagnetic radiation sensors comprises an array of similarly oriented nanoantennas, all oriented so as to be primarily sensitive to electromagnetic radiation polarized along the axis of the sensor and all electrically coupled together to generate the proportional signal for the sensor.

14. A star tracker according to claim 10, wherein:

at least one sensor of the plurality of electromagnetic radiation sensors is tiltable;

the star tracker further comprising;

a motor mechanically coupled to the at least one tiltable sensor, so as to tilt the at least one sensor from a first plane to a second plane; and

wherein;

the at least one tiltable sensor generates at least a first portion of the proportional signal, based on a magnitude of unpolarized electromagnetic radiation detected by the at least one tiltable sensor while the at least one tiltable sensor is in the first plane, and the at least one tiltable sensor generates a second portion of the proportional signal, based on a magnitude of unpolarized electromagnetic radiation detected by the at least one tiltable sensor while the at least one tiltable sensor is in the second plane; and

the ratio detector generates at least a first portion of the ratio signal based on the first portion of the proportional signal, and the ratio detector generates at least a second portion of the ratio signal based on the second portion of the proportional signal.

15. A computer program product for determining a direction to a source of unpolarized electromagnetic radiation, the computer program product comprising a non-transitory computer-readable medium having computer readable program code stored thereon, the computer readable program code, when executed by a processor, causes the processor to:

receive a first signal from a first sensor exposed to the unpolarized electromagnetic radiation, the first sensor being primarily sensitive to electromagnetic radiation polarized along a first axis and configured to generate the first signal proportional to a magnitude of the unpolarized electromagnetic radiation detected by the first sensor;

receive a second signal from a second sensor exposed to the unpolarized electromagnetic radiation, the second sensor being primarily sensitive to electromagnetic radiation polarized along a second axis, different than the first axis, and configured to generate the second signal proportional to a magnitude of the unpolarized electromagnetic radiation detected by the second sensor;

determine a ratio of the first signal to the second signal; and

use the ratio to calculate an angle of incidence of the unpolarized electromagnetic radiation.

16. A computer program product according to claim 15, wherein the computer readable program code further causes a processor to:

automatically access a database that stores a star catalog; and

automatically calculate a location in space based at least in part on the angle of incidence and information in the star catalog.

17. A computer program product according to claim 15, wherein:

the first sensor is exposed to the unpolarized electromagnetic radiation by exposing a first array of nanoantennas to the unpolarized electromagnetic radiation, wherein all nanoantennas of the first array of nanoantennas are similarly oriented so as to be primarily sensitive to electromagnetic radiation polarized along the first axis, and all the nanoantennas of the first array of nanoantennas are electrically coupled together to generate the first signal; and

the second sensor is exposed to the unpolarized electromagnetic radiation by exposing a second array of nanoantennas to the unpolarized electromagnetic radiation, wherein all nanoantennas of the second array of nanoantennas are similarly oriented so as to be primarily sensitive to electromagnetic radiation polarized along the second axis, and all the nano antennas of the second array of nanoantennas are electrically coupled together to generate the second signal.

18. A computer program product according to claim 15, wherein the computer readable program code further causes a processor to:

store a value representing the second signal;

tilt the second sensor;

expose the tilted second sensor to the unpolarized electromagnetic radiation; and

generate a third signal proportional to a magnitude of the unpolarized electromagnetic radiation detected by the tilted second sensor;

wherein;

determine the ratio of the first signal to the second signal and use the ratio to calculate the angle of incidence of the unpolarized electromagnetic radiation comprises using the first signal, the stored value representing the second signal and the third signal to calculate the angle of incidence of the unpolarized electromagnetic radiation.

19. A computer program product according to claim 15, wherein the computer readable program code further causes a processor to compensate for a degree to which the unpolarized electromagnetic radiation is polarized.

20. A star tracker for determining a direction to a source of unpolarized electromagnetic radiation, the star tracker comprising:

a first electromagnetic radiation sensor configured to generate a first signal proportional to a first polarization component of unpolarized electromagnetic radiation impinging on the first electromagnetic radiation sensor;

a second electromagnetic radiation sensor configured to generate a second signal proportional to a second polarization component of unpolarized electromagnetic radiation impinging on the second electromagnetic radiation sensor, the second polarization component being oriented differently than the first polarization component;

a ratio detector that receives the first and second signals and generates a ratio signal that represents a ratio of the first signal to the second signal; and

an angle of incidence calculator that receives the ratio signal and generates therefrom a signal representing an angle of incidence of the unpolarized electromagnetic radiation.

21. A star tracker, comprising:

a plurality of electromagnetic radiation sensors, each sensor of the plurality of electromagnetic radiation sensors configured to generate a respective signal in proportion to magnitude of a different polarization component of unpolarized electromagnetic radiation impinging on the sensor; and

a ratio detector configured to receive the signals generated by the plurality of electromagnetic radiation sensors and calculate an angle of incidence of the unpolarized electromagnetic radiation.